D ie Casting Dies are subject to the most exacting of
service conditions and wherein they are subject to
severe thermal and mechanical cyclic loading.
There are, thus, a number of phenomena, which restrict die life.
The most important of these are:
IMPORTANT DIE STEEL PROPERTIES
Die steels by their very nature must be capable of possessing
a wide variety of properties. They need to satisfy several very
important physical parameters simultaneously and yet perform
at their best under the most trying of conditions. The important
-Thermal Fatigue (Heat Checking) die steel properties are as follows:
-Corrosion / Erosion Temper Resistance
-Cracking (Die Failure) The steel must have a very high temper resistance. Temper
-Indentation resistance of the steel is a measure of its ability to retain its
The number of shots achievable in a die casting die is hardness even at very high operating temperatures and not
strongly influenced by the working temperature i.e. the casting soften during the casting process. As the steel softens, its yield
alloy. The die life for a specific alloy can also vary considerably point reduces and thermal fatigue sets in leading to excessive
due to the casting design, surface finish, production rate, heat checking. Thus the higher the Temper resistance, the better
process control, die design, die material and it's heat treatment is the resistance to heat checking.
and the tolerances on the finished products. Broad guidelines Hot Yield Strength
for productivity for various alloys are given in Table I. The steel must have a high Hot Yield Strength. As the die is
However, certain factors are critical in the selection of steels subject to cyclic loading during its operation, there is a time
for die casting applications, which have to be complied with when the steel is subject to temperatures in excess of its
irrespective of the alloy and the casting. The main parameters, tempering temperature. At these temperatures the steel should
which determine die life, are: not yield prematurely leading to deformation and this is
-Cleanliness of the Steel determined by the Hot yield strength and the higher it is the
-Chemistry of the Steel better the steel resists deformation.
-Heat Treatment of the Steel.
SELECTING STEELS FOR
DIE CASTING DIES
- Dr. Kumar M. Iyer, Assab Sripad Steels Ltd., Delhi.
Pressure Die Casting Dies are subject to the most rigorous of conditions
amongst the various tools used in industry. The steels are subject to a high
degree of Mechanical Stresses, Thermal Stresses and Plastic Strain, all of
which combine to produce a very high demand on the steel. With larger and
larger components being produced by the Pressure Die Casting route, the
demands on toughness and ductility of the dies are also ever increasing. These
myriads of factors have combined to put an exceedingly high demand on the
steels in terms of quality and physical properties. Producing Steels designed
for Pressure Die Casting Dies has become an exacting operation and process
steps such as Vacuum Processing and Electro Slag Refining (ESR), which are
otherwise considered redundant, are considered as normal for their
Dr. Kumar M. Iyer, an expert in his own field, imparts his understanding on
the required steel properties, importance of alloying elements and the necessity
procedures to be considered while melting, refining and manufacture of steels.
He has developed a detailed template, correlating the various aspects of
manufacture with the properties in die steels.
08 January 2008
Casting Alloy Casting Factors Limiting Normal Life, Number of Shots
Temperature oC Die Life Cavity Core
Zinc ~430 Erosion 0.5 – 2 Million 0.5 – 2 Million
Magnesium ~650 Heat Checking
Erosion 100,000 to 400,000 50,000 to 200,000
Aluminium ~700 Heat Checking
Erosion 60,000 to 200,000 40,000 to 150,000
Copper / Brass ~970 Heat Checking
Erosion 5,000 to 50,000 1,000 to 5,000
Creep Strength steel. Tough and ductile steel can prevent heat check marks
The steel must have a good creep strength. Die casting dies from growing and can drastically reduce the tendency to gross
are subject to eccentric thermo mechanical fatigue wherein the crack. It also reduces the plastic deformation of the material as
peak of the thermal loading and the peak of the mechanical a higher toughness & ductility usually means a higher elasticity
loading are not the same. Hence, even as it possesses a high and resilience and hence is desirable in the die steel.
mechanical fatigue properties due to its hardness, its ability to Thus a wish list of properties in the die steel can be
resist the thermal loading is directly dependent on its creep summarized in Picture 1.
strength and the higher it is the better suited the steel is for die IMPORTANCE OF STEEL MAKING PROCESS ON
casting applications as it improves its ability to resist heat
checking and deformation.
