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This handbook is intended to establish a ‘platform’ of practicable knowledge (or
methodology) for machining of P/M materials. In particular, the chapters on specific
machining techniques provide cutting parameters for materials, tools and methods. Also
shown is the influence of alternative approaches such as with additives, oil impregnation,
and selection of tool, which aim to improve cutting operations. Thus this handbook
should serve as a practical guide and handy reference for those working with machining
of P/M materials.

Chapter three was written in cooperation with Sandvik Coromant and the Swedish
Institute of Metal Research. Chapters four and five were written in cooperation with
Dormer Tools and IVF. All tests were sintered in a production furnace at GKN Sinter
Metals AB, Sweden.

Höganäs AB
Table of contents

     1. Machinability . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
         1.1. Application of powder metallurgy . . . . . . . . . . . . . . . . . 6
         1.2. History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
         1.3. Metal Powder Production . . . . . . . . . . . . . . . . . . . . . . . 7

     2. Metal alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
         2.1. Alloying methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
         2.2. The powder metallurgy process . . . . . . . . . . . . . . . . . 17
         2.3. Materials development . . . . . . . . . . . . . . . . . . . . . . . . 19
         2.4. Sintered Iron-based materials . . . . . . . . . . . . . . . . . . . 20
         2.5. Alloying system: Microstructures and
              mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . 23

     3. Turning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
         3.1. Analysis of the Machining Process . . . . . . . . . . . . . . . . 39
         3.2. Tool Wear and Tool Life . . . . . . . . . . . . . . . . . . . . . . . 44
         3.3. Classification of P/M Materials for Turning . . . . . . . . . . 51
         3.4. Influence of material, properties and
              machining processes. . . . . . . . . . . . . . . . . . . . . . . . . 55
         3.5. Surface Finish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
         3.6. Cutting Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
         3.7. Summary: Machining of P/M Materials . . . . . . . . . . . . . 82
         3.8. Turning tool recommendations and cutting data . . . . . . 84

     4. Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
         4.1. Quality and Performance in Machining . . . . . . . . . . . . . 87
         4.2. Increasing machinability . . . . . . . . . . . . . . . . . . . . . . . 88
         4.3. Classification of P/M Materials for Drilling . . . . . . . . . . 88
         4.4. Influence of Properties of P/M Materials . . . . . . . . . . . 91
         4.5. Tool Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
         4.6. Tool Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
         4.7. Selection of Drill Type . . . . . . . . . . . . . . . . . . . . . . . 102
         4.8. Drill Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
         4.9. Use of Cutting Fluids . . . . . . . . . . . . . . . . . . . . . . . . 107
         4.10. Hints for Optimal Drilling . . . . . . . . . . . . . . . . . . . . . 109
         4.11. Economy and Productivity in Drilling . . . . . . . . . . . . 111
         4.12. Setting Machine Limits . . . . . . . . . . . . . . . . . . . . . . 112

       4.13. Formulae for Cutting Forces . . . . . . . . . . . . . . . . . . 113
       4.14. Drill Recommendations and Cutting Data . . . . . . . . . 115

    5. Tapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
       5.1. Classification of P/M Materials for Tapping . . . . . . . . 119
       5.2. Influence of Additives . . . . . . . . . . . . . . . . . . . . . . . . 121
       5.3. Selection of taps . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
       5.4. Hints on Optimal Tapping . . . . . . . . . . . . . . . . . . . . . 127
       5.5. Tapping Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . 128
       5.6. Tap and Cutting Data Recommendations . . . . . . . . . . 129

    6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 135

1     Machinability

Powder metallurgy (P/M) is known for producing complex parts to very close tolerances
without the necessity of machining operations, yet machinability is still important for
some applications. Many components require surface-finish machining to reach final
shape due to particular geometries (such as holes perpendicular to the pressing direction,
bevels, slots and threads), and also due to demands for even finer tolerances.
    What then is machinability? One useful definition is: “The material with the best
machinability is the one permitting the fastest metal removal rate with reliable and
satisfactory tool life and surface finish.” Thus machinability focuses on efficiency with
finishing processes for metal products.
    Investigation of the P/M market reveals that about 60% of all components need
some kind of machining operation. Turning is by far the most frequently occurring
operation, but tapping, drilling, and discontinuous turning, are considered the most
difficult. Machining operations can account for up to 20% of the total production cost
of a component.
    The machinability of a P/M component is dependent on the work-piece and tool
material properties, cutting conditions, and machine and cutting tool parameters.
Chemical composition, porosity, free machining additives, and production process
parameters such as compaction and sintering methods, also collectively influence
    Optimization of machinability is limited by the mechanical properties of a given
component. The selection of a material grade for a component is mainly based on its
required mechanical properties. Consequently, there are limitations on chemical
composition adjustment for machinability. The addition of free machining additives,
and oil impregnation, remain as alternatives. New technologies, such as warm
compaction, can increase machinability due to an increase in density.
    Since developments with more highly alloyed materials have increased the
mechanical strength of components, P/M technology requires guidelines for the
selection of tool material and cutting conditions. If certain guidelines are followed,
production costs are reduced and P/M is able to compete strongly with other
manufacturing processes.


1.1 Application of powder metallurgy

As a metal-working technology, powder metallurgy (P/M) has the advantage compared
to other processes, that it can produce complex parts of high quality with close
tolerances, in an economical way. This is due to low energy consumption, high material
utilization and low capital cost for the technology. Additional advantages are its high
flexibility and particular mechanical properties, related to microstructure, and the
possibility for development of new materials. Accordingly, the market share for P/M has
been increasing rather rapidly.

1.2 History

Industrial production of iron powder began in 1937 on the incentive of the General
Motors Corporation in the USA. In Europe, Höganäs was active from 1922 in
producing high-quality sponge iron for the Swedish steel industry. This product was
used as high-purity melting stock for the production of special steels, such as tool steels
and stainless steel. Sponge iron, owing to its high porosity, could easily be comminuted
(reduced) to iron powder. However, due to its high content of reducible oxygen (2%)
and carbon (0.15%), the compressibility for this material was poor. After 1940, Höganäs
introduced an annealing procedure, with which the residual oxygen and carbon content
were considerably lowered.
    During World War II, iron metallurgy was dramatically developed in Germany.
Mainly due to a shortage of copper, artillery driving bands were produced from iron
powder, and 30,000 metric tons were produced in 1944. The iron material for this
purpose was mainly made by grinding wire cuttings and sheet clippings in hammer mills
of the type ‘Hametag’. Wartime innovations meant important iron powder processes
were developed. For example: gas atomization of a desulphurized cast iron melt, later
converted to the now commonly used water atomization process, and the electrolytic
process for production of iron, became practicable. Today water atomization is the
dominating process because it produces high compressibility in conjunction with high

                                                                Metal Powder Production

1.3 Production - Introduction

Iron and steel powders for the manufacturing of sintered structural components
(including sintered porous bushings) are produced in many parts of the world. The
worldwide consumption of such powders has been growing increasingly fast over the last
three decades and reached 770 000 metric tons by the end of 2002.

Over the last thirty years, the quality of iron and steel powders has been continuously
improved and the spectrum of available grades has been widened. During the same
period, compacting- and sintering-techniques have become more and more
sophisticated. This development has lead to a substantially widened range of applications
for sintered iron and steel parts.

From Table 1, it can be seen that in 1965 iron powders were used almost exclusively for
low- and medium-density applications, i.e. for parts having pressed densities from 5.5 to
7.0g/cm3 . First after about 1970, increasing quantities of iron powders were used for
high-density applications, i.e. for parts having pressed densities higher than 7.0g/cm3.
Between 1975 and 1985, low-alloyed iron powders appeared on the market and have
since been used in growing quantities for medium- and high-density applications, i.e. for
parts having densities higher than 6.7g/cm3.

This development of high density products has continued after 1995. It must be
mentioned that, at present (2004), the worldwide production capacity of iron and steel
powders is considerably larger than the consumption. Thus, there is no risk of shortage
for many years to come.


    At present there are two basically different production methods which together
account for more than 90% of the world production of iron and steel powders, viz. the
Höganäs sponge-iron process and the water-atomizing process. The former process is based
on reduction of iron ore, yielding a highly porous sponge-iron which subsequently is
comminuted to powder. The latter process is based on atomization of a stream of liquid
iron (or steel) by means of a jet of pressurized water. Both processes will be described in
detail further below.
    In the manufacturing of sintered parts, iron powders are always used admixed with a
small amount of lubricant in powder form in order to minimize the friction in the
compacting tool. In many cases, they are also blended with alloying elements in powder
form, like graphite, copper, nickel, molybdenum and others (in order to achieve
increased strength properties).
    Since powder blends tend to segregate when transported and handled, Höganäs AB
has developed special blending processes in which the alloying additives are safely bound
to the iron powder particles. Powdermixes produced according to these processes are
known as the trade names Distaloy™ and Starmix. These two processes are treated in
detail further below.

                                                          Metal Powder Production

1.   Coke breeze/Limestone mix.              10. Coarse crushing.
2.   Magnetite (Fe3O4).                      11. Silo storage.
3.   Drying ovens.                           12. Separation.
4.   Crushing.                               13. Fine crushing
5.   Screening.                              14. Grinding and screening.
6.   Magnetic separation.                    15. Belt furnace.
7.   Ceramic Tubes.                          16. Homogenizing silo.
8.   Tunnel Kilns,                           17. Automatic packing.
9.   Tube discharge.                         18, 19. Magnetite, reduction mix.

Figure 1. The Höganäs sponge iron process.


The sponge iron process starts with two raw materials: (1) a reduction mix consisting of
coke breeze blended with ground limestone, and (2) a pre-processed fine magnetite. The
magnetite and the reduction mix are dried separately in two rotary ovens (3). The
slightly agglomerated dried reduction mix is crushed (4) and screened (5), and the dried
magnetite is passed through a magnetic separator (6).
    Both materials are then charged, with an automatic charging device, into tube-like
ceramic retorts, as illustrated (7), (18), (19). These retorts consist of four tube segments
of silicon carbide stacked on top of each other. The retorts stand on rail-bound cars. The
cars travel slowly through a tunnel kiln, approximately 260m long (8), within which the
retorts are gradually heated to a maximum temperature close to 1200°C. As the
temperature inside the retorts increases, the coke breeze begins to burn, forming CO,
which in turn reduces the magnetite to metallic iron, while itself oxidizing to CO2.
    The generated CO2 reacts with the remaining coke breeze forming new CO, which
again reduces more magnetite to metallic iron. This reaction cycle continues until all
magnetite has been reduced to metallic iron and the major part of the coke breeze is
burned off. Parallel to this reduction cycle, the limestone in the reduction mix binds the
sulfur arising from the burning coke breeze.
    After reduction is completed, the retorts are slowly cooled to approximately 250°C
before leaving the kiln. Inside each retort there is now a tube-like sponge iron cake with
a porosity of about 75%, a residue of unburned coke breeze, and a sulfur-rich ash. At an
automatic discharging station (9), the sponge iron tubes are removed for cleaning and
the remaining coke breeze and ash is exhausted from the retorts. Thereafter, the retorts
are ready to be recharged for a new trip through the kiln.
    The sponge iron tubes are, in several steps, crushed and comminuted to a particle
size below 3mm (10). The thus-obtained crude powder is then put into temporary silage
to await further processing. From the intermediate silo (11), the crude powder is passed
through a specially designed chain of magnetic separators (12), mills (13) and screens
(14), in order to be refined to a particle size below 150µm and a well-defined bulk
    Subsequently, the powder is passing a belt furnace (15) where it is soft-annealed at
800-1000°C in based atmosphere, and the remaining content of carbon and oxygen is
reduced to a minimal level. During annealing, the powder agglomerates to a very
crumbly cake which is gently comminuted again in a special mill. The so-treated powder
has good compressibility and high green strength. Powder from several belt furnaces is
collected in a special silo (16), where it is homogenized in lots of 60 or 120 tons. Each
lot is carefully checked with respect to specified properties, and packaged and stored,
ready for shipment (17).

                                                                      Metal Powder Production

Figure 2. Two views of sponge iron powder NC100.24 as produced by Höganäs: external particle
shape (SEM view) and cross section.

Höganäs Water-Atomising Process
Raw materials for the Höganäs water-atomizing process are carefully selected iron scrap.
The water-atomizing process is illustrated below in figure 3.
    Scrap (1) is melted down in an electric arc furnace of 50 tons capacity (2) where, if
desired, alloying elements can be added. The melt is teemed, slag-free, through a bottom
hole into a ladle (3) where it is refined (4). The ladle is then transferred to the atomizing
station (5) and the liquid iron (or steel) is again teemed, slag-free, through a bottom hole
in the ladle into a specially designed tundish (A).
    From the tundish, the liquid iron (or steel) flows in (B) through the center of a ring-
shaped nozzle (D), where it is hit by jets of highly pressurized water (C). The stream of
liquid iron (or steel) explodes into fine droplets (E).


Figure 3. The Höganäs water-atomizing process.

                                                                    Metal Powder Production

Air, swept along by the water jet and water vapor arising in the atomizing process, causes
superficial oxidation of the small metal droplets. The solidified droplets and the
atomizing water are collected in a container, where they settle as a mud. This mud
powder is de-watered (6) and dried (7). The dried powder is magnetically separated from
slag particles (8), screened (9), homogenized (10), and eventually transported in special
containers (11) for further processing.
    On leaving the atomizing plant, the metal particles are not only superficially
oxidized, but also very hard. Due to the extremely high cooling rates residing in the
atomizing process, the particles solidify in a martensitic state - despite their low carbon
content. The particles are therefore soft annealed, and their surface oxides and residual
carbon are reduced in a belt furnace, as occurs in the sponge iron process.

                 SEM                               Cross Section

Figure 4. Two views of water-atomized iron powder ASC100.29 as produced by Höganäs: external
partical shape (SEM view) and cross section.


                                                                          Alloying methods

2     Metal alloys

Alloyed materials often have markedly different physical properties from those of pure
metals. In particular, alloys can significantly increase hardness, and mechanical strength
has obvious benefits for both component applications and machining possibilities.

2.1 Alloying methods

In order to achieve hardenable sintered ferrous materials, carbon and other suitable
alloying elements (e.g. copper, nickel, molybdenum) must be introduced. While carbon
is normally admixed to the iron powder in the form of graphite, metallic alloying
elements are commonly introduced by either of the following methods:
1. Admixing of the alloying element to the iron powder.
2. Diffusion bonding of the alloying elements to the iron powder.
3. Coating of the alloying element to the iron powder.
4. Manufacture of fully pre-alloyed powder, e.g. atomisation.

The methods most frequently used today are 1 and 2, mainly because they do not
deteriorate compressibility, unlike method 4, which has mostly been used in powder
forging applications. Method 3 is seldom applied in ferrous P/M, probably because the
cost of the coating operation is too high.
    Method 1 represents the easiest and most flexible way of alloying in P/M. Mixes of
this kind, however, consist of particles which often differ considerably in size, shape and
density. This means that it can be difficult to prepare a mix with uniform distribution,
and to avoid segregation and dusting during handling. During a production run this
might lead to variations in the chemical analysis from sample to sample. These variations
will influence the tolerances of a sintered part in a negative way.

Pre-alloyed powders
The partially pre-alloyed technique (method 2), was developed by Höganäs during the
1960s. Diffusion bonding of the alloying elements reduces the risk of segregation
without decreasing the compressibility of the material. Powders manufactured by this
technique are known as Distaloys to all producers of P/M metal parts, and have proved
to give substantially improved tolerances of the sintered properties. The alloying powder
particles are bonded to the surface of iron powder particles during heat treatment in a
reducing atmosphere. Improved homogeneity results in closer tolerances compared to a
plain mix.

Metal alloys

Graphite, one of the most commonly used alloying elements, is very sensitive to
handling due to its tendency to dusting. It has a considerable influence on dimensional
change and the other mechanical properties achieved during sintering, and thereby also,
the tolerances of the sintered parts. In order to improve tolerances, Höganäs has
developed a technique called STARMIX, in which graphite and other fine particles are
bonded to iron powder during blending.

Summary of advantages and disadvantages of alloy powder mixes:

Powder mixes, methods 1-3.
•    Have higher compressibility.
•    No additional mixing operation is required as the powder has to be admixed with a
     lubricant anyway.
•    The composition of a powder mix can easily be changed or corrected by re-mixing it
     with additional amounts of either iron powder or alloying elements.
•    Yield less homogeneity of alloyed sintered parts, because the admixed alloying
     elements (except carbon) diffuse very slowly in the solid iron.
•    Alloying elements tend to segregate when the powder mix is transported and

Homogeneously alloyed powders, method 4.
•    Alloying elements do not segregate when the powder is handled.
•    Yield fully homogeneously alloyed sintered parts.
•    Have low compressibility, because their particles are solution-hardened.
•    In order to change or correct the composition of a fully alloyed powder, if ever so
     little, a new melt has to be atomised.

                                                           The powder metallurgy process

2.2 The powder metallurgy process

The production of P/M parts, as illustrated in figure 5, consists of three main process
steps: mixing, pressing, sintering. Initially, iron powder and various alloying powders are
blended together with a lubricant. The powder mix is then compacted in a press to
produce components of a desired shape. In this operation the volume of the powder
mass is decreased to more than 50% of the original volume, depending on the applied
compacting pressure. The compacted components are sintered in the protective
atmosphere in a furnace for 20-40 minutes at about 1130°C. In order to improve the
dimensional accuracy of the components, a second pressing operation (or coining) is
often performed. Finished components can be hardened, machined, plated, et cetera, in
the same way as ordinary steel components.

Metal alloys




Figure 5. Three main steps in the production of P/M parts: (a) mixing, (b) pressing, (c) sintering.

                                                                                     Materials development

2.3 Materials development

Development of materials has to a large extent contributed to new P/M applications.
Highly compressible powders have made possible the manufacture of high density
components without double pressing and double sintering. This has also minimized tool
wear and tool breakage, as well as making it possible to reduce the required compacting
force and thereby to utilise available press capacity for large components.
    The advantage of an increased sintered density is clearly demonstrated by the relation
between mechanical properties and porosity. See figure 6.

                                                                   Density, g/cm3
       Properties, % of theoretical maximum

                                                      le Str

                                                                  g   th

                                                              g   atio

                                                               Relative density, %

Figure 6. Increase of sintered properties with sintered density. Schematically:
a = compacting + sintering; a’ = warm compacting+sintering; b = compacting+sintering+re-pressing
+re-sintering; c = powder forging.

