Effects of Surface Induction Hardening on the Torsional Fatigue Performance of Previously Carburized Modified 4121 Steel

Carburizing and induction hardening are two commonly used surface heat treatments that increase fatigue life and surface wear resistance of steels without sacrificing toughness. It is hypothesized that induction hardening following carburizing could yield further increased torsional fatigue performance through reducing the magnitude of the tensile residual stresses at the carburizing case-core interface. If successful, manufacturers could see gains in part performance by combining both established approaches. A carburizing heat treatment with a case depth of 1.0 or 1.5 mm and an induction hardening heat treatment with a case depth of 0, 2.0, or 3.0 mm were applied to torsional fatigue specimens of 4121 steel modified with 0.84 wt pct Cr. The as-carburized samples served as a baseline for comparison. Material property characterization consisting of torsional fatigue testing, radial cross‑sectional hardness testing, and X-ray diffraction (XRD) for residual stress profile analysis was conducted. The hardness profiles confirm that the designed case depths were achieved for all conditions, and XRD versus depth profiles indicate reduced compressive residual stresses on the surface of the carburized plus induction hardened conditions compared to the as-carburized parts. Fatigue testing indicates that the as-carburized conditions exhibit higher endurance limits than the carburized plus induction hardened conditions.