Coefficient of Thermal Expansion
• Temper resistance
The coefficient of thermal expansion of the steel determines • Hot yield strength
the extent of increase in volume of the steel as a function of
temperature. All metals expand when heated and contract when • Creep strength
cooled. However, in the case of die casting dies, if the die
expands too much then it can have an adverse effect on the • Coefficient of thermal expansion
component as well as it can affect the dimensional tolerances of
the finished product. Hence the lower the coefficient of thermal • Heat conductivity
expansion the better is the steel suited for a die casting die
application. • Ductility / Toughness
Picture 1. Thermo Mechanical Properties of Die Steels
Die casting dies are subject to a constant thermal cycling and
if the dies get heated up due to the heat being transferred to it STEEL PROPERTIES
from the molten metal as it solidifies. The main sources of The Steel making process has got a major role to play in the
extraction of heat from the die are through the cooling channels determination of properties in the die steel. Along with the
and through the spray of the die release agent. However, in a cleanliness of the steel, the absence of physical defects in the
good die operating practice it is desired that the maximum steel also plays a significant role in determining the mechanical
amount of heat is extracted through the cooling channels and properties of the steel. Each step in the manufacturing process
there is no excessive spraying of the release agent on the surface plays a crucial role in determining the properties of the steel.
as it can have a negative impact on the die life. To this effect a Once the steel is in the molten condition, proper processing
high thermal conductivity of the die steel will ensure that the in the ladle furnace is essential to ensure that the optimum
heat is conducted quickly away from the die surface and the conditions for casting have been achieved in the steel. Also the
need for external spray cooling is kept at optimum levels. adjustment of chemistry and temperature is usually done in the
Toughness / Ductility ladle furnace because one the steel solidifies and it is usually not
Toughness / Ductility of the die steel become a very possible to make any major chemistry changes in the steel. The
important parameter. All the three predominant modes of casting process also is very important as any gases trapped
failure i.e. Heat Checking, Gross Cracking or Plastic during the process can induce porosities in the steel, which are
Deformation, arise out of poor toughness / ductility in the die very difficult to eliminate during subsequent processing.
09 January 2008
Usually a Vacuum Degassing operation is also carried out to Another aspect, which can seriously affect the life of the die
reduce the trapped gases in the steel, most notably oxygen as it steel, is the post ESR operations. It is mandatory that the die
can severely reduce the toughness of the steel. The effect of blocks be forged to at least 50% size reduction prior to any
oxygen on the steel properties is shown in Picture 2. rolling operations. The heavy reduction, which is done in the
Electro Slag Refining (ESR) process is a very important step forging operation, breaks down any remaining carbide network
and provides homogeneity to the steel. This also increases the
isotropy of the die steel thus allowing the cavities to be sunk in
any orientation and providing uniformity of properties in all
directions. Also post forging annealing of the blocks is
necessary to homogenize the chemistry and provide a good
carbide network and uniform chemistry. This treatment is
necessary to ensure good machinability of the die blocks and
also to minimize the carbide segregation in the steel.
Thus it can be seen that the steel making process has a very
important role in the properties of the dies as the basis of
chemistry, cleanliness and isotropy are ingrained in the steel
during the steel making process itself. Some of the defects
caused by inadequate manufacturing practices are shown in
Picture 4. These could arise due to improper melting practice or
no secondary refining or incomplete forging or improper post
Picture 2. Effect of Oxygen ppm on Impact toughness and the production of the as cast blocks.
progress over the years.
in the manufacture of the steel. Die steels are usually
characterized by small hard carbides which are in the size range
of 2 to 20 microns. The smaller the carbides the better are the
properties of the steel especially its creep resistance and
toughness and ductility. The Presence of large inclusions in the
steel either as a trapped slag particle carried over from the steel
melting stage or oxide or sulphide inclusions can severely
reduce the toughness of the steel. In the ESR process the steel
ingots are remelted drop-by-drop and resolidified in a very
shallow melting zone leading to much lower carbide
segregation and cleaner steel as most of the trapped inclusion
particles are floated out in the slag which is present in the ESR
unit during the processing. A schematic diagram of the ESR Picture 4. Effect of manufacturing defects on die steel properties.
process is shown in Picture 3.
IMPORTANCE OF HEAT TREATMENT OF THE DIE
Good heat treatment is always a corner stone in achieving
good die life. Adequate heat treatment can make a good steel
better but a good heat treatment practice can even make a poor
steel act better. Thus the quality of the dies depends a lot on the
heat treatment practices. While hardening the dies, it is always
important that the manufacturers instructions with regard to
various heat treatment parameters are followed strictly so that
the optimum tool life can be extracted for the die.
One aspect that needs to be remembered clearly is the
difference between the hardness and the hardenability. While
hardness can be achieved in several ways, the hardenability of
the steel gives the idea of cooling rates to be employed so that
the best possible microstructure can be achieved in the steel.
