Study on the fatigue properties of plasma nitriding S45C with a pre-ultrasonic nanocrystal surface modification process

Graded microstructures with different grain sizes and depths were produced on the surface layer of S45C steel by ultrasonic nanocrystal surface modification (UNSM). Plasma nitriding at 500 °C for 8 h and 48 h was employed to process the UNSMed sample surface and investigate the surface properties. Compared with the un-UNSMed samples, the enhancement of the surface hardness, the effective nitrogen diffusivity, and the fatigue strength were obtained after UNSM treatment. For the UNSM68000 (68,000 mm− 2) sample, a nitrogen enrichment region of approximately 30 μm (for nitriding 8 h) and 80 μm (for nitriding 48 h) was observed in the surface zone. Little effect on the internal nitriding is found because the quick nitriding reaction of nitrogen with the nano-grain sized ferrite and the nitrides weaken the effective nitrogen diffusion within the sample materials. The fatigue limit of the UNSMed sample with nitriding for 8 h improved approximately 78 MPa (for 34,000 mm− 2) and 125 MPa (for 68,000 mm− 2) compared with the un-UNSMed sample for the enhancement of nitrogen diffusion. However, a decrease of the fatigue limit of the UNSMed sample with nitriding for 48 h was induced by the surface defects (small voids) that were produced by the prolonged nitriding. A prolonged nitriding phenomenon was observed for the nitriding of nano-grain sized surface structures, and this phenomenon promoted the formation of a thicker compound layer and induced nitrogen gas to form voids in the sample surface region.