Revealing the evolution behavior of multiple carbides precipitation and mechanical response in M54 secondary hardening steel

As a new type of secondary hardening ultra-high strength steel, the evolution behavior of multiple carbides and its effect on the strength–toughness of M54 steel have been studied by subjecting to tempering duration (2– 10 h) at 520 °C. Within tempering of 2– 4 h, there is incompletely dissolved cementite with intra-granular distribution; however, it does not deteriorate the impact toughness. Without the observation of cementite, it shows the largest precipitation density of M2C carbides after 8 h-tempering, with the maximum density of 650 μm−2 and minimum inter-particle distance of 12.3 μm. Prolonged tempering of 10 h does not lead to coarsening due to the low diffusivity of constituent W element and coherent relationship of M2C/matrix interface. However, there is obvious segregation of MC carbides along the long-axis of M2C carbides in growth. The favorable precipitation of M2C carbides mostly contributes to the highest yield strength of 1747 MPa and ultimate tensile strength of 2037 MPa in case of 8 h-tempering. The stable ultrafined martensite blocks (~0.30 μm) and high ratio of high-angle grain boundaries (~50%) guarantee the desirable V-notched impact absorbed energy of ~30 J for the 2– 8 h-tempering. Unfortunately, due to the insufficient ability of arresting propagating cracks, the possible preferential MC carbides-distributed sub-grain boundaries leads to an obvious degradation of impact toughness (~18 J) in the 10 h-tempered samples.