Effects of Peening Direction on Reverse Transformation Induced by Shot-Peening for Fe-33%Ni Alloy

Effects of peening direction on the reverse transformation induced by the shot-peening for the Fe-33 mass%Ni alloy with large amount of martensite (α’) are investigated. When the angle between the peened surface and the peening direction (Hereafter, peening angle) is 90 o, the reverse transformation occurs and subsequently martensitic transformation is induced by the shot-peening. On the other hand, in case of the peening angle of 30 o, only reverse transformation occurs. Furthermore, the volume fraction of austenite (γ) in the specimen after the shot-peening increases as the peening angle decreases. This means that the reverse transformation induced by the shot-peening is enhanced by decreasing the peening angle. Moreover, residual compressive stress around the peened surface increases as the peening angle decreases. Since the hydrostatic compressive stress decreases phase transformation temperature, the phase transformation temperature around the peened surface would be decreased by the shot-peening. Therefore, the reverse transformation behavior depending on the peening angle can be explained by the residual compressive stress due to the shot-peening.