Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 6M

A Certain amount of retained Austenite may always be found in hardened steel. Retained Austenite reduces the hardness, wear resistance and thermal conductivity of steel and makes its dimensions unstable.

‘A Sub Zero Treatment’ has been devised to reduce the retained Austenite in hardened steel. Itconsists in cooling the metal to Sub Zero Temperature. This treatment is suitable only when the temperature at which the Martensite transformation is completed (Mf) is below Zero.

Cooling to Mf transforms the retained austenite into Martensite. This increases the hardness of the part and its dimensions will become more stable. There is no purpose in cooling below Mf since no additional transformation of retained austenite occurs below this temperature.

Sub Zero Treatment is usually conducted in the defined temperature and holding time at this temperature from 1 to 1.5 hrs. Prolonged holding at room temperature after hardening will stabilize the austenite of many grades of steels and reduce the effect if subzero Treatment directly following the hardening operation.

Sub Zero Treatment is most frequently used for HSS tools, measuring tools and also for carburized gears and other machined components.