For the design of cyclically loaded components, such as roller bearings, the influence of microstructural defects, i.e., non-metallic inclusions, and thus, the defect tolerance of the material volume must be considered. Preliminary work on the bearing steel 100Cr6 has shown that the defect tolerance increases with higher contents of retained austenite, whereby a deformation-induced phase transformation from austenite to α’-martensite has a beneficial effect on the defect tolerance. However, an excessive phase transformation can also lead to dimensional deviations, which are inacceptable for common applications. Moreover, the austenite stability increases with increasing temperature, which might lead to less pronounced phase transformation, and hence, defect tolerance, for applications at elevated temperatures.

To analyze the influence of the fraction and the stability of the retained austenite on the defect tolerance and the resulting fatigue behavior, specimens made of a modified 100Cr6 with variable contents of Si (0.6 and 1.5 wt.-%) or Al (1.5 wt.-%) were investigated. To realize a bainitic microstructure with relatively high contents of retained austenite with a sufficient stability as well as a pronounced defect tolerance, for each alloy optimized heat treatment parameters were used. These parameters were determined using a statistical design of experiments (DoE). Thereby, instrumented cyclic indentation tests (CIT) were performed at ambient and elevated temperature of 100 °C, being relevant for application of roller bearings, to determine the cyclic hardening potential, which correlates with the defect tolerance [1]. Moreover, the results obtained at different temperatures enable an estimation of the austenite stability.

To determine the influence of the austenite stability on the defect tolerance, for each condition (one condition per alloy) woehler curves were determined at ambient temperature as well as at 100 °C. The fracture surfaces, and especially the crack initiation sites were analyzed and the defect tolerance was evaluated via the √area approach [2]. The results are compared to the cyclic hardening potential obtained in CIT. Furthermore, at defined numbers of load cycles the phase distributions were measured using X-ray diffraction during test interruptions, to investigate the evolution of the phase transformation at both temperatures.

Based on these investigations the interrelation of the cyclic hardening potential obtained in CIT, the austenite fraction, austenite stability as well as defect tolerance was determined. Moreover, the variants of 100Cr6 investigated exhibit excellent fatigue properties, which confirms the applied alloying and heat treatment concepts.

[1] Goerzen et al.: Int. J. Fat. (144), 2021.
[2] Murakami, Metal Fatigue: Effects of Small Defects and Nonmetallic Inclusions, 2002.