Hardening and Tempering

Hardening and Tempering

Hardening is process in which steel is heated to a temperature above the
critical point, held at this temperature and quenched (rapidly cooled) in
water, oil or molten salt baths.
As earlier mentioned that if a piece of steel is heated above its upper
critical temperature and plunged into water to cool it an extremely hard,
needle-shaped structure known as martensite is formed. In other words,
sudden quenching of steel greatly increases its hardness.

After hardening steel must be tempered to:

1.reduce a brittleness,
2.reliev e the internal stresses, and
3.obtain pre-determined mechanical properties.
The hardening process is based on a very important metallurgical reaction
of decomposition of eutectoid.

This reaction is dependent upon the
following factors:
1.Adecuate carbon content to produce hardening.
2.Austenite decomposition to produce pearlite , bainite and martensite
structures.
3.Heating rate and time.
4.Quenching medium.
5.Quenching rate.
6.Size of the part.
7.Surface conditions.

 The rapidly with which the heat is absorbed by the quenching bath
has a considerable effect on the hardness of the metal. Clear, cold
water is very oftenly used, while the addition of salt still increases
degree of hardness.oil, however , gives the best balance between
hardness toughness and distortion for standard steels.

 In order to increase the cooling rate the parts may be moved around
the quenching bath, either by hand, or by passing them through the
tank in basket attached to mechanical conveyer. Large parts may be
lowered into the tank by a crane and kept moving while cooling.
It is often cheaper and more efficient, however , to circulate the
cooling liquid around the hot part.

 The heating rate and heating time depend on the composition of the
steel, its structure, residual stresses, the form and size of the part to
be hardened, the more the intricate and large the part being
hardened, the slower it should be heated to avoid stresses due to
temperature differences between the internal and external layers of
the metal, warping, and even cracking. The practically attainable
heating rate depends upon the thermal capacity of the furnace, the
bulk of the changed parts, their arrangement in the furnace, and
other factors. The heating rate is usually reduced, not by reducing
the furnace temperature but by preheating the articles.

 The heating time for carbon tool steels and medium-alloy
structural steels should be from 25 to 30% more than for carbon
structural steels. The heating time for high-alloy structural and tool
steels should be from 50 to 100% higher.
 When steel is exposed to an oxidizing atmosphere, because of the
presence of water vapor or oxygen in the furnace, a layer of iron
oxide called (scale) is formed. Thin layer of scale has very little
effect on cooling rate, but that a thick layer of oxide (0.005 in.
deep) retord the actual cooling rate.

Quenching media
The quenching media in general use are :
Water, Brine, Oils, Air, Molten salt.
Water : it is probably the most widely used as it simple and effective, it
cools at the rate of 982°C per second. It tends, however, to form bubbles
on the surface of the metal being quenched an causes soft spots, so a
brine solution is often used to prevent this trouble.


Brine :
 it is very rapid cooling agent and may tend to cause distortion of
the parts , as will water.
Oil : it is used when there is any risk of distortion although it is more
suitable for alloy steels than plain carbon steels.
Air blast : when the risk of distortion is great, quenching must be carried
out air blast. Since the rate of cooling is then lower, more hardening
elements must be added to the steel , forming an air-hardening alloy. The
air blast must be dry, since any moisture in the air will crack the steel.

Molten salts :
high speed steels are often quenched in molten salt to
hardened them.
Note : hypo-eutectoid steel containing very little carbon, say less than
0.25%, cannot be easily hardened by sudden quenching because of large
amount of soft ferrite which is contains and all of which cannot be
retained in solution even on very quick cooling. The hardening capacity
of steel increases with carbon content.

Hardening methods

The most extensively used method is conventional hardening or
quenching in a single medium. The disadvantage of this method,
however, is that the cooling rate in the transformation range will be very
high. It will differ only slightly from the rate on the upper zone of supercooled
austenite of low stability and, therefore, cracks, distortion and
other defects may occur in this method.

Conventional Heat, Quench and Temper Process:

In this process, Austenite is transformed to Martensite as a result of rapid
quench from furnace to room temperature. Then, martensite is heated to a
temperature which gives the desired hardness. One serious drawback is
the possibility of distorting and cracking the metal as a result of severe
quenching required to form Martensite without transforming any of the
austenite to pearlite. During quenching process, the outer area is cooled
quicker than the center. Thinner parts are cooled faster than parts with
greater cross-sectional areas. What this means is that transformations of
the Austenite are proceeding at different rates. As the metal cools, it also
contracts and its microstructure occupies less volume. ]


Extreme variations in size of metal parts complicate the work of the heat
treater and should be avoided in the designing of metal parts. This means
there is a limit to the overall size of parts that can be subjected to such
thermal processing.