Metal alloys

Due to the highly favourable effects obtained through high densities, it has always been
important for the P/M industry to find ways to further improve the density of produced
parts. An important innovation in this field was warm compaction. As developed by
Höganäs, warm compaction is a cost effective method compared to double pressing/
sintering and powder forging. The cost advantage, combined with the simplicity and
stability of the warm compaction process, has already enabled mass production of P/M

2.4 Sintered Iron-based materials

There are several ways to achieve desired strength properties with iron-based sintered
materials. The most important parameters of influence are:
•    Density
•    Sintering conditions
•    Alloying elements
•    Heat-treating conditions
These parameters should be controlled to within the closest possible limits, because even
small variations may cause an unacceptably wide scatter of dimensional changes during
sintering, and thus spoil the dimensional stability of the sintered parts.

Density is of prime importance with respect to the physical properties of sintered
structural parts. The influence of density was reviewed above in Materials Development.
See figure 6. Tensile strength and fatigue strength increase in approximately linear
proportions, and elongation and impact strengths increase exponentially, with increases
in sintered density.

Sintering conditions
The following outcomes depend on what sintering conditions are used:
1. How fast and efficiently powder particles in the compact weld together and pores
   become rounded.
2. How quickly homogenisation of alloying elements takes place.
3. The extent of oxidation of sensitive elements.

Alloying elements
Alloying elements, dissolved in a base metal, give rise to the formation of various
microstructures and increase the material’s resistance to deformation. Tensile strengths

                                                                  Sintered Iron-based materials

arising from various alloys are shown in figure 7. The use of alloying elements also
influences the dimensional change of structural parts during sintering. Alloying elements
are indispensable with respect to the hardenability of conventional as well as sintered
steels. Hardenability factors arising from various alloys are shown in figure 8.
   Tensile strength, N/mm2

                                     Alloying element, wt. -%

Figure 7. Influence of alloying elements upon tensile strengths.

Metal alloys

     Multiplication factor

                                   Alloying element, wt. -%

Figure 8. Influence of alloying elements on hardenability.

In principle, alloying elements have the same effect on sintered steels as on conventional
steels. However, not all elements commonly alloyed with conventional steels can be used
in sintered steels. Some of them (e.g. Mn and V) are too easily oxidized in commercial
sintering atmospheres. On the other hand, elements undesirable in conventional steels
(e.g. phosphorous or “blue brittleness”) can have beneficial effects in sintered steels.
     Alloy compositions of sintered steels for structural parts have to be carefully selected
not only with respect to desired strength, but also with respect to dimensional stability
during sintering. With alloy compositions yielding hardness levels above 150 - 180 HV,
it is important that dimensional changes of the structural parts during sintering are as
small as possible and, even more importantly, that the scatter of these dimensional
changes is kept within the closest possible limits.
     While parts with hardnesses up to 150 HV can be sized or coined fairly easily, as
hardness increases beyond this threshold, sizing or coining becomes increasingly difficult
and eventually impossible. In the mass production of high-strength high-precision parts,
it is therefore of utmost importance that dimensional changes during sintering (and
subsequent heat-treatment) are insensitive to small unavoidable variations in process
parameters and material composition.

                               Alloying system: Microstructures and mechanical properties

Heat-treating conditions
Heat-treating conditions, when applied to sintered steel components, must be especially
well controlled to ensure the highest possible degree of dimensional stability of the
component in the hardening and tempering procedure. Asymmetric cooling during
quenching of a sintered component, especially with parts of complex shape, may lead to
distortions so severe that the part must either be rejected or subjected to expensive re-
machining. This would wipe out the cost advantage of P/M technology over
conventional production methods.

2.5 Alloying system: Microstructures and mechanical properties

Microstructures of sintered alloyed steels, produced from powder mixes, are typically
much more heterogeneous than those of conventional alloyed steels. While carbon
diffuses rapidly in iron lattice and reaches equilibrium during sintering, other alloying
elements like copper, nickel, and molybdenum diffuse slower and reaches equilibrium
after longer sintering times. Hence, when produced under commercially acceptable
sintering conditions, these materials will always exhibit a certain degree of heterogeneity.
    The following experimental alloying system has been investigated regarding

•   Plain Iron
•   Iron - Carbon
•   Iron - Copper
•   Iron - Copper - Carbon
•   Iron - Phosphorus
•   Iron - Molybdenum
•   Iron - Molybdenum - Carbon
•   Iron - Copper - Nickel - Molybdenum
•   Iron - Copper - Nickel - Molybdenum - Carbon
Microstructure and selected properties for the above alloys are presented below. (For
more detailed information see Höganäs AB Iron and Steel Powders for Sintered
Components - Handbook.)

Metal alloys

Plain iron and iron-carbon systems
Plain iron has a pure ferritic microstructure. Formation of pearlite is a consequence of
carbon addition, and varies with the amount added. The microstructures produced with
additions of different amounts of carbon in iron-carbon alloys can be seen in figure 9.

                                   Alloying system: Microstructures and mechanical properties

                                                                  50 µm

                                                                  50 µm

Figure 10. Microstructure of two iron-carbon alloys: (a) ASC100.29+0.2% C, (b) ASC100.29+0.5% C.
Both materials compacted with 589 N/mm2 and sintered 30 min. at 1120° C.
Sintered densities: (a) 7.15 g/cm3 and (b) 7.10 g/cm3.

Metal alloys

Parameters such as grain size will influence the physical properties of P/M alloys. With
decreasing grain size, strength increases, but with increasing grain size, ductility is
improved. A very efficient way to boost tensile strength and hardness of sintered iron is
to alloy it with carbon. Most conveniently, this is achieved by adding graphite powder to
the iron powder, before compacting and sintering. Being an interstitial alloying element,
carbon dissolves rapidly in the lattice during sintering.
    Pure ferritic material is difficult to machine, due to smearing on tools. If carbon is
added, the microstructure will be a mix of ferrite and pearlite. This microstructure has
the best performance regarding machinability.
    The presence of carbides (cementite) decreases machinability markedly. Apart from
the presence of pores, these microstructures are practically identical with those of
corresponding conventional plain carbon steels. The effect of dissolved carbon on tensile
strength and elongation of sintered iron is shown in figure 10.
  Tensile strength, N/mm2

                                                                                                Elongation, %
                                       ile   stre


                               0.2                0.4              0.6                0.8
                                          wt. -% Carbon

Figure 10. Influence of carbon alloy content on tensile strength and elongation of two sintered iron

                               Alloying system: Microstructures and mechanical properties

Iron-copper and iron-copper-carbon systems
Mixtures of iron and copper powder have a two-fold benefit:
•   Copper melts at 1083°C (i.e. below sintering temperature) and rapidly infiltrates the
    pore system of a compact powder, from where it diffuses relatively easily into the iron
•   Copper is dissolvable in γ-iron (austenite) up to approximately 9 wt.-%, but only up
    to 0.4 wt.-% in α-iron (ferrite); consequently, iron-copper alloys can be precipita-
    tion-hardened by low-temperature annealing after sintering – and they actually do so
    to a certain extent anyway, when passing the cooling zone of the sintering furnace.
Copper is added to the basic iron powder usually in amounts from 1.5 to 4 wt.-%.

Microstructures of two iron-copper materials (containing 2 wt.-% copper, and 0.2 and
0.6 wt.-% carbon, respectively) are shown in figure 11. These materials were compacted
to densities of 6.9 g/cm3 and sintered for 30 minutes at 1120°C in endogas. In the
micrograph with 0.2 wt.-% carbon, it appears that dissolved copper has concentrated in
carbon-rich areas of the iron structure where it has partly disintegrated the pearlite.

Metal alloys

Figure 12. Microstructure of (a) SC100.26+2% Cu+0.2% C and (b) SC100.26+2% Cu+0.6% C, both
materials compacted with 490 N/mm2 and sintered 30 min at 1120°C. Sintered densities 6.84 and
6.86 g/cm3.

                                  Alloying system: Microstructures and mechanical properties

The addition of graphite to iron-copper alloys has the very useful effect of increased
strength. The graph in figure 12 shows the effect of carbon additions on tensile strength
and elongation of sintered iron-copper materials.
  Tensile strength, N/mm2

                                                ile   stre

                                                                                             Elongation, %

                                                2                                  4

                                        wt. -% Copper

Figure 13. Influence of carbon content on tensile strength and elongation of sintered iron-copper

Iron-phosphorus system
In conventional steel-making, phosphorus is a most undesirable element since it
provokes irreparable segregation during solidification, making the steel brittle. In iron
powder metallurgy, however, phosphorus has shown much potential as a strengthening
alloy. Phosphorus is normally added to iron powder as a very finely ground Fe3P
powder which, compared with other phosphorus compounds, is relatively soft and less
harmful to compacting tools. During sintering, phosphorus and iron form a eutectic
melt (10.1wt.-% P; 1048°C) which rapidly infiltrates the pore system of the compact
and enhances the sintering process, due to sintering in the α-phase.
    As an effect of activated sintering, the pore structure of phosphorus material is more
spherical, and thus impact strength is increased. This phenomenon can be explained in

Metal alloys

terms of a substantially reduced notch effect. See figure 13 for micrography of a
phosphorus alloy, showing ‘rounded-off ’ pores of medium size and an absence of small

Figure 14. Microstructure of a phosphorus alloy (NC100.24+0.45% P); compacted with 690 N/mm 2
and sintered 30 min at 1120°C. Note well-formed spherical pores of good size.

As expected with alloys, there is a relationship seen in the mechanical properties found in
sintered materials. The graph in figure 14 shows the influence of phosphorus and carbon
upon tensile strength and elongation of iron compacts. The addition of 0.3 to 0.6 %
phosphorus has a similar effect on tensile strength and elongation as has addition of 2 to
4 % copper (cf. figure 14 vis-à-vis figure 12).

                                 Alloying system: Microstructures and mechanical properties

                                                      tre   ngt
  Tensile strength, N/mm2

                                          s  i   le s

                                                                                           Elongation, %

                                     0.2                            0.4         0.6

                                      wt. -% Phosphorus

Figure 15. Influence of phosphorus and carbon additions upon mechnical properties of sintered iron

Iron-molybdenum and iron-molybdenum-carbon systems
Molybdenum (0.85% or 1.5%) pre-alloyed to iron alloys has only a minor effect on
compressibility compared to plain iron. The advantages are increased dimensional
stability and hardenability. Figure 15 shows the influence of the cooling rate on the
microstructure. Where at a cooling rate of 8°C/s the microstructure consists of
Martensite and a low amount of Bainite and at a cooling rate of 0,5°C/s the
microstructure is completely bainitic.
The influence of copper and carbon additions to a pre-alloyed iron-molybdenum
material is shown in figure 16.

Metal alloys

Figure 16. Microstructure of sintered Astaloy Mo + 0.6% C, cooled from 1120°C.

                                 Alloying system: Microstructures and mechanical properties

  Tensile strength, N/mm2


                                                                                             Elongation, %


                               0.2                 0.4              0.6
                                       wt. -% Carbon

Figure 17. Influence of copper and carbon additions upon the properties of sintered iron materials
pre-alloyed with molybdenum (Astaloy Mo).

Iron-copper-nickel-molybdenum-carbon system
This system covers a wide range of iron alloy powders available for P/M applications.
Two of the most useful grades from this range are described here: a diffusion bonded
nickel, molybdenum and copper (known as Distaloy AE), and a grade based on pre-
alloyed molybdenum with diffusion bonded nickel and copper (Distaloy HP).
See figure 17 for micrography of Distaloy AE +0.5% C and figure 18 for micrography of
Distaloy HP+0.5% C.

Metal alloys

Figure 17. Microstructure of sintered Distaloy AE+0.5% C, cooled from 850°C.

Figure 18. Microstructure of sintered Distaloy HP+0.5% C, cooled from 1120°C.

                                     Alloying system: Microstructures and mechanical properties

Influence of carbon on sintered properties for these two grades are presented in figure 19,
figure 20 and figure 21. The addition of appropriate amounts of graphite is important in
the yield of high strength and other desirable mechanical properties.

MPa                    Tensile and Yield strength     HV10                                         Hardness
800                                                   300

700                                                   250

600                                                   200

500                                                   150

400                                                   100

300                                                    50

200                                                     0
      6.6   6.8      7.0      7.2      7.4      7.6      6.6           6.8       7.0     7.2      7.4    7.6

  J                                 Impact energy      %                                Dimensional change
 60                                                   0.3

 50                                                   0.2

 40                                                   0.1

 30                                                   0.0

 20                                                   -0.1

 10                                                   -0.2

  0                                                   -0.3
   6.6      6.8      7.0      7.2      7.4      7.6          6.6       6.8       7.0     7.2      7.4    7.6
                                                                             Sintered density (g/cm3)

 %                                     Elongation
                                                             0% C
  7                                                          0.2% C

  6                                                                 Tensile strength
                                                                    Yield strength



   6.6      6.8      7.0      7.2      7.4      7.6
                  Sintered density (g/cm3)

Figure 20. Distaloy AE+C. Sintered properties versus sintered density.

Metal alloys

MPa                    Tensile and Yield strength     HV10                                        Hardness
800                                                   300

700                                                   250

600                                                   200

500                                                   150

400                                                   100

300                                                    50

200                                                     0
      6.6   6.8      7.0      7.2      7.4      7.6      6.6          6.8       7.0     7.2      7.4    7.6

  J                                 Impact energy      %                               Dimensional change
 60                                                   0.3

 50                                                   0.2

 40                                                   0.1

 30                                                   0.0

 20                                                   -0.1

 10                                                   -0.2

  0                                                   -0.3
   6.6      6.8      7.0      7.2      7.4      7.6          6.6      6.8       7.0     7.2      7.4    7.6
                                                                            Sintered density (g/cm3)

 %                                     Elongation
                                                             0.5% C
  5                                                          0.8% C

  4                                                                Tensile strength
                                                                   Yield strength



   6.6      6.8      7.0      7.2      7.4      7.6
                  Sintered density (g/cm3)

Figure 20. Distaloy AE+C. Sintered properties versus sintered density.

                                     Alloying system: Microstructures and mechanical properties

 MPa                   Tensile and Yield strength     HV10                                        Hardness
1200                                                  400

1050                                                  350

 900                                                  300

 750                                                  250

 600                                                  200

 450                                                  150

 300                                                  100
    6.5     6.7      6.9      7.1      7.3      7.5      6.5          6.7      6.9      7.1      7.3     7.5

   J                                Impact energy       %                              Dimensional change
  35                                                   0.0

  30                                                  -0.1

  25                                                  -0.2

  20                                                  -0.3

  15                                                  -0.4

  10                                                  -0.5

   5                                                  -0.6
    6.5     6.7      6.9      7.1      7.3      7.5       6.5         6.7      6.9      7.1      7.3     7.5
                                                                            Sintered density (g/cm3)

  %                                    Elongation
                                                             0.2% C
   5                                                         0.5% C

   4                                                              Tensile strength
                                                                  Yield strength



    6.5     6.7      6.9      7.1      7.3      7.5
                  Sintered density (g/cm3)

Figure 22. Distaloy HP+C (1120°C). Sintered properties versus sintered density.

Distaloy AE and Distaloy HP are often used in the production of structural parts for
high-strength and high-accuracy applications. This is based on the fact that the
microstructure can be changed considerably by processing conditions. The variation in
microstructure achieved with manipulation of cooling rates is illustrated in figure 22 and

Metal alloys

figure 23. Both materials respond very well to subsequent heat-treatment and can be
used as non-segregatable materials since the alloy elements are diffusion bonded to the
iron particles.

Figure 22. Microstructure of sintered Distaloy AE + 0.5% C, cooled from 850°C.

Figure 23. Microstructure of sintered Distaloy HP + 0.5% C, cooled from 1120°C.

                                                          Analysis of the Machining Process

3     Turning

This chapter was written in cooperation with Sandvik Coromant and the Swedish
Institute of Metal Research.
Machinability is not a universally defined property. Generally, it is the ability of a
workpiece material to be machined, meaning how easy or demanding it is to shape a
workpiece with a cutting tool. Machinability can be divided into three main aspects
[Ref 1]:
1. Tool life.
2. Surface finish.
3. Power required to cut.

In order to state cutting parameters for machining of P/M materials these three aspects
must be investigated.

3.1 Analysis of the Machining Process

The machining process is complex. In order to select the right tool and machining
parameters, knowledge is necessary of the loads on the tool material and the properties of
the tool material, together with an analysis of wear mechanisms.
    Machining performance is determined by the physical properties and condition of
the workpiece and the cutting operation. P/M technology contributes with almost
infinitely possible modifications to material microstructures. Tool concept (i.e. chip
breaker profile, stability and geometry of tool holder, insert style, etc.) defines the wear
process which can be divided into four main load factors [Ref 2]: Mechanical, Thermal,
Chemical, and Abrasive.
    To determine cutting parameters, the load factors acting on the tool must be
controlled according to the active wear mechanisms. Typically, the loads on the edge of a
cutting tool are different at various locations. Consequently, different wear mechanisms
are activated and proceed at different rates at each location. Wear mechanism maps can
be constructed to describe the area of constant wear (or ‘safe zone’). The primary input
factors are feed rate (or pressure) and cutting speed (or velocity). The interaction of these
forces is shown schematically in the wear mechanism map at figure 24.

Machinability: Turning

                         Severe deformation and fracture limit
                                                                                                   Melt wear
  Pressure/feed rate

                                        Safe zone

                                                                           Mild oxid./diff. wear

                                                                                                           Severe oxid./diff.
                                   Plasticity-dominated wear

                       Built-up edge


Figure 24. Wear mechanism map for machining: pressure (feed rate) versus velocity.

Regarding the wear mechanism map, it is apparent that the main wear mechanism for
continuous turning of P/M materials is abrasive flank wear.
    Understanding of the metal cutting process involves prediction of the behavior of
various types of metals as they are formed into chips. Partly, this means predicting the
effect of deformation, temperature and mechanical forces, as these play a dominant role
in the quality of machining operations. Temperature affects the turning process and high
temperatures will negatively alter the cutting material. Cutting forces affect the power
and strength needed to perform the operation. Designing an accurate cutting edge
means controlling temperature, cutting forces and chip formation under known
machining conditions. The effect of the process on tool life and the security of the edge
are important factors in design of a cutting geometry.

Dynamics of the cutting process
In cutting P/M material to an edge, as occurs during turning, the tool deforms some of
the workpiece material which then separates as a chip. Large stresses build up as the layer,
which is to become the chip, approaches the cutting edge. Elastic and plastic
deformation of the metal occurs as the cutting forces reach the yield strength of the
material. Chips vary considerably with the type of workpiece material, but if the metal is
sufficiently tough, the process resembles a continuous flow of plate-like elements which
are sheared consecutively.

