PSI - Issue 23

Ulrich Krupp et al. / Procedia Structural Integrity 23 (2019) 517–522 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

The very high cycle fatigue (VHCF) strength of high strength steels is limited by the occurrence of metallurgical defects. Of particular harmfulness are non-metallic inclusions, like Al, Ca oxides or Ti carbonitrides, which can be rated by their structure, size and shape as well as by their geometrical arrangement within a loaded component. Differences between the elastic properties of the inclusions and the steel matrix give rise to stress concentrations and local plasticity, which eventually lead to crack initiation and fracture even at numbers of cycles exceeding 1 billion. The estimation of the fatigue limit based on the properties of the inclusions is difficult. In general, it can be concluded that the higher the static strength of a material the more intense is the detrimental effect of inclusions. According to Murakami and Endo (1994), the fatigue limit  FL is a function of the inclusion size (evaluated by the projected area within a metallographic cross section or within the fracture surface) and the material's strength, expressed by the Vickers hardness HV:     FL 1 6 C HV 120 area    (1) Here, the constant C refers to the geometric arrangement of the inclusion (C=1,43 (surface), C=1,56 below surface) und C=1,41 (bulk). Eq. (1) neglects the influence of stochastics and microstructure inhomogeneities. In the VHCF regime, the fatigue strength is strongly dependent on the component size, since the probability of the occurrence of critical inclusions increases with increasing sample size. As shown schematically in Fig. 1, the dendritic solidification during continuous casting of alloy steels leads to macro and micro segregations, leading to pronounced banding, cf. Offerman et al. (2002), which affects the fatigue behavior of the respective material.

Fig. 1. Schematic representation of the microstructure inhomgeneities resulting from the process steps continuous casting, rolling and heat treatment .

Furthermore, the Murakami concept (eq.1) does not refer to the material's ductility, which is determined by the grain size, as well as by the crystallographic misorientation and chemistry of grain boundaries. Non-equilibrium quenching of alloy steel leads to a diffusion-less martensitic transformation of the fcc austenite to a tetragonally distorted, C supersaturated ferritic structure. The martensitic transformation is accompanied by strong lattice strains. These strains are accommodated by the formation of thin lathes being separated by dislocations or twinning. This causes a complex microstructure composed of prior austenite grains, martensite pakets and and martensite blocks that