Monitoring fatigue degradation in austenitic stainless steels

During cyclic loading of austenitic stainless steel, microstructural changes occurred, which affected both mechanical and physical properties. For certain steels, a strain-induced martensitic phase transformation was observed. The investigations showed that for the given material and loading conditions the volume fraction of martensite depended on the cycle number, temperature and initial material state. It was found that the martensite content continuously increased with the cycle number. Therefore, the volume fraction of martensite was used for indication of the fatigue usage. The temperature dependence of the martensite formation was described by a Boltzmann function. The martensite content decreased with increasing temperature. Two different heats of the austenitic stainless steel X6CrNiTi18-10 (AISI 321, DIN 1.4541) were investigated. The martensite formation rate was much higher for the cold-worked material than for the solution-annealed one. All applied techniques, neutron diffraction and advanced magnetic methods allowed the detection of martensite in the differently fatigued specimens.