                                                              Analysis of the Machining Process

The basic dynamic of the cutting process (i.e. chip formation) is illustrated in figure 25.
The boundary between the chip and workpiece, the zone which separates the deformed
and undeformed metal is called the shear plane (SP). This has an angle to the workpiece
called the shear plane angle (∅). In figure 25, the metal to the right of the plane is the
deformed chip, with thickness (h2), and the metal to the left is the undeformed chip,
thickness (h1).
    Chip deformation is related mainly to three factors: the thickness of the undeformed
chip, the rake angle (γ) between the chip face and the normal workpiece surface, and the
mechanical properties of the workpiece material. These factors also affect the shear plane
angle during the cutting process.

Figure 25. Fundamentals of cutting process: shear plane (SP) and chip-forming factors. (See text in
this section for key with details of each factor.)

During cutting a stagnation zone follows the tip of the edge. Softened metal protects the
tool by sticking or sliding on the surface. A flow zone takes over after the shear plane and
is visible at the division of undeformed and deformed material. Thus the principal
cutting action occurs at the shear plane, determined to a great extent by the cutting ratio
between the undeformed and deformed chip thickness. Microstructure at the shear plane
of a P/M material is shown in figure 26.

Machinability: Turning

Figure 27. Etched microstructure of Distaloy AE +0.5%C, showing shear plane, porosity, and chip for-
mation, during turning.

Plasticity during turning
Metal cutting involves considerable plastic deformation. Most of the energy needed in
the metal turning process is expended at the shear plane; a concentrated shear occurs
when the workpiece metal is forced against the cutting edge. There is a flow along the
face of the tool: flow lines appear behind the deformed chip, after the shear plane, and
the surface turns rough due to the varying strains in the metal.
    The plastic behavior of the metal through the shear plane is influential in the process
since it affects the strain hardening of the chip and the cut surface. The metal is
plastically deformed at a high temperature through ‘hot working’. Structural change and
work hardening are the main results. Work hardening increases the cutting force,
reducing the shear angle, and produces a thicker flow over the rake face of the insert. The
degree of deformation depends considerably upon the size of the rake angle of the tool.
When the shear plane is small, the shearing force is high. The size of this angle and the
area of the shear plane are thus influential to cutting performance. In practice, factors
such as the rake angle and cutting data also affect the conditions of the shear.

                                                          Analysis of the Machining Process

The created shear plane is the turning point for the metal being cut; it is the surface
where materials yield and the chip is born. The strain and stress occurring at this point
depends on dynamics in the shear plane and the contact between the chip and tool. The
chip formation process is affected by the shearing angle which in turn influences the
contact length.
    The character of the movement of the chip along the contact length with the tool
face is another important factor in metal cutting. Friction is a factor in the process as the
metal is forced along at great pressures and high temperature. The flow zone is thus
created when the surface seizes but the movement between chip and tool continues. The
speed of the chip material increases from zero at the interface, to higher levels further
from the tool. Thus the chip moves along the tool face through a shear movement.
Consequently, more heat is generated in this zone of both stationary and flowing metal.
The pattern of movement is to a large extent characteristic of the material being cut as
well as the cutting data in use.

Built up edge
The meeting between chip and tool along the contact length can be divided into three
areas where different reactions take place during the turning process:
1. Sticking.
2. Adhesion and diffusion.
3. Abrasion, where with higher temperatures, diffusion and adhesion increase.

With certain conditions and materials, successive layers of the flow zone material will
build up and harden on the tool face. The flow zone moves up and along with the top of
the formed layer and, in this way, a built up edge (BUE) is formed. Metal is pressure-
welded continuously on the rake angle of the tool and eventually becomes unstable. It
breaks off at a certain point in the process whereupon the build up of a new layer
commences. BUE is a negative factor appearing in various forms and conditions in
machining. It can usually be eliminated by altering the conditions of the machining
process on which it thrives.
    Often there is a certain temperature and cutting speed range which promotes the
growth of BUE and certain workpieces and tool materials are more prone than others.
High speeds soften the layer and replace it with a flow zone. The BUE can also take some
of the cutting edge with it when it breaks off, and it may itself be hard enough to
function as a cutting edge. Some forms of BUE particularly affect the rake angle and also
the chip thickness. Formation of BUE increases with larger rounding of the cutting edge
(ER) but decreases with more positive rake angles. The strength of the BUE decreases
with increasing temperatures.

Machinability: Turning

3.2 Tool Wear and Tool Life

Analyzing tool wear and taking measures to ensure that there is optimal, balanced wear is
important for the quality and performance of turning operations. Wear affects how
much and in what way the cutting edge deviates from the nominal dimension and the
surface finish required.
    In turning for surface finishing, flank wear dominates as the wear type. A balanced,
predictable development of wear over a long time is the normal goal. The way in which
this development takes place determines the fluctuation of the machined dimensions
within the tolerance area and thereby the frequency of positional adjustments of the
cutting edge. There are five main wear mechanisms which dominate in metal cutting (as
illustrated in figure 27):
1. Abrasion.
2. Diffusion.
3. Oxidation.
4. Fatigue.
5. Adhesion.

These wear mechanisms combine to attack the cutting edge in various ways depending
upon the tool material, cutting geometry, workpiece material and cutting data. A basic
analysis of the tool wear is an important strategy in optimizing performance and quality
in finish turning. In the following summary, eight forms of tool wear are related to the
above wear mechanisms.

                                                                          Tool Wear and Tool Life


                                            2.                                             3.

                                            4.                                             5.
Figure 28. Schematic representation of five forms of tool wear during metal cutting: 1. Abrasion, 2.
Diffusion, 3. Oxidation, 4. Fatigue, 5. Adhesion.

Forms of tool wear
1. Flank wear takes place at the flank or clearance face of the cutting edge along the
   length of engagement. In this way it affects the position of the edge and the insert
   geometry. Its development can be positive in that it makes the edge sharper as
   machining progresses, however, after a certain amount of wear, increasing friction
   against the machined surface deteriorates performance of the edge. The main cause is
   the abrasive wear mechanism and the effect grows with higher cutting speeds.
2. Crater wear takes place on the chip face, where high temperatures and pressure pre-
   vail. Diffusion and abrasion wear mechanisms cause tool material to be worn away

Machinability: Turning

     and if this is allowed to develop excessively, a change in cutting geometry can occur,
     affecting chip formation and cutting forces, and weakening the cutting edge.
3. Plastic deformation can take place as a result of a combination of high temperatures
   and high pressures on the cutting edge. High cutting speeds and feeds along with
   hard workpiece materials lead to heat and high compression. ‘Hot hardness’ is a nec-
   essary property for the tool to stand up to this effect. Once the deformation starts to
   take place, further deterioration follows as a result of even higher temperatures, lead-
   ing to an escalation of critical effects between the edge and workpiece.
4. Thermal cracking is mainly a type of fatigue wear due to intermittent heat effect.
   With tool materials having a smaller coefficient of thermal expansion, cracks can
   form on the cutting edge, leading to edge weakness and the risk of rapid edge break-
   down due to failure. The application of cutting fluid is often a negative factor if tem-
   perature fluctuations are allowed to amplify during machining.
5. Chipping of the cutting edge takes place when the edge-line breaks, rather than
   wears, due to load pressure from the cutting action or due to material adhesion.
   Intermittent cutting is a frequent cause of edge chipping or uneven breaking, and
   ultimately fracturing. There are various degrees of this wear type with either micro to
   macro pieces breaking away from the cutting edge. In many cases the tool material
   type or grade is not suitable for the operation in question, indicating that more
   strength is required.
6. Built up edge formation is a wear type occurring largely as a function of temperature
   and cutting speed interaction. The workpiece material plays an important role as
   does the tool material, with some types being much more prone than others. Low
   carbon steels generally have a severe tendency to smear. Surface finish is first to be
   affected negatively, followed by a change in cutting geometry and then edge break-
7. Notch wear on the trailing edge is to a great extent an oxidation wear mechanism
   occurring where the cutting edge leaves the machined workpiece material in the feed
   direction. But abrasion and adhesion wear in a combined effect can contribute to the
   formation of one or several notches. There is a state of tension where the cutting edge
   leaves the workpiece material and the surface can be deformation-hardened leading
   to concentrated notch wear. Notch wear has a considerable effect on surface finish as
   the notch leaves high peaks and burrs which rapidly exceed the permitted profile
   height in finishing. Excessively high cutting speeds for the tool material in question,
   combined with a large entering angle, are typical causes of notch wear. Notch wear
   may involve the effect of squeezing, also called side-flow. This involves the material
   from the machined peaks being partly pulled away on either side of the chip. The
   side-flow affects the surface finish, depending upon chip flow direction, in that the

                                                                    Tool Wear and Tool Life

   newly cut and deformed material is hard and the peaks wear abrasively against the
   cutting edge. Notches are worn due to concentrated wear, spaced at a distance equal
   to the feed per revolution.
8. Flaking involves the coating being damaged at an early stage, usually because of
   demanding machining conditions and/or inferior coating. Poor adhesion of the
   coated layer on the grade, as well as plastic deformation, lead to flaking when the
   workpiece material is smearing or if the cut is intermittent. If the exposed substrate is
   more susceptible to wear, this is also a critical factor. (Modern coated GC 1525 and
   GC 4015 have been developed to resist flaking tendencies).

Speed and feed rates and tool wear
Cutting speed and cutting depth do not affect the tension build up in the workpiece to
any great extent, but the feed rate does. Both small and large feed rates can give rise to
material tension. This is another reason for testing to find the median, optimum feed
rate for an application. Edge sharpness and a positive geometry help to keep material
tension from building up during machining. A smaller entering angle gives rise to less
build up of material tension through a more even magnitude of the cutting forces.
    Increased velocity (cutting speed) and temperature has varied influences on different
forms of tool wear. The tendencies for several different forms of wear are shown in
figure 28.

Machinability: Turning

Figure 28. Influence of cutting speed and temperature on four types of tool wear: 1. abrasive (flank
and crater), 2. diffusion (crater), 3. oxidation (notch), 4. built up edge.

Tool life criteria
The tool life for a cutting edge is determined according to its ability to satisfy demands
such as:
•    Maintaining tolerances.
•    Obtaining the required surface finish.
•    Satisfactory chip breaking.
Tool life determination is a key factor for setting productivity levels. Often, experience is
the only available guide in judging what degree of wear to allow on a cutting edge before
machining is discontinued. Degree of wear on the flank is considered a reasonably
reliable criterion on which to base decisions about economical tool life.
    Tool wear is determined by observing and measuring the degree of wear as it occurs
and specifying the effective cutting time (T) which elapses before a stipulated degree of
wear is reached. This formula is represented in figure 29: the curve Vc1 indicates the
highest cutting speed which produces the shortest tool life (T1).

                                                                            Tool Wear and Tool Life

Figure 30. Relationship between cutting speed (m/min) and tool life (min), as indicators of tool wear.

Figure 30. Tool life test (vT-curve) based on cutting speed values as shown in the above (figure 29).

Machinability: Turning

In figure 30 a curve, which is practically a straight line, known as ‘Taylor’s curve’ and
often referred to as the ‘vT-curve’ describes a relationship used to determine optimal tool
life. The first generally accepted single point turning test using “Taylor ’s tool life test”
was conducted in 1975 for an ISO standard. The details of the test procedure and tool
life evaluation data are explained in ISO document No. 3685 of 1977.

Cutting forces in balance
Metal cutting requires a lot of power to separate chips from the workpiece. An
understanding of cutting forces will lead to well balanced cutting edges through positive
cutting actions and good cutting edge strength.
    There is a relationship between the power needed for the cutting process and the
cutting forces involved. Seen from an orthogonal point of view, a state of equilibrium
exists with the forces involved and in relation to the shear plan. The forces at work on
the workpiece and chip, along the shear plane, and between the tool face and chip are, in
principle, equal.
    Seen in three dimensions, the cutting forces can be divided into three components:
tangential or main force (FC), radial or passive force (FCN) and axial or feed force (FP), as
shown in figure 31.

Figure 31. Forces at work to produce cutting during machine turning: there is a dynamic equilibrium
between tangential (FC), radial (FCN) and axial (FP) forces.

The main is to a great extent dependent upon the contact and friction between, not only
the workpiece and tool, but also the condition of contact between chip and rake face of

                                                 Classification of P/M Materials for Turning

the cutting edge. The quality of the actual chip formation and breaking, affects the main
force considerably. There is also a direct relationship between the undeformed chip
thickness (h1) and the magnitude of this force. For most workpiece materials, increasing
cutting speed leads to lower cutting forces. The higher temperature in the flow zone and
reduced contact area contribute to this effect. The decrease in forces varies with material
and the range of cutting speeds in question.

3.3 Classification of P/M Materials for Turning

In order to state cutting data regarding turning, cylinders (∆out: 64 mm, ∆in: 35 mm,
H ≈ 62 mm) compacted to 7.0 g/cm3 were produced from the mixes shown in Table 2
on page 52). Influence of density was evaluated at 6.7 , 7.0 and 7.1 g/cm3. Carbon
enriched components were sintered in endogas with 0.3% CO2 ; dissociated ammonia
was used for carbon free components. All components were sintered in a production
furnace at GKN Sinter Metals AB, Sweden. Sintering was at 1120°C for 30 minutes. The
main turning evaluation was performed by the Swedish Institute for Metal Research, in a
SMT lathe.

Machinability: Turning

Table 2. Description and classification of three groups of P/M materials
investigated in turning performance tests.
                       Machina-                    Ferrite    Tensile   Hardness   Elong.
                         bility                     (%)      strength    (HV10)     (%)
                        groups                                (MPa)
 Iron base

 ASC100.29                                          100        180         50       16

 ASC100.29 + 0,45% P                                100        400        100       12

 ASC100.29 + 2% Cu                                  100        280         70       10

 ASC100.29 + 2% Cu + 0,25% C                         85        400        120       5.4

 ASC100.29 + 2% Cu + 0,5% C                          30        490        130       4.3

 ASC100.29 + 2% Cu + 0,8% C                          1         600        150       3.4

 Diffusion bonded

 Distaloy AE (4% Ni, 1,5% Cu, 0,5% Mo)               90        360        100        8

 Distaloy AE (4% Ni, 1,5% Cu, 0,5% Mo) + 0,25% C     35        550        160        4

 Distaloy AE (4% Ni, 1,5% Cu, 0,5% Mo) + 0,5% C      8         620        200       2.6

 Distaloy AE (4% Ni, 1,5% Cu, 0,5% Mo) + 0,8% C      2         590        230        2

 Pre-alloyed with 1,5% Mo

 Astaloy Mo + 0,5% C                                           500        160       1.8

 Distaloy HP (4% Ni, 2% Cu) + 0,5% C                 <1        900        270       2.2

A wide range of tools is used in cutting operations with P/M components. In this
investigation tools of the following types were selected: Cermet, PVD TiN coated HM,
CVD - multi coated HM and CBN. To determine tool life (i.e. when ‘worn out’), a
criteria of 0.3 mm flank wear was chosen. For the main investigation a depth of cut of
0.5 mm was used. All tests were performed in dry conditions (i.e. no cutting fluid
    Since P/M technology provides many alternative ways to add alloying elements, a
large number of mixes is on the market. The classification of P/M materials is necessary
so that machining recommendations can be given for specific materials and operations.
    Classification for turning was mainly based on cutting performance at 15 minutes
tool life, with a PVD-TiN coated tool (GC1015MF). The results for the mixes from the

                                                       Classification of P/M Materials for Turning

three groups, measured in terms of cutting speed, are shown in figure 32. Influence from
additives, oil impregnation, cutting tool can displace a material from one group to
another. However, taking all factors into account the classification determined was found
to be acceptable. Influence of alloy elements strongly affect performance (e.g. see Astaloy
Mo+0.5% C and ASC100.29+0.45% P in figure 32). Partition of the main groups with
regard to the largest amount of alloy element, is open to discussion.

Figure 33. Ranking of a range of P/M materials by cutting speed with tool life at 15 minutes, feed rate
0.1 mm/r. (a= depth of cut).

A remarkable quality of P/M materials is the small amount of scatter between test runs
made at the same cutting data. When two or more tools were tested for the same data
they performed almost identically. It was only when cutting data came close to the limits
of load and temperature that the scatter increased to a level more common in
machinability testing. The small test pieces and the consistency of their mechanical
properties could account for this effect.
    To determine machinability of a conventional steel, the correlation to hardness is
mainly used. For P/M material this relationship can be used if consideration is taken to
the amount of ferrite present in the microstructure. (See figure 33 in which materials
with more than 90% ferrite lie outside the standard hardness/tool life correlation.) This

Machinability: Turning

relationship is valid for tool GC1015 evaluated at a cutting speed of 200 m/min.
Investigation of other types of tools under equivalent conditions revealed a similar

Figure 34. Influence of hardness on tool life during turning (facing with machined surface: velocity
200 m/min, feed rate 0.1 mm/r, tool life criteria 0.3mm flank wear).

The slope of the vT-curve must be taken into account if comparison is made under other
conditions. This is based on the fact that ferrite materials differ rather much in Alfa
value. MnS addition will also similarly change the slope of the vT-curve (see Figure
figure 36 below). Influence of density should also be considered (see under section
“Density” below). Note too, the difference between valves operations, particularly
continous versus intermittent cutting. Still, it is possible to get an indication of overall
machining performance for different types of P/M materials.

                                Influence of material, properties and machining processes

3.4 Influence of material, properties and machining processes

This section deals with the main material and process influences on machinability,
particularly in relation to turning.

Alloy elements
The diffusion rate during sintering can be used as an indication of machining
performance for P/M materials. Sintered plain iron has a ferritic iron particle. If copper,
phosphorous, nickel or molybdenum, et cetera, is added, the central part of the iron
particle is also ferritic, due to the slow diffusion rate of these alloying elements in the
iron lattice at the investigated sintering condition. A zone is formed around the central
part of the iron particle (see micrographs in Chapter 2). As a consequence of this ferritic
microstructure, machinability is decreased.
    The amount of ferrite present in the microstructure of a P/M material roughly states
its machining performance. (See Table 2 for proportions of ferrite in a range of mixes).
Ferrite in amounts of 90% or more can act as a rule of thumb to indicate decreased
performance, relative to the hardness correlation (cf. figure 33).

