Effect of the phase transformation sequence on microstructure and low-temperature impact toughness in the martensite-bainite complex phase steel

Abstract

In this study, the effect of the phase transformation sequence on microstructure and low-temperature impact toughness in the martensite-bainite complex phase steel was investigated. The specimens after the low-temperature Charpy impact test were analyzed to determine the cause of the difference in low-temperature impact toughness between two transformation sequences: martensite transformation prior to bainite transformation and bainite transformation prior to martensite transformation. When martensite is transformed before bainite, micro-cracks were hardly observed and the fracture mode is a trans-granular fracture. On the other hand, when bainite is transformed before martensite, relatively many micro-cracks were observed, and both inter-granular and trans-granular fractures occur. Also, packet size is finer in the case of martensite transformation after bainite transformation, whereas block width is finer in the case of bainite transformation after martensite transformation. The difference in the size of the substructure causes a difference in the (100) misorientation distribution.