A B S T R A C T 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.Keywords austenitic stainless steel; lifetime extension; low-cycle fatigue; non-destructive testing; nuclear safety; strain-induced martensite. N O M E N C L A T U R Ea = lattice constant A, B, t o = parameters of the Boltzmann function for data analysis bcc = body centred cubic lattice B x , B z = axial and radial components of the magnetic induction c α , (c α ) o = volume fraction of martensite for the fatigued (strained) and initial material state D = usage factor, ratio of the applied cycle number to an averaged cycle number representing the technical crack initiation DMC = neutron diffractometer for cold neutrons used for determination of the volume fraction of martensite EC = eddy current fcc = face centred cubic lattice FERROMASTER ® = device for measurement of the magnetic susceptibility or permeability FLUXGATE = high sensitive magnetometer for measuring the remaining magnetic induction (remanence) GMR = Giant Magneto-Resistance sensor used for eddy current impedance measurement with high local resolution ICP-OES = inductive coupled plasma emission photometry used for the determination of the chemical composition of steels LCF = low-cycle fatigue M d = starting temperature for the strain-induced martensite formation

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