Transformation Mechanism for the Blocky Microstructure of Nuclear Power Used SA508-3 Steel

A type of abnormal blocky microstructure (B-C), which is responsible for low or unstable low-temperature impact toughness, is observed in some low-alloy steels used for heavy forgings. However, the specific formation conditions and transformation mechanism of the B-C in low-alloy steels are unclear. In this study, SA508-3 steel heavy forgings, which are used for nuclear power, were investigated. Based on thermal-mechanical coupling simulation (TMCS), the formation conditions of the B-C were investigated during solid-state phase transformation. Furthermore, the composition, crystallographic variants, and fine structures of the B-C were thoroughly characterized using electron probe microanalysis, electron backscatter diffraction, and transmission electron microscopy. The results showed that B-C comprises massive ferrite (alpha) and acicular Mo2C carbides that usually form a mixed structure with bainite and lath martensite; this explains the bimodally distributed alpha grain size. B-C ferrite exhibited a preferred variant selectivity and a near K-S orientation relationship with the parent austenite phase. TMCS experiments indicated that an applied stress is essential for the transformation of the Bc, and that coarser prior gamma grains increase the propensity to form the Bc. Because of the low transformation temperature (350 degrees C to 550 degrees C) and the precipitation of high-density needle-like Mo2C, the microhardness of the B-C is significantly higher than that of pro-eutectoid ferrite and approaching to lower bainite and martensite counterparts.