Effect of isothermal quenching on the amount and stability of residual austenite and on the properties of structural steels

1. The mechanical and service properties of structural steels can be substantially improved by isothermal quenching; as a result, lower bainite forms, and, in addition, a certain amount of metastable residual austenite (up to 20%) is retained in the structure; then, in tests, this austenite is gradually transformed into martensite. 2. The largest amount of residual austenite is found in steels after brief holding (up to 15 min) with all isothermal temperatures, and when the holding time is extended, the amount decreases. In subsequent deformation the degree of stability of residual austenite increases with rising temperature of isothermal holding. 3. The best combination of strength, ductile properties, and impact toughness is attained only when the amount of residual austenite and the degree of its stability are optimal for the kind of steel involved. 4. Industrial tests of pneumatic hammers heat-treated in the corrected regime showed that their operational life was doubled.