Pore Shape Evolution in Nitrided SCM440H Compound Layer and its Influence on Pitting Resistance during Rolling-Sliding Contact Fatigue

Nitriding is a beneficial surface hardening heat treatment to reduce CO2 emissions and shows a potential partially replace carburized steels adopted for sliding parts such as bearings and gears. Both the bending fatigue strength and pitting fatigue strength under high contact pressure were evaluated for nitrided JIS-SCM440H materials using two types of controlled-nitridings, i.e. γ' and ε, and conventional gas nitriding. Not only the thickness of the compound layer but also its phase structure such as γ' and ε significantly affected the 107 cycles bending fatigue strength, where the γ'-nitrided material was the highest. On the other hand, almost no effects of the thickness and phase structure on the 107 cycles pitting fatigue strength were detected in the materials. Compared to the carburized material, the γ'-nitrided material exhibited higher 107 cycles fatigue strength in bending and pitting, but poor fatigue strength in the lower cycles range. The shape and size of the pores in the compound layer were altered during roller-pitching tests, although there was almost no reduction in the thickness of the compound layer. The pore size was reduced by pore closure, especially at higher contact pressures. Pore closure was more pronounced in the γ'-nitrided materials than in the ε-nitrided materials at lower contact pressures. The 107 cycles pitting fatigue strength was improved after the pore closure, resulting from fewer defects.