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

Abstract

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, Ƴmax ∼ 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 M23(C, N)6 and M2(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 100Cr6 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 (M23(C, N)6 ∼ 22 GPa, M2(N, C) ∼ 20 GPa) and increased thermodynamic stability in comparison to M3C cementite (hardness ∼ 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.