Effects of multi-cycle quenching-partitioning-tempering on the microstructure, quasi-static and dynamic mechanical performance of the high-strength armour steels

In this work, an innovative strategy of multi-cycle quenching-partitioning-tempering (QPT) procedure was adopted in the large-scale armour steel to achieve excellent strength, ductility and toughness combinations. Compared with the conventional quenching-tempering (QT) treatment, the MQPT sample possessed a hierarchical multiphase constitution of the tough lath martensite, nano-lamellar metastable austenite (MA) and nano-scale carbides. A higher water-cooling rate could refine the lath and block boundaries of the matrix, and the self-tempering effect during the water-air intervals resulted in abundant carbon diffusion and strong austenite stabilisation. Furthermore, compared to the QT samples, the MQPT sample exhibits a greater yield strength (YS: ∼1501 MPa) and an unusually high tensile elongation (TEL: ∼16%). The higher YS mainly results from the higher dislocation and grain boundary strengthening from the MA, and the large ductility is closely associated with the coordinate deformation of constituent phases and the transformation-induced plasticity (TRIP) effect of the MA. Furthermore, the MQPT sample also shows a high fracture strain (∼37.5%) and large dynamic absorption energy (∼2.8*109 J·m-3) under dynamic impact deformation. The overcome of strength-toughness trade-off can be attributed to the synergistic effects of mild dislocation generation in martensite laths and sustainable TRIP effect of nano-film MA, which can absorb the strain energy and alleviate the local strain concentration in the neighbouring martensite phases without the detrimental effects of transformation products.