In this paper we demonstrate that the residual stress introduced by several different surface finishes affects the critical current density for passivation and the passive current density in the anodic polarization curve of austenitic stainless steel and that those critical current densities can be reduced by controlling the residual stress by applying a cavitating jet to the backs of specimens. The results show that the current density either increased or decreased depending on the surface finish, and that was decreased by introducing compressive residual stress for all surface finishes.

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The objective of this study is to evaluate fatigue crack propagating behavior in surface modification layer treated by peening. A load-controlled plate bending fatigue tester has been developed in this study. This makes possible to evaluate relation between stress intensity factor range and fatigue crack growth rate, i.e., da/dN-K diagram, in the surface modification layer, since the tester keeps the load constant differently from a displacement-controlled tester. Although the displacement-controlled fatigue tester which keeps displacement constant has been widely used to evaluate the fatigue crack propagating behavior, an applied load, i.e., stress ratio, changes along with crack propagating. In this study, the fatigue crack propagation test was conducted on specimens treated by cavitation peening, which is one of the surface modification techniques, and mechanical properties were also evaluated, including residual stress, yield stress and Vickers hardness, so as to reveal the effect of such mechanical properties varied by the peeing on the fatigue crack propagating behavior. It was concluded that the da/dN-K diagram determined by the tester follows the Paris law and K at da/dN = 10-9 ~ 10-8 was increased more than twice by cavitation peening. The increment has a relation with the mechanical properties varied by the peening.

Introducing compressive residual stress by a cavitating jet into the sub-surface of components used in nuclear power plants can mitigate stress corrosion cracking in these components. Although applying the jet is an effective method for this purpose, it should be used without causing damage to the surface from water jet droplets arising from high-pressure injection of the water jet. Thus, in introducing compressive residual stress, the injection pressure needs to be optimized. In this paper, in order to determine the optimum injection pressure, the residual stress of stainless steel treated by a jet at various injection pressures was measured using an X-ray diffraction method. The injection pressure of the jet was varied from 5 MPa to 300 MPa, and the diameter of the nozzle throat of the jet was varied from 0.35 mm to 2.0 mm. The variation of residual stress with depth was measured by alternating X-ray diffraction measurements with electropolishing. It was revealed that a cavitating jet at an injection pressure of 10 MPa with a nozzle diameter of 2.0 mm can introduce higher compressive residual stress to deeper into stainless steel compared with a jet at 300 MPa with a nozzle diameter of 0.35 mm when the downstream pressure of the nozzle was constant.

In this paper, a method that uses the Vickers hardness to estimate the yield stress of a metallic material with taking account of residual stress is proposed. Although the yield stress of bulk metal can be evaluated by a tensile test, it cannot be applied to local yield stress varied by surface modification methods, such as the peening technique which introduces high compressive residual stress at the surface. Therefore, to evaluate the local yield stress employing a relatively easy way, the Vickers hardness test was conducted in this paper. Since the Vickers hardness depends on both the residual stress and the yield stress, the relationship between the residual stress and the Vickers hardness was experimentally examined. It was concluded that the yield stress of the surface treated by several peening techniques can be estimated from the Vickers hardness once this has been corrected for residual stress.