Effects of martensite-austenite constituent on crack initiation and propagation in inter-critical heat-affected zone of high-strength low-alloy (HSLA) steel

Crack susceptibility of martensite-austenite constituent (MA) in inter-critical heat-affected zone (ICHAZ) is varied with properties of adjacent matrix, hardness of MA, and MA/matrix interfacial characteristics, but reasons why MAs are voided at MA/matrix interfaces or cracked at themselves still remain to be addressed. Effects of microstructural features including MA and matrix composed of granular bainite (GB) and quasi-polygonal ferrite (QPF) on crack initiation and propagation behavior, which affected critical crack tip opening displacement (CTOD), were investigated in this study. In the ICHAZ mainly composed of GB (75.2 vol%) and QPF (19.5 vol%), together with 5.3 vol% of MA, QPF areas were more deformed than GB areas during the interrupted three-point bending tests, while some MAs were voided or cracked as MA areas were also highly deformed. The Nano-indentation test data indicated that the hardness of MA depended on boundary characteristics of nearby matrix, e.g., QPF having high-angle boundaries vs. GB having low-angle boundaries. According to detailed analyses on MA/QPF interface by using an atom probe, C, Mn, Mo, Ni, and Cr atoms were segregated at the MA/QPF interface, whereas Si, Cu, Al, and P were hardly segregated. These results indicated that major alloying elements (C, Mn, Ni, and Cr) were segregated at the MA/QPF interface, which influenced the deterioration of critical CTOD by reducing the mean free path for the crack initiation and propagation.