The strengthening and ductilization mechanisms of D406A high-strength steel treated by quenching and partitioning process

Abstract As a low-alloy high-strength steel, D406A steel functions as a crucial structural material for solid rocket motor casings. With the rapid advancement in solid rocket motor technology, conventional quenching and tempering (Q&T) heat-treated D406A steel exhibits insufficient mechanical performance to satisfy the increasingly demanding strength-ductility balance criteria for advanced solid rocket motor casings. This study systematically investigates the strengthening and ductilization mechanisms induced by quenching and partitioning (Q&P) treatment to elucidate the enhanced mechanical performance of D406A steel. It was found that the Q&P process induced a multiphase microstructure in D406A steel, consisting of lath martensite, retained austenite, martensite-austenite (M-A) constituents and bainite, resulting in a synergistic improvement of strength and ductility. The tensile strength of D406A steel increased slightly with the rise of the partitioning temperature, but the elongation decreased at higher partitioning temperatures. As the partitioning temperature rose, the dislocation density increased, resulting in the formation of high-density dislocation tangles. At the partitioning temperature of 400 °C, the strength initially increased and then decreased with prolonged partitioning time. As the partitioning time extended, the film-like retained austenite gradually transformed into lath-like and blocky morphologies, accompanied by carbide precipitation, while the martensite laths coarsened and their boundaries became blurred. In this paper, the optimal partitioning process was as follows: holding at 400 °C for 60 min, which resulted in a tensile strength of 1574.61 MPa, an elongation of 16.28%, and a product of strength and elongation (PSE) of 25.63 GPa·%, the synergistic improvement of strength and ductility was achieved.