Silicide-Strengthened Steel - A New Method of Wear Protection within Nuclear Environments

Cobalt-based (Co) steel alloys are extensively used in nuclear reactors, particularly in the valves and pumps employed in water-cooled reactors. This is because they provide outstanding wear resistance, and so improve component life and reduce maintenance requirements. However, these alloys are expensive, and also raise 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 in terms of health and safety and cost, as well as long-term decommissioning. To date, however, alternative materials such as iron (Fe)- and nickel (Ni)-based alloys have not been able to match the outstanding properties of Stellite 6, a well-known Co-based alloy for hardfacing applications (i.e. applications where wear is of particular importance).

In response to the EPSRC call for feasibility studies related to energy research, we propose to explore the manufacturability of novel silicide-strengthened stainless steels in order to deliver a new class of Co-free hardfacing materials for nuclear components, with high galling- and corrosion-resistance. We have discovered a new class of Fe-based alloys for hardfacing applications only very recently, providing a real and valuable opportunity to finally have a material system that can match Co-based alloys in their wear and corrosion performance, while also offering great cost advantages and reduced exposure risk for nuclear utility workers. Success in demonstrating the viability of this class of material, as proposed here, could further have great impact in many other areas where high-strength stainless steel solutions are required (such as in highly corrosive environments in the petrochemical industry).

This project is concerned with the development of novel silicide-strengthened stainless Fe-based alloys and identifying possible manufacturing routes. It is a potential route to a new generation of highly wear-resistant materials for use in very demanding environments, and has the potential 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, as well as to their significant cost. Finding a replacement for current alloys, such as Stellite, which offer increased safety and lower-cost, without loss of performance and durability, has been a long-term priority for industrial stakeholders and academic partners in this area.

Previous work at Manchester (in collaboration with the University of Mainz, Germany) 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. Based on this discovery, this study now intends to address the feasibility of repeat manufacture of these silicide-strengthened alloys, with control of stoichiometry in order to understand how minor variations in composition can affect the properties and microstructure of the resultant alloy and provide strategies for manufacturing and optimising such material.

The proposed research contributes to two key EPSRC themes, Energy and Manufacturing the Future. It particularly focuses on applications in the nuclear sector, with the aim of developing a novel and breakthrough Co-free hardfacing material. Hence, it focuses on the research area Nuclear Fission although silicide-strengthened stainless steels could potentially find applications in many other sectors (for instance the petrochemical industry where materials must retain high-temperature strength in highly corrosive environments). 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.

The key novel scientific aspect of the development of silicide-strengthened steel is that large Si additions drive Fe-Cr system away from the formation of the detrimental sigma phase and instead silicides are formed; these are also Cr-rich but do not deplete the metal matrix of Cr. A silicide-strengthened steel may enable a very corrosion resistant material, due to the high Cr additions, in combination with high strength, through the formation of a silicide phase, but without the issue of sigma phase formation.