Fatigue-induced martensitic transformation in metastable stainless steels

In metastable stainless steels deformation-induced formation of martensite gives rise to drastic changes in the material's monotonic and cyclic deformation properties. The objective of the present study was to characterize the effect of sample geometry, austenite stability and testing mode on the cyclic deformation-induced austenite/martensite transformation in AISI 304 (German designation X5 CrNi 18 9, 1.4301) and AISI 304L (X2 CrNi 18 11, 1.4306) at room temperature. For this purpose the behaviour of bulk samples was compared with that of thin foils of thickness down to 25 mu m. These tests indicated that in both geometries a similar tendency to form martensite exists. Furthermore, the carbon content was changed by a carburization and decarburization annealing treatment, respectively. The corresponding results confirmed the austenite-stabilizing effect of carbon. Tests which were carried out under block loading conditions showed that a linear martensite accumulation rule based on data from corresponding constant amplitude fatigue tests gives reasonable results in certain cases only.