Conventional quenching and tempering process.
Other hardening method, which shall be briefly described, are generally
employed to avoid these defects ad to obtain the required properties.
The various hardening method are:
1. Quenching in two media.
2. Hardening with self tempering.
3. Stepped quenching or martempering.
4. Isotermal quenching or austempering.
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1.Quenching in two media:
Articles hardening by this method are first quenched in water to a
temperature from 300°C to 400°C and then quickly transferred to a less
intensive quenching medium (for example oil or air) where they are held
until they are completely cooled. The purpose of the transfer to the
second quenching is to reduce internal stresses associated with the
austenite to martensite transformation. It is not advisable to quench first
in water and then in oil as this may lead to partial decomposition of the
austenite in its zone of the least stability (500C to 600°C) and to the
development of high residual stresses due to rapid cooling in martensite
transformation range.
Quenching in two media is widely employed in the heat treatment of
carbon steel tools (taps, dies, milling cutters etc.) of a shape unfavourable
as cracking and warping.
2.Hardening with self tempering:
Here the article is held in the quenching medium until it is completely
cooled but is withdrawn to retain a certain amount of heat in core which
accounts for the tempering (self tempering). Frequently, more heat is
retained in the core than is required for tempering and, when the
tempering temperature is reached, the article is reimmersed in the
quenching liquid.
This hardening is applied for chisels, sledge hammers, hand hammers,
centre punches, and other tools that require a high surface hardness in
conjunction with tough core.
3.Stepped quenching or martempering:
After heating the steel to a hardening temperature, it is quenched in the
medium having a temperature, from 150°C to 300°C. the article is held
until it reaches the temperature of medium and then its cooled further to
room temperature in air and sometimes in oil, the holding time in the
quenching bath should be sufficient to enable a uniform temperature to be
reached throughout the cross section but long enough to cause austenitic
decomposition. Austenite is transformed into martensite during the
subsequent period of cooling to room temperature.
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This treatment will provide a structure of martensite and retained
austenite in the hardened steel. (the cooling is stopped at a point above
the martensite transformation region to allow sufficient time for the
center to cool to the same temperature as the surface. Then cooling is
continued through the martensite region, followed by the usual
tempering) [Figure 2.25 ] .
Figure 2.25 Martempering process.
Retained austenite there is a large volume expansion when
martensite forms from austenite. as the martensite plates form during
quenching, they surround and isolate small pools of austenite (Figure
2.26), which deform to accommodate the lower density martensite.
However, for the remaining pools of austenite to transform, the
surrounding martensite must deform. Because the strong martensite resist
the transformation, either the existing martensite cracks or the austenite
remains trapped in the structure as retained austenite.
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Figure 2.26 Retained austenite (white) trapped between martensite
needles (black) ( 1000).
Retained austenite can be a serious problem. Martensite softens and
become more ductile during tempering. After tempering, the retained
austenite cools below the Ms and Mf temperatures and transforms to
martensite, since the surrounding tempered martensite can deform. But
now the steel contains more of the hard, brittle martensite. A second
tempering step may be needed to eliminate the martensite formed from
the retained austenite. Retained austenite is also more of a problem high
carbon steels.
The martensite stars and finish temperatures are reduced when the carbon
content increases (Figure2.27). High carbon steels must be refrigerated to
produce all martensite.
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Figure 2.27 Increasing carbon reduces the Ms and Mf temperatures in
plain-carbon steels.
Residual stresses and cracking residual stresses are also
produced because of the volume change or because of cold working. A
stress relief anneal can be used to remove or minimize residual stresses
due to cold working. Stresses are also induced because of thermal
expansion and contraction. In steels, there is one more mechanism that
causes stress.
When steels are quenched, the surface of the quenched steel cools rapidly
and transforms to martensite. When the austenite in the center later
transforms, the hard surface is placed in tension, while the center is
compressed. If the residual stresses exceed the yield strength, quench
cracks form at the surface (Figure 2.28)
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Figure 2.28 Formation of quench cracks caused by residual stresses
produced during quenching. The figure illustrates the development of
stresses as the austenite transforms to martensite during cooling.
Martempering has the following advantages over conventional
quenching:
1. less volume changes occur due to the presence of a large amount of
retained austenite and possibility of self tempering of the martensite.
2. less warping since the transformations occur simultaneously in all parts
of the article.
3. less danger of quenching cracks appearing in the articles.
On the other hand, the extremely low solubility of austenite in this range
from 500 to 600°C requires a cooling rate of 200 to 500°C per second in
this range to obtain supercooling. at the same time, cooling in hot media
is much slower than in water or oil at room temperature therefore,
austenite in carbon steel can be cooled through the zone from 600 to
500°C, without decomposition, only in thin articles (upto 5.8 mm I
thickness). Such articles are expediently hardened by this method. Alloy
steel articles hardened by this method, may be considerably thicker.
