Monitoring of low-cycle fatigue degradation in X6CrNiTi18-10 austenitic steel

The microstructural changes in the pre-crack stage of low-cycle fatigue damage (LCF) in austenitic piping steels were investigated by neutron and X-ray diffraction (XRD). The LCF damage evolution in the metastable austenitic steel causes a deformation-induced phase transformation from austenite to martensite. Thresholds exist for the formation of martensite as a function of accumulated plastic strain. Magnetic stray field and eddy current measurements were chosen to transfer the results of material characterisation to NDT methods. The density and distribution of martensite obtained by neutron diffraction experiments were used to adjust the NDT signals. Both NDT techniques were able to detect the very low amount of martensite in the different fatigued specimens (0.5–3.1 vol.% martensite) with usage factors from 0 up to 1.0.