Sub zero treatment or cryogenic treatment:
The desired microstructure after hardening is martensite. But in actual practice it is very difficult to have a complete martensitic structure. Some amount of austenite is present in the hardened steel and it is referred as retained austenite. Retained austenite reduces mechanical properties to a great extent and it is found difficult to achieve higher degree of hardness even after the cooling rates employed are greater than the critical cooling rate. Retained austenite may convert in to martensite on subsequent thermal cycles, which may lead to dimensional instability of the steel part. Hence it is essential to transform the retained austenite in to martensite to improve the strength of the hardened steel.
A process known as subzero treatment is used for this purpose. Retained austenite is fully converted in to martensite in this process, thus improving hardness, wear resistance and dimensional stability. In this process steel is cooled to sub zero temperature, which is lower than the Mf. For most of the steels this lies between –30 to -70— C. When the steel is cooled to sub zero temperatures from higher temperature ranges, the metal produces a loud noise or “cry”. The name cryogenic is originated from this phenomenon. Considerable amount of internal stresses are developed during this process and hence to be followed with tempering immediately. Transformed martensite also gets tempered during this tempering treatment.
Retained austenite gets stabilized if the hardened steel is kept at room temperature for several hours. Heating the hardened steel mildly (to 100-150— C) also will result in the stabilization of retained austenite. In such cases sub zero treatment is found to be less effective. The reason is stabilized austenite does not transform completely to martensite. There fore the treatment has to be performed immediately after hardening. Mechanical refrigeration units, dry ice or liquefied gases such as liquid Nitrogen is used for cooling in this process. This process is employed for parts, which require high impact strength and fatigue strength along with high degree of dimensional stability. Case hardened steels also are subjected to sub zero treatment.
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| Image form https://www.researchgate.net/profile/Sitki_Akincioglu2/publication/271732446/figure/fig2/AS:295003684392967@1447345767731/Fig-2-Temporal-temperature-variations-of-the-cryogenic-treatment-process.png |
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