Deep Cryogenic Treatment is process of treating various materials at extremely low temperatures (-1960C). It is also called as Cryogenic tempering or Secondary Heat Treatment. This process consists of 3 stages.
Gradually cooling the material. This slow cooling process from ambient temperature to soaking temperature (-1960 C) is important in avoiding thermal shocks.
Once Soaking temperature (-1960 C) is achieved, the cycle shift to Soaking stage. This stage consists of holding the soaking temperature for various soaking periods depending upon the material.
Soaking stage is followed by Ascend in which material is brought to ambient temperature from soaking temperature.
Deep Cryogenic Treatment is followed by Low Temperature Tempering.
Why Deep Cryogenic Treatment ?
Cryogenic treatment is a supplementary process to conventional heat treatment process in steels. It is an inexpensive, one time permanent treatment affecting the entire cross section of the material unlike coatings.
The purpose of cryogenic treatment is to transform retained austenite into martensite. Austenite (a soft form of iron) is a solid solution of carbon and iron that is formed during the quenching phase of metal production. Austenite is weak and undesirable because it contains few molecular interfaces to help hold the metal together. There may be as much as 30-35% residual austenite in heat-treated ferrous metals. This untransformed austenite is brittle and lacks dimensional stability, which allows the metal to break more easily under loads.
When metal is cryogenically treated, the austenite structure is transformed slowly into a highly organized grain structure called martensite, a body centered tetragonal crystal structure. Martensite is a finer and harder material that brings high wear resistance that is very desirable in carbon steels.
This martensite is decomposed & large quantities of dispersed ultrafine carbides are diffusively precipitated out. This is the key factor contributing to the increase in red hardness, strength & toughness of the ferrous material. Those ultrafine carbides precipitated out owing to volume concentration. The crystalline lattice tends to decrease. The crystal deformation of martensite in the super saturated solid solutions tends to increase in Deep Cryogenic Treatment which grow larger during subsequent tempering but are apparently smaller in size than after conventional Heat Treatment. The carbides are more diffused & evenly distributed.
In addition, better dimensional stability is often achieved along with minimizing residual stresses. This is especially important for progressive dies, where cumulative tolerances are critical. Deep Cryogenic Treatment increases wear resistance, improves bending & fatigue life. Stress is the enemy of steel, if it is not imparted in a uniform manner. Stress boundary areas are susceptible to micro cracking which leads to fatigue and eventual failure. Residual stresses exist in parts from the original steel forming or forging operations which is followed by many different machining operations to finish the part.
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