Effect of martensite content and geometry of inclusions on the VHCF properties of predeformed metastable austenitic stainless steels

Abstract

The effect of inclusions on the VHCF properties of a metastable austenitic stainless steel in the predeformed condition was studied. The material contains an inhomogeneous distribution of oxide inclusions with an elongated geometry in the rolling direction. Samples were monotonically predeformed at -80°C start temperature at a strain rate of 0.1 %/s resulting in a martensite content of about 60 vol-% and subsequently fatigued by means of high frequency testing machines. On the one hand the high martensite content results in an increase of the HCF strength, on the other hand fatigue failure occurs even beyond 107 loading cycles. The higher notch sensitivity of the martensite phase leads to internal crack initiation from inclusions accompanied by the formation of a fine granular area (FGA). A direct correlation between the size of the FGA-and the number of cycles to failure can be shown. In order to find the optimal martensite content for both HCF and VHCF regime, the notch sensitivity of the material in different predeformed conditions was investigated. The results show a significantly higher notch sensitivity for 30 vol% α’ martensite, whereas higher martensite contents do not show any further significant increase. Since mechanical components are in practice subjected to complex cyclic loading situations, samples were extracted and tested both parallel and transversal to the rolling direction, in order to reproduce the relation between rolling and loading direction. The change in testing direction perpendicular to the rolling direction results in a larger projection area of the inclusions and reduces the number of cycles to failure due to the increased stress intensity factor. In this case, the area of the inclusions (but not of the FGA) correlates with the number of cycles to failure. These findings are discussed on the basis of a detailed microstructural characterization of the material focusing on the effect of martensite content, the inclusion morphology with respect to the rolling direction and the load axis applied.