Tracing the evolution of hierarchical microstructures and its effect on the mechanical properties of a secondary hardening ultra-high strength steel

As one of the most potential materials for structural components, the secondary hardening ultra-high strength steel (SH-UHSS) with hierarchical microstructure exhibits an optimum mechanical properties. The present study focused on the effect of tempering temperature on the relationship between mechanical performance and multiple-phase in M54 SH-UHSS. When temperature increases from 480°C to 620°C, there is a continuous refinement of martensite matrix featured by the decreased size of blocks and increased ratio of high-angle grain boundaries (HAGBs). Starting from 560°C, there is formation of reversed austenite with fraction of 5∼12 vol.%. Cementite is retained at 480°C accompanied by the initial precipitation of M 2 C (M = Mo, Cr, W and V) carbides. It follows a desirable precipitation at 520°C by fine size (diameter: 1.14 nm, length: 11.20 nm), maximum density of 650 μm -2 and minimum inter-particle distance of 12.3 nm. The higher concentration of W element in the M 2 C carbides enables its resistance of coarsening at 560°C, while tempering at 620°C leads to dissolution of M 2 C carbides. Correspondingly, the 520°C-tempering exhibits an optimum combination of ultra-high strength (yield strength: 1731 MPa, ultimate tensile strength: 2018 MPa), desirable toughness ( A KV = 30 J) and ductility (total elongation: 12%). The lowest inter-particle distance of M 2 C carbides guarantees the dominant precipitation strengthening effect. The refined matrix, higher ratio of HAGBs can postpone the propagation of cracks to improve the toughness and ductility. However, the intra-granular yet larger-sized distribution of M 2 C carbides leads to a deterioration of toughness-ductility by the embrittlement of matrix at 560°C.