A Comparison of Steels for Nuclear Bolting Applications

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At present, the bolts are produced from SA540 steel, which is very similar in composition to the common AISI 4340 steel. Previous work on SA540 has suggested that its composition is not well optimised for use in very large bolts - results suggest that, when oil quenched, through-thickness transformation to 100% martensite is unlikely to be achieved on a consistent basis. If processing is performed perfectly then the grade is capable of an exceptional combination of strength and toughness; however, the grade is not robust to variability in processing. The component size is at the limit of the hardenabilty of the composition. Hence, properties are non-uniform through the section. Furthermore, it has been demonstrated that small differences in cooling rate around a critical value can lead to significant changes in microstructure evolution, which in turn result in significant scatter in hardness values. This project will seek to compare SA540 to another grade of steel, A723, which is likely to be more hardenable in thick sections. It will involve applying industrial heat treatment processes to both SA540 and A723 and monitoring the evolution of microstructure and properties. A number of techniques will be utilised, including dilatometry, electron microscopy (both scanning and transmission) and mechanical testing (which is likely to extend to Charpy and fracture toughness tests).