Understanding the materials performance of additive manufactured stainless steel components in high temperature water

Austenitic stainless steels and Ni base alloys are extensively used in the primary circuit internals of pressurized water reactors (PWR) due to their high corrosion resistance properties. However, it is also well known that materials processing can have a strong impact on the susceptibility to stress corrosion cracking (SCC) of these materials when exposed in high temperature aqueous environment water coolant under active loading. Historically, components have been manufactured via conventional manufacturing routes, such as forging and welding; however, there is the desire to produce near net shape components via additive manufacturing thanks to the reduce machining costs, more agile manufacturing, and shorter lead times. However, there is currently insufficient knowledge on the impact of the metallurgical quality of the material produced by such processes on the materials performance. It is critical, therefore, to have a fundamental understanding of the relationship between manufacturing via modern near-to-net-shape manufacturing technologies, such as laser powder bed fusion, so that potential degradation caused by changes to current manufacturing practices can be judged. This, in turn, requires a scientifically-based understanding of the various underlying mechanisms influencing/controlling the environmental degradation and their linking to the end effects.
SCC is one of the most insidious forms of materials degradation and its initiation behaviour in as manufactured components are major technical challenges. Although the SCC performance of stainless steels, Ni-base alloys in light water reactors environments has been studied extensively, the SCC data are not available for components produced using near-net-shape technologies.

The overall aim of this project is to characterise the microstructure of additively manufactured (AM) stainless steels produced via laser powder bed fusion, and compare the mechanical properties (tensile strength, fracture toughness) and susceptibility to environmentally assisted cracking (EAC) of material in the three conditions of interest: forged, AM and heat treated. The secondary aim is to develop an understanding of the processing-microstructure-mechanical property relationships at work, and hence suggest process alterations to optimise material performance.

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