The Effects of Initial Microstructure on the Microstructural Evolution and Mechanical Behavior of Intercritically Quenched Low‐Alloy NiCrMo Steel

The microstructural evolution and mechanical properties of a low‐alloy NiCrMo steel with initial lath martensite (LM) and tempered martensite (TM) microstructures after intercritical quenching were systematically investigated. The results showed that the tensile properties of both the initial microstructures exhibited similar trends when quenched at different intercritical temperatures. However, the initial microstructure significantly influenced the impact toughness after the intercritical heat treatment. In the low‐temperature intercritical region (720°C), the TM specimens exhibited lower impact toughness than the LM specimens. This is because the blocky reversed austenite (γ G ) in the TM microstructure nucleates preferentially at alloy carbides along the prior austenite grain boundaries and forms a necklace‐like M–A constituents. These brittle phases act as stress‐concentration sites that facilitate the microcrack initiation and propagation of microcrack. In contrast, in the high‐temperature intercritical region (≥780°C), the LM specimen exhibited inferior impact toughness compared with that of the TM specimen. This can be attributed to the microstructural inheritance effect in the LM specimen, which limited the grain refinement and resulted in poorer crack resistance.