Effects of Induction Surface Hardening Following Carburizing on the Torsional Fatigue Performance of a 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 carburized samples without further induction processing, the 0 mm induction case depth, serve as a baseline for comparison. The as-received microstructure of the alloy is a combination of polygonal ferrite and upper bainite with area fractions of approximately 27% and 73% respectively. The case microstructure of the heat treated conditions is primarily tempered martensite and transitions to the as-received microstructure around the deepest carburized or induction hardened case depth.

Material property characterization consists of torsional fatigue testing, radial cross‑sectional hardness testing, and electropolishing and X-ray diffraction for subsurface residual stress measurements. The hardness profiles confirm that the designed case depths were achieved for all conditions. Torsional fatigue testing will be conducted using a Satec® SF-1U Universal Fatigue Tester, and the results will be presented to assess the effect of induction hardening on the fatigue life of the carburized modified 4121 steel. Of the six tested conditions, the condition with the deepest case depths, i.e. carburized to 1.5 mm and induction hardened to 3.0 mm, is expected to have the greatest increase in fatigue performance.