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4. Isothermal quenching or austempering:
This is the second method that can be used to overcome the restrictions of
conventional quench and tempering. The quench is interrupted at a higher
temperature than for Martempering to allow the metal at the center of the
part to reach the same temperature as the surface. By maintaining that
temperature, both the center and the surface are allowed to transform to
Bainite and are then cooled to room temperature ( Figure 2.29).
Advantages of Austempering:
(1) Less distortion and cracking than martempering,
(2) No need for final tempering (less time consuming and more energy
efficient)
(3) Improvement of toughness (impact resistance is higher than the
conventional quench and tempering)
(4) Improved ductility
Limitations of Austempering:
Austempering can be applied to parts where the transformation to pearlite
can be avoided. This means that the section must be cooled fast enough to
avoid the formation of pearlite. Thin sections can be cooled faster than
the bulky sections. Most industrial applications of austempering have
been limited to sections less than 1/2 in. thick. The thickness can be
increased by the use of alloy steels, but then the time for completion of
transformation to bainite may become excessive.
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Figure 2.29 Austempering process.
In Austempering process, the end product is 100% bainite. It is
accomplished by first heating the part to the properr austenitizing
temperature followed by cooling rapidly in a slat bath which is
maintained between 400 and 800 oF. The part is left in the bath until the
transformation to bainite is complete. The steel is caused to go directly
from austenite to bainite.
Quench rate In using the TTT diagram, we assume that we could
cool from the austenitizing temperature to the transformation temperature
instantly. because this does not occur in practice, undesired
microconstituents may form during the quenching process.
For example, pearlite may forms as steel cools past the nose of the curve,
particularly because the time of the nose is less than one second in plain
carbon steels.
The rate at which the steel cools during quenching depends on several
factors. First, the surface cools faster than the center of the part. In
addition, as the size of the part increases, the cooling rate at any location
is slower. Finally, the cooling rate depends on the temperature and heat
transfer characteristics of the quenching medium (Table 2.2 ).
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Quenching in oil, for example, produces a lower H coefficient, or slower
cooling rate, than quenching in water and brine.
The H coefficient is equivalent to the heat transfer coefficient. Agitation
helps break the vapor blanket (e.g., when water is the quenching medium)
and improves overall heat transfer rate by bringing cooler liquid into
contact with the parts being quenched.
Sub-zero treatment
The resultant microstructure of a fully hardened steel should consist of
martensite. In practice, it is very difficult to have completely martensitic
structure by hardening treatment. Some amount of austenite is generally
present in the hardened steel. This austenite existing along with
martensite is referred to as retained austenite.
The presence of retained austenite greatly reduced mechanical properties
and such steels do not develop maximum hardness even after cooling at
rates higher than the critical cooling rate. The amount of retained
austenite depends largely on the chemical composition of steel. For plain
carbon steels, the amount of retained austenite increases with the rise in
carbon contents.
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The problem of retained austenite is more complex in alloy steels. Most
of the alloying elements increase the content of retained austenite.
In hardened steels containing retained austenite, the strength can be
improved by process known as sub-zero treatment or cold treatment.
Retained austenite is converted into martensite by this treatment.
This conversion of retained austenite into martensite results in increased
hardness, wear resistance and dimensional stability of steel.
 The process consist of cooling steel to sub-zero temperature which
should be lower than Mf temperature of the steel. Mf temperature
for most steels lie between -30°C and -70°C.
During the process, considerable amount of internal stresses are
developed in the steel, and hence tempering is done immediately
after the treatment. This treatment also helps to temper martensite
which is formed by decomposition of retained austenite during subzero
treatment.
 Sub-zero treatment must be performed first after the hardening
treatment. Mechanical refrigeration units, dry ice, and some
liquefied gases such as liquid nitrogen can be used for cooling
steels to sub-zero temperature.
 This treatment is employed for : high carbon and high alloy steels
used for making tools, bearings, measuring gauges and components
requiring high impact and fatigue strength coupled with
dimensional stability-case hardened steels.
Example 2.4 Design of a Quench and Temper Treatment
A rotating shaft that delivers power from an electric motor is made from a
1050 steel. Its yield strength should be at least 145,000 psi, yet it should
also have at least 15% elongation in order to provide toughness. Design a
heat treatment to produce this part.

Solution:
We are not able to obtain this combination of properties by annealing or
normalizing . however a quench and temper heat treatment
produces Aa microstructure that can provide both strength and toughness.



shows that the yield strength exceeds 145.000 psi if the steel
is tempered below 460oC, whereas the elongation exceeds 15% if
tempering is done above 425oC.
The A3 temperature for the steel is 770oC. A possible heat treatment is:
1. Austenitize above the A3 temperature of 770oC for 1 h. An
appropriate temperature may be 770 + 55 = 825oC.
2. Quench rapidly to room temperature. Since the Mf is about 250oC,
martensite will form.
3. Temper by heating the steel to 440oC. Normally, 1 h will be
sufficient if the steel is not too thick.
4.Cool to room temperature.