Effects of nitrocarburizing on fatigue property of medium carbon steel in very high cycle regime

In this study the medium carbon steel was nitrocarburized in gas and salt bath, forming three layers (oxide layer, compound layer and diffusion layer) on the surface of the specimen. The very high cycle fatigue (VHCF) tests were carried out and the results show that fatigue limits of the specimens treated in gas and salt bath are improved by 120% and 100%, respectively, compared to that of the untreated specimens. For the specimens treated in gas, from which the oxide layer was removed or both the oxide layer and compound layer were removed, the S–N curve of the former shows a step-wise shape while the latter continuously decreases and both of them show no fatigue limit in VHCF regime. However, all the specimens treated in salt bath have the same fatigue strength and show the fatigue limit in VHCF regime whether both the oxide layer and compound layer are removed or only the former is removed.