Fatigue short crack behaviour in metastable austenitic stainless steels with different grain sizes

The martensitic transformation (gamma --> alpha') induced by cyclic strains in a metastable austenitic stainless steel leads to significant modifications of fatigue mechanisms compared with stable alloys. in particular at the mesoscopic scale (short cracks). The present study has been carried out on a metastable high purity Fe-17Cr-13Ni steel with two different grain sizes. From the very beginning of cycling, the microstructural instability induces a strong modification of the plastic behaviour of the material by limiting the slip activity at surface. At same applied plastic strain amplitude. the grain size has a decisive influence on the amount of martensite formed and on the fatigue life which is almost twice longer in the fine microstructure with 20% alpha' than in the coarse one (2% alpha'). Whatever the grain size and the martensite content, short cracks nucleate in transformed alpha' regions; gamma --> alpha' transformation in front of the crack tip precedes further propagation which takes place exclusively in the martensite. The beneficial effect of the grain size on the fatigue resistance of metastable steels is explained by the indirect influence of grain boundaries on crack propagation rate, providing an original contribution to the problem of microstructural barriers in fatigue. (C) 2001 Elsevier Science B.V. All rights reserved.