Study of microstructure evolution and fatigue crack extension properties of 42CrMo steel strengthened by induction hardening

To elucidate the fatigue crack propagation behavior in the gradient microstructure of induction-quenched 42CrMo steel, this paper first examines the variation in microstructure and mechanical properties concerning surface depth, followed by an analysis of crack propagation behavior at the microscale. The results show that the specimens with a 0.6 mm hardened layer, following induction quenching, exhibit a refined grain structure in the hardened layer and display enhanced mechanical properties. Microanalysis of the fatigue fractures in both the stable propagation zone and the transient fracture zone reveals that induction-quenched specimens exhibit more pronounced zigzag fatigue steps, a higher number of secondary cracks, denser fatigue striations, and deeper toughness nests, among other morphological features. These observations provide insight into the mechanism by which induction quenching enhances the fatigue resistance and performance of the material. A detailed microscopic analysis has been conducted on the mixed-mode propagation within the hardened layer, specifically along the <111> orientation through the crystal and the <100> orientation along the crystal. The analysis reveals that the direction of propagation of the through-crystal cracks is parallel to the extension of the {011} crystalline surface. Based on these findings, the mechanism by which induction hardening influences crack growth has been identified, providing a theoretical foundation for subsequent related strengthening processes.