Incompleteness of bainite transformation in quenched and tempered steel under continuous cooling conditions

Abstract

The austenitic transformation kinetics of quenched and tempered 22CrMnNiMo steel was investigated by dilatometric measurement at different cooling rates of 0.1∼20 K/s. It was found according to the dilatometric curves that the bainite transformation process of super-cooled austenite exhibited incompleteness during continuous cooling. The incomplete characteristics under different cooling rates were analyzed by determining both changes of ferrite fraction and formation rates with decreasing temperature. It indicated that the three-stage transformation during austenite decomposition was gradually transferred to a distinct two-stage process with increasing the cooling rate. On the basis of microstructural observation and transformation kinetic models, it was further demonstrated that the incompleteness of bainite transformation occurring at low cooling rate was mainly ascribed to the formation of displacive-controlled acicular ferrite, while that during intermediate or fast cooling was attributed to martensite transformation.

Keywords

• Austenitic transformation kinetics
• Incomplete bainite transformation
• Acicular ferrite
• Martensite transformation