James Nygaard, M.Phys. (Hons) Peterhouse University of Cambridge Materials Science and Metallurgy Pembroke Street, Cambridge CB2 3QZ, U. K. jrn36@cam.ac.uk +44 1223 334495

Aerospace bearing steels under rolling contact fatigue

Secondary hardened steels are used in the manufacture of bearings for aircraft turbine engines; here, working temperatures exceed 220°C and rotation speeds up to 12,500 rpm can lead to fatigue failure.

The ball elements themselves are a high-carbon M50 tool steel which in the heat-treated condition has a hardness of 850 HV (64 HRC). The raceways are low carbon, case-carburised M50NiL steel where the tough core is able to cope with tensile stresses arising from mainshaft fitting and centrifugal effects.

After tens of millions of cycles, rolling contact fatigue failures can initiate at imperfections on the meeting surfaces and also beneath the point of contact where the shear stresses are maximum. Here in the subsurface cracks nucleate at stress raising defects; traditionally at interfaces between non-metallic inclusions and the matrix. To compensate, aeroengine bearing steels are produced under vacuum for high purity and low oxygen content (< 5 ppm), and so fatigue failures during service are rare yet unpredictable.

The aim is to gain a better understanding of these rare failures through characterisation of engine-run bearings.

This PhD research is carried out in collaboration with Rolls-Royce and SKF Group, and funded by the EPSRC and Rolls-Royce.

Together with my academic work I am also an active member within Peterhouse; holding the position of Social Secretary on the graduate committee, and rowing in our men's second division racing eight.