Dilatometric Investigation of Phase-Transformation Kinetics in HP 9-4-20 Steel

摘要

Dilatometry has been used to investigate the phase transformation behavior of HP 9-4-20 steel. The decomposition of austenite to either martensite or bainite was monitored during continuous cooling or isothermal holds. The martensite start temperature, M/sub s/, was found to be 290 +- 3 exp 0 C for cooling rates between 0.01 exp 0 C/s and 20 exp 0 C/s. Isothermal decomposition of austenite to bainite occurred both above and below M/sub s/ and was accelerated if some athermal martensite was formed first. The previously published TTT (time-temperature-transformation) diagram for HP 9-4-20 steel has been modified to include the sub M/sub s/ formation of bainite. To avoid the formation of bainite during cooling through its formation range of 324 exp 0 C to 224 exp 0 C, a critical cooling rate of 0.05 exp 0 C/s must be exceeded. Partial time-dependence reversion of martensite to austenite took place during tempering at temperatures as low as 600 exp 0 C. The extent and kinetics of reversion increased with increases in tempering temperature above 600 exp 0 C. The linear coefficients of thermal expansion of austenite, bainite, and martensite in HP 9-4-20 steel have been determined. (ERA citation 08:057361)