Insight into the impact of microstructure on crack initiation/propagation behavior in carbide-free bainitic steel during tensile deformation

Abstract

The impact of microstructural characteristics on the crack initiation/propagation behavior of carbide-free bainitic steel was investigated during austempering at 320 and 280 °C. The primary mechanism for causing damage to the material was via of the formation of micro-voids and cracks. The results demonstrated that an uneven distribution of carbon content in the blocky martensite/retained austenite (M/RA) led to decreased carbon content at grain interiors. This caused splitting and decohesion of both blocky M/RA and M/RA-BF phase boundaries. During tests at 280 °C, the blocky M/RA content was decreased, and the microstructure refined. The selection and arrangement of bainite variants also affected the deformation compatibility of bainite ferrite (BF), M/RA and the density of high-angle boundaries (HAGBs). As the covariant boundary fraction of the CP (CP1) variant (V1/V2 – V6) in bainite was much higher than that measured in other covariant boundaries, the ability of covariant deformation between the two phases was greater, and the delayed initiation of micropores was observed. The effect of packet boundaries and block boundaries for hindering crack propagation was different, the most effective barrier for crack propagation was high-angle misorientation boundaries between packet boundaries.