Heat Treatment Of Steel by anurag12

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									FET-Maturaausarbeitung               Heat Treatment                         Potpetschnigg
                                        of Steel


Heat Treatment of Steel

1 INTRODUCTION
Heat treatment is the operation of heating and cooling a metal in its solid state
to change its physical properties. An important factor is the time the cooling
takes.
According to the technique used, steel can be made hard to resist cutting
action and abrasion, or it can be softened to permit further machining.

Further, internal stresses may be removed, grain size reduced, toughness
increased or a hard surface produced on a ductile interior.

Alloyed steels owe their properties to the presence of one or more elements
other than carbon, so I will discuss mainly the techniques of heat treatment of
ordinary comercial steels.



2 IMPORTANT WORDS
Ferrite       pure iron (e.g. no carbon is present), the shape of the crystal
and the              ammount of iron atoms that form a crystal depends on the
heat

Austenite     face-centered iron (γ-iron) with one carbon atom in the center

Cementite     also called Iron Carbide (Fe3C), the most hard constituent of
steel;        occurs naturally at high carbon contents

Pearlite    mixture of Ferrite and Cementite; steel has a 100% pearlite
composition       at the eutektoid point (0,8% carbon)

Martensite very hard content of steel; it is obtained in hardening treatments



3 ALLOTROPIC CHANGES IN IRON AND STEEL
If a piece of carbon steel is slowly and uniformly heated, and then cooled, and
its temperature recorded, an inverse rate curve is obtained.

The three changes in each curve point out that structural changes occur.
These points are called critical points. On the heating curve they are
designated with c for ‘chauffer’ (french) which means to heat, on the cooling
curve the r stands for ‘refroidir’ (french) which means to cool.


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The changes that occur at these temperatures are a change in the atomic
structure, electrical resistance (and loss of magnetism at one point).
Let’s heat a piece of steel for example (for cooling the process is reverse).
At Ac1 the steel becomes non-magnetic, at Ac2 the body-centered α-iron
changes to the face-centered γ-iron, at Ac3 this structure changes back to a
body-centered form called δ-iron, further heating will melt the steel (at 1536°C
iron melts for example).

The temperatures at which these changes happen are different for every steel
and should be known, because most heat treatments require heating above
this range .
So, for example, steel can not be hardened unless it is heated to a point
above the lower, or even the upper, critical point.



4 THE IRON-CARBIDE-DIAGRAMM
This diagram shows us the temperatures at wich the allontropic changes
occur for every carbon content a steel may have.
Three carbon contents are to be mentioned here.
First, at 0,8% carbon the steel consists of a 100% pearlite structure (eutektoid
point), at 2,08% the alloy is no longer steel, but cast-iron and at 4,3% the
iron-carbon alloy has its lowest melting-point (→Eutektikum).

At 2,08% the cementite content is at about 11%, at 6,67% carbon the steel
has a 100% cementite structure.


5 TIME-TEMPERATURE-TRANSFORMATION DIAGRAM
The iron-carbide diagram is useful in selecting temperatures for parts to be
heated in various treating operations, and it also shows the type of structure
to expect in slowly cooled steels.
Although very useful in all heat-treating operations, it does not give much
information concerning effects of cooling rate, grain structure, or structures
obtainable with various quenching techniques.

This information is provided by the TTT-diagram or Isothermal transformation
diagrams (also called S-curves because of their appearance).
The diagram shows how the structure of an austenitized steel changes when
quenched in a given time.

To obtain a (in most cases desired) martensitic structure, the steel has to be
cooled with sufficient rapidity, so the cooling curve does not intersect the
nose of the transformation curve.




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The less carbon the steel contains the more the curve will move to the left
side, making the steel more difficult to harden. Most alloying elements move
the curve to the right → easier to harden.

Fine-grained steels also displace the curve to the left, but on the other hand
are coarse-grained steels are more apt to crack or distort during quenching.

6 GRAIN SIZE
Molten steel, upon cooling, starts to solidify at many small centers. The atoms
in such groups tend to be positioned similarly. The irregular grain boundaries
are the outlines of each group.
The size of these grains depends on a number of factors, but the principal
one is the furnace treatment it has received.

Coarse-grained steels are less tough and have a greater tendency for
distortion than those having fine grain, but they have better machinability and
greater depth-hardening power.

Fine-grained steels, in addition to being tougher, are more ductile and tend
less to distort or crack during heat treatment.

The control of grain size is possible through regulation of composition in the
initial manufacturing process, but after the steel is made, the control is
through proper heat treatment.
Heating the steel until the upper critical point Ac1 is reached, results in an
average grain size of a minimum.
Further heating results in an increase of the size of the grains. So quenching
from Ac1 leads to fine grains, while quenching from a higher temperature
would yield a coarser grain.



7 HARDENING
This is the process of heating a piece of steel to a temperature within or
above its critical range and then cooling it rapidly.

If the carbon content is known, the prober austenitization temperature may be
obtained by refering to the iron-carbide diagram.

