On Isothermal Transformation of Steel Below Ms Temperatures

To clarify the transformation phenomena of steel below Ms temperatures during isothermal transformation treatment and isothermal treatment during tempering, named isothermal tempering, the authors have investigated the electric resistivity, the microstructure, the dilatation during transformation and the mechanical properties of steel specimens during such a treatment. The results obtained were as follows: (1) In isothermal transformation treatment below Ms temperatures, the untransformed Austenite→Bainite transformation occurs, and its beginning and finishing times are placed on extrapolated curves of the values obtained over Ms temperatures in TTT-diagram. (2) The authors have found that the retained Austenite→Bainite transformation occurs isothermally during tempering,and its beginning and finishing times coincide with those obtained by isothermal transformation treatment. (3) Steel samples isothermally transformed below Ms temperatures are superior in toughness to the isothermally tempered and conventionally quenched-and-tempered ones, provided that their hardness is equal.