Hydrogen embrittlement of a roller bearing outer ring

The following article has been extracted from a recent publication by Dr Paul Roffey in the Journal of Failure Analysis and Prevention.

Although rolling contact fatigue (RCF) is a common bearing failure mechanism, it is not the only mechanism observed in bearings. Others include wear and abrasion, contamination related failures (etching, corrosion or fretting corrosion), inadequate lubrication related failures (peening, overheating, smearing) and false brinelling, true brinelling or electrical damage.

All of the mechanisms above are identified by characteristic features such as pitting, spalling or blueing on the raceways and rollers. The current study however describes an investigation of an axle roller bearing which failed due to a full width axial crack of the outer ring initiating from the outer surface and not the raceway; an uncommon failure mode in roller bearings.

The bearing failed due to full width axial cracking of the outer ring, an uncommon occurrence in roller bearings, Figure 1 and Figure 2. Typically, this mechanism of failure is more consistent with inner rings that have been installed with excessive hoop stress from an incorrect interference fit on a rotating shaft.

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Two fatigue spalls were present on the raceway of the inner ring, along with polishing and secondary mechanical damage on the guide flange. The cage was intact but had undergone some plastic deformation. Several of the rollers had catastrophically failed, although the majority remained in relatively good condition with slight indentation damage from entrained debris.  Subsequent metallography and electron microscopy, confirmed all the features on the cage, roller and inner ring were secondary failure related, with the axial crack in the outer ring being the primary root cause of failure.

Fractography of the axial fracture on the outer ring revealed a shallow intergranular crack initiating from the surface of an external corner, Figure 3 and Figure 4. The remainder of fracture propagation was high cycle fatigue.  Detailed metallographic analysis revealed hydrogen embrittlement on all four corners of the ring. Final machining processes, such as grinding and polishing performed on the side faces, raceway and outer surface had removed the embrittled material in these areas, Figure 5. This indicated the embrittlement process was not service related, but a result of manufacturing heat treatment, most likely following the first rough cut of the ring, prior to finishing.

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