Influence of Cu precipitates and C content on the defect tolerance of steels

Abstract Understanding the effects of microstructural defects, which can have a high impact on service life, is of great importance in the design of cyclically loaded components. For this, the size of the defect as well as the material’s ability to counteract microstructural defects have to be considered. In this study, the defect tolerance of differently heat-treated Cu alloyed steels with two different C contents (0.005 and 0.21 wt % C) has been investigated by testing specimens with artificial defects. From the results of fatigue experiments, defect tolerance is assessed based on different approaches: i) a comparison of stress intensities, derived from Murakami’s √area approach, ii) comparing fatigue lifetime for a given defect size and stress amplitude, iii) the reduction of fatigue strength by a defect for a given defect size, and iv) using the Kitagawa-Takahashi diagram, enabling the calculation of the critical defect size √area0 as well as fatigue crack propagation threshold ΔKth,calc. The results show that for the 0.005 wt % C steel an increase in aging time is accompanied by an increase in defect tolerance, which correlates with a higher cyclic hardening potential induced by the formation of Cu precipitates. However, the results show that strength and ductility also influence defect tolerance. By using the short-time procedure PhyBaLCHT, which is based on cyclic indentation tests, all these factors influencing defect tolerance can be determined efficiently. The 0.21 wt % C steel does not reveal an improved defect tolerance which is presumably caused by the dominating pearlite phase.