The effect of nitriding, severe shot peening and their combination on the fatigue behavior and micro-structure of a low-alloy steel

Fatigue strength of mechanical components can be greatly enhanced by generating compressive residual stress, increasing the hardness and reducing the grain size. It is well known that while the use of mechanical treatments is able to generate an effective field of compressive residual stresses and, if severe parameters are used, to cause grain refinement, thermochemical treatments are able to increase the surface hardness. This justifies the interest in developing combined treatments, able to achieve all the just mentioned factors. In this study, the effect of combination of severe shot peening and nitriding on the fatigue limit of a low-alloy steel is investigated. Severe shot peening was conducted by using particular processing parameters to obtain ultra-fine/nano-structured surface layers. Micro-structural observation, micro-hardness, surface roughness and XRD measurement of residual stress were performed on single and hybrid surface treated specimens including nitrided, severely shot peened, nitrided plus severely shot peened and severely shot peened plus nitrided specimens. The fatigue limit of all series was experimentally determined and compared with the as-received specimens. Severe shot peening and Nitriding improved the fatigue limit by 11.6% and 51.3% respectively. Combination of severe shot peening and nitriding improved hardening, residual stress and nitrogen diffusion with respect to the single treatment. Nevertheless, it could not guarantee further improvement in the fatigue limit as compared with the nitrided smooth specimens. The results are critically assessed by considering the local fatigue limit concept

The effect of nitriding, severe shot peening and their combination on the fatigue behavior and micro-structure of a low-alloy steel

Fatigue strength of mechanical components can be greatly enhanced by generating compressive residual stress, increasing the hardness and reducing the grain size. It is well known that while the use of mechanical treatments is able to generate an effective field of compressive residual stresses and, if severe parameters are used, to cause grain refinement, thermochemical treatments are able to increase the surface hardness. This justifies the interest in developing combined treatments, able to achieve all the just mentioned factors. In this study, the effect of combination of severe shot peening and nitriding on the fatigue limit of a low-alloy steel is investigated. Severe shot peening was conducted by using particular processing parameters to obtain ultra-fine/nano-structured surface layers. Micro-structural observation, micro-hardness, surface roughness and XRD measurement of residual stress were performed on single and hybrid surface treated specimens including nitrided, severely shot peened, nitrided plus severely shot peened and severely shot peened plus nitrided specimens. The fatigue limit of all series was experimentally determined and compared with the as-received specimens. Severe shot peening and Nitriding improved the fatigue limit by 11.6% and 51.3% respectively. Combination of severe shot peening and nitriding improved hardening, residual stress and nitrogen diffusion with respect to the single treatment. Nevertheless, it could not guarantee further improvement in the fatigue limit as compared with the nitrided smooth specimens. The results are critically assessed by considering the local fatigue limit concept

Experimental assessment and simulation of surface nanocrystallization by severe shot peening

Surface nanocrystallization is an effective approach to bypass the difficulties of synthesizing bulk nanocrystalline material and yet exploit its unique advantages in service. This study uses air blast shot peening over a wide range of coverage (peening time), from conventional to severe, to generate nanostructured surface layers on high strength low alloy steel. Electron microscopy observations were carried out to systematically study the degree and the mechanism of grain refinement as the severity of deformation increases. A model linking finite element simulation of severe shot peening to dislocation density evolution due to the accumulated plastic strain was developed to predict the resultant grain/cell size gradient in the surface layers. The proposed framework establishes a physical connection from processing parameters such as media size, velocity and peening coverage to the resultant structure, opening the possibility of designing a severe surface peening process to achieve a desired nanostructure.Massachusetts Institute of Technology. Institute for Soldier NanotechnologiesScuola Interpolitecnica di DottoratoPolitecnico di Milano (International Fellowship)National Research Foundation of Korea. Basic Science Research Program (2009-0093814