Abstract
Shot peening processes are commonly used for improving the fatigue properties of steels. Shot peening introduces a compressive residual stress field in the near surface of steel, which can reduce or stop the growth of fatigue cracks and improve fatigue properties. This study experimentally investigated the effect of shot peening on the fatigue properties of 50CrV4 steel alloys with different artificial surface defects. Drilling tools were used to introduce different artificial defects with root radii of 0.585 mm and 0.895 mm on the surface of unpeened samples. The shot peening was applied to the drilled and undrilled samples. Scanning electron microscopy (SEM) observations, micro-hardness and X-ray diffraction residual stress measurements were conducted to analyse the characteristics of the shot-peened and unpeened samples. The results show that the shot peening leads to the transformation of the retained austenite to martensite in the near-surface microstructure. The hardness rates of the surface and near surface both increase by 8% after the shot peening. The peened samples exhibit compressive residual stresses with a high degree of isotropy in the near surface. The fatigue properties of samples were experimentally evaluated by conducting 3-point bending tests. The results indicate that the shot peening improves the fatigue life of drilled and undrilled samples. For the defects with the root radius of 0.895 mm, the shot peening leads to a 500% improvement in the fatigue life compared to unpeened samples regardless of defect depth. For the defects with the root radius of 0.585 mm, the improvement in fatigue life is 40% for the defect depth of 0.2 mm compared to unpeened samples. The improvement increases to 60% and 200% by increasing the defect depths to 0.4 mm and 0.6 mm. The fatigue properties are linked to the changes in the features of defects mainly caused by the deformation hardening and compressive residual stress after shot peening.
Highlights
High-strength 50CrV4 steel alloys are widely used for loadbearing applications where both toughness and resistance to abrasion are key requirements, including gears, pinions, shafts and axles [1]
The surface defects have negative effect on the fatigue properties of components. These defects can be due to manufacturing processes such as machining or during service originated from foreign object damage
This study aims to demonstrate that shot peening can be used to repair these defects and improve their fatigue properties
Summary
High-strength 50CrV4 steel alloys are widely used for loadbearing applications where both toughness and resistance to abrasion are key requirements, including gears, pinions, shafts and axles [1]. Small surface defects and scratches can be produced due to manufacturing processes such as casting, forging and welding, installation and foreign object damages during the service. When these parts are subjected to cyclic stresses at high loads, fatigue cracks can initiate and propagate from these surface defects [2, 3]. When stress is below the fatigue limit, these defects are rarely the origins of crack propagation and may become nonpropagating cracks [5].
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