Bearing steel microstructures after aircraft gas turbine engine service

Abstract

The microstructure of aircraft gas turbine engine bearing steel has been characterised after service in Rolls-Royce Trent™ engines, with the focus on surface condition and the consequences of sliding contact. Carbide populations at the surfaces of rolling elements are found depleted by 30% after 30 000 h engine service. A single ball failure occurred after this period, leading to fatigue spalling that initiated below the contact surface. Comparisons between unused bearing raceways and those that have experienced service revealed that the microstructures resulting from secondary hardening remain remarkably stable. Plastic flow along the direction of rolling is confined to a shallow zone <2 μm beneath the contact surfaces. Transmission electron microscopy has revealed a new deformation mechanism in these bearing steels, in the form of mechanical twinning at interfaces and prior austenite grain boundaries. It is demonstrated that workhardening occurs to a depth of 1 mm in the raceway that has experienced 30000 h service.

Keywords:

• Bearing steels
• Aircraft gas turbine engines
• Rolling contact fatigue
• Surface initiated failures
• Deformation twinning
• Carburised M50NiL
• Sliding shear
• Workhardening

Acknowledgements

The authors would like to thank Rolls-Royce for their donations of full scale bearings used in this work and would also thank Professor L. Greer and Professor M. Blamire for their provision of laboratory facilities at the Department of Materials Science and Metallurgy, University of Cambridge. Additional thanks are paid to E. Vegter from the SKF Engineering and Research Centre, Neiuwegein for contributions and test material.