Effects of boundary characteristics on resistance to temper embrittlement and segregation behavior of Ni–Cr–Mo low alloy steel

SA508 Gr.4N Ni–Cr–Mo low alloy steel has higher strength and fracture toughness than those of commercial SA508 Gr.3 low alloy steel, due to its tempered martensitic microstructure as well as the solid solution effect and its higher contents of Ni and Cr. Hence, several studies have been performed on SA508 Gr.4N for nuclear application. In this study, the effects of microstructure on temper embrittlement and segregation behaviors in Ni–Cr–Mo low alloy steel were evaluated from the viewpoint of grain boundary characteristics. To evaluate the microstructural effect while excluding chemistry effects, the same heat was used but different microstructure samples were prepared by changing the cooling rate after austenitization. The increased volume fraction of martensite reduces the resistance to temper embrittlement, showing an increased transition-temperature shift (TTS) and increased P segregation at prior austenite boundaries. The segregation occurred intensively at prior austenite grain boundaries in tempered martensite, while the segregation occurred simultaneously at both prior austenite boundaries and packet boundaries in tempered bainite. In the EBSD results, most of the packet boundaries in tempered martensite are special boundaries such as ∑3 coincident site lattice (CSL) boundaries. The differences in P segregation between tempered martensite and tempered bainite are mainly caused by different portions of low energy special boundaries among the sub-grain boundaries. The reduction of temper embrittlement resistance in tempered martensite could be explained by the increased fraction of low energy CSL boundaries, which leads to a concentrated segregation of P at prior austenite grain boundaries.