Effects of Surface Compound Layer on Bending Fatigue Strength of Nitrided Chromium-Molybdenum Steel

Carburized and quenched materials with high fatigue strength are often used for motorcycle engine parts. Nitrided materials exhibit less deformation during heat treatment than carburized and quenched materials, so if the same or higher fatigue strength can be achieved with nitrided materials as with carburized and quenched materials, the geometric precision of parts can be increased and we can reduce engine noise as well as power loss. When the fatigue strengths of a nitrided material with its compound layer surface put into γ’ phase through nitriding potential control (hereafter, G), and a nitrided material put into ε phase (hereafter, E) were measured, the results showed the fatigue strength of the G to be about 11% higher than that of carburized and quenched materials. It was inferred that the strength of the compound layer determines fatigue strength. The reason the fatigue strength of the G is higher is that initial cracks do not readily form, and it can be inferred that when cracks do form, they progress readily and lead to final fracture. In the case of the E, it is thought that when the stress intensity factor, ΔK, due to initial cracks exceeds the threshold of the stress intensity factor range, ΔK_th ($\simeq 5.9\mathrm{M}\mathrm{P}\mathrm{a}\sqrt{m}$), it leads to fatigue fractures. While the G has higher fatigue strength than carburized and quenched materials, it is likely to have a big effect on microcrack fatigue strength. This is a factor we believe requires consideration when designing engine parts for strength and in planning part manufacturing.