a: © Polizeiinspektion Stade b: © Fa. Schaeffler d:  ©RWTH Aachen c und e: © Max-Planck Institut für Eisenforschung GmbH

Understanding the outstanding durability of the high nitrogen martensitic bearing steels

By characterizing the high N alloyed martensitic stainless bearing steel X30CrMoN15-1 in-depth, we rationalize the exceptional white etching crack resistance of this complex technical alloy in terms of the different grain boundary segregation behavior between nitrogen and carbon, the mechanical and thermodynamic stability of the precipitates, and the cleanliness of the steel.

White etching cracks bearing failure causes worldwide roughly one billion euros of costs per year. Despite the tremendous effort devoted to the investigation of the white etching matter, elimination or mitigation of white etching matter in bearing applications still remains an ongoing challenge. The high N alloyed martensitic stainless bearing steel X30CrMoN15-1 shows superior white etching crack resistance compared to conventional high C bearing steels. Currently, it is considered as a sustainable solution to prevent white structure flaking. Despite of the fact that the high N bearing steel X30CrMoN15-1 has been used since the late 1990s, the microstructure details of this complex technical alloy, together with the reason why it has high white etching crack resistance, is still not yet fully understood. In this project we characterize this complex technical alloy in-depth by state of the art microscopy methods from SEM over APT to nano-indentation and elucidate the microstructural reasons behind its extraordinary performance. The origin of the exceptional white etching crack resistance of this alloy is rationalized in terms of the different grain boundary segregation behavior between nitrogen and carbon as indicated by the atom probe results, the mechanical and thermodynamic stability of the precipitates, and the cleanliness of the steel [1]. Our findings are crucial for the bearing steel community to design new steels with similar properties but lower costs.

(a) Atom probe analysis of the martensite matrix of the high N bearing steel X30CrMoN15-1 shows different grain boundary segregation behavior between N and C; (b) and (c) Comparison of average nano-hardness, elastic moduli and formation enthalpies (∆Hf) of the matrix/precipitates in the high N bearing steel X30CrMoN15-1 and the high C bearing steel 100Cr6.

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