One must understand the material science involved. Take for instance carbon steel. As the alloy is rapidly cooled (quenched) it undergoes a diffusion-less transformation known as martensitic transformation. The cooling rate is so high that the carbon atoms do not get enough time to re arrange themselves into a stable structure and hence form a metastable structure which is very hard.This hardness comes at a price. The material becomes brittle due to the presence of residual stresses in the steel. How easily can this alloy form martensite is also hardenability or the ease with which martensitic tranformation takes place is hardenability.
The rapid cooling or 'quenching' has two factors that effects its potency; the ability of the material to dissipate the heat from the interior to the surface and the other being the ability of the quenching medium to remove the heat from the surface. What happens is that in dissipation step the role of thermal diffusivity comes into play, higher the diffusivity lesser the time the material bar retains heat.This factor can not be controlled or always considered because it is material dependent. While the second factor is something we can control so it is mostly considered as the important one. It can briefly be described in three stages, considering the material to be steel:
- A layer of vapor/steam forms adjacent to the surface of the steel. The steam insulates the surface and produces a cooling rate.
- The heat transfer makes the continuous production of vapors to break down upon bubble formation thus allowing more of the water or medium to come in contact with the surface, cooling it this way. Rapid cooling takes place here.
- As the medium lowers the temperature of the steel down below the medium's boiling point the vapor formation stops and the surface now is in contact with medium directly as it starts to cool through convection and conduction.
Quenching Media:
We discuss a few of the common quenchants used in industry and laboratories here.Brine:
It is a severe quenchant. The reason for the severity is that brine does not dissolve the atmospheric gases which inherits less bubble formation. As a result the surface is continuously kept wet so the metal piece cools rapidly. Its rapid cooling character comes with a drawback as the severity causes distortion. Because we know the transformation is rapid so the crystal does not get time to relieve itself from the thermal stresses introduced. Also the solution is corrosive to many non ferrous metals hence it is not preferred to use it with them.Water:
It is a very common quenchant and probably the oldest too. It is less severe than brine though still a good quenchant. Its severity does cause distortion and cracking as well. But another trait to it is that it follows an uneven manner in wetting and rewetting the surface of the metal. This non-uniformity causes thermal gradient over the surface and thus promoting distortion that may lead to cracking.Air:
The metal piece should be placed in an open rack where the air can cool it uniformly from all sides. It does not harden the metal to the standards of other quenched media but still a property of mildly hardened and high ductility can be achieved.Oil:
It is the preferred quenchant in industry as it gives an intermediate severity. Which results in achieving less distortion and compromised hardenability.*Apart from the mentioned media we will highlight other less common quenchants that are used in heat treatment process for hardenability in an another blog.
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