This study evaluated the fatigue performance of nitrided H13 steel with and without a compound layer (CL), using two nitrogen potentials (K_N = 0.8, designated as LN, and K_N = 2.0, designated as HN). Fine particle peening (FPP) was applied prior to gas nitriding to introduce a refined microstructure and compressive residual stress (CRS) in the peened zone. After gas nitriding at 540 °C for 8 h, the refined structure remained on the outermost layer of all samples, regardless of the nitrogen potential. A CL primarily composed of Fe₃N formed on the external surface of the HN sample, whereas the LN sample remained free of CL. A higher K_N promoted CL formation and slightly increased the case depth in the HN sample compared to the LN sample. Fatigue cracks initiated at the external surface of the H13 steel substrate (SB). Overall, the LN and HN samples exhibited similar residual stress fields and, consequently, comparable fatigue performance. In the high-cycle fatigue regime, fatigue cracks originated from subsurface inclusions, resulting in significantly improved fatigue strength and life for both the LN and HN samples compared to the SB sample. Under cyclic stresses at or above 1100 MPa, the crack initiation site in the HN sample tended to shift from subsurface inclusions to the external surface. Throughout the fatigue tests, no multi-cracking or spalling of the CL was observed in the HN sample, regardless of the cyclic stress.

Gas nitriding was implemented in the current work at a constant nitrogen potential (K_N) of 2.0 for 8 h to enhance the fatigue properties of SCM 440 steel, and the results were compared with those of the substrate tempered at the nitriding temperature (475 °C). Fine particle peening (FPP) prior to nitriding imposed a refined structure and induced compressive residual stress (CRS) in the near-surface peened zone. The fine-grained structure provided numerous paths to enhance nitrogen diffusion inwards during nitriding. The compound layer formed on the nitrided SCM 440 steel primarily comprised a mixture of Fe₃N and Fe₄N; however, the pre-peened and nitrided (SPN) specimens exhibited a higher proportion of Fe₃N and a thicker compound layer than the non-peened and nitrided (NPN) counterparts. In addition, FPP prior to nitriding increased both the case depth and the magnitude of the CRS field compared with nitriding alone. The fatigue limits of the substrate (SB), NPN, and SPN samples were approximately 750, 1050, and 1400 MPa, respectively. Gas-nitriding at 475 °C significantly improved the fatigue performance of SCM 440 steel. Moreover, pre-peening prior to nitriding further enhanced fatigue strength and life of the treated SCM 440 steel by introducing a deeper case depth and higher CRS field. Multiple cracks initiation at the outer surface of the SB sample accounted for its lowest fatigue limit among the tested samples. Surface microcracks and pits on the surface of the NPN specimen would be crack initiation sites and harmful to its fatigue resistance. These surface dents were considered to be responsible for fatigue crack initiation in the SPN specimens. Therefore, polishing after nitriding to reduce surface roughness and/or microcracks was expected to further increase the fatigue resistance and the reliability of nitrided SCM 440 steel.