Effect of prior austenite grain size on isothermal bainite transformation in 65Cr steel

Abstract Isothermal bainite transformation was conducted on both ultrafine-grained (UFG) and normal-grained (NG) 65Cr steels. Both bainite morphology and transformation rate are greatly influenced by prior austenite grain size. Bainite transformation rate is greatly retarded in the UFG steel with average grain size of 4.6 μm. Lower bainite was obtained for both UFG and NG steels when austempered at 250 °C and 300 °C; however, the bainite morphology changed from lower to upper banite for UFG steel when austempered at 400 °C, and lower bainite was still obtained in NG steel. This phenomenon is caused by austenite yield strength increase due to grain refinement.