Abstract
Impact toughness values of martensite and bainite formed at 350°C and below in 0.34%C- and 0.54% CNi-Cr-Mo steels were compared at equivalent hardness levels, and factors which affect the toughness of both structures were discussed. Transition temperature of bainite lowered with decreasing transformation temperature over a wide range of hardness. Transition temperature of bainite which was formed just above M5 was similar to that of martensite at high and low hardness levels. But at intermediate tempering temperatures martensite showed lower impact transition temperature than bainite. Upper shelf energy value of bainite transformed at low temperature was slightly higher than that of martensite. Increase of carbon content increased appreciably the transition temperature of bainite, as compared at the same transformation temperature, but the transition temperature of martensite was scarcely varied except at low hardness level. Decrease of transition temperature of bainite was observed, when bainite coexisted with martensite by isothermal transformation below M5 or by up-quenching treatment to the temperature above M5. The transition temperature of mixed structures, however, seemed not to decrease below that of martensite. In a 0.54%CNi- Cr-Mo steel, increase of prior austenite grain size caused an increase in the transition temperature of bainite but no change was observed for martensite, although upper shelf energy value was decreased with increase of austenite grain size in both structures. Good correlation was obtained between υTγs and cleavage facet size in the various microstructures.
