1.)To harden the steel to the maximum level by austenite to
2.)To increase wear resistance and cutting ability of steel.
Steels can be hardened by following methods:
1.)Conventional hardening:The conventional hardening process
consists of heating the steel to above A3 temperature for
hypoeutectoid steels and above A1 temperature for hypereutectoid
steels by 500C, austenitising for sufficient time and cooling with a
rate just exceeding the critical cooling rate so that steel to room
temperature or below room temperature. Due to this ,the usual
diffusion transformations are stopped and the austenite transforms
to martensite by a diffusionless process.
All the time hypeutectoid steels are hardened from above A3
temperature.They are not hardened between temperatures between
A1 and A3, because the phases which exist at this temperature are
austenite and proeutectoid ferrite and only austenite gets
transformed to martensite with no change in ferrite. Such steels
show free ferrite in their microstructures and since ferrite is a soft
phase, the hardness of hardened steel gets reduced.
• On the other hand hypereutectoid steels are always hardened from
temperatures between A1 and Acm. At this temperature, austenization is
not complete and some proeutectoid cementite will exist along austenite at
the temperature of heating. Such steels after hardening show free Fe 3C
along with martensite in their microstructures. Since Fe 3C being a hard
phase the hardness of hardened substance doesnot get reduced.
Moreover, this free Fe3C does not increase the britttleness of steels
because usually it is fine, well distributed and partially spheroidised. Also,
the grain size remains fine because the Fe3C particles do not allow to
coarsen the austenite.
• However if these steels are hardened above Acm temperature, the
following drawbacks are observed:
1.)Since Acm line is steep, higher temperatures are required to cross
the Acm line. Due to this and absence of Fe3C above Acm temperature,
heavy grain coarsening occurs during austenitization and results in coarse
grained martensite which is extremely brittle.
2.)Quenching from such a high temperature results in more
distortions and may lead to cracking of the components.
3.)Due to higher temperature oxidation and decarburization are more.
4.)The amount of retained austenite increases because of higher
• Hence it is not heated above Acm and also the free carbides present in
the structure increases wear resistance and cutting ability of these steels.
• A proper quenching medium should be used such that the
component gets cooled at a rate just exceeding the critical cooling
rate of that steel.Faster cooling than the above also produces
martensite but the tendency of warping and cracking is more and
hence should be avoided.The critical cooling rates depend largely
on the alloying elements and to a lesser extent on the carbon
present in the steel. Alloy steels have less critical cooling rate and
hence some of the alloy steels can be hardened by air cooling.
• High carbon steels have slightly more critical cooling rate and has to
be hardened by oil quenching. Medium cooling rates have still
higher critical cooling rates and has to be hardened by water or
brine quenching. Low carbon steels are having still high critical
cooling rates and cannot be hardened by quenching(even brine).
• For high carbon steels containing more than 0.7% carbon and for
some alloy steels Mf is below room temperature. If these steels are
cooled only upto room temperature all austenite will not tranform to
martensite but a part of it will appear as retained austenite.This
retained austenite decreases the hardness of the hardened
steel.Therefore, the steel should be cooled below room temperature
to eliminate part of retained austenite.
2.)The timed quench (Interrupted quench)
• For plain carbon steels of low to medium carbon,critical cooling rates
are high and therefore,very fast cooling from austenizing temperature
is necessary to prevent the formation of pearlite or bainite at
temperatures near the nose of the TTTdiagrams.
• Once this region of rapid transformation has been passed,the
transformation of austenite becomes slow.Therefore it is possible to
obtain a completely martensitic structure in a steel of low hardenability
by cooling it rapidly to a temperature below the nose of the ttt diagram
and then cooling it more slowly through the temperature range
martensite is formed.
• Since the cooling rate between Ms and Mf is reduced,cracking
tendency also gets reduced.
• The process consists of heating the steel to the austenization
temperature,quenching for a short period in cold water or brine to a
temp between the nose and Ms and then cooling in some other
medium like oil to room temperature.
• For steels of slightly high hardenability (i.e. of slightly less critical
cooling rate)like high carbon steels and low alloy steels,the initial
quench may be in oil with subsequent cooling in air.
• In this process, the austenitized steel is cooled rapidly
avoiding the nose of the I.T diagram to a temperature
between the nose and Mg, soaked at this temperature
for a sufficient time for the equlization of temperature but
not long enough to permit the formation of bainite and
then cooled to room temperature in air or oil.
• Since the component has to be held for some time for
equalization of temperature, the process will be
applicable to steels of slightly high hardenability such as
high carbon steels and low alloy steels.The process
produces martensitic structures with the following
1.)It results in less distortions and wraping, since the
martensite formation occurs at the same time throughout
the cross section of the component.
2.)There is less possibility of quenching cracks appearing
in the component.
This is a hardening process named so because of
• In this process, austenitized steel is
cooled with a rate exceeding the critical
cooling rate of that steel to a temperature
between the nose and Ms,forged or rolled
at this temperature and cooled to room
temperature in oil.Also, this results is
increased dislocation density in martensite
and a finer distribution of carbides on
tempering.Ausformed structures in
tempering at low temperatures show better
combination of T.S and ductility.Steels
with sufficient hardenability can be only be