Cryogenic treatment

TREATING of steel is widely accepted within the metallurgical profession as a supplemental treatment that can be used to enhance the transformation of austenite to martensite and to improve stress relief of castings and machined parts.Common practice identifies -84 °C (-120 °F) as the optimum temperature for cold treatment. There is evidence, however, that cryogenic treatment of steel, in which material is brought to a temperature of the order of -190 °C (-310 °F),improves certain properties beyond the improvement attained at cold-treatment temperatures.

This discussion will explain the practices employed in the cold treatment of steel and will present some of the experimental results of using cryogenic treatment to enhance steel properties.
Cold Treating of Steel
Cold treatment of steel consists of exposing the ferrous material to subzero temperatures to either impart or enhance specific conditions or properties of the material. Increased strength, greater dimensional or microstructural stability,improved wear resistance, and relief of residual stress are among the benefits of the cold treatment of steel.

Generally, 1 h of cold treatment for each inch of cross section is adequate to achieve the desired results.
All hardened steels are improved by a proper subzero treatment to the extent that there will be less tendency to develop grinding cracks and therefore they will grind much more easily after the elimination of the retained austenite and theuntempered martensite.
Hardening and Retained Austenite
Whenever hardening is to be done during heat treating, complete transformation from austenite to martensite is generally desired prior to tempering. From a practical stand-point, however, conditions vary widely, and 100% transformationrarely, if ever, occurs. Cold treating may be useful in many instances for improving the percentage of transformation and thus for enhancing properties. During hardening, martensite develops as a continuous process from start (Ms) to finish (Mf) through the martensite formation range. Except in a few highly alloyed steels, martensite starts to form at well above room temperature. In manyinstances, transformation is essentially complete at room temperature. Retained austenite tends to be present in varying amounts, however, and when considered excessive for a particular application, must be transformed to martensite and then tempered.