Enhancement of damage tolerance of 52100 bearing steel by influencing the static and dynamic cold working features due to defined stabilized retained austenite

• Applicants: Professor Dr.-Ing. Tilmann Beck; Professorin Dr.-Ing. Brigitte Clausen; Professor Dr.-Ing. Ulrich Krupp, since 2/2021
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: from 2019 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 420401443

• Final Report Year: 2023

Final Report Abstract

Since high-strength steels tend to have a big lifetime scatter due to defects, increasing the defect tolerance can improve their performance. Hence, in this research project two modified 100Cr6 alloys were developed on laboratory scale, having, in relation to the standard chemical composition, an increased aluminum or silicon content of 1.5 wt.-%, respectively. These alloys were heat treated to realize a bainitic microstructure with a relatively high content of retained austenite, i.e., approximately 20 vol.-%. Note that these heat treatment parameters were defined based on a parameter analyses. Besides the increased deformability of the austenitic phase, local austenite-α’-martensite transformations in the vicinity of defects can increase the defect tolerance. However, excessive deformation-induced phase transformations must be avoided, as they can lead to undesirable changes in the shape of the components. Consequently, a well-defined phase stability is required. Despite the finer microstructure of the steel alloyed with Si, the monotonic properties of the two steels show only small differences, whereas the fatigue strength of the Si variant is substantially higher. In contrast to the Al variant, the steel alloyed with Si revealed a decrease in the fatigue strength due to an elevated temperature of 100 °C. Using the √area-approach, it was shown that this results from a temperature-induced decrease in defect tolerance of the Sivariant, while the Al-variant shows no differences in defect tolerance between the tests performed at AT and 100 °C. Since higher temperatures impedes deformation-induced phase transformations, this indicates a stronger phase transformation of the Si-Variant at ambient temperature and hence, a lower phase stability of this steel. These results are also confirmed by rolling contact fatigue (RCF) tests performed on roller bearings. Additional electron probe (EPMA) and atom probe (APT) analyses showed that the austenitic phase in the Si variant is distributed more finely and has a lower carbon content, and thus, a lower phase stability. Since macroscopic phase analyses done by X-ray diffraction did not show any transformation due to the cyclic loading, it can be assumed that the phase transformations occur only locally at the defects. Because of the finer distribution and the lower stability of the austenitic phase in the Si variant, in this steel the effect of the phase transformation is more pronounced. In summary, a 100Cr6 variant was developed, which exhibits an increased defect tolerance due to a fine distribution and defined stability of the retained austenite, without showing macroscopic dimensional changes due to massive phase transformations. This is in accordance with the research goal defined for the presented project.

Publications

• A New Approach to Estimate the Fatigue Limit of Steels Based on Conventional and Cyclic Indentation Testing. Metals, 12(7), 1066.
  Görzen, David; Ostermayer, Pascal; Lehner, Patrick; Blinn, Bastian; Eifler, Dietmar & Beck, Tilmann
  
• Evaluation of the plastic deformation behavior of modified 100Cr6 steels with increased fractions of retained austenite using cyclic indentation tests. Materials Testing, 64(9), 1298-1312.
  Ostermayer, Pascal; Burkart, Klaus; Blinn, Bastian; Surm, Holger; Clausen, Brigitte & Beck, Tilmann
  
• Effect of retained austenite on the fatigue behavior of modified bainitic 100Cr6 steels considering local phase transformation. Materials Science and Engineering: A, 877, 145204.
  Ostermayer, Pascal; Allam, Tarek; Shen, Xiao; Song, Wenwen; Burkart, Klaus; Blinn, Bastian; Clausen, Brigitte; Bleck, Wolfgang & Beck, Tilmann
  
• Influence of the Inherited Structure Induced by Al and Si Alloying on Microstructure Evolution and Mechanical Properties of 100Cr6 Steels. steel research international, 94(6).
  Allam, Tarek; Ostermayer, Pascal; Blinn, Bastian; Beck, Tilmann & Bleck, Wolfgang