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Materials • Processes • Finishes


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 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. Given the track records of both SKF and the Department


Fig. 4. Tata Steel has produced around 300 tonnes of superbainite for evaluation by means of testing and microscopy.


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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