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Hardening

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					                         Hardening
• Purpose:
         1.)To harden the steel to the maximum level by austenite to
   martensite transformation.
         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
  thermal stresses.
• 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.
  3.)Martempering(marquenching)
• 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
   advantages:
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
   martensite temperaing.
              Austempering
• 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
  ausformed.

				
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posted:8/18/2010
language:English
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