Contrary influences of carbon addition
Interstitial elements like carbon generally diffuse very rapidly in the iron lattice. As a
consequence pearlite forms in the central part of the iron particle. This structure, from a
machining point of view, is “easy” to work with. Depending on the type of tool and
machining operation, an optimum amount of carbon can be stated for particular
    In figure 34 the influence of carbon is shown for two mixes (viz. Distaloy AE and
ASC100.29+2% Cu). It is apparent that these two materials have distinct, but very
differing, optimum levels of carbon content in relation to effect on tool life during

Machinability: Turning

Figure 35. Influence on tool life in a cutting operation of carbon content in two materials (i.e. Distaloy
AE and ASC100.29 +2%Cu). Tool: GC1015MF; velocity: 150m/min; feed rate: 0.1 mm/r).

In turning ASC100.29+2% Cu without carbon addition, the influence from the
proportion of ferrite is obvious. But note that as the amount of added carbon is increased
beyond 0.25%, tool life decreases towards that occurring without added carbon.
Regarding the effect on Distaloy AE, the same positive influence from carbon addition
could be expected due to the fact that the microstructure contains 95% ferrite, but this is
not the case. The hardness of Distaloy AE is 100 (HV10); deviation from the hardness
relationship shown in figure 33 is apparent. Amount of martensite increases with carbon
addition. This particular phase can explain the decrease in machinability for the
Distaloy™ grade when any amount of carbon is added.

Surface oxides which are more or less present in P/M materials clearly influence
machining performance. The influence of oxides is well known for conventional material
[Ref 1]. Oxides present in the workpiece structure cause notch wear. Turning the ‘skin’
surface of an oxidized component causes a sharp notch to immediately form on a new

                                   Influence of material, properties and machining processes

Investigations of the influence of additives on turning are detailed here. Based on its
negligible effect on mechanical properties, MnS additive has been investigated regarding
performance in continuous and intermittent cutting. For the test of intermittent cutting,
turning of a synchronizing hub was used (see figure 35).

Figure 35. Synchronizing hub component turned with intermittent cutting, as used in tests of MnS
addition to P/M materials.

The effect of MnS can be interpreted according to different ‘roles’ it can play [Ref 1]. In
particular it can:
1. Reduce strain on the shear plane during chip formation.
2. Increase tool face friction.
An intermittent cutting test of a hub component (pictured above) found that MnS
added to a Distaloy AE+0.5% C mix gave substantially longer tool life irrespective of
changes in cutting speed (see figure 36).

Machinability: Turning

Figure 37. Influence on tool life of MnS addition to a Distaloy™ mix, compacted as a hub component
and turned intermittently with a GC3025 tool, shown as vT-curves. Feed rate 0.1 mm/r; depth of cut
0.5 mm; entry angle 95°; dry cutting; criterion: Vb of =0.25 mm.

One possible explanation for the large effect found in the cutting operation described
above is reduced strain in the shear plane as a result of added MnS. Tests of the forces
acting on the cutting tool were conducted with a comparison of MnS addition, and oil
impregnation, of Distaloy AE+0.5% C. Figure 37, figure 38, and figure 39 show the
values found for passive force, feed force, and main force, respectively. The effect of MnS
on a continuos cutting operation is shown in figure 40.

                                    Influence of material, properties and machining processes

Figure 37. Influence of MnS addition and oil impregnation on passive force during dry cutting, at vari-
ous feed rates, of a hub component compacted from Distaloy AE +0.5%C. Cutting speed
200 m/min; depth of cut 1.0 mm.

Figure 39. Influence of MnS addition and oil impregnation on feed force during dry cutting, at various
feed rates, of a hub component compacted from Distaloy AE +0.5%C. Cutting speed
200 m/min; depth of cut 1.0 mm.

Machinability: Turning

Figure 39. Influence of MnS addition and oil impregnation on main force during dry cutting, at various
feed rates, of a hub component compacted from Distaloy AE +0.5%C. Cutting speed
200 m/min; depth of cut 1.0mm.

Figure 40. Influence on tool life of added MnS with continuous dry cutting, at various speeds, of a hub
component compacted from Distaloy AE +0.5%C. Tool GC1015MF; feed rate 0.1mm/r; depth of cut

                                    Influence of material, properties and machining processes

In summary regarding the above tests, we can see that increased cutting speeds give an
increase in the MnS benefit. This can be expected if the strain on the shear plane is
reduced, as appears the case according to the force tests. Increased feed rates reveal an
interesting effect. Added MnS actually decreased tool life at a feed rate of 0.2 mm/r,
while at 0.3 mm/r the effect from MnS was negligible (see figure 41).

Figure 42. Influence of feed rate on tool life with continuous dry cutting of a hub component com-
pacted from Distaloy AE +0.5%C, and with alternatively, added MnS, and oil impregnation. Tool
GC1015MF; cutting speed 200m/min; depth of cut 0.5mm.

The reason for the result shown in figure 41 is not clear. The influence of MnS on built
up edge (BUE) could be one explanation for the negative outcome at feed rate
0.2 mm/r. Surface roughness (Ras /Ral) measurements with MnS addition reveal higher
values compared with no addition. This indicates that a BUE is formed on the tool. To
solve this the temperature on the tool must be changed. This can be achieved by
increased cutting speed or by alternative tool selection. Use of cutting fluid could avoid
BUE formation, but cutting fluid is not recommended for turning of P/M materials.
This situation demonstrates how complex determinations of machinability can be due to
the interaction of several mechanisms which occur simultaneously during a particular
operation. (See below for more on surface roughness, Section 3.5)

The influence of density on tool life is shown in Table 3. This investigation was made
with ASC100.29+2% Cu+0.5% C in the density range 6.7 to 7.1 g/cm3.

Machinability: Turning

Table 3. Influence of density on tool life with ASC100.29+2%Cu+0.5%C, at
cutting speeds of 200 and 300 m/min.

                                           Density              Density             Density
     Cutting speed (m/min)
                                          6.7 g/cm3            7.0 g/cm3           7.1 g/cm3
                 200                        31.8 min.           33.3 min.           34.1 min.
                 300                        7.6 min.             9.6 min.            8.5 min.

Improvement with longer tool life was achieved at higher densities. This is despite an
increase in hardness which logically should act in the opposite direction (see figure 33).
The effect of variation in the density range 6.7 to 7.1 g/cm3 is small. Evaluation of
cutting forces revealed no significant difference with the investigated mix.
    The influence of feed rate revealed improved tool life for high densities at higher feed
rates (in tests with the same ASC100.29+2% Cu+0.5% C workpieces). The increase in
tool life was apparent when the feed rate was increased above 0.2 mm/r, particularly with
the material of density 7.1g/cm3 (see figure 42). This result suggests that vibrations may
be a negative influence on cutting performance.

Figure 42. Influence of density on tool life with turning, at three feed rates, of ASC100.29+2% Cu+
0.5% C compacted and sintered to densities of 6.7, 7.0 and 7.1 g/cm3.

Use of coolant during cutting is common with conventional steel. The situation is
different with P/M materials since those have pores in the micro structure. Porosity
combined with the presence of water can be detrimental to mechanical properties due to
the resulting oxidation. The effect on performance in cutting a Distaloy AE+0.5% C

                                     Influence of material, properties and machining processes

alloy when dry, wet, and dry with oil impregnation, is shown in figure 43. Flank wear
increased markedly with continued wet cutting; oil impregnated workpieces cut dry
performed somewhat better than dry-cutting. An obvious conclusion is that applied
liquid is detrimental for turning of P/M material.

Figure 44. Influence on cutting performance (in flank wear) of applied liquid, and oil impregnation, with
turning of Distaloy AE 0.5%C. Tool: GC1015 MF; cutting speed 200m/min; feed rate 0.1 mm/r; depth
of cut: 0.5 mm.

The obvious deterioration in tool performance with cutting fluid can be traced back to
the severe thermo-mechanical load cycle associated with the test mode. When cutting
fluid is introduced to machining of P/M materials, the effects can include:
•   Severe fluctuations in the tool temperature due to the better heat transfer characteris-
    tics of the water based cutting fluid, leading to severe thermal cycling.
•   Probable reaction between wet workpieces and the atmosphere during the interrup-
    tion period of the machining cycle.

Oil impregnation
Internal lubrication on the cutting edge is possible with P/M technology, an advantage it
can offer compared with conventional steels. Porous P/M materials can be readily oil
impregnated to good effect, as shown in the above tests. Improvement in dry cutting by
a factor between 1.48 and 2.21 was achieved at various feed rates with oil impregnation
of Distaloy AE +0.5% C (see figure 41). This can be explained by the reduction in
cutting forces as found in the tests with Distaloy AE alloys, discussed above in Section
3.3 (see figure 37, figure 38 and figure 39).

Machinability: Turning

With the mix ASC100.29 +2%Cu +0.5%C the effect from oil impregnation decreased
with lower feed rates compared with Distaloy AE 0.5%C (see figure 44, figure 45, and
figure 46). This indicates that oil impregnation is more effective with high strength
material. Since the main wear mechanism for P/M material is abrasive flank wear and
considering cutting forces versus flank wear, lubrication on the cutting edge for high
strength material can explain its better performance.

Figure 44. Influence of oil impregnation on passive force at various feed rates for turning of
ASC100.29+2% Cu+0.5% C. Tool GC1015 MF; cutting speed 200 m/min; depth of cut 1.0 mm.

Figure 46. Influence of oil impregnation on feed force at various feed rates for turning of ASC100.29
+2% Cu +0.5% C. Tool GC1015 MF; cutting speed 200 m/min; depth of cut 1.0 mm.

                                    Influence of material, properties and machining processes

Figure 46. Influence of oil impregnation on main force at various feed rates for turning of ASC100.29
+2% Cu+0.5% C. Tool GC1015 MF; cutting speed 200 m/min; depth of cut 1.0 mm.

Machinability: Turning

3.5 Surface Finish

An important quality outcome of machining is surface finish, and in particular, degrees
of roughness of turned surfaces. The influence of various P/M parameters and
machining conditions was investigated to determine influences on surface roughness.
    Machined P/M items were tested for surface profiles using a Rodenstock RM600
laser measuring station. Ra values were evaluated using two combinations of cut-off
lengths (Lc) and measuring lengths (L), each set reflecting one range of the surface
roughness. The small Lc of 0.025 mm (L = 1 mm) was four times less than the feed level.
Ra values from this run describe the surface between the feed marks according to micro
surface roughness, while the standard parameter set of Lc = 0.8 mm, L = 5.6 mm,
includes the feed marks in the roughness test and gives an idea of the macro surface
roughness (Ral). See figure 47.
    The tool used in the surface roughness investigation was a CNMG 120408,
PVD-TiN coated HM with the depth of cut set at 0.5 mm.

Figure 47. Schematic structure of turned metal surface profile, indicating dimensions of Ral and Ras
measures of surface roughness.

Surface roughness and feed rate
Assessment in mean values of all 12 P/M materials included in the turning tests (see
Table 2), showed a decrease in surface roughness within single feed marks (Ras), with
increased feed rates. The variation (max.- min., excluding extreme values) for Ras is
shown in figure 48. The variation in micro surface roughness decreased with increased
feed rate.
    Influence from the tool on surface roughness can be described by Ral. As a result of
increased feed rates, macro surface roughness (Ral) will increase (see figure 48).
    A conclusion regarding the influence of feed rates on surface roughness is that the
possibility of using a high feed rate is limited by the tool geometry. To obtain the
required limit in surface roughness, an additional machining operation is needed in
many cases.

                                                                                     Surface Finish

Figure 48. Influence of feed rate on micro (Ras) and macro (Ral) surface roughness of various cut off
lengths of P/M material. Tool: CNMG 120408 MF, GC 1015; cutting speed 200 mm/m; depth of cut
0.5 mm.

Surface roughness and cutting speed
Tests of the influence of cutting speed on surface roughness revealed that at speeds of
100-120 m/min, roughness was minimal. These tests were made with a feed rate of
0.1 mm/r. When turned at this rate and at speeds above 100 m/min, all 12 investigated
P/M materials had surface roughness of less than 0.4 µm (Ras) and 0.9 µm (Ral), as
shown in figure 49.

Figure 50. Influence of cutting speed on micro (Ras) and macro (Ral) surface roughness of various cut
off lengths of P/M material. Tool: CNMG 120408KF, CVD-TiN coated HM.

Machinability: Turning

3.6 Cutting Forces

Cutting forces were measured for a range of 18 P/M materials turned with a
GC1015MF tool at three feed rates (viz. 0.1, 0.2 and 0.3 mm/r). In addition, a special
study was made of the influence of oil impregnation on two P/M materials: Distaloy
AE+0.5% C and ASC100.29+2% Cu+0.5% C. A minor test was conducted showing the
influence of increasing tool wear. It is important to consider the effect of tool wear in
regard to cutting forces when reviewing the following tests since they were all done with
new tools.
The 18 materials investigated for cutting forces are listed in Table 4.

Table 3. Key to materials in figure 51: 18 materials as investigated for tests of
cutting forces.

       Key                                        Material
        A          ASC100.29
        B          ASC100.29+2% C
        C          Distaloy AE
        D          ASC100.29+2% Cu+0.25% C
        E          ASC100.29+2% Cu+0.5% C, ρ: 7.1 g/cm3
        F          ASC100.29+0.45% P
        G          ASC100.29+2% Cu+0.5% C, ρ: 6.7 g/cm3
        H          ASC100.29+2% Cu+0.8% C
        I          Reference: OVAKO 234S (16MnCr5)
        J          Astaloy Mo+0.5% C
        K          ASC100.29+2% Cu+0.5% C, ρ: 7.0 g/cm3
        L          Distaloy AE+0.5% C, oil impregnated
        M          ASC100.29+2% Cu+0.5% C, ρ: 7.0 g/cm3 (second evaluation)
        N          Distaloy AE+0.25% C
        O          Distaloy AE+0.5% C+0.5% MnS
        P          Distaloy AE+0.5% C
        Q          Distaloy AE+0.8% C
        R          Distaloy HP+0.5% C
The variation in cutting forces during turning of Material P (Distaloy AE +0.5% C) are
shown in figure 50.

                                                                                     Cutting Forces

Figure 51. Passive, feed and main force, in raw values, for Distaloy AE with 0,5% C material turned with a
 GC1015MF tool at a speed of 200 m/min and feed rate at 0.1 mm/r.

Variation in mean cutting forces
Mean values were calculated for each cutting force at each of three feed rates (0.1, 0.2
and 0.3mm/r). When the mean cutting forces are compared at each feed level the
differences between the materials are small. The force values are closely related to
hardness, above a certain hardness level, and the ranking for all three force dimensions is
very similar with only the hardest materials (viz., Distaloy HP+0.5% C, Distaloy
AE+0.8%C and Distaloy AE+0.5% C) being distinct with considerably higher forces.
Among the remainder of the materials shown in figure 51, ASC100.29, ASC100.29+2%
Cu and Distaloy AE gave the lowest cutting forces.

Figure 52. Mean cutting force values for 18 P/M materials turned with a GC1015MF tool at a speed
of 200 m/min and feed rate at 0.1 mm/r. See Table 4.

Machinability: Turning

The variation in the force at steady state is of interest if it differs between materials,
creating a significant amplitude in forces and possibly higher maximum forces and
increased fatigue loads. The variation in the forces was evaluated as the standard
deviation at steady state.
    The oscillation in the cutting forces at steady state was greater for the P/M materials
than for the wrought reference material, OVAKO 234S (16MnCr5). Measured as the
standard deviation, the oscillation was about 10 to15 % of the mean value, at the feed
levels 0.05 and 0.1 mm/r for all three force dimensions. At a feed rate of 0.1 mm/r, the
variation in absolute values was almost the same for all P/M materials. A force level
closer to the maximum measured force was evaluated by adding the scatter to the mean
value. However the rank order of the materials shown in figure 51 was not altered.
    For the higher feed rates (0.2 and 0.3 mm/r), the scatter in the feed force and passive
force can become considerable, in some instances up to 30 or 40% of the mean value
while it remains at 10 to 15% of the mean value for the main cutting force.

Hardness and cutting forces
Cutting forces were found to be relatively limited (in the range 70-110 N) for all but the
hardest materials. Figure 52 shows the relationship between hardness and cutting forces
Fx (feed force) and Fy (radial or passive force) for a range of P/M materials. The curve
for each cutting force is almost horizontal, except for the two hardest materials.
    Of the three force dimensions, passive force shows the clearest differences between
materials. Feed force also shows material dependent variations, while with main cutting
force (Fz) there is only quite small differences between materials.

Figure 52. Influence of hardness on feed force (Fx) and passive force (Fy) for a range of P/M

                                                                                    Cutting Forces

Tool wear and passive force
Of the three cutting forces, passive force is most effected by increased tool wear. It can
surpass both the feed force and the main cutting force, when the flank wear reaches a
certain level, as shown in figure 53. (See Section 3.2 for types of tool wear.)

Figure 53. Influence of flank wear on three dimensions of cutting force during dry turning of Distaloy
AE+0.5% C. Tool GC1015MF, velocity 200 m/min, depth of cut 0.5 mm.

The strong influence of flank wear seems to be very apparent in these tests of P/M
materials with a cutting situation of small cutting depths and lower feed rates, during
radial turning. It is also consistent with general principles which stress the importance of
a sharp tool edge.
    Increased feed rates lead to increased cutting forces. With a higher feed rate, the
absolute and relative increase in forces is greater for the main cutting force, than for feed
force and passive force. However, when considering differences between materials,
comparisons show up most clearly with passive force and feed force.

Influence of changes in feed rate
Cutting forces were measured for the 18 materials listed above with feed rates varied
from 0.05 mm/r to 0.30 mm/r. Figure 54, figure 55 and figure 56 show the mean force
measurements for feed, passive and main forces, respectively. All tests were for dry
cutting with tool GC1015MF.
    Some of the P/M materials have shown particularly good results with higher feed
rates, in terms of reduced wear and longer tool life, while also producing improved
surface finish. Therefore it is interesting to investigate how the cutting forces are altered
with increased feed rates. It is possible, for example, to determine if a smaller than

Machinability: Turning

average increase can be correlated with a greater than average increase in tool life. This is
indicated by a steady incline in the curve (i.e. less change in values) in these three figures.
The distribution of the forces on the cutting edge is altered when the feed is increased.

Figure 55. Influence of feed rate on feed force for eight Distaloy materials and the reference OVAKO
234S (16MnCr5). Dry cutting with tool GC1015MF; cutting speed 200 mm/min.
Materials: a= Distaloy HP+0.5% C, b= Distaloy AE+0.8% C, c= Distaloy AE+0.5% C, d= Reference
OVAKO 234S (16 MnCr5), e= Distaloy AE, f= Astaloy Mo+0.5% C, g= Distaloy AE+0.5% C+ 0.5%
MnS, h= Distaloy AE+0.25% C, i= Distaloy AE+0.5% C, oil impregnated.