In any heat-treating operation, the rate of heating is important. Heat flows
from the exterior to the interior of steel at a defined rate.
If the steel is heated too fast, the outside becomes hotter than the inside and
thus a uniform structure cannot be obtained.

If the piece is irregular in shape slow heating is even more important due to
the danger of warping and cracking.


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The hardness obtained by a given treatment depends on the quenching rate,
the carbon content, and the work size of the piece.

As I stated before, steel cannot be heated instantly throughout, because of
this it is logical that the other way round steel cannot be cooled down in a
moment, thus the hardening depth is limited because the inner parts of the
workpiece take longer to cool down.
In large workpieces even the surface hardness will decrease slightly, because
a lot of heat flows from the interior to the exterior, and so the surface cannot
be cooled that fast.

Steels with a carbon content up to around 0,6% increase their hardenability
with the ammount of carbon. Above this point hardness can only be increased
slightly, because the steels are now made up entirely of pearlite and
cementite in the annealed state. Pearlite responds best to hardening
treatments.


7.1 DIFFERENT QUENCHING MEDIA
The main difference between th media in use is the speed in which they cool
a piece to the desired temperature.

Air is the slowest media, only high alloy steels can be hardened by air-
cooling.
Oil is faster than air but by far slower than water, the advantage of oil is that
the piece is less likely to crack or warp during quenching.



7.2 CONSTITUENTS OF HARDENED STEEL
It has been stated that above the upper critical point all carbon steels are
composed of austenite, a solid solution of carbon in γ-iron.

If such a steel is cooled down slowly it will obtain a structure as laid out in the
iron-carbide diagram. But if it is quenched fast then a martensite structure will
form itself.

Martensite is a body-centered γ-iron with an additional carbon atom in it’s
middle - this causes internal stresses that make this structure very hard but
brittle.

Through control of the cooling time other structures may be produced that are
not as hard, but more ductile.

Sorbit, Trostit and Bainite are very similar to Martensite with the exception
that they are a little bit softer an more ductile.


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8 TEMPERING
I have already mentioned the brittleness of martensite. By tempering or
‘drawing’, the hardness and brittleness may be reduced to the desired point
for service conditions.
As these properties are reduced, there is also a decrease in tensile strength
and an increase in the ductility and toughness of the steel.

The operation consists of the reheating of the quench-hardened steel to some
temperature below the critical range (aprox. 150-300°C), followed by any rate
of cooling. Although this process is similar to annealing it softens the steel not
to such an extent. The final structure obtained from tempering is called
tempered martensite.

9 ANNEALING
The primary purpose of annealing is to soften hard steel so that it may be
machined or cold-worked.
This is usually acomplished by heating the steel slightly above the Ac3 Point,
holding the temperature until the piece is uniformly heated, and then cooling it
slowly and controlled so that the temperature in the interion and on the
surface are approximately the same.
This is called full anealing, because it completely refines the crystalline
structure and so softens the metal. Annealing also relieves internal stresses
set up in the metal.



10 SURFACE HARDENING

10.1 CARBURIZING
This is the oldest known method of producing a hard surface. This process is
merely heating iron or steel above Ac1 while in contact with some
carbonaceous material, which may be solid (coke or charcoal), liquid (cyanide
salt bath with less nitrogen) or gaseous (propane or any natural hydrocarbon
gases).
Iron, at temperatures close around its critical temperature, tends to absorb
carbon. This carbon forms a solid solution with the iron and converts the outer
layers into a high-carbon steel. How deep the carbon is absorbed depends on
the time and the temperature.


10.2 NITRIDING

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In this process the metal is heated to the temperature of around 500°C and
held there for a period of time while in contact with ammonia gas or liquid
cyanide salt.

Nitrogen from the gas is introduced into the steel, forming a very hard, but
thin layer (for example from 0,02 to 0,6mm if liquid cyanide salt (in this case
with less carbon) is used, the depth for ammonia gas is lower).
The surface effectively resists even corrosive action of water, salt-water
spray, alkalies, crude oil and natural gas.




11 INDUCTION HARDENING
Here a high-frequency alternating current is used to heat the metal. For low
hardening depths frequencies of about 500.000 Hz will be used, the
frequency is lower if the depth is higher.

Since the hardening is accomplished by an extremely rapid heating and
quenching of the surface, the interior is not affected. The hardness obtained
is the same as that obtained in conventional treatments and depends on the
carbon content.

The equipment for induction hardening is expensive but this is offset by the
advantages of the process, like fast, clean operation, little distortion, freedom
of scaling and low treating cost.



12 VOCABULARY
                                   abrasion     Verschleiß, Abrieb
                                  ammonia       Ammoniak
                                 annealing      Glühen
                            body-centered       Raumzentriert
                                       brine    Salzbad, Salzlauge
                               constituents     Bestandteile
                                  distortion    Verzug
                                equilibrium     Gleichgewicht
                             face-centered      Flächenzentriert
                                      lattice   Gitter
                                 tempering      Anlassen
                           tensile strenght     Zugfestigkeit
                                toughness       Zähigkeit
                                       warp     verzerren, verziehen
                                        yield   bringen, hervorbringen


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13 APPENDIX




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