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| Heat Treatment of Steel
Heat Treatment 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.
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.
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.
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.
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
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
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abrasion
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Verschleiß, Abrieb
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ammonia
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Ammoniak
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annealing
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Glühen
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body-centered
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Raumzentriert
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brine
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Salzbad, Salzlauge
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constituents
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Bestandteile
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distortion
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Verzug
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equilibrium
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Gleichgewicht
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face-centered
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Flächenzentriert
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lattice
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Gitter
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tempering
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Anlassen
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tensile strenght
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Zugfestigkeit
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toughness
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Zähigkeit
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warp
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verzerren, verziehen
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yield
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bringen, hervorbringen
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13 APPENDIX
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