                                                                                   Cutting Forces

Figure 56. Influence of feed rate on passive force for eight Distaloy materials and the reference
OVAKO 234S (16MnCr5). Dry cutting with tool GC1015MF; cutting speed 200 mm/min.
Materials: a= Distaloy HP+0.5% C, b= Distaloy AE+0.8% C, c= Distaloy AE+0.5% C, d= Reference
OVAKO 234S (16 MnCr5), e= Distaloy AE, f= Astaloy Mo+0.5% C, g= Distaloy AE+0.5% C+0.5%
MnS, h= Distaloy AE+0.25 C, i= Distaloy AE+0.5% C, oil impregnated.

Figure 57. Influence of feed rate on mainforce for eight Distaloy materials and the reference OVAKO
234S (16MnCr5). Dry cutting with tool GC1015MF; cutting speed 200 mm/min.
Materials: a= Distaloy HP+0.5% C, b= Distaloy AE+0.8% C, c= Distaloy AE+0.5% C, d= Reference
OVAKO 234S (16 MnCr5), e= Distaloy AE, f= Astaloy Mo+0.5% C, g= Distaloy AE+0.5% C+ 0.5%
MnS, h= Distaloy AE+0.25 C, i= Distaloy AE+0.5% C, oil impregnated.

Machinability: Turning

Estimated friction on rake face
The ratio between the feed force and the main force varies with increased feed rate
during cutting of the different materials. The change in ratio across four feed rates is
shown in figure 57.
     The ratio between feed and main forces can be used as a very rough estimate of the
friction on the rake face. At f = 0.05 where the edge radius constitutes a great part of the
contact length on the tool, the Fx/Fz ratio is quite high, between 0.5 and 0.9. As the feed
is increased more and more force is distributed to the rake face and purely geometrical
factors make the curves look very similar for all materials. The relative difference
between the materials is however maintained and thus should somehow be material
     It is interesting to note the beneficial effect on this estimated friction with the MnS
and oil impregnated materials, where especially the latter gives a substantial decrease at
the highest feed rate. The lowest feed rate (0.05 mm/r) is clearly at the limit of the useful
feed range and was not included in subsequent evaluations.

Figure 57. Influence of feed rate on the relationship between feed and main forces, during cutting of a
range of P/M materials. Two of the hardest materials, Distaloy HP+0.5% C and Distaloy AE+ 0.8% C, are

                                                                                       Cutting Forces

Influence of feed rate on passive force
The 18 materials investigated in the tests of cutting forces were ranked according to
absolute increase in passive force when feed rate was increased from 0.1 to 0.3 mm/r.
This rank is given in figure 58, with the relative increase in passive force. See Table 5 for
key to the materials as ranked from this test.
    The materials showing the largest absolute force increase when the feed rate was
increased from 0.1mm/r to 0.3mm/r were Distaloy HP+0.5% C, ASC100.29 and
ASC100.29+2% Cu. To some extent the low density material ASC100.29+2% Cu+
0.5% C (density 6.7 g/cm3), showed a different behaviour to the other ASC100.29+ 2%
Cu +0.5% C variants. The Distaloy HP+0.5% C material is unique in that it had such
high cutting forces and absolute increases in passive force. This effect was even more
evident with main and feed force. The two machinability enhanced materials (i.e. oil
impregnated and MnS added) and Distaloy AE+0.25% C showed the lowest absolute
increases in passive force, much less than the Distaloy AE+0.5% C.


Figure 58. Influence of increased feed rate in terms of relative and absolute increases in passive force
during turning of 18 materials, including reference. (See Table 5 for key to materials as ranked in this

Machinability: Turning

Table 5. Key to P/M materials investigated for cutting forces, as ranked in figure
58 (above).

       Key                                          Material
        A           Distaloy AE+0.5% C+0.5% MnS
        B           Distaloy AE+0.5% C
        C           Distaloy AE+0.25% C
        D           ASC100.29+2% Cu+0.8% C
        E           ASC100.29+2% Cu+0.5% C
        F           ASC100.29+2% Cu+0.5% C (second evaluation)
        G           Reference: OVAKO 234S (16MnCr5)
        H           Astaloy Mo+0.5% C
         I          Distaloy AE+0.5% C
         J          ASC100.29+2% Cu+0.5% C, ρ: 7.1 g/cm3
        K           Distaloy AE
        L           Distaloy AE+0.8% C
        M           ASC100.29+0.45% P
        N           ASC100.29+2% Cu+0.25% C
        O           ASC100.29+2% Cu
        P           ASC100.29
        Q           ASC100.29+2% Cu+0.5% C, ρ: 6.7 g/cm3
        R           Distaloy HP+0.5% C

Increase in passive force and tool life
When the relative increase in the number of cuts before tool failure was plotted against
the relative increase in passive force, most of the materials showed predictable behaviour.
Values for a range of P/M materials, the reference OVAKO 234S (16MnCr5), and pure
ferrite are shown in figure 59.
     With greater increases in passive force there is a ‘reasonable’ reduction in tool life for
nearly all the P/M materials tested. A few fall out of the normal range found with these
materials. The high ferrite variants of ASC100.29 and Distaloy AE show increasing tool
life with increased passive force. ASC100.29+2% Cu+0.5% C and ASC100.29+2% Cu
+0.8% C both perform similarly to the wrought reference material.

                                                                                        Cutting Forces

Figure 60. Relative tool life as a function of relative increase in passive force for a range of P/M mate-
rials, indicating the ‘normal’ behaviour curve (with exceptions Distaloy AE, ASC100.29 variants and the
wrought reference material).

Influence of carbon content
The proportion of carbon content in P/M materials is known to be critical to
machinablity. Figure 60, figure 61 and figure 62 show the effect of carbon content on
passive force for a range of feed rates.
    With ASC100.29+2% Cu, carbon content variants showed a clear ranking at
f=0.1mm/r (see figure 60). With increased feed rate the rank order was completely
reversed (cf. values at f=0.1 and f=0.3 mm/r). This effect was not the case for the
Distaloy AE carbon variants (see figure 61) where the carbon free Distaloy AE has a
slightly higher increase but not enough to alter the fact that a higher carbon content (and
hardness) gives a higher passive force. The ranking of materials with the same carbon
level (0.5%) but different microstructures and hardness levels, remains the same at 0.3 as
for 0.1 mm/r with the harder materials giving higher passive forces (see figure 62).

Machinability: Turning

                                         Feed rate (mm/r)

Figure 61. Effect of carbon content on passive force at three feed rates for cutting of ASC100.29 +
2% Cu.

Figure 62. Effect of carbon content on passive force at three feed rates for cutting of Distaloy AE.

                                                                                      Cutting Forces

Figure 63. Influence of feed rate on passive force at three feed rates for cutting of five materials of
various hardness and microstructure. Materials: A= Distaloy HP+0.5% C, B= Distaloy AE+0.5% C,
C= ASC100.29+2% Cu+0.5% C, D= Astaloy Mo+0.5% C, E= Reference OVAKO 234S (16 MnCr5).

Influence of alloys
Passive forces were investigated in relation to ASC100.29 variants which included
copper and phosphorus. The differences between the pure ferrite material with and
without additions (2%Cu or 0.45%P) were very small (see figure 63). All three variants
gave great increases in the cutting force with increased feed rates. The hardest material
showed the smallest increase. However, the increased forces do not seem to influence the
tool life or surface finish in any negative way with these variants.

Figure 64. Influence on passive force of addition of copper and phosphorus to ferrite material, at
three feed rates. (Key to materials: A = ASC100.29; B = ASC100.29+0.45% P; C = ASC100.29 +
2% Cu.)
Machinability: Turning

Influence of density
Increased density did not influence the cutting forces in any clear way over the interval of
feed rates investigated (see figure 64). The density variant (d=6.7, Hv=110) showed a
greater increase than harder, high density variants.

Figure 65. Influence of density on passive force during cutting of ASC100.29+2% Cu+0.5% C, at
three feed rates.

Machinability enhancement
Machinability enhancing additive MnS and oil impregnation gave good results, lowering
cutting forces considerably at f=0.1 mm/r. The effect was even more accentuated at high
feed rates. Oil impregnation was found to be so interesting with regard to tool life and
cutting forces that additional testing was performed.
    The impregnated variants lead to substantial cutting force reductions as shown in
figure 65 and figure 66. The reductions range between 10 and 20% and seem to increase
with higher feed rates for Distaloy AE+0.5% C, while they are reduced for ASC100.29+
2% Cu+0.5% C when the feed is increased.

                                                                                     Cutting Forces

Figure 65. Reduction in cutting forces (in %) with oil impregnation of Distaloy AE+0.5% C. Fx: feed
force; Fy: passive force; Fz: main force.

Figure 66. Reduction in cutting forces (in %) with oil impregnation of ASC100.29+2% Cu +0.5% C. Fx:
feed force; Fy: passive force; Fz: main force.

Machinability: Turning

3.7 Summary: Machining of P/M Materials

The machinability of materials produced with powder metallurgy varies considerably.
The various types of P/M materials impose different demands on the cutting edge; some
have excellent machinability, while others are rather demanding to machine.
    At one end of the scale are materials with high demands in abrasive wear resistance
on the tool. Soft low carbon materials are also demanding since they produce a high
degree of built up edge formation and smearing on tools, which in turn causes chipping
of the edge especially with interrupted cutting.
    The pores which are characteristic of P/M materials, give rise to micro-fatigue and
poor thermal conductivity. In many cases hard phases occur in a relatively soft matrix,
creating very high demands on the tool. Some P/M materials are very sensitive to burr
    However, by choosing proper tool material/insert geometry and working with the
specific behaviors of P/M materials, in many cases it is possible to reach a productivity
level as high as that of standard steel. In order to achieve the best possible machining
economics, care must be taken with several specific factors. These are summarized below.

Insert geometry
Use a geometry with positive rake and small edge rounding. The smaller the feed, the
more important is to ensure appropriate insert geometry. In many cases this applies to
interrupted cutting, especially when the selected grade has sufficient toughness.

Cutting tool
The type of cutting tool used should be selected carefully. Coated cemented carbide
(CVD and PVD) as well as Cermet and CBN (high CBN) tools can be used in P/M
machining. CBN should preferably be used with the high abrasive alloys of Distaloy AE
and Distaloy HP. Uncoated cemented carbide is seldom the best alternative unless a very
low cutting speed is used.

Cutting data
Cutting speed recommendations depend upon the grade of tool material used and the
type of P/M material being worked. Speeds can range from around 100 m/min for an
uncoated grade, up to 500 m/min with CBN.
    Selection of feed rate is of utmost importance with P/M materials, more so than with
most other workpiece materials in fact. With the exception of high speed applications, in
particular CBN, most operations are limited by flank wear development. An increase of
feed rate (i.e. chip thickness) does not seem to have any negative effect on flank wear. It
seems that flank wear is more dependent on the turned distance. Obviously the highest
feed rate possible should be selected; unnecessarily low rates should be avoided.

                                                       Summary: Machining of P/M Materials

There are now inserts on the market (e.g. Sandvik Coromant WF and WM) which
permit an increased feed rate while keeping surface roughness at a low level.

Oil impregnation
The use of oil impregnation will invariably improve machinability. Oil impregnation
also increases the possibility of obtaining the greatest benefit from wear resistant, coated
CVD tools.

Whenever possible the use of coolant and cutting fluid should be avoided. From a tool-
life point of view, it is usually better to machine dry. This is in particularly true with high
performance grades such as coated CBN and CVD tools.

Difficult materials
It is best to avoid working with pure iron. Whenever possible, include a small portion of
carbon which will change the material from one of the worst to one of the best to
machine within the P/M group. Have in mind also that a standard low- carbon steel
gives rise to problems due to smearing and built up edge. In pure iron there is no carbon
(i.e. ‘much less’ than in a low - carbon steel).

Machinability: Turning

3.8 Turning tool recommendations and cutting data

Table 6. Recommended Sandvik Coromant insert geometry and grade for
turning P/M materials.

                3005     3025         5015         1025         H13A

     UF           X        X            X            X            X

     UM           X        X            X            X            X

     WF           X        –            X            X            –

     WM           X        –            –            X            –

     KF           X        X            –            –            –

     KM           X        X            –            –            –

     MF           –        –            –            X            –

                                               Turning tool recommendations and cutting data

Table 6. Recommended start values for selected turning inserts (dry cutting and
15 min. tool life).
                       Specific   Recommended Cutting Speed, start value, m/min.
                       cutting   Based on labtests, VB crit.=0.30 mm, Tool Life: 15 min.
 Materials groups      force
                       Kc=0.20   GC3005      GC3025      GC1025      CT5015      H13A       CB7050

                                                            Feed, mm/r
                                 0.10-0.30   0.10-0.30   0.10-0.30   0.05-0.20   0.05-0.3    0.10

 Iron base
 ASC100.29               1900     220-230     195-200     195-200     115-125    115-120
 ASC100.29+0.45% P       2150     130-165     115-145     115-145     80-100     80-100

 ASC100.29+2% Cu         2050     175-210     155-185     155-185     100-120    100-120
 ASC100.29+2% Cu         2100     200-260     175-230     175-230     195-250    110-145
 +0.25% C

 ASC100.29+2% Cu         2150     220-250     195-220     195-220     205-235    120-130
 +0.5% C

 ASC100.29+2% Cu         2300     170-235     150-205     150-205     170-230    100-130
 +0.8% C

 Diffusion bonded
 Distaloy AE (4%Ni,      1900     200-210     175-185     175-185     150-150    95-100
 1.5% Cu, 0.5% Mo)

 Distaloy AE (4% Ni,     2100     165-170     145-150     145-150     135-140     85-90      465
 1.5% Cu, 0.5% Mo)
 +0.25% C

 Distaloy AE (4% Ni,     2200     150-165     130-145     130-145     115-125     75-85      430
 1.5% Cu, 0.5% Mo)
 +0.5% C
 Distaloy AE (4% Ni,     2200     120-135     105-120     105-120     105-115     65-75      375
 1.5% Cu, 0.5% Mo)
 +0.8% C

 Pre-alloyed with 1,5% Mo
 Astaloy Mo+0.5% C       2250     175-200     140-160     140-160     160-180    95-105

 Distaloy HP (4%         2500     95-120      85-105      85-105       75-95      50-60      300
 Ni,2% Cu)+0.5% C

Machinability: Turning

                                                      Quality and Performance in Machining

4     Drilling

This chapter is written in cooperation with Dormer Tools and IVF.
Drilling is a term covering all methods of making cylindrical holes in metal products.
The term can be divided into two categories: short hole and deep hole drilling. The
difference between the two is not restricted to the relationship between depth and
diameter. Parameters such as chip evacuation (i.e. quality and removal rate) also form the
basis for differentiating short and long hole drilling methods. The term drilling usually
also covers subsequent machining, including reaming, counter boring, and various forms
of finishing operations. For P/M components most drilling involves short holes.
     Drilling is a combination of two movements: a main rotating motion plus a linear
feed motion. With short hole drilling in conventional machines the most usual form of
working is that the tool does both rotating and feeding motion. The use of universal NC
and CNC controlled lathes for short hole drilling has lead to an increase in the
combination of a rotating workpiece and non-rotating drill.

4.1 Quality and Performance in Machining

Approximately 60% of all P/M-derived parts need some kind of machining. Geometric
qualities such as annular grooves, threads, undercuts and re-entrant angles, and the
demands of tight tolerances, are all reasons which may make a machining operation
necessary. It is common in machining of P/M-components to have small metal removal
    A stable set up, the right cutting tool, and accurate cutting data, are all prerequisites
for acceptable machining. A stable machine and set up, does not in itself, ensure good
machining, but it is essential for work of a consistent quality.
    Quality and productivity in machining operations rely on a combination of several
1. Machinability of P/M materials.
2. The machine performance.
3. Tool life: performance and quality of the cutting tool.

See figure 67 for a summary of vital considerations for approved cutting operations, and
an indication of the way these factors are related.

Machinability: Drilling

                                                 Workpiece setup,        Machine performance
                                                 max load.               stability.

Machinability         Choice of tool           Cutting forces:          Restrictions
P/M material.         and cutting data.        Torque.


Figure 67. Flow diagram of vital factors for approved machining operations.

4.2 Increasing machinability

P/M materials have a very wide machinability area - wider than that can be found in
ordinary steel materials. The choice of material is not simply a material factor but also a
machinability factor. Machinability enhancive additives give lower cutting forces in
drilling and lower torque when tapping, and in general they provide higher productivity.
Examples of additives are MnS and MnX. With such additives, the machinability of a
material can be substantially improved.
     The addition of MnX means that in drilling the cutting data can be increased and
still the same tool life can be achieved, or if the cutting data is unchanged, increased tool
life can be obtained. Alloyed materials and P/M materials with additives give better
possibilities for the modern HSS and even for carbide tools when machining dry.
     Carbon content is an important factor for machinability. Alloys without carbon
produce tough chips that are hard to break, while carbon contents over 0.5 % accelerate
abrasive wear on the tool and cause lower tool life.

4.3 Classification of P/M Materials for Drilling

Since P/M technology allows many possible alternatives with additions of alloying
elements and additives which enhance machinability, there is a large range of mixes on
the market. The classification of P/M materials is necessary to allow specific machining
recommendations. In this section a system is presented to classifying materials according
to recommended drilling operations.

Test materials
Prior to classification of materials, experimental parameters were established. To set a
standard for cutting data with a drilling operation, the relationship between cutting

                                                  Classification of P/M Materials for Drilling

speed and hardness was demonstrated. Test materials were cylindrical plates (∅ 80mm,
H = 10mm) compacted to 7.0 g/cm3 produced from the materials indicated below.
Density was evaluated at 6.7, 7.0 and 7.3 g/cm3. Carbon enriched components were
sintered in endogas with 0.3% CO2, and for carbon-free components dissociated
ammonia was used. All components were sintered in a production furnace at GKN
Sinter Metals AB, Sweden. Sintering was at 1120°C for 30 minutes. The range of P/M
materials investigated is shown in Table 8.
    The main drilling evaluation was performed by IVF, Sweden, in a MODIG-CNC
machine center. As tool life criteria, total failure of the drill was used [Ref. 1]. For the
main investigation a drill diameter of 4 mm was used. Performance was evaluated in dry
condition (no cutting fluid applied), unless otherwise stated. In all evaluations the
vT-curve test was used. (For more on the vT-curve test see Section 3.2. in the previous
chapter or ISO document No. 3685 from 1977).

Machinability: Drilling

Table 7. Ferritecontent of investigated materials.

