Materials & Processes
elevated temperatures and aggressive lubrication. Aero engine bearings are also unusual in that the thin section of the outer race (designed to reduce weight and increase fuel efficiency) and the high rotational speed result in high centrifugal hoop stresses. The bearing’s inner ring also has to withstand high hoop stresses, as this is often press-fitted on the shaft to prevent relative movement and consequent fretting. Another application in which bearings suffer due to unfavourable operating conditions is wind turbines.
Bearing steel development
Fig. 2. Dr Harry Bhadeshia, director of the SKF University Technology Centre for Steels.
Enhanced understanding of steels, improved purity and sophisticated processing techniques have resulted in the current generation of bearing steels being far superior to those previously available, especially for challenging applications such as aero engine bearings. But, of the millions of tonnes of steel that are manufactured annually for bearing applications, almost all is based on compositions developed from those first used for tool steels, containing around 1 per cent carbon and 1.5 per cent chromium, with small additions of alloying elements to improve certain characteristics or counteract undesirable side-effects caused by other alloying elements. These steels have microstructures consisting of undissolved carbides in a matrix of either mildly tempered martensite or bainite generated by isothermal transformation (heat treatment at a constant temperature). Although it would be possible to make further improvements in purity and inclusion
Photo: Steve Penny.
content to improve ductility, in fact there would be little gain because other factors would then determine the steel’s brittleness. Dr Bhadeshia’s team at the SKF UTC for Steels has
developed the theory for solid-state phase changes to create a new steel that contains no carbides. Very closely spaced interfaces result in high hardness and also maintain the separation between the ferrite and highly stable austenite. Furthermore, the new steel is cheap to manufacture, requiring no complex processing or heat treatment, is homogeneous and can be produced in quantity. Known as superbainite, the nanocrystalline steel has slender plates of ferritic bainite, just 20-40 nm in thickness, within a matrix of finely divided, high- carbon austenite (Fig. 3). Distortion caused by phase change is minimal.
Hiding hydrogen
At the same time, the researchers are investigating ways to capture hydrogen at carefully designed locations so as to render it harmless. This is important because hydrogen can readily diffuse into the steel during manufacture and operation
Fig. 4. Tata Steel has produced around 300 tonnes of superbainite for evaluation by means of testing and microscopy.
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through a number of different mechanisms, and as little as one part-per-million of hydrogen can cause embrittlement in steels.
Already Tata Steel has produced around 300 tonnes of superbainite to enable testing and microscopy to be used in evaluating the progress to date (Fig. 4). This steel has proven to be very strong, have uniform ductility, no residual stresses, and is uniform in large sections. Dr Bhadeshia is eager to see superbainite bearings manufactured and put into service so performance can be evaluated, but SKF will not say when this might happen. Indeed, SKF is content to let the research proceed to see where it leads, rather than driving it with product-related goals. SKF sees the UTC for Steels as a long- term partnership with the University of Cambridge, having committed to five years but stating that university-based research is five-times better value than in-house research.
Fig. 3. Superbainite, the world’s first bulk nanocrystalline metal, has slender plates of ferritic bainite within a matrix of finely divided, high-carbon austenite.
Given the track records of both SKF and the Department of Materials Science and Metallurgy at the University of Cambridge, there is every possibility that truly ‘disruptive’ developments will emerge from the UTC for Steels, resulting in substantial progress in the design of rolling element bearings. l
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