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Medium-carbon ultra-high-strength steels processed through the Direct Quenching and Partitioning (DQ&P) route have emerged as promising structural materials for lightweight engineering applications owing to their exceptional combination of strength, ductility, and toughness. The DQ&P process produces a predominantly martensitic microstructure containing stabilized retained austenite through controlled carbon partitioning, enabling enhanced strain hardening via the transformation-induced plasticity effect. While the monotonic mechanical properties of DQ&P steels have been extensively investigated, their cyclic deformation and fatigue behaviour, particularly under different stress ratios, remain insufficiently understood despite their significance for fatigue-critical structural components.
In the present study, the low-cycle fatigue behaviour of a DQ&P processed 0.4 wt.% C medium-carbon ultra-high-strength steel was systematically investigated at room temperature under total strain amplitudes of 0.8%, 1.0%, and 1.2%, employing stress ratios of R = −1, −0.5, and 0. The cyclic stress response was analysed to establish the influence of stress ratio on cyclic hardening/softening behaviour, fatigue life, and damage evolution, and the results were correlated with the quasi-static tensile properties.
The cyclic stress amplitude–fatigue life relationship revealed an initial cyclic hardening stage followed by stabilization or softening, depending on the applied loading conditions. The observed behaviour is governed by the interplay between dislocation accumulation, retained austenite transformation, and microstructural damage evolution during cyclic deformation. To elucidate the governing fatigue mechanisms, fracture surfaces were examined using laser scanning microscopy and scanning electron microscopy, while the evolution of microstructural constituents and retained austenite fraction was characterized using electron backscatter diffraction and X-ray diffraction. The findings demonstrate that the unique DQ&P microstructure, characterized by high-strength martensite and mechanically stable retained austenite, provides excellent resistance to cyclic deformation and fatigue crack initiation, while the applied stress ratio significantly influences fatigue life and failure mechanisms.
Keywords: Medium-carbon ultra-high-strength steel; Direct quenching and partitioning; Retained austenite; Low cycle fatigue; Stress ratio