                                        Iron base                                   Ferrite
 ASC100.29                                                                            100
 ASC100.29 +0.5% C                                                                    55
 ASC100.29 +0.45% P                                                                   100
 ASC100.29+0.45% P +0,5% C                                                            45
 ASC100.29+2% Cu                                                                      100
 ASC100.29+2% Cu +0.25% C                                                             80
 ASC100.29+2% Cu +0.5% C                                                              30
 ASC100.29+2% Cu +0.8% C                                                               1

                                    Diffusion bonded                                Ferrite
 Distaloy AE (4%Ni, 1.5% Cu, 0.5% Mo)                                                  90
 Distaloy AE (4% Ni, 1.5% Cu, 0.5% Mo) +0.25% C                                        35
 Distaloy AE (4% Ni, 1.5% Cu, 0.5% Mo) +0.5% C                                          8
 Distaloy AE (4% Ni, 1.5% Cu, 0.5% Mo) +0.8% C                                          2

                             Pre-alloyed with 1,5% Mo                               Ferrite
 Astaloy Mo+0.5% C                                                                     –
 Distaloy HP (4% Ni,2% Cu)+0.5% C                                                      1

Initial evaluation
An initial evaluation, of the relationship between cutting speed and hardness, was made
with a tool life of 200 holes and a feed rate of 0.06 mm/r with a High Speed Steel (HSS)
drill. As shown in figure 68, the speed/hardness relationship is insufficient to describe
machinability of P/M materials. Material with a microstructure containing more than
70% ferrite diverged from the standard relationship, therefore the amount of ferrite
present in materials must be taken into account when describing machinability. The
effect of feed rate on tool life and different drill types must also be taken into account, as
discussed later in this chapter.

                                                          Influence of Properties of P/M Materials
Cutting speed at tool life
200 hole

                                            Hardness (HV10)

Figure 69. General influence of hardness upon cutting speed for sintered iron materials. Divergence
from this relationship is seen in materials with more than 70% ferrite evident in microstructure or with
specific additives.

4.4 Influence of Properties of P/M Materials

Substitutional elements
The diffusion rate of alloying elements during sintering can be used as an indication of
the machining performance of a mix. For plain iron, the iron particle is ferritic. This is
also the case with many alloy materials. If copper, phosphorous, nickel, molybdenum,
et cetera, is added, a zone is formed around the central part of the iron particle due to the
slow diffusion rate compared to carbon at normal sintering conditions for these alloying
elements in the iron lattice. As a consequence, machinability is decreased. This is not
surprising: it is generally acknowledged that soft ductile materials have an inferior
cutting performance.
    The amount of ferrite present in the microstructure determines the performance of a
material in drilling (Refer Table 8 for 1-100 range of %-ferrite in test materials). Less
than 70% ferrite in the microstructure can be taken, as a rule of thumb, to mean
decreased performance compared to the hardness correlation (see figure 68).

Interstitial elements
Interstitial elements like carbon diffuse very rapidly in iron lattice. The formation of
pearlite of the central part of the iron particle is a consequence. From a machining point
of view this structure (alternate plates of pure iron and iron carbide) is “easy” to machine.
Dependent on the type of tool, an optimum amount of carbon can be identified. Shown

Machinability: Drilling

here is the influence of carbon addition for ASC100.29+2% Cu and Distaloy AE (see
figure 69).

                     Effect of carbon addition on relative tool life.
                        Feed rate = 0.06 mm/r, Dry, HSS drill

Figure 69. Influence of carbon content on relative tool life for two sintered iron materials.

Surface oxides which are more or less present in various P/M materials influence cutting
performance. Generally, oxides decrease the machining performance.
    The influence of oxides is well known for conventional materials [Ref. 4]. For P/M
materials steam treatment is used for some components in order to improve machining.
Here the oxide layer can act as chip breaker for ductile material. The tendency for
smearing can be avoided, which explains the improvement that is usually reported when
oxides are removed.

A large number of additives are reported to improve the machinability of P/M material
(e.g. MoS2, S, Se, Te, Bi, Pb, MnS, MnX, BN, glasses, plastic impregnation, and
compounds containing group V11B elements). These additives must fulfil two main

                                                          Influence of Properties of P/M Materials

1. Mechanical properties retained at same values as for material without additive.
2. Improved machinability.

Investigation of the influence of additives reveals that nearly all decrease the mechanical
properties of P/M materials. Additives as MnS, MnX, and BN, can produce satisfactory
influences on mechanical properties, for the sake of improved machinability.
    If decreased mechanical properties can be accepted, sulfur, selenium and tellurium
will improve machinability. The benefit is progressively better, from sulphur to selenium
and to tellurium, respectively [Ref. 5]. Additional effects such as rounded pore shape was
found for sulfur, tellurium (due to low boiling point), and MoS2 (due to dissociation
into elemental molybdenum and sulfur).
    Due to the marked effect on mechanical properties produced by other additives, only
MnS and MnX have been investigated regarding machinability.

MnS used as an additive, is commonly known to improve machinability. Addition of
0.5% is acceptable, taking into account the effect on mechanical properties. Evaluation
of the performance at a tool life of 200 holes and feed rate of 0.06 mm/r for a 4 mm
HSS drill revealed a large influence from MnS with plain iron. The effect decreased with
the amount of alloying elements (see figure 70). Lubrication on the cutting edge,
improved chip breaking, and a reduction in cutting force, are believed to be likely
reasons for the improvement seen with addition of MnS.
Relative tool life (%)

                         ASC100.29              ASC100.29+                       Distaloy AE+
                                               2% CU+0.5% C                         0.5% C

Figure 71. Influence of additive MnS on relative tool life for three P/M materials.

Investigation of the cutting force and torque when drilling, as influenced by MnS
addition, reveals a decrease in both cutting force and torque (see figure 71 and figure 72,
    With hole quality, MnS has a large influence. From investigation of Distaloy SA
(with density 6.2 and 6.6 g/cm3, sintering time 20 minutes at 1150°C in endogas

Machinability: Drilling

[Ref. 6]), torque and thrust increased with cutting speed for plain Distaloy SA, but
decreased with cutting speed when MnS was added. Increased feed rate caused increases
in both torque and thrust. Measured variations in surface finish and roundness of the
hole for Distaloy SA, with and without MnS addition, revealed strong relationships
regarding cutting speed and feed rate (see figure 73 and figure 74, respectively). MnS was
seen to make a marked difference, especially with roundedness. It should be noted that
in this investigation, a rather wide 95% confidence band was found with both variation
in surface finish and roundedness.

Figure 72. Influence at three feed rates of additives MnS and MnX on drill feed force, with Distaloy AE
+0.5% C.

                                                         Influence of Properties of P/M Materials

Figure 73. Influence at three feed rates of additives MnS and MnX on drill torque, with Distaloy AE +

                                                                                                   Roundedness (µm)
 Surface finish (µm)

                                       Feed rate (m/min)
Figure 73. The effect of drilling feed rate on variations in surface finish and roundedness with Distaloy
SA (with and without additive MnS).

Machinability: Drilling

                                                                                                   Roundedness (µm)
Surface finish (µm)

                                  Cutting speed (m/min)

Figure 75. The effect of drill cutting speed on variations in surface finish and roundedness with
Distaloy SA (with and without additive MnS).

MnX added at 0.3% has a smaller effect on mechanical properties compared to MnS
[Ref. 7]. Improved results were achieved compared to 0.5% MnS addition, upon testing
with Distaloy AE+0.5% C. For high strength materials, the effect of MnX is greater.
With the mix ASC100.29+2% Cu+0.5% C, the effect is in the same range as 0.5%
MnS. (See figure 75; compare with figure 70.)
    The effect of MnX is in the same range or better compared to MnS, in regard to feed
force (thrust) and torque (see figure 71 and figure 72, respectively).

                                                       Influence of Properties of P/M Materials

Figure 75. Effect of MnX additive on relative tool life for two P/M materials. A = ASC100.29+2% Cu+
0.5% C+0.3% MnX. B = Distaloy AE+0.8% C+0.3% MnX.

The additive MnX prevents the decreases in machinability which otherwise occur when
machining feed rates are increased (see figure 76). In this case machinability is measured
by relative tool life.
    To fully utilize the benefits of MnX, the machine feed rate has to be increased. The
shorter tool life at higher feed rates does not balance the apparent increase in
productivity. Another conclusion is that MnX has an improved effect for high
performance grades of P/M mixes. It must be stated that drilling Distaloy AE +0.8% C
(or 0.5% as per figure 76) with an uncoated HSS drill is, from a productivity point of
view, not to be recommended without addition of MnX.

Machinability: Drilling
Relative tool life (%)

                                        Feed rate (mm/r)

Figure 76. Influence on relative tool life of MnX added to two P/M materials, at various feed rates.
A=ASC100.29 +2% Cu+0.5% C. B=ASC100.29+2% Cu+0.5% C+0.3% MnX. C=Distaloy AE+0.5% C.
D=Distaloy AE+0.5% C+0.3% MnX.

The main investigated mix (ASC100.29+2% Cu+0.5% C) revealed an increase of 5% in
tool life for the density range of 6.7 to 7.3 g/cm3. It is reported that a better machining
response, in terms of reduced drill torque and thrust levels, is obtained with increased
densities [Ref. 6].

Oil impregnation
The option of oil impregnation is one advantage that P/M technology can offer
compared with conventional steel. Internal lubrication on the cutting edge is possible
when material has been impregnated with oil. As a consequence, the cutting force
required and the variation of cutting forces are decreased. Performance for oil
impregnation compared with cutting fluid and dry cutting of Distaloy HP+0.5% C is
shown in figure 77.
    In recent years an alternative with minimal lubrication has been introduced. This
involves applied oil suspension on the tool every time it leaves the work material. Initial
tests reveal improvements in the same range as cutting fluid and oil impregnation.

                                                                                        Tool Materials

                  Dry f=0.2 mm/r                 Liquid coolant                Oil impregnation
                                                 f=0.12 mm/r                   f=0.12 mm/r

Figure 78. Cutting performance in terms of relative tool life for three conditions with Distaloy
HP + 0.5% C: dry cutting, fluid-aided cutting, material oil-impregnated.

4.5 Tool Materials

Several grades of high speed steel and cemented carbide materials have been vital in the
development of machining tools for P/M components. Some of these materials have
themselves been produced with P/M technology. In a later Section 4.7 on drill selection
the application of these materials in the manufacture of drills will be apparent. The
general properties of these materials are covered in this section, along with a comparison
of two important tool materials. Treatment processes used with alloyed steels to achieve
high quality tools are discussed in the next section.

HSS - High speed steel
Since its introduction at the end of the 19th century high speed steel has become one of
the most important materials in the manufacture of cutting tools. High speed steels
exhibit hardness, toughness and wear resistance, characteristics which make them useful
in a wide range of applications.

HSCo - Cobalt high speed steel
Over the years new types of high speed steels have been developed. The principal
improvement in the field of alloying has been use of cobalt. This development has led to
grades that feature excellent high temperature strength properties without compromise
to wear resistance and toughness. Drills, milling cutters, reamers and taps manufactured
from these grades meet the exacting demands of high productive machining.

Machinability: Drilling

HSCo XP - Sintered cobalt high speed steel
HSCo XP is a cobalt high speed steel produced using powder metallurgy technology.
High speed steel produced by this method exhibits superior toughness and grindability.
The use of XP steels is particularly advantageous when machining materials that are
difficult to cut or when the material is extremely hard. Taps and milling cutters have
particular advantage when made from XP grade steel.

Cemented carbide
The carbide grades P40 and K10 are most often used in the manufacture of solid carbide
drills. Micrograin K10 grades are today most common thanks to their good combination
of hardness and toughness. In the hardest and most abrasive P/M materials carbide drills
are essential to attain acceptable productivity and tool life. K10 micrograin grade
cemented carbide typically consists of 10% cobalt and 90% tungsten carbide (WC).

Comparison of two tool materials
In figure 78 comparison is shown between two materials used to manufacture high
performance tools. K10 grade cemented carbide rated higher than HSCo on three of the
four physical properties tested. The maximum value for each property was set at 100%
for purposes of the comparison.

            Relationship between HSCO and K10 regarding properties.

Figure 78. Relative ratings of two high performance tool materials on four physical properties.

Alloy content
Alloys have, of course, frequently been used in the production of high performance
steels. The alloy contents of those grades of steel most often used in drills and taps are
shown in Table 9.

                                                                              Tool Treatments

Table 8. Alloying proportions in four high speed steels.

     Grade           C%          W%          Mo%          Cr%          V%            Co%
       M2            0.83        6.0          5.0          4.0         2.0
       M35           0.80        6.0          5.0          4.0         2.0           5.0
       M42           1.10        1.5          9.5         3.75         1.15          8.0
    HSCo - XP        1.27        6.4          5.0          4.0         3.1           8.5

4.6 Tool Treatments

Tools materials may be treated with a variety of surface treatments and coatings. These
allow tools to better meet the demands of more economical machining and improved
quality of machined surfaces. The following surface treatments and coatings have been
used to good effect, particularly with P/M materials.

Heat treatment
The highly alloyed high speed steels used today in the manufacture of cutting tools
require precision heat treatment. Extensive experience in heat treatment utilising the
most modern equipment, such as vacuum furnaces, ensures that the optimum
combination of properties essential to the efficient performance of a tool can be achieved
consistently. Meticulous attention to detail in all aspects of heat treatment is the only
guarantee of high and consistent quality in tool production.

Steam tempering
Steam tempering gives a strongly adhering blue oxide surface that acts to retain cutting
fluid and prevent chip-to-tool welding, and thereby counteracts the formation of built
up edge. This is an advantage particularly in softer, less abrasive P/M materials
(ASC100.29, ASC100.29+2% Cu and Distaloy AE). Steam tempering can be applied to
any bright tool but its most useful applications are with drills and taps.

Nitriding is a process that is used to increase the hardness and wear resistance of the
surface of a tool. It is particularly suitable for taps used on abrasive P/M materials. Used
on twist drills when it is desirable to increase the strength and wear resistance of the
cylindrical lands.

TiN Titanium Nitride is a gold colored ceramic coating applied by physical vapor
deposition (PVD). High hardness combined with low friction properties ensure

Machinability: Drilling

considerably longer service life or alternatively, better cutting performance, with tools
that have been TiN coated. TiN is used mainly for coating of drills and taps. It gives in
most P/M materials longer tool life and allows tools to be used at higher cutting speeds.

AlTiN Aluminum Titanium Nitride is a multi layer ceramic coating applied by PVD
technology, which exhibits high toughness and oxidation stability. These properties make
it ideal for higher speeds and feed rates whilst at the same time improving tool life. It is
recommended to use AlTiN when machining abrasive P/M materials and for dry

4.7 Selection of Drill Type

Dormer Tools, as used in the tests reported here, are available in an extensive range of
standard and special drills. Materials and geometries are optimised to take account of the
cutting behaviour of particular work-pieces. When drilling a particular material at a
given speed and feed rate, drill performance is governed by the drill quality and a range
of other related factors. These are summarised below.
Factors influencing drilling performance:
•   P/M-Material Drilled
•   Choice of Tool Holding
•   Depth of Hole
•   Stability of Work-piece Hold
•   Through or Blind Hole
•   Horizontal or Vertical Drilling
•   Dry or Cutting Fluid
•   Stationary or Revolving Drill
•   Condition of the Machine
•   Swarf Control
•   Machine Power Capacity
Most drills could be used to produce a hole in any material, but at what cost?
Commercial production requires skilful matching of tools, drills, materials and
machining processes. For maximum productivity in terms of greater tool life, hole
accuracy and other indicators of optimum performance, careful selection of drills, drill

                                                                               Selection of Drill Type

speeds and feed rates is essential. The tables given in the final section of this chapter
provide recommendations of drill types for different materials and applications (see
Section 4.14, figure 88 and Table 14).
    The selection of drill type should be related to both the hardness of the material
being machined and productivity. This is shown schematically in figure 79.

Figure 79. Three drill types as a function of productivity and hardness of material to be drilled.

A range of High Speed Steel (HSS), High Speed Cobalt (HSCo) and solid carbide drill
types has been tested with P/M materials. Those drills listed in the recommendations
given later in this chapter (see figure 88 on page 117 ), are presented in the following
sections. For details of drills with other than cylindrical shanks see Dormer catalogues.

HSS standard drills
Standard HSS jobber drills for general applications and hole depths down to 4xD are the
A100, and for depths to 2.5xD, the A120. The shorter A120 has a special split point that
ensures easy starting and accurate drill location. Both drills have a standard cylindrical
shank (see images A and B in figure 80).
    Standard HSS drills are useful with a large variety of P/M materials. They show best
performance in soft materials where an open point geometry ensures the cut material
will cleanly leave the flutes.

Machinability: Drilling

                          (a)                           (b)

Figure 80. Standard HSS drill, (a) A100 and (b) A120.

HSS/HSCo high performance drills
The high performance family of ADX drills, which includes A510 and A520, has a
patented design. The design features include a quick helix, a 130 degree point with
special thinning, optimised flute space, and a bowed convex lip shape. The benefits
include excellent swarf removal and accurate holes normally to H9. The major
advantage, however, is high attainable productivity. The special design combined with
the wear resistance of the TiN-coating substantially performs better than the uncoated
drills in most P/M materials. The A510 (Image C in figure 81) can be used down to 4xD
and the A520 (Image D in figure 81) down to 2.5xD. Both have cylindrical shanks.

                          (c)                            (d)

Figure 82. High performance drill, (c) A510 and (d) A520.

                                                                            Selection of Drill Type

High performance solid carbide drills
Coated high performance solid carbide drills are known as the CDX family of drills.
They include R510 (Image E in figure 82) and R520 (Image F in figure 82). Their rigid
design with its special convex lip shape ensures excellent swarf removal. Hole size
generally within H8 limits is produced, with good hole surface finishes and excellent
positional accuracy.
    Solid carbide drills are the only acceptable choice for machining of high carbon
content Distaloy AE and Distaloy HP materials.

                        (e)                                        (f)

Figure 83. High performance coated solid carbide drill, (e) R510 and (f) R520.

Machinability: Drilling

4.8 Drill Dimensions

Various dimensions of a drill and drilling task must be considered to determine optimal
match of tool and operation. The effects of changes in drill length, hole diameter, and
feed and speed rates, are addressed in this section.

Drill length
One of the demands for an approved operation is good stability. The presence of pores in
the microstructure of a material leads to vibrations during drilling. Vibrations may be
minimized if a short drill length is used. This has been demonstrated with a cemented
carbide tool of different sizes (R510/R520). The effect increases with hardness, and was
significant for Distaloy AE 0.5% C in figure 83. With Distaloy HP+0.5 % C the effect is
also large.

