In order to investigate the interior-induced fatigue crack propagation behavior of spring steel (SUP7), fatigue tests in axial loading were performed for 4 kinds of specimen with different hardness (tempered at 450 ℃ or 386 ℃) or residual stress (finished by grinding, electrochemical polishing or shot peening). Harder specimen has indicated longer fatigue life resulting from its mechanical properties of matrix. Ground specimen and shot-peened specimen also provided longer fatigue life than electrochemical-polished specimen, and the fatigue life of shot-peened specimen was almost same as one of ground specimen. After fatigue tests, fracture surfaces were observed using a scanning electron microscope (SEM). In some cases, non-metallic inclusion was not observed at interior fracture origin, but granular microstructure was observed. Profile analysis and crack-path analysis with FRASTA (Fracture surface topography analysis) method were performed to investigate the fatigue fracture mechanism induced by the granular microstructure. As results of these analyses, it was clarified that a facet of the granular microstructure was inclined from 33 to 42 degrees to the fracture surface. It was another finding that small countless cracks emanated discretely inside the granular microstructure during fatigue process.
In order to investigate the interior-induced fatigue crack propagation behavior of high cleanliness valve spring steel (JIS SWOSC-V), rotating bending fatigue tests were performed for various kinds of specimens with different hardness or surface finishings. The harder specimen with higher compressive residual stress showed longer fatigue life. The electrochemical polished specimen pre-treated with shot peening showed almost same fatigue life as the shot-peened specimen in spite of the difference in surface roughness. After fatigue tests, fracture surfaces were observed using a scanning electron microscope (SEM) to evaluate the fatigue fracture mechanism. Most specimens failed in surface-induced fracture mode due to high cleanliness; however, some specimens failed in interior-induced fracture mode in the very high cycle regime. Although non-metallic inclusions were not observed at interior fatigue crack initiation sites, 2 types of significant microstructures (with smooth surface or granular surface) were observed. EBSD analysis, profile analysis and computational simulation using a fracture surface topographic analysis (FRASTA) method were performed to investigate the mechanism of the interior-induced fatigue fracture caused by the microstructure at defect without any inclusion.
Round and spiral-shaped coil springs enable various peening angles that correspond to the surface location, and the directional shot angles may lead to a non-uniform residual stress on the coil spring surface. It is commonly known that a material under directional deformation exhibits non-linear 2θ-sin 2 ψ diagram (ψ-split) due to the triaxial stress state. In this study, the residual stress distributions of spring materials deformed by shot peening at different angles were measured, and the microstructure for carbide precipitates was examined using a field emission scanning electron microscope (FE-SEM). The non-linearity in the 2θ-sin 2 ψ diagram for the shot peening samples was revealed. The extent of the ψ-split increased with increasing shot peening angles, and was dependent not only on the mass fraction of carbide particles but also on the size distribution of the carbide particles.
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