Adjusting the very high cycle fatigue properties of a metastable austenitic stainless steel by means of the martensite content

The effect of deformation induced martensite on the HCF and VHCF properties of austenitic stainless steel sheets in different conditions was studied. In the undeformed condition true VHCF fatigue limit exists even though microplasticity (slip markings) could already be found at a stress amplitude of less than a third of the VHCF fatigue limit and global plastic behavior at less than a half of the VHCF fatigue limit. Fatigue tests with monotonically predeformed specimens in one direction containing different martensite volume fractions showed that the martensite content enhances the HCF and VHCF properties and the fatigue limit remains independent of the cycle number up to 27 vol-%. At 54 vol-% martensite the more brittle behavior and the high notch sensitivity of the martensite phase becomes predominant. This leads to crack initiation at inclusions and a fatigue limit decreasing with the number of cycles in the VHCF regime. By comparing these tests with the behavior of specimens that were monotonically predeformed in two directions, it could be concluded that the VHCF properties are not determined predominantly by the dislocation density, direction of predeformation or amount of predeformation, but by the martensite content, notch sensitivity and the purity of the material. The results lead to the recommendation that automotive stainless steel structures undergoing cyclic loads beyond 2⋅106 cycles should not exceed a deformation-induced martensite content of 27 vol-%.