Fatigue behaviour of austenitic stainless steels in the VHCF regime

The focus of this work lies on the investigation of the fatigue properties of austenitic Cr-Ni steels in the HCF and VHCF regime. This class of steels is characterized by low stacking fault energies which determine the main characteristics of deformation mechanisms (dislocation gliding, phase transformation). Three austenitic stainless steels, having distinct stacking fault energies and correspondingly distinct stabilities of the austenitic phase, were used in this study (304L, 316L and 904L). Other factors that influence the propensity of the materials for α’ martensite formation were also considered (e. g. temperature, strain rate/frequency). The metastable austenitic stainless steel 304L shows a very pronounced transient behaviour and a true durability without failure beyond 106 cycles. A comprehensive description of the microstructural changes governing the cyclic deformation is presented. The 316L steel has higher stacking fault energy and its cyclic deformation is much less pronounced. The plastic shear is more localized and the topography investigations show the formation of deep intrusions where microcracks can be initiated. The propagation of such microcracks however is impeded by the α’ martensite formed within the slip bands. The fatigue tests using different frequencies show higher fatigue strength for samples tested at 20 kHz compared to those tested at 140 Hz. This can be explained by the dependency of the plastic strain amplitude on the strain rate which was experimentally demonstrated in this study. The highly stable steel 904L exhibits a decrease in fatigue strength in the VHCF regime with failures up to 5.5∙108 cycles. Microcracks initiate from twin boundaries with extremely few signs of plasticity and grow very inhomogeneously due to the strong barrier effect of neighboring grains. The effect of predeformation on the HCF and VHCF properties was also investigated in the case of the metastable grade 304L. The amount of the α’ martensite phase obtained by means of monotonic predeformation was deliberately adjusted in order to influence the fatigue properties of the material. The results show that the initial martensite content should be kept below 30 vol-% in order to obtain optimal HCF and VHCF properties. For very high martensite contents (e.g. 60 vol-%) internal cracks are initiated and the fatigue life is strongly determined by the local stress and the geometry of the crack initiating inclusions.