At high levels of stresses the initiation of cracks occurs in the surface nitrided layer.
The maximum improvement of 96% in fatigue strength was developed for 3 h, 630[degrees]C gas nitrided samples, which had highest (850 [HV.
The results of this study show that growth of fatigue limit of nitrided samples is not directly related to the thickness of compound layer and case depth.
In some earlier studies [8, 9, 22], it was shown that the presence of a compound layer did not have any significant effect on high cycle fatigue strength of nitrided steel since a fatigue crack initiated under a strengthened nitrided layer.
The samples nitrided at 630[degrees]C had the highest fatigue limit in comparison with samples nitrided at 550[degrees]C and 700[degrees]C respectively.
It was determined that a deep case depth was not needed to achieve maximum growth of fatigue limit of nitrided steel.
Examination of the fatigue fracture surfaces showed that the samples had a higher fatigue resistance when the fatigue crack began to develop under the nitrided (hardened) sample surface at the centre of "fish eye" from structural stress concentrator--non-metallic inclusion.
Structural surface characterization of ion nitrided AISI 4340 steel, Materials and Design 36: 741-747.