Effect of retained austenite on the fatigue behavior of modified bainitic 100Cr6 steels considering local phase transformation

Since highly loaded components, such as roller bearings, tend to have relatively high lifetime scatter caused by microstructural defects, an increased defect tolerance of the material can improve the performance as well as the reliability of these highly stressed components. To increase the defect tolerance of the roller bearing steel 100Cr6, two different laboratory melts with either 1.5 wt.-% Al or 1.5 wt.-% Si in addition to the standard composition were developed. After an adapted heat treatment, both steels exhibited a bainitic microstructure with a relatively high content of retained austenite with more than 20 vol.-%. Considering the effects of the retained austenite, a defined mechanical stability is essential to avoid excessive deformation-induced transformation from the retained austenite into α′-martensite, which can lead to shape deviations in components. However, besides the increased deformability of the austenitic phase, local phase transformations in the vicinity of defects can increase the defect tolerance. The fatigue tests performed at ambient temperature (AT) and at T = 100 °C, show a higher fatigue strength of the Si-alloyed steel due to a refined microstructure and revealed for both steels a relatively high austenite stability, since no macroscopic phase transformation was observed. The high austenite stability can be explained by a relatively high carbon content of the austenitic phase. However, in contrast to the Al-alloyed steel, a decrease in the fatigue strength and in the defect tolerance due to the increased temperature were observed for the Si-alloyed steel, which indicates a more pronounced phase transformation at AT. The lower carbon content and the finer distribution of the retained austenite, when compared to the Al-alloyed steel, are supposed to be the reason. While a smaller carbon content decreases the phase stability, the refined austenite distribution results in a higher probability of retained austenite presence in the vicinity of a defect.