Improvement in Surface Integrity and High-Cycle Fatigue of 42CrMo4 Steel Axles and Shafts by Single-Toroidal-Roller Burnishing

This article presents an optimised deep rolling process, implemented using a single-toroidal-roller burnishing method, to improve the surface integrity and high-cycle fatigue strength of 42CrMo4 steel axles and shafts. A second-order composition plan, regression analyses, and multi-objective optimisation were employed. The solution was based on a non-dominated sorting genetic algorithm (NSGA-II) and Pareto front search approach. The compromise optimal solution yields an average roughness Ra of 0.190 μm, average surface microhardness of 503 HV, and average surface residual axial stress of –855 MPa. Deep rolling conducted using the selected optimal values (a burnishing force of 1000 N and feed rate of 0.11 mm/rev) of the governing factors achieves stable surface integrity characteristics under multiple repetitions of the process. Rotating bending fatigue tests showed that the positive effect of deep rolling begins to manifest itself after 104 cycles, i.e., in the second half of the high-cycle fatigue field and in the mega-cycle region, where the fatigue strength increases from 390 (after turning and polishing) to 440 MPa (after turning and subsequent deep rolling).