Revealing the interaction of residual strain – mechanical performance using in situ neutron diffraction at high temperature for similar and dissimilar Eurofer97 joints

Nuclear fusion is a potential source of electricity which can address the environmental problems posed by
fossil fuels. The UK’s Spherical Tokamak for Energy Production (STEP) has progressed to the second
stage, as of 2024, from the concept design to the engineering design. Eurofer97 steel is a primary structural
material for in-vessel components in fusion Tokamaks. Assembling and maintaining works are necessary
for the structural integrity of these in-vessel components in high-temperature working conditions. This
requires joining techniques, forming similar and dissimilar Eurofer97 joints and inducing immense residual
stress. The interaction of the residual strain and the heterogeneous microstructure degrades the
mechanical performance and reduces the lifetime of these critical components. To mitigate the detrimental
effects of residual strain within the Eurofer97 similar and dissimilar joints, research must be conducted to
reveal the fundamental understanding of (i) the origins of residual strain inherited from the joining process
and its interaction with the high temperature, and (ii) underpinning deformation mechanism associated with
the residual strain at high temperature. The results obtained from the current study will enhance the design
protocols and structural integrity assessment of the EU DEMO and UK STEP (Spherical Tokamak for
Energy Production) fusion power plants.

核聚变是一种极具潜力的电力来源，可有效解决化石燃料引发的各类环境问题。截至2024年，英国能源生产用球形托卡马克（Spherical Tokamak for Energy Production, STEP）项目已从概念设计阶段推进至工程设计第二阶段。Eurofer97钢是聚变堆堆内构件的核心结构材料。在高温工况下，为保障这类堆内构件的结构完整性，组装与维护作业必不可少，这便需要用到连接工艺，通过该工艺可制备同材质与异材质Eurofer97接头，但同时会引入显著的残余应力。残余应变与非均匀微观结构的交互作用会劣化构件的力学性能，缩短这类关键部件的服役寿命。为缓解Eurofer97同、异质接头中残余应变带来的不利影响，亟需开展相关研究以阐明两大核心问题：一是连接过程中引入的残余应变的起源及其与高温环境的交互机制；二是高温下与残余应变相关的核心变形机制。本研究所得成果将助力欧盟演示堆（EU DEMO）与英国STEP聚变电站的设计规范优化及结构完整性评估。