Picture 3. Photograph and Schematic Diagram of the ESR Process. The steel manufacturer usually provides these data for Heat
10 January 2008
Treatment in the form of CCT diagrams and these provide a Figure 5, shows the microstructures of some good and bad heat
guide for the quench rates and also about the hardenability of the treated steel specimens and it is important to compare these
steels. structures with the NADCA approved standards for the same. It
CCT diagrams for a few of the common grades used in die should be noted that any of the nine structures which comprise
casting is given in Figure 4. the acceptable standards, may be achieved in the heat treatment
As can be seen from these diagrams, even a small variation process and the number is not a measure of quality of the heat
in chemistry of the steel can have a significant effect on the treatment. NADCA approved list of heat treated structures is
hardenability of the steel grades and hence the study and proper shown in Figure 6.
understanding of these diagrams assumes great importance. It is to be noted in the above figure that all the nine structures
Figure 4. CCT curves for four different Hot Die Steels.
(a) Premium H13 Die Steel (b) Patented Hot Die Steel (Dievar) (c) Modified H10 Die steel (d) Premium H11 Die Steel.
Figure 5. Good (a) and Poorly (b) heat treated microstructures.
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Though the carbon percentage is
more or less similar in all the four
grades, if we look at the CCT curves
shown above, we can see a marked
difference in their heat treatment
behaviour. The presence of alloy
elements greatly disturbs the balance
of the iron carbon diagram leading to
different behaviour during
quenching. Lowering the silicon
content of the steel has a direct
impact on its V notch energy with a
lower silicon content translating into
a higher impact energy thereby
providing toughness and ductility.
Of the alloying elements V forms
very strong and hard MC type
carbides which are small and hard
carbides. However, increasing the
Figure 6 : NADCA approved list of acceptable and unacceptable microstructure for die casting dies presence of V in the steel, drastically
AS1 – AS9 are equally acceptable and there is no such reduces its hardenability as it promotes the formation of grain
distinction as one is superior to the other. Similarly, all the nine boundary carbides during the quenching process. This is
structures AS10 – AS18 are equally unacceptable and there is no clearly seen in the CCT curves for the H10 type steel shown
marginal or borderline acceptance case as far as microstructure above. So it is mandatory to oil quench the steel so that we get
is concerned. the desired martensitic structure. However, the advantage of
Another aspect which needs to be remembered is that there is such a steel is its resistance to heat checking. It has got an
always going to be some distortion during heat treatment due to excellent fatigue and creep strength and also an excellent
the phase transformations that take place. So a finished die thermal conductiity, temper resistance and hot yield strength.
should never be sent for heat treatment irrespective of whether But it has got a very poor toughness and ductility and is highly
the same is being done under vacuum or in a salt bath or in an succeptible to gross cracking. Hence, it is recommended that
electrically heated furnace. So it is recommended that the this steel be used for smaller inserts and also for fixed cores
advise of the heat treaters need to be listened to regarding where cooling is difficult to arrange for. It is also recommended
allowances for heat treatment so that the optimum to use this steel in the gate area of the dies where higher temper
microstructure can be achieved in the dies and tool life can be resistance is required.
maximised. Mo on the other hand forms slightly larger M2C type
IMPORTANCE OF STEEL CHEMISTRY IN STEEL carbides. These carbides while less hard than the VC, they are
PROPERTIES equally adept in providing good hardness to the steel. However,
The chemistry or chemical composition of the steel has a by substituting V with Mo, we increase the hardenability of the
major role to play in determining its suitability in the steel. The absence of strong carbide former like V pushes the
manufacture of Die Casting Dies. Not all steel grades are suited carbide curve to the right reducing the tendency for grain
for all application areas and small variations in alloying element boundary carbide precipitation thus allowing for air quenching.
content can affect the properties substantially. The important However, with a lower V content, the Austenitising temperature is
alloy elements considered in this exercise are C, Si, Mo, V & Cr. reduced as there are no hard carbides to dissolve. Thus in the case
The chemical composition of four different Uddeholm grades of H13 the austenitising temperature is at around 1020 – 1030 oC,
for Die Casting is shown in Table 2. while for H11, the austenitising temperature is around 990 – 1000
oC. Besides, the lower V content while increasing
Steel Grade AISI C Si Cr Mo V the toughness and ductility, reduces the resistance
Vidar Superior Premium H11 0.38 0.3 5.0 1.3 0.5 to heat checking. Hence, H11 has a lower
Orvar Supreme Premium H13 0.39 1.0 5.2 1.4 0.9
QRO 90 Supreme Premium H10 type 0.38 0.3 2.6 2.3 0.9
resistance to heat checking as compared to H13.
