Browsing by Author "Baris, F"

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    Effects of shot peening and artificial surface defects on fatigue properties of 50CrV4 steel
    Saklakoglu, N; Bolouri, A; Irizalp, SG; Baris, F; Elmas, A
    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.
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    Effects of Shot Peening Process on Fatigue Strength of 50CrV4 Steel Contaning an Artifical Surface Defect
    Baris, F; Saklakoglu, N; Irizalp, S
    It is well known that surface defects have a very significant effect on the fatigue strength of engineering materials. The shot peening process is a mechanical surface treatment to enhance surface properties and fatigue strength. In this study, it is aimed to investigate the effects of surface defects on the fatigue strength of 50CrV4 spring steel and the effect of shot peening process effects. Samples including a semicircular slit sized 0,2 mm or a hole sized 3 mm throughout the sample were shot-peened by S230 steel balls. 3point bending tests were performed on defected and non-defected samples. It was found that the defects acted as a pre-existent crack and samples with hole and semicircular slit significantly reduced the fatigue strength by 85% and 80% (for 672 MPa), respectively. While shot peening enhanced the fatigue strength of the semi -circular slitted sample by 126%, it had almost no effect on the holed sample. Although shot peening increased the fatigue strength of the semi -circular slotted sample, it could not reach that of the non-defected sample. According to the results, it can be concluded that 3 mm hole and 0,2 mm scratch defects cannot be made harmless by the Shot Peening method.
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    Effect of Shot Peening on Residual Stress Distribution and Microstructure Evolution of Artificially Defected 50CrV4 Steel
    Irizalp, SG; Saklakoglu, N; Baris, F; Kayral, S
    The aim of this paper is to clarify the effects of shot peening on fatigue life and strain hardening behavior of spring steel samples containing artificial surface defect. Artificial defect acting as pre-cracks is introduced and the effects of this defect on the material surface and the distribution of residual stress away from the defect are investigated before and after shot peening. Shot peening has provided the following modifications on the surface: (1) homogenous compressive residual stress, (2) high dislocation density, (3) superficial strain hardening, (4) retained austenite reduction. In this paper, shot peening is applied to 50CrV4 steel and its effect on surface roughness, microhardness, surface residual stress, crystallite size and dislocation density calculation is determined and SEM observations are used to reveal the properties severely strain-hardened layer. It can be shown that the shot peening can modify the crystallite size, however, the main effect is achieved by increasing the dislocation density and inducing the highly compressive residual stress. The effect of the surface modification to the 3-point bending fatigue life are analyzed.