From the testing laboratory: damage analysis of wheelset bearings

1) Background

Unusual pitting was found on both the rolling elements and the rings of freight wagon wheelset bearings. The customer commissioning the damage analysis suspected that electrical current passing through the bearings had caused the damage.

2) Material and operating conditions

The rolling elements and bearing rings were manufactured from 100Cr6 bearing steel. The damage occurred both in bearings with low mileage and in those with a long service history. There were no obvious indications of possible current sources, such as electric welding work on the freight wagons.

3) Damage analysis

The investigation focused on metallographic analysis and electron microscopy. Scanning electron microscopy (SEM) revealed dendritic solidification structures within the pits on both the rolling elements and the bearing rings. Droplet-shaped deposits of material that were chemically identical to the base material were also found.

Chemical analysis was carried out in the SEM using energy-dispersive X-ray spectroscopy (EDX).

The rolling elements and bearing rings are forged, quenched and tempered components. In their as-supplied condition, however, they do not have a dendritic microstructure. Dendrites are characteristic of solidified melts, as found in cast microstructures. Their presence therefore indicates that the material had locally melted and subsequently resolidified. Such temperatures can arise in service when electrical current passes through a bearing.

4) Conclusions: damage analysis of wheelset bearings

The damage analysis results confirm the suspicion that electrical current passing through the bearings caused the damage. The metallographic analysis also supports this interpretation. The experts identified a series of pits on a rolling element and a geometrically identical series on the corresponding bearing ring.

This mirror-image pattern demonstrates that the rolling element and bearing ring were damaged simultaneously. It also shows that the damage must have occurred while the freight wagon was moving. This is the only way to explain the identical but mirror-image arrangement of the pits.

The most likely cause is thought to be stray currents that may have originated from the electric locomotive pulling the freight wagons.