Effect of Compound Layer Thickness Composed of γ'-Fe₄N on Rotated-Bending Fatigue Strength in Gas-Nitrided JIS-SCM435 Steel

Abstract

The rotating bending fatigue strength of gas nitrided JIS-SCM435 steel was evaluated to find the thickness that gives the highest bending fatigue strength. Four specimens were used that had different compound layer thicknesses composed of γ' (-Fe₄N) phase: 0, 2, 10, and 20 μm. The composition of γ' phase was more than 70% of the volume of the compound layer in the specimens. The increment of the fatigue strength at 10⁵–10⁷ cycles was remarkably high when the compound layer thickness was from 0 to 10 μm and gradually increased from 10 to 20 μm. The increment of the fatigue strength was caused by the increment of the γ' phase thickness and the compressive residual stress depth as the compound layer thickness increased. The gradually increasing fatigue strength of more than 10 μm thickness was responsible for the porous layer thickness increasing and the compressive residual stress reducing on the surface as the compound layer thickness increased. These findings show that the fatigue strength of gas-nitrided JIS-SCM435 steel composed of γ' phase depends on not only the compound layer thickness but also the porous layer thickness and the compressive residual stress of the compound layer.