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.

临界热影响区（ICHAZ）中的马氏体-奥氏体成分（MA）的裂纹敏感性随相邻基体的性质，MA的硬度以及MA /基体的界面特性而变化，但是为什么MAs在MA /基体中空化的原因界面或自身破裂仍然有待解决。研究了MA和由颗粒贝氏体（GB）和准多边形铁素体（QPF）组成的基体的微观结构特征对裂纹萌生和扩展行为的影响，裂纹扩展的临界开裂位移（CTOD）受到影响。在主要由GB（75.2％（体积））和QPF（19.5％（体积））组成的ICHAZ中，在MA的5.3％（体积）的情况下，在中断的三点弯曲试验中，QPF区域比GB区域变形更大，而一些MAs则被排空了或破裂，因为MA区域也高度变形。例如，具有高角度边界的QPF与具有低角度边界的GB。根据使用原子探针对MA / QPF界面的详细分析，C，Mn，Mo，Ni和Cr原子在MA / QPF界面处偏析，而Si，Cu，Al和P几乎不偏析。这些结果表明，主要的合金元素（C，Mn，Ni和Cr）在MA / QPF界面处偏析，从而通过减少裂纹萌生和扩展的平均自由程来影响临界CTOD的劣化。