Research on the evolution of material surface characteristics and anti-fatigue based on thermal field assisted rolling strengthening

Aiming at the challenge of enhancing the fatigue life of vehicle torsion bars under higher strength requirements, a rolling strengthening method based on thermal softening was proposed. This technique employs heat-assisted deep rolling (HTDR) to improve the surface characteristics of the material. Following quenching and low-temperature tempering, a high-strength martensitic structure was achieved. With increasing temperature, the flow stress of the material decreased, indicating notable thermal softening behavior. Experimental results show that, compared to room temperature rolling, the HTDR process reduces the average surface roughness and enhances the magnitude of residual compressive stresses. The modified surface layer features a refined grain structure, increased dislocation density, a higher fraction of recrystallized grains, and greater plastic deformation. Additionally, the HTDR process produced a nano-gradient surface layer with fine grains and abundant precipitates distributed along grain boundaries. The optimal treatment at 160°C-DR resulted in the highest fatigue life, while the HTDR process overall effectively reduced crack propagation rates. These findings offer valuable technical insights for surface modification and fatigue-resistant manufacturing of difficult-to-machine ultra-high-strength steels.