Effect of Geometry and Distribution of Inclusions on the VHCF Properties of a Metastable Austenitic Stainless Steel

The effect of inclusions on the VHCF properties of a metastable austenitic stainless steel in undeformed and predeformed condition was studied. The material contains an inhomogeneous distribution of elongated oxide inclusions. TEM investigations of foils extracted by means of FIB technique show that the stress concentration at the inclusions is compensated by plastic deformation in the austenite phase preventing internal crack initiation in the VHCF regime for the non-predeformed, i.e., almost martensite-free condition. The effect of the spatial distribution and geometry of the inclusions on the VHCF strength was systematically investigated for the predeformed condition. Samples were monotonically predeformed at -80 degrees C resulting in a martensite content of about 60% and then fatigued in high frequency testing machines. Since mechanical components are in practice subjected to complex cyclic loading situations, samples were tested both parallel and transversal to the rolling direction, in order to cover a broad field of applications. The higher notch sensitivity of the martensite phase leads to internal crack initiation from inclusions supported by the formation of a fine granular area (FGA). The change in testing direction perpendicular to the rolling direction reduces the number of cycles to failure due to the increased stress intensity factor at inclusions which leads to internal crack initiation without the formation of a fine granular area. 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.