Figure 84. Difference in relative tool life with use of short (R520) and long (R510) drill for drilling of
Distaloy AE+0.5% C. Drilled dry at 0.12 mm/r.

Hole diameter
Nominal hole diameters vary somewhat between different types of drills. CDX and ADX
drills can, under the right cutting conditions and with a stable set up, produce holes with
a tolerance of H8 and H9, respectively. Standard drills reach a tolerance of H12. Actual
nominal hole diameters (in mm) for a range of drills are shown in Table 10.

                                                                     Use of Cutting Fluids

Table 10. Selection of four drills showing nominal hole diameters: Deviations in

   Diameter       CDX = H8          ADX = H9          PFX = H9         A100 = H12
    (∅ mm)
       ≤3           0 / +0.014        0 / +0.025       0 / +0.040         0 / +0.100
    >3<6            0 / +0.018        0 / +0.030       0 / +0.048         0 / +0.120
    > 6 < 10        0 / +0.022        0 / +0.036       0 / +0.058         0 / +0.150
   > 10 < 18        0 / +0.027        0 / +0.043       0 / +0.070         0 / +0.180

Hole depth and speed and feed rate
The depth of holes to be drilled has an influence on the setting of optimal speed and feed
rates. As the ratio of hole depth to diameter increases, speed and feed rates should be
proportionally decreased. The recommendations given in Table 11 can be used as a
guideline for calculating reductions in machine rates when drilling holes deeper than
twice their diameter.

Table 10. Recommended cutting speeds and feed rates at four hole depths.

   Hole depth              Percentage of                       Percentage of
                       recommended speed, Vc                recommended feed, f
       2xD                        100 %                              100 %
       3xD                         90 %                               90 %
       4xD                         80 %                               80 %
       5xD                         70 %                               70 %

4.9 Use of Cutting Fluids

The application of cutting fluids or liquid coolants generally improves machining
performance (see figure 84). If the additional operation to prevent oxidation can be
performed, taking the increased cost into account, the use of cutting fluids has a
beneficial effect. However, all recommendations given at the conclusion of this chapter
are for dry cutting conditions. This is due to the fact that oxidation of the component in
most cases cannot be tolerated.

Machinability: Drilling

Figure 85. Influence of cutting fluid on relative tool life with three P/M mixes. A=Distaloy AE+0.5% C,
A510 f=0.2 mm/r. B=ASC100.29+2% Cu+0.5% C, A510 f=0.2 mm/r. C=ASC100.29+2% Cu +
0.5% C, A100 f=0.06 mm/r.

The effect of cutting fluids is strongly dependent on the cutting speed. The α-value for
cutting speed (vT-curve) is lower if a fluid is used (see figure 85). For Distaloy AE the
same behaviour has been found [Ref. 8].

Figure 85. Effect of cutting speed on tool life (vT-curve) for drilling with and without cutting fluid.

Using cutting fluid when drilling with HSS-drills in P/M materials increases tool life at
factors approximately between 1.1 and 1.5 (see Table 12). These factors are valued for a
tool life of 300 hole. When the α-value is taken into account, large differences can be
expected if an evaluation is made at a different tool life.

                                                                   Hints for Optimal Drilling

Table 11. Correction factors for drill life increase with use of cutting fluid.

                 Drill                                        Factor
                 A100                                          1.25
                 A510                                        1.1 - 1.5

An additional conclusion for machining of plain iron (>70% ferrite; see figure 68) with a
PVD-TiN coated HSS drill (A510), is that there is much to gain by use of cutting fluid
and an increased cutting speed. With these materials, cutting fluid will have more of a
lubricating than cooling effect. Taking into account the additional operation required to
protect the component from oxidation, the benefit of cutting fluid should be considered.
    The total cost of the use of fluids, work piece cleaning and the required changes to
the work environment should be compared with the cost of modern drilling tools,
especially those made of coated HSS and solid carbide. Coated carbide tools may be used
so that they have an impressively long tool life, even with low-speed machines, if special
attention is given to the control of vibrations. (See below for more on cutting data with
specific drills.)

4.10 Hints for Optimal Drilling

1. Select the best type of drill for your application. The tables at the end of this chapter
   indicate the best drill to suit the P/M-material being drilled and give recommended
   speeds and feed rates.
2. The work piece must be held rigid and the machine spindle should have no runout.
   Rigidity can also be helped by using as short drill as possible.
3. The holder in which a straight shank drill is held must be of good quality. If the drill
   slips in the holder and the feed is automatic, breakage of the drill can be the result.
   With drills up to ∅6 mm a runout in the holder of up to 0.015 mm can be accepted.
4. If possible use recommended lubricants for cutting fluid to enhance the life of the
   tool. Ensure lubricants reach the drill point.
5. Do not allow the flutes of a drill to become choked with swarf, especially on holes
   deeper than 4X diameter. Withdrawal to clear the swarf may be required. Use special
   parabolic flute drills for deeper holes.
6. Do not use 2-fluted drills to open out existing drilled or cored holes as they are not
   designed for this application.

Machinability: Drilling

7. When the drill is reground ensure that all wear is removed and check that the correct
   point geometry is produced. Keep drills sharp.

Hints for carbide drilling
1. Use shortest possible drill.
2. Use machines with adequate stability.
3. Avoid unstable or weak tool-holders and work-pieces.
4. When vibrations are present, carbide tools are more prone to chipping than
5. Use shortest possible overhang for the application. It is better to use modular
   adapters than to have a long overhang on the tool.
6. The tool-holder is very important so use holders of good quality and with small
   runout (up to 0.01 mm).

                                                              Economy and Productivity in Drilling

4.11 Economy and Productivity in Drilling

Machining cost and productivity are probably the two most important parameters to
consider when deciding which tool that should be used and how to set up a drilling
process. The information in this section gives an indication of what can be expected
from different tools with four P/M materials.

Cost per hole
Economy in drilling has been determined according to cost per hole for a standard
machining procedure. Calculations of cost per hole were made on cutting data giving a
tool life of 15 minutes, no regrinding and a drill depth of 2xD. Tests were conducted to
allow comparisons with a standard iron powder and three P/M alloy materials when
machined with appropriate drill. (See figure 86 and refer to Section 4.7 above regarding
drill types.)

Figure 87. Comparative cost per hole with recommended drills (from a selection of three drills) in four
sintered iron materials. A=ASC100.29. B=Distaloy AE+0.5% C. C=Distaloy AE+0.8% C.
D=Distaloy HP+0.5% C.

The results shown in figure 86 indicate the economic limits of different tools. Material
classified as group 1 (i.e. ASC100.29 alloys) can be machined economically by a HSS
tool (A100). There is a clear difference for Distaloy AE+0.5% C: a coated HSS drill
(e.g. A510) should be selected. For Distaloy HP+0.5% C, the cemented carbide tool
(R520) replaces the coated HSS (A510) as the best choice.
    The limits shown in figure 86 are only indications of drilling-material combinations
with different tools, and are likely to be the best choice from an economic point of view.

Time per hole
Productivity in drilling was determined according to time per hole for a standard
machining procedure. Calculations of seconds per hole were made on recommended

Machinability: Drilling

cutting data and a drill depth of 2xD. See figure 87 for productivity results relevant to
the tool and material combinations as used in the above test for economy.

Figure 87. Comparative time for drilling single holes with recommended drills (from a selection of
three drills) in four sintered iron materials. A=ASC100.29. B=Distaloy AE+0.5% C.
C=Distaloy AE+0.8% C. D=Distaloy HP+0.5% C.

4.12 Setting Machine Limits

The drilling machine and the set up for a work piece are required to conform to
particular limits in order to maintain a certain quality of output and tool life. The limits
set should provide for the cutting forces and power input required, according to the
choice of tool and cutting data. The traditional choice of shortest possible drill is
important when machining P/M materials, as is a well-centering holder and a good
fixture of the work piece.
    It is difficult to set a stiffness value for a drilling machine in the same way as for
turning and milling operations. One procedure to reach a good reference for drilling
machines, is a follows:
1. Choose a P/M material similar to one listed in Table 8.
2. Use a drill for which specific feed force and cutting force values are indicated.
3. Determine the data from the cutting data chart (see figure 88).
4. Use the formulae shown below to calculate actual feed and cutting forces.
5. Test the drilling operation with the machine in question.

                                                                   Formulae for Cutting Forces

6. Evaluate as follows:
    i. If the operation is running well: Repeat the test with higher cutting data until
    there are vibrations or unacceptable work quality.

    ii. If the operation fails: Repeat the test with reduced cutting data until the opera-
    tion proceeds smoothly.

The cutting force levels thus determined, should be considered as the limit for the
machine. These limits, in cutting and feed forces, can then be used in planning new
operations and other diameters. (It is important to note that drilling with some tool wear
will increase the cutting force by up to 25%.).

4.13 Formulae for Cutting Forces

The following formulae are useful for calculating cutting data.

      Factor            Symbol                 Formula                     Measurement
 Cutting speed             Vc              Vc = (π·Dc·n) / 1000                m/min
 Rotational speed           n             n = (Vc·1000) / (π·Dc)               rev/min
 Feed rate                  f                    f = fn·n                      mm/min

The maximum rotational speed (rpm) and the machine stability affect tool choice and
the choice of cutting data. Specific feed force and torque for some significant P/M

Machinability: Drilling

Table 12. Specific feed force and cutting force for A100 and R520 drills and a
range of P/M materials.

                     Specific feed force, kf                  Specific cutting force, kc

      Material groups            A100            R520          Fn=0.16-0.06 mm/rev
                              Fn=0.16-0.06    Fn=0.12-0.20
                                mm/rev          mm/rev
 Iron base
 ASC100.29                      2800-4600                            4600-5800
 ASC100.29+0.5% C               1900-3100                            2800-3500
 ASC100.29+0.45% P              2000-3300                            3800-3500
                                3100-5000                            2800-3500
 P+0.5% C
 ASC100.29+2% Cu                2600-3800                            4600-5800
 ASC100.29+2% Cu+
                                2400-3500                            4100-5200
 0.25% C
 ASC100.29+2% Cu+
                                2400-3500                            2900-3700
 0.5% C
 ASC100.29+2% Cu+
                                2900-4200                            2800-3500
 0.8% C
 Diffusion bonded
 Distaloy AE (4% Ni+1.5%
                                2200-3500                            3700-4600
 Cu+0.5% Mo)
 Distaloy AE (4% Ni+1.5% Cu
                                3600-5000                            3200-4000
 + 0.5% Mo)+0.5% C
 Distaloy AE (4% Ni+1.5% Cu
                                                3500-4400            2700-4600
 + 0.5% Mo)+0.8% C
 Pre-alloyed with 1,5% Mo
 Astaloy Mo+0.5% C              2700-4400                            3500-4400
 Distaloy HP (4% Ni+2% Cu)
 + 0.5% C

The formulae below can be used for approximate calculations of feed force and net
power consumption. (NB: Wear on tool and machine efficiency is not considered in
these calculations.)

                                                    Drill Recommendations and Cutting Data

         Factor             Symbol                 Formula                      Measurement
 Drill diameter                Dc                                                   mm
 Feed per revolution           fn                                                 mm/rev
 Specific feed force            kf                from Table 12                     N/mm2
 Specific cutting force         kc                from Table 12                     N/mm2
 Feed force                    F                F = (Dc·fn·kf) / 2                  N
 Power                         P          P=   (D2·f·k                240·106       kW
                                                         c·n·π)   /

4.14 Drill Recommendations and Cutting Data

In the following tables recommendations are given for drills to match materials in three
P/M alloy groups. In the drill table (figure 88), each recommended drill application is
rated as either excellent or acceptable. The speed and feed rates given are start values for
dry cutting of P/M material with a density of approximately 7.0 g/cm3. For each drill
application, numerical data represent recommended cutting speeds for a tool life of 15
minutes. With each cutting speed is a letter (T - Y) indicating recommended feed rate
for the cutting operation. The key to the letter notation is given in Table 14 on page 118,
where the code is transformed to numerical data (i.e. feed rate recommended for various
diameter drills).

Machinability: Drilling

                  Selection of Drills                      Standard drills              Applica
                                                          A120         A100      A520
             Recommendations and cutting data                          A001      A521
 ■ Excellent for application.
 ● Acceptable for applications

 50 = Peripheral speed m/min. mid range +/- 10%.

 V = Feed per rev – See seperate Feed chart drills.

 All recommendations dry, when machining with emulsion
     recommendations dry, when machining with
 speed by 10 50 %.
 cutting fluid -increase speed by 10-50%.

                                                          HSS         HSS      HSS/HSCo
                     Material groups                     2.5 x D      4xD       2.5 x D
                                                          Blue        Blue        TiN
 Iron base
 ASC 100.29                                              ■ 50 V       ■ 50 V
 ASC 100.29 + 0.5% C                                     ■ 64 U       ■ 64 U
 ASC 100.29 + 0.45% P                                    ■ 38 V       ■ 38 V
 ASC 100.29 + 0.45% P + 0.5% C                           ■ 24 U       ■ 24 U    ■ 41 Y
 ASC 100.29 + 2% Cu                                      ■ 24 V       ■ 24 V
 ASC 100.29 + 2% Cu + 0.25% C                            ■ 50 U       ■ 50 U    ■ 87 Y
 ASC 100.29 + 2% Cu + 0.5% C                             ● 45 T       ● 45 T    ■ 78 Y
 ASC 100.29 + 2% Cu + 0.8% C                             ● 25 T       ● 25 T    ■ 44 Y
 Diffusion bonded
 Distaloy AE (4% Ni, 1.5% Cu, 0.5% Mo)                   ■ 31 U       ■ 31 U    ● 45 X
 Distaloy AE (4% Ni, 1.5% Cu, 0.5% Mo) + 0.25% C         ■ 31 U       ■ 31 U    ■ 48 Y
 Distaloy AE (4% Ni, 1.5% Cu, 0.5% Mo) + 0.5% C          ● 20 T       ● 20 T    ■ 31 Y
 Distaloy AE (4% Ni, 1.5% Cu, 0.5% Mo) + 0.8% C                                 ■ 20 X
 Pre-alloyed with 1,5% Mo
 Astaloy Mo + 0.5% C                                     ■ 30 U       ■ 30 U    ■ 47 Y
 Distaloy HP (4% Ni, 2% Cu) + 0.5% C                                             ●8X

                                                                   Drill Recommendations and Cutting Data

pplication drills, HSCo                          Application drills solid carbide
         A510             A577               R520                R522                R510
         A511                                R550                R552

o     HSS/HSCo            HSCo              K10                  K10                 K10
        4xD               6xD              2.5 x D              2.5 x D              4xD
         TiN              AlTiN              TiN                 AlTiN               TiN

        ■ 41 X            ■ 41 V

        ■ 87 X            ■ 87 V
        ■ 78 X            ■ 78 V
        ■ 44 X            ■ 44 V

        ● 45 W            ● 45 U
        ■ 48 X            ■ 48 V
        ■ 31 X            ■ 31 V           ■ 80 W               ■ 80 W               ■ 60 V
        ■ 20 W            ■ 20 V           ■ 80 W               ■ 80 W               ■ 60 V

        ■ 47 X            ■ 47 V
        ●8W               ●8U              ■ 83 W               ■ 83 W               ■ 50 V

        Figure 88. Recommended drill selection from eight drill types for a range of P/M materials. Cutting data
        is given for each recommended application (i.e. peripheral speed in meters/minute and feed rate code as
        per Table 14).
Machinability: Drilling

Table 14 indicates recommended feed rates for six drill sizes. The feed rates given (mm/r)
apply to drill applications as recommended in the drill table shown in figure 88.

Table 14. Drill feed chart.

   Feed code
                     3 mm     5 mm        8 mm       10 mm        12 mm         16 mm
        T             0.04     0.06        0.09        0.11         0.13         0.17
        U             0.07     0.09        0.14        0.17         0.20         0.23
        V             0.10     0.13        0.20        0.25         0.28         0.32
        W             0.13     0.17        0.26        0.33         0.38         0.43
        X             0.15     0.21        0.33        0.42         0.48         0.55
        Y             0.18     0.26        0.43        0.55         0.70         0.70

                                                 Classification of P/M Materials for Tapping

5     Tapping

This chapter was written in cooperation with Dormer Tools and IVF.
Tapping is one of the most difficult machining operations, according to a study among
Höganäs customers. This would explain the reported use of oil impregnation to assist the
operation. Straight flute taps of coated or uncoated high speed steel (HSS) are most
often used.
    Tapping is closely related to drilling. There is an obvious need for a hole to perform a
tapping operation. The diameter of the hole is very important factor in determining
both thread quality and the useful life of a tap. A correct thread can only be made if the
hole is round and straight.
    The recommendations given regarding drilling also apply to obtain a good thread.
(See previous chapter 4.)

5.1 Classification of P/M Materials for Tapping

Materials investigated for performance with tapping were the same as those evaluated for
drilling (see Chapter 4, Table 8). The tap used for the main evaluation was a straight
flute tap (E500).
    With tapping in the dry condition, chip clamping severely influenced performance
and therefore it was decided to perform the main comparison of materials with use of
applied emulsion.
    To classify P/M material regarding tapping, evaluation of tool life of the tap is
necessary. The only investigation made was of torque. For this reason the classification
stated in the drilling and turning evaluations was used. Torque evaluated at a speed of
6.3 m/min (straight flute tap, applied fluid) showed a correlation to hardness
(see figure 89).

Machining of P/M materials regarding Tapping

Figure 90. Correlation of torque in tapping and hardness of a range of P/M materials (as per Table 8).
Tapping with E500 straight fluted tap, applied emulsion.

High strength material such as Distaloy AE + 0.8% C was only possible to tap if 0.3%
MnX was added. Materials with superior properties were impossible to tap. A further
conclusion from this evaluation is that density has a strong influence on torque during
tapping. Additives as MnS and MnX and minimal lubrication decrease the torque.

Wet versus dry tapping
The turning and drilling evaluations (see previous chapters) demonstrated the
fundamental influence in machining of the amount of ferrite present in the
microstructure of materials. However, tapping with emulsion revealed no such
relationship (see figure 89). With dry tapping, the effect of ferrite was apparent (see
Table 15).
    There is a clear difference between the two machinability groups identified (viz.
alloys of ASC100.29 and Distaloy AE). With Distaloy AE+0.5% C, chip length was
much smaller compared to ASC100.29+2% Cu+0.5% C. This would explain the
performance shown in Table 15, torque values for tapping under dry and wet conditions,
with comparison of P/M materials according to proportion of ferrite present in
microstructures. For dry tapping the amount of ferrite must be taken into account.

                                                                                 Influence of Additives

Table 14. Torque values for tapping under dry and wet conditions, with
comparison of P/M materials.

