Microstructural aspects of low cycle fatigued austenitic stainless tube and pipe steels

Material degradation of thermomechanically strained reactor coolant piping made of austenitic stainless steels is accompanied by the strain-induced martensitic transformation of the metastable austenite to varying extents. Besides the accumulated plastic strain, the volume fraction of the α′ martensite also depends on the chemical composition, processing and final heat treatment or cold work condition of the individual heat. The objective of our study was to investigate the microstructural changes in isothermally low cycle fatigued specimens made out of industrially processed tube and pipe steels and tested in air. The volume fractions of martensite, determined by optimized magnetic nondestructive test methods, were globally small but could be large in the vicinity of the crack tip where higher plastic strains are present. The martensite was found at the intersections of slip bands. It was shown that the Schaeffler diagram and M d30 temperature provide only qualitative information for the susceptibility to the transformation.