Cyclic deformation behaviour and stability of grain-refined 301LN austenitic stainless structure

Low cycle fatigue (LCF) behaviour of metastable austenitic 301LN stainless steel with different grain sizes - coarse-grained (13 mu m), fine-grained (1.4 mu m) and ultrafine-grained (0.6 mu m) - produced by reversion annealing after prior cold rolling was investigated. Fully symmetrical LCF tests with constant total strain amplitudes of 0.5% and 0.6% were performed at room temperature with a low constant strain rate of 2x10(-3) s(-1). Microstructural changes in different positions within the gauge part of the specimens were examined by optical microscopy (polarized light) and electron backscatter diffraction (EBSD) technique; for quantitative assessment of the volume fraction of deformation induced martensite (DIM) a Feritscope FMP 30 was adopted. The cyclic stress-strain response and specific changes of hysteresis loop shapes in the very early stage of cycling are confronted with the character of DIM formation and its distribution in the whole volume of the material. A possible effect of strain rate (frequency of cycling) on the destabilization of austenitic structure during cyclic straining of materials with different grain sizes is highlighted.