Microstructural investigations and detection of low-cycle fatigue degradation in metastable austenitic steel

The microstructural changes in the pre-crack stage of low-cycle fatigue damage (LCF) in austenitic piping steels are characterised by neutron and X-ray diffraction. 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 both, the load amplitude and the number of LCF cycles. 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 from 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 aged specimens (0.5 - 3.1 vol.% martensite, usage factors from 0 up to 1.0).