Development of Understanding of the effects of Laser Cladding on Novel Iron- Based Hardfacing Materials

Nuclear power plants are complex engineering structures with a multitude of different components required to make them operate. One area that rarely hits the headlines, but it fundamental to the operation of the plant, is tribological components such as valves and pumps. These components operate in a particularly aggressive environment of a high neutron flux, high load contacts and lubricated by just superheated water. Traditionally, there has only been one material that can survive this highly hostile environment: cobalt based hardfacing alloys called Stellites. However, there is a big problem with these alloys. The exposure of cobalt to a neutron flux results in the formation of cobalt-60 radioisotope, which causes radioactive exposure by site workers and contributes to radioactive waste. The obvious answer to this problem is to remove the cobalt from the hard facing. This is a very challenging materials problem. Our hypothesis is that iron based alloys can be designed that will meet the performance standards of the current Stellites.

This project will explore some exciting new opportunities in new iron-based hard facing alloys to replace the existing cobalt based Stellites. This includes both alloy development and exploring laser cladding as a novel route to manufacture. The principal failure mechanism of the hardfacing materials is through a mechanism called galling, where the two surface effectively weld together. There are several important components to the material’s microstructure which are key to make it successful in this application and be resistant to galling. The alloy design will need to fundamentally understand the metallurgy of these materials in order to design a new material. Firstly, the alloy must have excellent corrosion and oxidation resistance. Secondly, the alloy must have a matrix that is highly resistant to plastic deformation. Thirdly, the alloy must contain a uniform dispersion of hard, tough, second phase particles that provide load support. Overall, the alloy must have good thermal conductivity. These factors, and the need for low radio activation elements, limits the alloy additions that can be used. Previous work at Rolls-Royce has laid the basic alloy design principles. The current project will extend this work with the clear aim of designing a microstructure that will deliver a material that matches the current Stellites. In addition to the novel alloy design, the process route must also be investigated. The hardfacings will be manufactured through a novel process route of laser deposition. This route should give much greater flexibility in the alloy design.

The experimental programme will bring together a wide range of state-of-the-art techniques. The microstructure of new hard facings will be extensively characterised and quantified through the latest electron microscopy techniques. The wear behaviour will be investigated using the state-of-the-art suite of test instruments at UoS and a new environmental galling rig at the Royce Institute at Manchester. Stellite will be used as the base-line to compare the new alloys. The aim here will be to develop a fundamental mechanistic understanding of the inter-relationship between microstructure and the friction and wear behaviour. This will focus understanding on what is required for the ideal microstructure. This understanding will be used to further alloy design and manufacturing process development. This will provide a major step forward in in bringing these alloys into service for the next generation nuclear power plants.

Applications can be made using the information on this page https://www.sheffield.ac.uk/postgraduate/phd/apply/applying