Hardness convergence enables ductilization of flash-heated high-carbon martensitic steel via two-stage tempering

Abstract

Flash quench-tempering is an efficient route for producing ultrahigh-strength high-carbon martensitic steels by refining the martensitic framework and retaining a high defect density. However, further improving ductility without sacrificing ultrahigh strength remains challenging because non-equilibrium carbon distribution generates microscale mechanical heterogeneity and promotes strain localization. Here, we propose a two-stage tempering strategy to ductilize a flash-heated high-carbon low-alloy martensitic steel without sacrificing strength. Compared with the flash quench-tempered (QT) steel, which exhibited a yield strength of ∼2067 MPa, an ultimate tensile strength of ∼2254 MPa, and a total elongation of ∼7.5%, the secondary-tempered steel, hereafter referred to as QTT steel, maintained comparable strength while increasing the total elongation to ∼11.1%. Microstructural characterization revealed continued carbide evolution from η-Fe2C to θ-Fe3C, while the martensitic block size and dislocation density were both largely retained. Nanoindentation mapping demonstrated a clear convergence of local hardness after secondary tempering, as evidenced by the decrease in the fraction of high-hardness indents above 12 GPa from 19% to 3.5% and the reduction in hardness standard deviation from 3.1 to 1.8 GPa. High-resolution micro-digital image correlation (μ-DIC) further revealed reduced microscale strain heterogeneity after secondary tempering, indicating improved deformation compatibility. This enhanced ductility is attributed to hardness convergence and improved deformation compatibility rather than global martensite softening. These findings establish two-stage tempering as a practical strategy for ductilizing flash-heated ultrahigh-strength martensitic steels by tailoring local mechanical gradients.