             Material groups                         Ferrite    Torque Liquid fluid      Torque dry
                                                      (%)             (Nm)                 (Nm)
 Iron base
 ASC100.29+2% Cu                                      100                  –                     –
 ASC100.29+2% Cu+0.25% C                               80                 0.75               1.57
 ASC100.29+2% Cu+0.5% C                                30                 0.85               1.26
 ASC100.29+2% Cu+0.8% C                                1                  0.98               1.47
 Diffusion bonded
 Distaloy AE (4% Ni, 1,5% Cu, 0,5% Mo)                 90                 0.7                1.81
 Distaloy AE (4% Ni, 1,5% Cu, 0,5% Mo)+0,25% C         35                  1                 1.19
 Distaloy AE (4% Ni, 1,5% Cu, 0,5% Mo)+0,5% C           8                 1.17               1.36
 Distaloy AE (4% Ni, 1,5% Cu, 0,5% Mo)+0,8% C           2                  –                     –

5.2 Influence of Additives

Different types of additives used with P/M materials are described in Chapter 4 (see
Section 4.3). Investigated in relation to tapping was the effect of 0.5% MnS and 0.3%
MnX (see Table 16). It is clear that these two additives affect the tapping operation
differently. They also have different effects on each machinability group.

Table 15. Torque values for tapping of two P/M alloy groups with addition of
additives MnS and MnX, under dry and wet conditions.

                Material groups                             Torque Liquid fluid         Torque dry
                                                                  (Nm)                    (Nm)
 Iron base
 ASC100.29+2% Cu+0.5% C                                            0.86                    1.51
 ASC100.29+2% Cu+0.5% C+0.5% MnS                                   0.91                    0.83
 ASC100.29+2% Cu+0.5% C+0.3% MnX                                   0.79                      1.05
 Diffusion bonded
 Distaloy AE (4% Ni, 1,5% Cu, 0,5% Mo)+0,5% C                      1.17                    1.36
 Distaloy AE (4% Ni, 1,5% Cu, 0,5% Mo)+0,5% C+0,5%                 0.96                      1
 Distaloy AE (4% Ni, 1,5% Cu, 0,5% Mo)+0,5% C+0,3%                 1.07                    1.37

Machining of P/M materials regarding Tapping

In the turning evaluation, the forces acting on the tool decreased with MnS addition (see
Chapter 3, figure 37, figure 38 and figure 39).
    Comparison of the MnS effect on torque for emulsion and dry tapping revealed a
large decrease in torque under the dry condition. In tapping of ASC100.29+2% Cu+
0.5% C with applied emulsion, there was a small increase in torque.
    With Distaloy AE+0.5% C, the torque decreased. The effect from MnS seems to
increase with hardness.

It is clear from the evaluation that MnX has no effect on tapping Distaloy AE +0.5% C
under a dry condition. For ASC100.29+2% Cu+0.5% C the effect is significant.
Improved chip breaking would explain the difference in performance.
     For material with a high amount of ferrite, MnX addition in combination with
lubrication by either oil impregnation or minimal applied lubricant, is believed to be
beneficial. In the case of tapping with a cold forming tap, the thread is formed by plastic
deformation of the material. Internal lubrication (oil impregnation) or MnS addition is
believed to be beneficial for the threading operation. Some kind of lubrication is very
important when using cold forming taps.

                                                                          Selection of taps

5.3 Selection of taps

A range of taps was investigated for use with P/M materials. The four main taps tested
are shown in Table 17. Different types of tool materials influence tap performance (see
Chapter 4, Section 4.5 for discussion of tool materials). Details of surface treatments and
coatings used with tool materials were also given in Chapter 4 (see Section 4.6).

Table 17. Images of four types of taps evaluated for use with P/M alloys.

  Spiral point tap       Spiral flute tap      Straight flute tap       Cold forming tap

Descriptive characteristics of taps used in these evaluations are shown in figure 90. The
illustrations serve as definitions for those characteristics that are important in the
performance of taps.

Machining of P/M materials regarding Tapping

Figure 90. Defining characteristics of quality taps.

Five main hole configurations were considered for tapping. Figure 91 illustrates these
hole types. Guidelines for tap selection and general recommendations for tapping of
P/M materials are given below.

             1                  2                 3         4             5

Figure 91. Five main type of holes in tapping operations.

                                                                          Selection of taps

Straight fluted taps
Straight flutes are the most commonly used type of tap. Suitable for use on most
materials, straight-fluted taps form the basis of most tapping operations. They are
recommended for hole types 1, 2 and 3. The E500 is the model of this type evaluated

Spiral point taps A tap with a straight, fairly shallow flute is often referred to as a gun
nose or spiral point tap. The gun nose or spiral point is designed to drive the swarf
forward. The relatively shallow flutes ensure that the section strength is maximised and
they also act to allow emulsion to reach the cutting edges. This type of tap is
recommended for threading through-holes and is used for hole types 1 and 2. It may also
may be used in blind hole applications where there is sufficient space to accommodate
the swarf, hole type 3. The model evaluated was the E509.

Spiral fluted taps
Taps with spiral flutes are intended primarily for threading in blind holes of type 3, 4 or
5. The helical flute transports the swarf back away from the cutting edges and out of the
hole thus avoids packing of swarf in the flutes or at the bottom of the hole. In this way
the danger of breaking the tap or damaging the thread is minimised. The model
evaluated was the E507.

Cold forming taps
Cold forming taps differ from cutting taps in that the thread is produced by plastic
deformation of the component material rather than by the traditional cutting action.
Thus no swarf is produced by cold forming taps. The application range is materials with
good formability: tensile strength (Rm) should not exceed 1200 N/mm2 and the
elongation factor (A5) should not be less than 10%. The model evaluated was the E565.
    Cold forming taps without flutes are suitable for normal machining and are
especially suitable when working with vertically tapped blind holes. Conversely, cold
forming taps with flutes are especially suited for applications in horizontal holes and
vertical through-holes. The flutes act to facilitate the supply of lubricant to the working
area. With difficult to machine materials, TiN coating has been shown to give
outstanding tool life.

Determination of hole width
The diameter of the hole prior to tapping is an extremely important factor in
determining both thread quality and the useful life of the tap. Normally, a drilled hole is
slightly larger than the diameter of the drill. The amount of ‘oversize’ depends upon the
material being drilled, the cutting conditions selected and the condition of the
equipment being used. Note:

Machining of P/M materials regarding Tapping

•     If material is pushed up at the thread entry by the tap and/or the life of the tap is too
      short - select a slightly larger drill diameter.
•     If on the other hand the profile of the thread formed is insufficient - select a slightly
      smaller drill diameter.

Torque comparison
A selection of four taps of various types was tested for torque values on a range of P/M
materials. A tap dimension of M4 was used and emulsion was applied in each case.
Results in Nm values are shown in Table 18.

Table 17. Comparison of torque in tapping of 11 P/M materials with a selection
of four taps.

          Material groups               E565           E973           E507           E500
    Iron base
    ASC100.29                                                         1.00           0.81
    ASC100.29+0.5% C                    1.81                          1.16           1.31
    ASC100.29+0.45% P+0.5% C                                                         1.10
    ASC100.29+2% Cu+0.25% C                                                          1.06
    ASC100.29+2% Cu+0.5% C                                                           1.19
    ASC100.29+2% Cu+0.8% C                                                           1.38
    Diffusion bonded
    Distaloy AE (4% Ni, 1,5% Cu,         1.50                         1.31           1.00
    0,5% Mo)
    Distaloy AE (4% Ni, 1,5% Cu,                                                     1.40
    0,5% Mo)+0.25% C
    Distaloy AE (4% Ni, 1,5% Cu,                       1.31                          1.62
    0,5% Mo)+0.5% C
    Distaloy AE (4% Ni, 1,5% Cu,                       1.66
    0,5% Mo)+0.8% C
    Pre-alloyed with 1,5% Mo
    Astaloy Mo+0.5% C                                  1.70

(Emulsion applied in each case; tap dimension M4; values in Nm.)

                                                                   Hints on Optimal Tapping

5.4 Hints on Optimal Tapping

The success of any tapping operation depends on a number of factors all of which affect
the quality of the finished product. Brief guidelines for optimal tapping are given here,
followed by discussion of some important factors affecting performance:
•   Select the correct design of tap for the component material and type of hole (i.e.
    through or blind). See cutting data recommendations for tapping in Section 5.6, at
    the end of this chapter.
•   Ensure the component is securely clamped: lateral movement may cause tap breakage
    or poor quality threads.
•   Select the correct size of drill from the tapping drill charts. Remember: drill sizes dif-
    fer between cutting and forming taps.
•   Select the correct cutting speed as shown in the cutting data recommendations for
•   Use appropriate cutting fluid for each application.
•   In NC applications ensure that the pitch value chosen for the program is correct.
    When using a tapping attachment, 95% to 97% pitch is recommended to allow the
    tap to generate its own pitch.
•   Where possible hold the tap in a good quality torque-limiting tapping attachment.
    This should ensure free axial movement of the tap and that it is presented squarely to
    the hole. It also protects the tap from breakage if it is accidentally ‘bottomed’ in a
    blind hole.

Core hole dimensions
A correct thread can only be obtained if the hole is round and straight. The diameter of
the hole must never be smaller than the minor diameter of the thread. The larger the
hole diameter, the lower the generated torque will be on the tap. Recommended core
sizes are given in the concluding section of this chapter (see Table 18).

It is important that the cutting part of the tap is ensured a good supply of lubricant or
cooling fluid. The need for good lubrication increases with the depth of the hole and the
hardness of the material being machined. Any ‘cooling fluid’ must have a lubricating
effect in order to reduce friction.
     One solution to avoid applied emulsion is to use minimal lubrication. A small
amount of oil, (20-40 ml/h) can then be directed to the thread part of the tap.

Machining of P/M materials regarding Tapping

Guidelines when using equipment for minimal lubrication are:
•   Direct the nozzles to the tap and the core hole.
•   Use only oil recommended for minimal lubrication.

Dry tapping
When selection of the most suitable cutting fluid is not possible, or when dry cutting,
the useful life of the tool will be shorter than normal. When tapping dry, cutting speeds
must be reduced.

Tapping attachment
In order to ensure the best possible result a high quality tapping attachment with axial
float should be used. See Dormer catalogue for available tapping attachments.

Clamping and centring
The tap must be carefully centred with the work piece for the best result to be obtained.
When thin materials are to be threaded it is important that the work piece is properly
secured in order to prevent the formation of oblong holes.

5.5 Tapping Guidelines

The following conclusions have been drawn from experimental and practical experiences
in tapping of P/M materials. These are ‘rules of thumb’ or a general guide to optimal
•   For materials of lowest hardness use high spiral taps.
•   For materials of medium hardness use straight fluted or low spiral taps.
•   For materials of highest hardness use surfaced-treated low spiral taps.
•   Lubrication should be used whenever possible. It is of particular importance with
    materials of lowest and greatest hardness, and with cold forming taps.

                                                 Tap and Cutting Data Recommendations

5.6 Tap and Cutting Data Recommendations

Recommendations for tap application and related cutting data are presented in figure 92.
The speed rates shown are start values for tapping with emulsion of P/M materials with a
density of 7.0 g/cm3.
   In Table 19 - Table 22 drill diameters recommended for cutting taps and cold
forming taps are stated. In each case drill diameter refers to ISO metric coarse thread.

Machining of P/M materials regarding Tapping

                         Selection of Taps                       Spiral point taps                    S
                                                     ISO         E509        E511        E507         E
   Recommendations and cutting data
   for thread form M coarse. For other thread        DIN 371     E214        E206        E208         E
   forms contact Dormer Tools.                       DIN 376     E265        E257        E259         E
   ■ Excellent for application.
   ● Acceptable for applications

   15 = Peripheral speed m/min. mid range +/- 10%.

   All recommendations with lubricants.

                                                                  HSS         HSS         HSS          H
                           Material groups                      -2.5 x D    -2.5 x D     -2 x D       -2
                                                                 Bright        TiN       Bright         T
   Iron base
   ASC 100.29                                                  ■ 20        ● 35        ■ 20       ● 35
   ASC 100.29 + 0.5% C                                         ■ 15        ■ 25        ■ 15       ■ 25
   ASC 100.29 + 0.45% P                                        ■ 10        ■ 20        ■ 10       ■ 20
   ASC 100.29 + 0.45% P + 0.5% C                               ■ 8         ■ 16        ■ 8        ■ 16
   ASC 100.29 + 2% Cu                                          ■ 15        ● 25        ■ 15       ● 25
   ASC 100.29 + 2% Cu + 0.25% C                                ■ 10        ■ 20        ■ 10       ■ 20
   ASC 100.29 + 2% Cu + 0.5% C                                 ■ 8         ■ 16        ■ 8        ■ 16
   ASC 100.29 + 2% Cu + 0.8% C
   Diffusion bonded
   Distaloy AE (4% Ni, 1.5% Cu ,0.5% Mo)                       ■ 10        ■ 20        ■ 10       ■ 20
   Distaloy AE (4% Ni, 1.5% Cu ,0.5% Mo) + 0.25% C             ■ 8         ■ 16        ■ 8        ■ 16
   Distaloy AE (4% Ni, 1.5% Cu ,0.5% Mo) + 0.5% C
   Distaloy AE (4% Ni, 1.5% Cu ,0.5% Mo) + 0.8% C
   Pre-alloyed with 1.5% Mo
   Astaloy Mo + 0.5% C                                         ■ 8         ■ 16        ■ 8        ■
   Distaloy HP (4% Ni, 2% Cu) + 0.5% C

                                                                  Tap and Cutting Data Recommendations

        Spiral flute taps                               Straight flute taps               Cold forming taps
7      E508          E506          E973         E500          E504           E961        E565         E561
8      E213          E207                       E200          E420           E201        E217         E426
9      E264          E258                       E250          E421           E252        E267         E427

S       HSS           HSS         HSCo           HSS          HSS            HSCo       HSCo          HSCo
D      -2 x D        -2 x D       -2 x D       -1.5 x D     -1.5 x D        1.5 x D     -2 x D        -2 x D
ht       TiN         Bright         nitr.       Bright         TiN            Nitr.     Bright          TiN

     ● 35                                                                             ■ 30         ● 45
     ■ 25                                                                             ■ 20         ● 35
     ■ 20                                     ■ 10         ■ 20                       ■ 20         ● 35
     ■ 16                                     ■8           ■ 16                       ■ 15         ● 25
     ● 25                                                                             ■ 20         ● 35
     ■ 20                                     ■ 10         ■ 20                       ■ 20         ● 35
     ■ 16                                     ■8           ■ 16                       ■ 15         ● 25
                   ● 5          ■ 5           ●5           ■ 10          ■ 5

     ■ 20                                     ■ 10         ■ 20                       ■ 20         ● 35
     ■ 16                                     ■8           ■ 16                       ■ 15         ● 25
                   ● 5          ■ 5           ●5           ■ 10          ■ 5
                                ■ 3

     ■ 16                                     ■8           ■ 16
                                ● 5                                      ● 5

     Figure 92. Recommended tap selection from four tap types, with speeds at starting values, for a range of
     P/M materials.

Machining of P/M materials regarding Tapping

Recommended drill diameters (ISO metric coarse thread).

Table 18. Drill diameter for cutting taps.

   Tap M          Pitch, mm             Max. Internal           Drill
                                        diameter, mm        diameter, mm
      1.8            0.35                      1.521               1.45
      2               0.4                      1.679               1.6
      2.5            0.45                      2.138               2.05
      3               0.5                      2.599               2.5
      3.5             0.6                      3.010               2.9
      4               0.7                      3.422               3.3
      5               0.8                      4.334               4.2
      6                1                       5.153                5
      8              1.25                      6.912               6.8
      9              1.25                      7.912               7.8
      10              1.5                      8.676               8.5
      12             1.75                    10.441                10.3
      14               2                     12.210                12
      16               2                     14.210                14
      18              2.5                    15.744                15.5
      20              2.5                    17.744                17.5

Table 19. Recommended diameters when using Dormer ADX and CDX drills, for
cutting taps.

       Tap M         Pitch, mm                              Drill
                                                        diameter, mm

            4                    0.70                       3.40
            5                    0.80                       4.30
            6                    1.00                       5.10
            8                    1.25                       6.90
            10                   1.50                       8.60
            12                   1.75                      10.40
            14                   2.00                      12.20
            16                   2.00                      14.20

                                                        Tap and Cutting Data Recommendations

Table 20. Drill diameters for cold forming taps.

      Tap M                     Max. Internal                                Drill
                                diameter, mm                             diameter, mm
         2                            1.679                                     1.8
        2.5                           2.138                                     2.3
         3                            2.599                                     2.8
        3.5                           3.010                                     3.2
         4                            3.422                                     3.7
         5                            4.334                                     4.6
         6                            5.153                                     5.5
         8                            6.912                                     7.4
        10                            8.676                                     9.3
        12                           10.441                                    11.2
        14                           12.210                                    13.0
        16                           14.210                                    15.0
 For cold forming taps the drill diameter is calculated on 65% of the theoretical thread profile

Table 21. Recommended diameters when using Dormer ADX and CDX drills, for
cutting taps.

       Tap M             Pitch, mm                                         Drill
                                                                       diameter, mm

          4                           0.70                                   3.40
          5                           0.80                                   4.30
          6                           1.00                                   5.10
          8                           1.25                                   6.90
         10                           1.50                                   8.60
         12                           1.75                                   10.40
         14                           2.00                                   12.20
         16                           2.00                                   14.20

Machining of P/M materials regarding Tapping

6     References

3 Turning
[1] M. C. Shaw. Metal Cutting Principles, Oxford, 1997.

[2] A Thelin, “Verschleissmechanismen und Leistungen von Zerspanwerkzeugen” VDI Berichte,
    No. 762, 1989, pp. 111-126.

4 Drilling
[1] O. W Reen: ‘The machinability of P/M materials’, Modern Developments in Powder
    Metallurgy, Vol 10, pp. 431-452 (1977).

[2] M. C. Shaw. Metal cutting principles, Oxford, 1997.

[5] U. Engström: Machinability of Sintered Steel, Powder Mettalurgy, Vol 26, No 3 1983, p.

[6] P. J. James: ‘The machinability of Sintered Steels’, PMI vol. 22 no. 6, 1990.

[7] O. Mårs. ‘Dynamic Properties of Warm Compacted High Strength Steels’ PM2
    TEC ’96, Washington, 1996.

[8] L. Hultman, H Thoors, B. Steen: ” Influence of machining parameters on the
    machinability of sintered steels”, PM2TEC’96, Washington 1996.


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