Fatigue Strength and Subsurface Crack Growth Properties of Plasma Assisted Duplex Surface Treated Tool Steel (SKD61).

Abstract

Numerous methods of surface treatment have been developed for purpose of the increase in high cycle fatigue strength and extension of lifetime of the engineering materials in various kinds of environment. In this study, cantilever-type rotating-bending fatigue test in air was conducted using the smooth specimens of alloy tool steel, SKD 61, with three kinds of surface treatment ; that is, TiN coating by PVD method, plasma assisted ion-nitriding and duplex treatment with the ion-nitriding followed by the TiN coating. Fatigue strength of the surface treated specimens was improved as compared with untreated specimen. The highest fatigue strength was the duplex surface treated specimen, followed by the ion-nitriding one and the lowest was the TiN coating one. This tendency was obvious at the region of high stress amplitude and disappeared at low stress amplitude region. Increase of fatigue strength of specimen by surface treatment was due to transition of fatigue crack initiation site from surface to subsurface by an effect of compressive residual stresses occurred at surface layer by ion-nitriding, and barrier to the formation of surface crack nuclei beneath the TiN coating film. The difference in fatigue strength between the duplex surface treated specimen and the ion-nitriding one, of which crack initiation site was subsurface, was discussed by the effect of TiN coating film on subsurface crack growth behavior. It was found that TiN coating film acts as barriers to formation of plastic zone at the front of subsurface crack and retards crack growth twoard surface.