Investigation into Hot Isostatic Pressing of a 12% Chromium Martensitic Steel and optimisation of the bond to Nickel Based Alloy 625

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This research aims to demonstrate and optimise a powder-based manufacturing route to allow Rolls-Royce continued use of Jethete M153 which is already commercially obsolete in its wrought form. Rolls-Royce rely on this alloy primarily for its magnetic performance for a single application but their limited order volumes puts the alloy at serious risk of total obsolescence through loss of a wrought supply chain. The volume and frequency of Rolls-Royce orders is better aligned to a powder supply route. This PhD will explore whether a powder hot isostatic pressing (HIP) manufacturing route can successfully be applied to Jethete M153 and will look further to understand if the component method of manufacture can be overhauled to improve the resulting performance and structural integrity. Rolls-Royce currently uses welding to join wrought Jethete M153 to nickel-based Alloy 625, already manufactured via powder HIP. Moving to a fully powder-based manufacturing route for the component potentially opens up an opportunity to remove this dissimilar metal weld if these materials can be joined via HIP bonding to give an equivalent or improved in-service performance. If successful, this will improve the component structural integrity and reduce the inspection burden during manufacture and assembly with the potential to provide time, cost and performance benefits to Rolls-Royce. In-service performance will be benchmarked against the current baseline of welded and post-weld heat treated quenched and tempered Jethete M153 and solution annealed HIP consolidated Alloy 625. Joining Jethete M153 and Alloy 625 via HIP will alter the in-service condition of the two materials as any post-HIP thermal treatment will need to be applied to both materials simultaneously. Since the magnetic performance of the Jethete M153 is the driving requirement for this component, it is expected that the currently employed quench and temper will also be applied to the HIP consolidated product, however optimisation of this thermal processing will be required to ensure acceptable mechanical performance of the Jethete M153 can be achieved as well as mechanical and environmental performance of the Alloy 625. HIP will alter the microstructure and chemistry of Jethete M153 which may influence its magnetic properties in the service condition. The extent to which this occurs will also be explored (via characterisation, DSC/dilatometry and B-H loop testing) and the HIP powder feedstock studied to understand any potential optimisations which may improve the performance of post-processed HIP Jethete M153.