Nitriding duration reduction without sacrificing mechanical characteristics and fatigue behavior: The beneficial effect of surface nano-crystallization by prior severe shot peening

• Ultrafine grained/nano-structured layer up to 10–12 μm from the surface was generated by sever shot peening.
• Nitriding at 15 h increased the fatigue limit of a low-alloy steel by 51.3%.
• Severe shot peening plus 50% shortened nitriding came up with 54.7% improvement.
• The same deep compound layer and surface micro-hardness was achieved in both cases.
• Nitriding time can be successfully shortened if prior severe shot peening is performed.

Generally a clear beneficial effect of nitriding duration on resultant mechanical characteristics is reported in the literature. Considering the high energy cost in the competitive business environment, this work explores any opportunities to reduce nitriding duration while not sacrificing the resultant mechanical characteristics and fatigue behavior. To this end prior shot peening is applied with particularly severe parameters to generate ultra-fine grains and nano-structures in the surface layers. It was recently shown that the local fatigue strength improvement by combination of severe shot peening and 15 h nitriding could not eventually contribute in further increasing the fatigue limit of high strength low alloy steel smooth specimens as compared to only 15 h nitriding. In the present research combination of severe shot peening with nitriding at 7.5 h is assessed. It is affirmed that improvement by hybrid treatment can be actively exploited in the form of duration reduction. The characterization is carried out by optical and scanning electron microscopy observation, micro-hardness test, surface roughness measurement and X-ray diffraction measurement of residual stress. Fatigue limit of the treated specimens is experimentally determined. A critical comparison between the hybrid process with 50% nitriding duration reduction and the original nitriding process is presented. Based on the result of this study, nitriding duration can be successfully reduced without losing improvements in mechanical characteristics and fatigue behavior if a suitable prior severe shot peening, aimed to surface nano-crystallization, is performed.