Cobalt-free Hard-facing for Reactor Systems

Cobalt-based (Co) alloys are extensively used in nuclear reactors, particularly in regions prone to wear and galling such as valves and pumps. This is because they provide outstanding wear resistance, and so improve component life and reduce maintenance requirements. However, these alloys are responsible for a large portion of the radiation exposure of workers at nuclear utilities, due to the formation of Cobalt-60, a gamma-emitting radioactive isotope. For this reason, replacing Co-based hardfacing alloys in future nuclear reactors is highly desirable. For the current Indian prototype fast breeder reactor (PFBR) Nickel-based (Ni) alloys have been used as an alternative, however these alloys are expensive and very susceptible to cracking. A new class of Iron (Fe) based, silicide strengthened systems have shown great potential for hardfacing application. In response to the scale of components that require hardfacing in the PFBR, plasma transfer arc (PTA) deposition is proposed, a highly flexible manufacturing technique, as an alternative to HIPbonding. To date, however, PTA manufacturing of Fe silicide strengthened alloys is yet to be explored. Indeed, the residual stress resulting from PTA processing nickel (Ni)-based alloys, a key driver in the formation of unacceptable cracks and defects, is still to be understood. Successful characterisation, optimisation and simulation of the PTA process and application to Fe based alloy systems is a key step in eliminating Co from nuclear plant.

This project is concerned with the development of plasma transfer arc (PTA) deposition and novel silicide-strengthened stainless Fe-based alloys to create opportunity to remove Cobalt from hardfacing applications. It is a potential route to a new generation of highly wear-resistant materials for use in very demanding environments across large scale components. The potential impact of replacing Co-based materials for hardfacing applications, which, although currently utilised extensively in nuclear reactor systems are particularly undesirable, due to their large contribution to radiation exposure by the formation of Cobalt-60, is to drastically reduce what is considered "as low as reasonably possible" for radiation dose resulting from hardfacing materials.

Previous work at Manchester has led to the discovery of a new type of silicide phase in steel, named pi-ferrosilicide, which promises to have excellent wear and corrosion resistance. To date, trials have only been conducted using HIP bonding which is limited in terms of maximum component size. The prototype, sodium cooled, fast breeder reactor being commissioned in India needs hardfacing to be applied to components in excess of 1000mm in diameter, way beyond the current HIP bonding capabilities. Therefore, producing and characterising the first PTA processed silicide strengthened alloys is an essential step for their widespread application. Residual stress and cracking is a limiting factor and providing an evidence based procedure for minimising both issues will be a key scientific impact of the project.

The proposed research contributes to two key EPSRC themes, Energy and Manufacturing the Future. It particularly focuses on applications in the nuclear sector with direct collaboration with IGCAR in India. Hence, it focuses on the research area Nuclear Fission and the application to the Indian PFBR, although silicide-strengthened stainless steels processed by the PTA method could potentially find applications in many other sectors (for instance the petrochemical industry where materials must retain high-temperature strength in highly corrosive environments) and the mineral processing industry. In light of the new nuclear build programme in the UK and the prospect of small modular reactors (SMRs), the proposed research is also extremely timely, and has the potential to make a significant contribution to these programmes.