Cyclic deformation behaviour and plasticity-induced martensite formation of the metastable austenitic steel X6CrNiTi1810

The aim of the present investigation is to determine the influence of plasticity-induced martensite formation on the cyclic deformation behaviour of the metastable austenitic stainless steel X6CrNiTi1810 (German material number 1.4541) in single- and multiple-step-tests at room temperature and T=300 C. Several specimen batches were tested under stress, total strain and plastic strain control with different R-values, a frequency of 5 Hz (HCF) respectively 0,1 Hz (LCF) and triangular load-time functions, σ,ε-hysteresis and nondestructive magnetic measurements were used to characterize the fatigue behaviour. At room temperature and T=300 C the cyclic deformation behaviour is characterized by cyclic softening and hardening phases, which are strongly influenced by the loading conditions. A certain amount of accumulated plastic deformation has to be exceeded to initiate the martensite formation. The martensite formation varies with the loading conditions, temperature and number of cycles. At T=300 C, only low fractions of plasticity-induced martensite were formed and therefore highly sensitive magnetic sensors are necessary to detect phase transformations in fatigued specimens. The experiments yield characteristic cyclic deformation curves and martensite fractions corresponding to the actual fatigue state. Cyclic creep was determined in experiments with positive and negative mean stresses. The martensite formation is enhanced by positive mean stresses. A significant influence of positive or negative mean strains on the martensite formation behaviour could not be determined, due to the fast and nearly complete relaxation of the mean stresses. Light, electron microscopic and X-ray diffraction investigations confirm the nondestructive magnetic results with the measured fatigue state of the material. (orig.)