Dievar Patented Grade 0.35 0.2 5.0 2.3 0.6 Cr as an alloying element has an effect on the
Table 2. Chemical Composition of Various Uddeholm bainite formation in the steel during the heat
steel grades for Pressure Die Casting treatment. In the case of Die Steels, Cr does not
play a major role as a carbide former but its
12 January 2008
presence is required to increase the hardenability of the steel. It the quench rates have to be in the order of 25 – 28 oC/min.
has a tendency to push the bainitic curve to the right thus, However, due to the lower V, the Austenitising temperature is
increasing the hardenability of the steel. This is again evident lower at around 1000 – 1010 oC and also it has a lower resistance
from the curves above wherein the lower Cr content in the H10 to creep and thermal fatigue,as compared to H10 or even H13 type
type material as compared to the H11, H13 or the patented steel steels. But the better toughness of the steel, ensures that even in
grade tends to increase the tendency to form Bainite the eventuality of any heat check marks, they will not grow and
necessiating Oil quench to achieve martensitic transformation. the marks will maintain the size for a longer period of time.
However, increasing the Cr content reduces the creep and Figure 7 gives a schematic description of the resistance to
thermal fatigue properties of the steel as the Cr tends to increase thermal fatigue and Gross Cracking as a function of properties
the dissolution of Fe in the carbide and as the Fe dissolves in the and important microstructural elements that are necessary in the
carbide, it increases its size and reducing its hot yield strength. steel. While some of the factors are associated with the steel
The effect of this migration is shown in Table 3. manufacturing process, others are determined by the chemistry
of the steel while the rest
Material Size(? m) V (at%) Cr (at%) Fe (at%) Mo(at%) Type are dependent on the heat
Before Creep testing treatment of the steel.
Prem. H13/Dievar/Prem. H11 2-5 20 - 29 ~35 ~10 ~25 MC Thus the factors which
Prem. H13/Dievar/Prem. H11 15 - 25 ~15 ~35 ~20 ~35 M2C affect die life are
QRO90 Supreme 2-5 ~30 ~30 ~10 ~30 M2C determined equally by the
After Creep Testing three most important steps
Prem. H13/Dievar/Prem. H11 20 - 110 ~10 ~10 ~40 ~40 M2C in the manufacture and
QRO90 Supreme 2-5 ~45 ~35 ~5 ~15 MC treatment of the die steel.
QRO90 Supreme 2 - 110 ~30 ~20 ~10 ~40 M2C The various desirable
Table 3. The composition and size of secondary carbides in Hot Die steels in creep testing. properties in the die steel
The patented steel grade Dievar has properties, which are are directly determined by the chemistry, the manufacturing
distinct from the others notably its better toughness and process and the heat treatment. Any discrepancy in any of the
ductility. This has been achieved primarily by reducing the three stages can have an adverse impact on the die life. Good die
carbon content but also by reducing the V content. Thus the life and excellent tool performance are achievable by combining
dual action of lower C and lower V has a tendency of pushing the right steel grade manufactured under exacting conditions,
the carbides curve in the CCT to the extreme right. Also the with good heat treatment and with proper die design and good
presence of Cr and Mo has a tendency to keep the Bainitic curve shop floor practices. One factor which needs to be noted is that
depressed and this helps in increasing the hardenability of the the best of dies can fail prematurely due to improper design or
steel substantially. Thus for larger, thicker dies where core inadequate shop floor practices. While the factors that need to be
quench rates become important, it is possible to achieve taken care of while choosing the die steel have been enumerated in
Martensitic structure in the core even at quench rates of as low this paper, the other two factors are equally important and
as 18 oC/min unlike for the conventional H11 and H13 where proper care has to be taken in their adherence.
Factors with impact Critical tool steel Important
on tool performance properties microstructural elements
Ductility Residual elements
Resistance to.. Toughness Slag inclusions
Cleanliness Micro segregations
Thermal fatigue Homogeneity Primary carbides
Cracking Thermal conductivity Grain boundary carbides
Hot yield strength Carbide size and distribution
Temper resistance Alloying elements
Creep strength Grain size
Figure 7. Schematic of Tool Performance as a function of steel properties & microstructural elemants.
(The author is a B. Tech. in Metallurgical Engineering, I. I. T, Powai and has a Ph. D. in Metallurgical Engineering, University
of Utah, USA. He is currently a Vice President – Technical with Assab Sripad Steels Ltd. He is also a Visiting faculty at various
colleges lecturing students on Metallurgy & Cryogenics. To his name are over 30 Presentations and Publications in various
National & International Conferences and also internationally referred Publications)
13 January 2008