Yukui Gao

Another mechanism for fatigue strength improvement of metallic parts by shot peening

Fatigue crack source in shot-peened specimens may be located either at the surface or in the interior, beneath the hardened layer. In this paper, the mechanism for fatigue strength improvement of shot-peened specimens with internal fatigue crack source was studied. Un-peened and shot-peened specimens made of quenched and low-temperature tempered 40CrNi2Si2Mo2V steel were used. The fatigue crack source in shot-peened specimen is found in the interior beneath the hardened layer. X-ray diffraction analyses of both kinds of specimen fatigue tested at stress equal to their apparent fatigue limit show that obvious changes have taken place in the surface layer for un-peened specimens, while for shot-peened specimens, such changes are observed in the sub-surface layer beneath the hardened layer. The calculated actual critical stress at the fatigue source position (the “internal fatigue limit”) for shot-peened specimen is about 138% of the (surface) fatigue limit of un-peened specimen. According to an analysis about the micromeso-processes of fatigue crack initiation in metals, a concept of “internal and surface fatigue limits of metal” has been proposed. It is believed that the fatigue crack source transfers into the interior, Also, the internal fatigue limit of metal is higher than its surface fatigue limit, and is another mechanism for the improvement of apparent fatigue limit of shot-peened specimen.

Effects of shot peening on fatigue properties of 0Cr13Ni8Mo2Al steel

Abstract The influence of shot peening on the fatigue properties of 0Cr13Ni8Mo2Al steel has been studied. Changes in surface roughness, surface topography and residual compressive stress field were determined by experiments. The experimental results show that shot peening improves the fatigue property and the fatigue crack sources are pushed to the region beneath the hardened layer. Low Almen intensities should be used when 0Cr13Ni8Mo2Al steel is shot peened because of its sensitiveness to the surface roughness.

Influence of carburization followed by shot peening on fatigue property of 20CrMnTi steel

In this study, the influences of carburization (followed by quenching and low-temperature tempering) followed by shot peening on “apparent” fatigue limits of 20CrMnTi steel specimens were studied and quantitatively analyzed according to the microstructure changes, induced residual stress fields, and position of fatigue crack sources, as well as a micro-meso-process theory for fatigue crack initiation previously proposed by the authors (Ref 6–8). The experimental results show that, although the hardness of the surface layer of carburized specimens is much higher than that of the pseudo-carburized specimens, the improvement effect of carburization on the apparent fatigue limits of specimens is uncertain. It should be related to the possible formation of nonmartensitic microstructure in the surface layer of carburized specimen. After the shot peening, the fatigue limit of specimens was improved and rose to a level about 40% higher than that of the pseudo-carburized specimens. Scanning electron microscopy fractographic analyses show that the fatigue sources, which indicate the weakest link of specimens, in pseudo-carburized and ascarburized specimens are all located at the surface, while after shot-peening, they appear in the interior beneath the hardened layer.