DMW-Creep: Influence of Inhomogeneity on Creep of Dissimilar Metal Welds

This research is part of a national UK programme through the RCUK Energy programme and India's Department of Atomic Energy. The research involves the Universities of Bristol, Oxford and Manchester together with the Open University. The research is focussed on understanding the characteristics of welded joints between austenitic stainless steels and ferritic steels that are widely used in many nuclear power generating plants and petrochemical industries as well as conventional coal and gas fired power systems. In the steam generator circuit of sodium cooled fast breeder reactors stainless steel pipes from the intermediate heat exchangers are required to join with a ferritic steel pipe of steam generators. In these welds a transition bond is formed where the chemical composition, microstructure, stress state, physical properties, and mechanical properties vary spatially. These welds are called dissimilar metal welds. To reduce the mismatch between the two steels created when operated at high temperature the thermal expansion coefficient across the joint is reduced with the insertion of a nickel alloy pipe section and using nickel alloy welds having thermal expansion coefficients intermediate between austenitic and ferritic steels. However, premature creep failure is also encountered in such dissimilar weld joints. To provide an understanding of the operating life of the weld it is essential to gain an understanding of the microstructural changes across the weld interface between ferritic and austenitic alloys and their effects on high-temperature creep deformation and fracture behaviour. The objectives of this research are summarised as follows: 1) to model and test the effects of metallurgical and structural aspects of welds, 2) to develop material models and 3) to develop reliable methods to monitor the material.

Beneficiaries: World-wide there is year-on-year increase in consumption of energy, whilst at the same time there is increasing pressure to reduce carbon emissions. Nuclear fission for current and advanced designs will play a role in meeting these demands and the outcomes of this project are aimed at creating an approach to ensuring that advanced materials can operate efficiently, safely and for extended periods of time. We have been actively involved in informing decision makers and regulators at the highest level in Government and industry, and assisting in the vital task of rebuilding public confidence in nuclear power. We also have strong track records for promoting the impact and benefits of our work to the non-academic community. This is evidenced by our number of strategic industrial partners, which include Rolls-Royce and EDF-Energy. The proposed research is also embedded in strategic work to ensure safe operation of the current and future fleet of civil and defence nuclear reactors systems. The benefits derived from this project are to be fed directly to these companies. Our outcomes will also be widely disseminated through the Technical Advisory Group for Structural Integrity (TAGSI) and BSI where the applicants have made extensive contributions.
Links created with NDA and NNL through the existing EPSRC Nuclear Fission programme enable us to extend our framework to materials research . In relation to strategic UK policy these organisations are also major stakeholders for the newly formed DECCactivities. Within Europe non-academic beneficiaries include AREVA (France and Germany), EDF and CEA through EURATOM research programmes such as NULIFE, PERFORM60 and STYLE. In the broader context of the rest of the world we envisage opportunities to inform and network companies we already work with and include in Japan; Hitachi Group and Toshiba; in USA, Westinghouse, EPRI and NRC together with several major engineering consultancies including EMC2, and Dominion Engineering.

Benefits: Specific impacts being sought through the networking workshops include establishing novel routes using complexity science in the nuclear community, developing strategic routes to underpin the development of material test programmes particularly in the context of materials for GEN IV high temperature reactors, and seeking techniques to handle complex interactions between potentially differing failure mechanisms contributing to structural integrity. Historically the community has been very discipline specific in its research themes and consequently we aim to create a community of niche researchers who will become experts in handling complexity and crossing disciplines. Finally, a significant impact will be the creation of trained and skilled researchers who will have an understanding of the nuclear industrial community.

Engagement: Our previous experience with users and beneficiaries has demonstrated that there are several effective routes for engagement and these will constitute the main basis for our communication and engagement plans. The plans (given in detail in the Impact Plan) will be managed by a Research Impact Group, chaired by the project PI, Prof David Smith, together with commercial support from the four Universities. The plans include routes for direct commercial exploitation, using University created companies for which we have knowledge transfer arrangements, through to networking workshops that we will conduct throughout the research project. It is envisaged that exploitable outputs will include techniques for handling complexity. We also see opportunities to engage with the wider programmes associated with Complexity Science and Systems Engineering, both doctoral training programmes at Bristol.