Comparative Study of Microstructure-sensitive Fatigue Crack Propagation in Coarse- and Fine-grained Microstructures Between Stable and Metastable Austenitic Stainless Steels Using Miniature Specimen

Microstructure-sensitive fatigue crack propagation was studied on coarse- and fine-grained stainless steels with different austenite stabilities using miniature compact-tension specimens. For coarse-grained 310S stable austenitic steel, the crack growth rate was increased by shear-localised bands formed ahead of the crack tip. For fine-grained 310S with an average grain size of similar to 0.25 mu m, the crack-tip plastic strain was concentrated on the grains favourable to dislocation multiplication, rather than being dependent on the distance from the crack surface, which led to discontinuous crack propagation. Consequently, the fatigue crack growth rate was lower in the fine-grained 310S steel than in the coarse-grained one. In 304 metastable austenitic steel, the fatigue crack propagated within the martensite that formed ahead of the crack tip, and the crack growth rate was lower than that in the 310S steel. The grain refinement of 304 steel to a similar to 0.99 mu m average grain size enhanced the crack growth resistance. Electron back-scatter diffraction analysis of the fracture surface revealed microstructural fragmentation due to single-variant transformation for each grain in the fine-grained 304 steel. These findings indicate that the microstructural evolution ahead of the crack tip dominates the rate of mechanically short fatigue crack propagation in austenitic stainless steels.