Improvement of fatigue limit by pre-fatigue deformation in 1.6 GPa-grade as-quenched martensitic steel

抄録

The fatigue limit of steel generally increases with tensile strength; however, when tensile strength exceeds ~1.4 GPa, further increases in tensile strength fail to elevate the fatigue limit or may even decrease it (fatigue limit ceiling). This ceiling poses a challenge for the widespread application of ultrahigh-strength steels. In this study, we successfully achieved a significant improvement in the fatigue limit through pre-fatigue deformation, thereby breaking through the fatigue limit ceiling in 1.6 GPa-grade as-quenched martensitic steel (Fe-3Mn-0.2C (wt.%)). The maximum stress corresponding to the fatigue limit increased from 675 MPa to 1300 MPa (stress ratio: 0.1, 10⁷ cycles). In contrast, a specimen subjected to pre-constant-loading at the same maximum stress as the pre-fatigue training showed only minimal improvement in the fatigue limit. These findings reveal that fatigue deformation, traditionally viewed as detrimental, can be beneficial for fatigue fracture resistance when properly applied. Moreover, the absence of surface cracks in all non-fractured specimens indicates that the fatigue limit of as-quenched martensitic steels aligns with the crack initiation limit, rather than the crack non-propagation limit. Therefore, the significant improvement in the fatigue limit is attributed to the improvement in the crack initiation limit achieved through the pre-fatigue deformation.