It was previously found that the fretting-fatigue crack propagation rate in type SUS 304L stainless steel increased with crack length, but at a certain length it decreased to a minimum value and then increased again for stress ratios of R=-0.33 and 0. This phenomenon has been interpreted based on the contacting of the crack surfaces. Since SUS 304L is a metastable austenitic stainless steel, the martensitic transformation induced by deformation strengthens the crack tip and may reduce the crack propagation rate. In order to clarify the effect of martensitic transformation on the crack propagation under fretting-fatigue, microbeam X-ray Fractography on the fractured surface was performed.
The martensitic transformation occurred both in the fretting- and in the unfretting-fatigue fractured surface. The half value breadth Δ(2θ) of the martensitic phase α'(211) varied with crack length a, stress ratio R, and the presence of fretting. The relation between a and Δ(2θ) was similar to the relation between a and the crack propagation rate da/dN. It was shown, however, that Δ(2θ) did not correspond to da/dN owing to the effect of crack surface contact. Optical metallography showed that the martensite transformation at a=2mm for R=-0.33 had occurred already to the depth of several hundreds micronmeter from the crack surface. Although the crack propagation may be affected to some extent by this strengthened zone at the crack tip, the retardation of crack propagation in fretting-fatigue was attributed to the heavy contacting of the crack surface in the stage of slantwise crack propagation.