Tempering and Stabilization:
Tempering of tool steel in heat treatment is again a significant step. The quenching of such steels reasons the existence of untempered martensite, retained austenite and carbide in the structure. This happen due to finer changes in composition does not permit to found correct austenising temperature. Tempering nears removing such structural deficiencies assist also induce secondary hardness. Following figure shows the secondary hardness presence manifested via increased hardness at specific tempering temperature in the condition of high alloy steels. Both intermediate and high alloy steel reduces in hardness respectively upto about 460oC and 500oC and thereafter they enhance in hardness. High alloy steels illustrate greatest enhance in hardness along with enhancing tempering temperature.
Figure: Tempering Characteristics of Tool Steels
After tempering primarily the treatment is repeated two or three instance in multi-tempering practice. That treatment stabilizes the microstructure via decreases the amount of retained austenite. Still now another stabilization method is to cool the tempered steel to subzero temperature via placing it in solid carbon dioxide (- 75oC) or dipping it in liquid nitrogen (- 196oC).
Normalized quenched, stabilized and tempered tool steel possesses maximum hardness, dimensional stability and wears resistance.