The purpose of cryogenic treatment is to transform retained austenite and raise the hardness of the as-quenched structure. 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. 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. There may be as much as 40% residual austenite in heat-treat ferrous metals. That percentage can be lowered to as little as 1% in some cases. Martensite is also formed during the quenching phase. There is always a certain amount of martensite present, but prior to cryo, the ratio of strong martensite to weak austenite is less than favorable. This untransformed austenite is brittle and lacks dimensional stability, which allows the metal to break more easily under loads. To eliminate austenite, the quenching temperature has to be lowered. At very low temperatures, austenite is unstable and readily becomes martensite. The result is a much improved part or tool with no cracking, warping or any other cryogenically imposed defect. Improvement in durability is around 100%. The typical increase in strength is 30-50%. Another advantage of cryo is the increase in efficiency to dissipate heat. Gears, engines, transmissions and disc brakes run cooler.
In addition, better dimensional stability is often achieved. This is especially important for progressive dies, where cumulative tolerances are critical. Sub zero treatments have as their ultimate goal an increase in wear resistance, improved bending fatigue life, and minimizing residual stress. Stress is the enemy of steel, if it s 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 and additional as a result of the many different machining operations to finish the part. They create a complex invisible random pattern in the steel. Residual stresses are uneven and located variously throughout the structure.
The grain structure changes in cryo treated materials were very significant.
Results of microanalysis were: