ABSTRACT

Shot peening is used in many industrial applications, e.g. steam turbine blades, to induce near-surface compressive residual stresses and reduce the likelihood of failure by fatigue, corrosion fatigue and stress corrosion cracking. On the whole, shot peening has proven to be very successful in increasing the life of structures and components. However, the depth of the compressive stress layer is typically only about 250 µm and this poses the question as to the retained benefit when corrosion pits develop to varying depth. In the first stage to addressing this issue we show that the fatigue limit of a 12 Cr martensitic stainless steel turbine blade material tested in air at varying pit depths, ranging from 50 µm to 320 µm, was still significantly enhanced by shot peening even for the maximum depth studied. Complementary measurement of the crack propagation rate from a corrosion pit showed that the propagation rate was retarded by the near-surface compressive stress for crack depths up to 0.9 mm, well beyond the depth of the compressive layer. Serial sectioning to identify the loci of crack initiation sites yielded the unexpected result that crack development occurred preferentially away from the pit base, especially for the smaller pit depths.

INTRODUCTION

Shot peening is commonly applied to engineering components to induce a residual compressive stress gradient into the surface of the metal and thereby increase the fatigue life. Most research has focused on the beneficial effect of the shot-peened surface1,2,3 including testing with mechanical notches.4 However, where cracks develop from corrosion pits there is significant uncertainty as to the residual benefit of shot peening, especially when the pit depth approaches that of the compressive stress layer. While there is awareness of this possibility, there has been surprisingly little research to investigate the effect of pitting corrosion on the fatigue life of shot-peened components. This poses challenges in evaluating the impact on remanent life of components of pits detected in service. The few studies that are in the literature tended to be based on testing of specimens with notches/artificial pits induced by electrical discharge machining5,6 or by drilling to a certain size followed by a corroding treatment with nitric acid7 and deductions made then about the location of crack initiation. However, artificial pits generated by these methods are not usually representative of the macro-geometry and microtopography of real pits.

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