2020 Volume 60 Issue 5 Pages 193-198
In order to clarify the effect of the crystal structure of the compound layer on the torsional fatigue strength of the nitrided parts used in commercial vehicle engines, etc., torsional fatigue tests were carried out for carbon steel（JIS-S50C）specimens with a compound layer mainly composed of a γ’ phase or an ε phase. As a result, specimens with γ’ phase showed higher torsional fatigue limit compared to that of specimens with ε phase. Fracture surface observation showed that the fatigue crack initiated at the diffusion layer mainly in the specimens with γ’ phase, wheres at the surface of the compound layer mainly in the specimens with ε phase. Vickers hardness of the diffusion layer in the specimens tested under the fatigue limit increased after fatigue tests, however the residual stress distribution did not changed after the fatigue test.
