Understanding the interaction of galling and corrosion in hard facing materials

Hardfacing alloys have been used for many years in applications that require wear and galling resistance, particularly in extreme environments. Historically, cobalt-based alloys such as Stellite-6 have been used for this purpose on valve sealings in nuclear power plants due to their outstanding tribological and environmental resistance. However, wear and corrosion products from these alloys are carried by cooling water towards the reactor core and irradiated resulting in the formation of gamma-emitter Co-60. This poses an occupational hazard for workers during refuelling thus Co-free replacements are sought.

Novel Co-free hard facing alloys are being developed with empirical success in an effort to reduce worker radiation exposure. These materials must be able to withstand the primary water reactor (PWR) environment i.e. at 300 C in pressurised water for the whole service life of the plant (around 40 years). These alloy matrices have been evolved from stainless steels and enhanced with strengthening precipitates to imitate cobalt alloy microstructures.

However, the fundamentals of galling particularly in pressurised water environment at elevated temperatures are little understood and consequently hard facing alloy design is lacking behind. To date, development alloys have been mainly tested at room temperature and in air, a very different environment to a PWR.

This project aims are to develop experimental protocols of a new set up at the University of Manchester that will, for the very first time, enable galling testing within a pressurised water environment simulating PWR conditions. Building on these protocols, the project will investigate the influence of alloy composition and metallurgical processing methods on wear and galling resistance and test new and existing hardfacing alloys in order to provide improved mechanistic understanding of galling in service relevant conditions. The findings will inform the design of new Fe-based Co-free hardfacing alloy.

The EPSRC Centre for Doctoral Training in Advanced Metallic Systems was established to address the metallurgical skills
gap, highlighted in several reports [1-3] as a threat to the competitiveness of UK industry, by training non-materials
graduates from chemistry, physics and engineering in a multidisciplinary environment. Although we will have supplied ~140
highly capable metallurgical scientists and engineers into industry and academia by the end of our existing programme,
there remains a demonstrable need for doctoral-level training to continue and evolve to meet future industry needs. We
therefore propose to train a further 14 UK based PhD and EngD students per cohort as well as 5 Irish students per
cohort through I-Form.

Manufacturing contributes over 10% of UK GVA with the metals sector contributing 12% of this (£10.7BN [4,5]) and
employing ~230,000 people directly and 750,000 indirectly. It is estimated that ~2300 graduates are required annually to
meet present and future growth [5]. A sizeable portion of these graduates will require metallurgical expertise and current
numbers fall far short. From UK-wide HESA data, we estimate there are ~330 home UG/PGT qualifiers in materials and
~35 home doctoral graduates in metallurgy annually, including existing AMSCDT graduates, so it is unsurprising that
industry continues to report difficulties in recruiting staff with the required specialist metallurgical knowledge and
professional competencies.

As well as addressing this shortfall, the CDT will also impact directly on the companies with which it collaborates, on the
wider high value manufacturing sector and on the UK economy as a whole, as follows:

1. Collaborating companies, across a wide range of businesses in the supply chain including SMEs and research
organisations will benefit directly from the CDT through:

- Targeted projects in direct support of their business and its future development and competitiveness.
- Access to the expertise and facilities of the host institutions.
- Involvement in the training of the next generation of potential employees with advanced technical and leadership skills
who can add value to their organisations.

2. The UK High-Value Manufacturing Community will benefit as the CDT will:

- Develop the underpinning science and advanced-level knowledge base required by future high technology areas, where
there is high expectation of gross added value.
- Provide an enhanced route to exploitation, by covering the full spectrum of technology readiness levels.
- Ensure dissemination of knowledge to the sector, through student-led SME consultancy projects, the National Student
Conference in Metallic Materials and industry events.

3. The wider UK economy will benefit as the CDT will:

- Promote materials science and engineering and encourage future generations to enter the field, through outreach
activities developed by the students that will increase public awareness of the discipline and its contribution to modern
life, and highlight its importance to future innovation and technologies.
- Develop and exploit new technologies and products which will help to maintain a competitive UK advanced
manufacturing sector, ensure an internationally competitive and balanced UK economy for future generations and
contribute to technical challenges in key societal issues such as energy and sustainability.

References:
1. Materials UK Structural Materials Report 2009
2. EPSRC Materials International Review 2008
3. EPSRC Materially Better Call 2013
4. The state of engineering, Engineering UK 2017
5. Vision 2030: The UK Metals Industry's New Strategic Approach, Metals Forum