Quenching
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Quenching:
May be Carried out in water, brine ( saltwater), oils, molten salts, or air as well as other solutions. (Book)
Severity of quench: because of differences in thermal conductivity, specific heat, and heat of vaporization the cooling rate or “severity of quench” will also be different. (book)
Cooling capacity from 5-0 : Agitated Brine, 5; still water, 1; still oil, .3; cold gas, .1; still air, .2; (book)
Agitation is a big factor in the of cooling.
Cooling rate also depends on the surface to area-to-volume ratio of the part. The higher the ratio the higher the cooling rates.
Water vapor is a common cooling medium, but metal may for a vapor blanket to form water-vapor bubbles, creates a barrier to heat conduction. Can be helped by agitating the water.
Brine is effective quenching medium, because salt helps to nucleate bubbles at the interfaces, improving agitation, but it can corrode the part.
Die quenching: describes the process of clamping the part to be heat treated to a die, which chills selected regions of the part, this way cooling rates and warpage can be controlled. (book)
The rate of cooling during quenching may not be uniform, particularly with complex shapes of varying cross sections and thicknesses, thus producing severe temperature gradients. These gradients lead to variations in contraction which induce thermal stress and may cause the part to crack. Also nonuniform coolingcauses the residual stresses in the part which can lead to stress-corrosion cracking; hence proper method and choice of quenching media and temperatures are important.
General guideline; internal or external sharp corners should be avoided. Stress concentrations at these corners