Decomposition-resistant carbonitride precipitates in X30CrMoN15-1 high-nitrogen bearing steel deformed by high-pressure torsion

The X30CrMoN15-1 high-nitrogen bearing steel is far more resistant against premature rolling contact fatigue (RCF) failures than the conventional 100Cr6 bearing steel. However, the origins of this steel's resistance against localized severe plastic deformation, which accompanies RCF failure, are still indefinite. In this work, we use multi-scale characterization techniques to investigate the microstructural changes in through-hardened X30CrMoN15-1 steel upon high-pressure torsion (HPT, maximum shear strain, gamma(max) similar to 14.14). HPT is used to ingress severe plastic deformation in larger bulk samples mimicking the localized severe plastic deformation induced by crack face rubbing in RCF samples just below the sample surface. Upon HPT, the M-23(C, N)(6) and M-2(N, C) carbonitride precipitates in the microstructure remain intact without decomposition, whereas the matrix martensite deforms severely. This is in contrast to pronounced M3C carbide decomposition observed in high-carbon 100Cr(6) bearing steels under equivalent ingress of severe plastic deformation. Apart from the cleanliness of the X30CrMoN15-1 steel avoiding crack-initiating inclusions, we conclude that the superior stability of carbonitride precipitates largely contributes to better RCF performance. We show that the carbonitrides exhibit higher hardness (M-23(C, N)(6) similar to 22 GPa, M2(N, C) similar to 20 GPa) and increased thermodynamic stability in comparison to M3C cementite (hardness similar to 15 GPa). We evaluate the differences in thermodynamic stability through density functional theory (DFT) calculations. Additionally, we discuss the influence of mechanical properties of the surrounding matrix microstructure on precipitate decomposition.

Automated summary

The X30CrMoN15-1 high-nitrogen bearing steel exhibits superior resistance against rolling contact fatigue failures compared to the conventional 100Cr6 bearing steel. Through multi-scale characterization techniques, it is found that the stability of carbonitride precipitates largely contributes to the better performance of the steel.