低活化钢搅拌摩擦焊超细晶组织演变行为与强化机制

Developing nuclear power is an important strategy of China in recent years. High quality welding for key structure materials in nuclear power generation is an important foundation for promoting the development of nuclear industry in safe and high efficiency. As an advanced solid-state joining technology, friction stir welding exhibits several advantages in welding reduced activation steels used in nuclear power plant. However, degradation in thermal stability and creep properties of the ultrafine grain structure can be adverse for the structural reliability, which is restricting its development and application. This project focuses on investigating the key issues for evolution behavior and strengthening mechanism of ultra-fine grain structure on reduced activation steel friction stir welds. Formation mechanism and evolution behavior of ultra-fine grain structure during welding process will be studied with self-adaptive coupled model simulation combined with experiment result. High temperature creep deformation mechanism and failure process will be revealed. Key influence factors and acting mechanism for high temperature creep property will be clarified.The mechanism for thermal stabilization high temperature strengthening of the ultra-fine grain structure will be discussed. And, theories and methods of optimizing microstructures and properties will be proposed. Aiming at friction stir welding of reduced activation steels, for enhancing the microstructure and properties at high temperatures, we will new methods for simulating formation and evolution mechanism of ultra-fine grain in friction stir welding, reveal the new law of creep deformation at high temperatures, and develop new technologies for high temperature properties. This project can be a fundamental for developing high quality friction stir welding of reduced activation steel in nuclear power industry.

发展核能发电是我国的一项重要能源战略，开发核电关键结构材料高品质焊接技术是促进核能产业安全、高效发展的重要基础。作为先进固相连接技术，搅拌摩擦焊在核电低活化钢焊接方面具有显著优势。但是，搅拌头作用区域超细晶组织的热稳定性和蠕变性能不足对构件可靠性有不利影响，是限制其发展和应用的瓶颈之一。本项目聚焦低活化钢搅拌摩擦焊超细晶组织演变与强化关键基础问题开展研究：采用多模型自适应耦合模拟与实验相结合研究超细晶组织形成机制与演化规律；揭示其高温蠕变变形机制和失效过程、澄清影响蠕变性能的关键因素及作用机制；阐明超细晶组织热稳定化和高温强化机理，提出组织性能优化调控的原理与方法。针对低活化钢搅拌摩擦焊提出超细晶组织演变研究新方法，揭示超细晶组织高温蠕变变形及失效新规律，发展超细晶组织高温性能优化新技术，最终达到高温性能优化、提升的目的。本研究可为发展核电低活化钢高品质搅拌摩擦焊接奠定重要基础。

低活化铁素体/马氏体耐热钢是目前先进快堆燃料组件和未来聚变堆包层模块等核心部件的关键结构材料。搅拌摩擦焊是一种先进固相连接技术，基于搅拌针周围材料热塑性流动而实现基材间的冶金连接，被认为是核电低活化钢高品质焊接的理想方法。但搅拌头作用区域晶粒超细化组织超细晶化对焊接接头高温性能有不利影响。因此，研究低活化钢搅拌摩擦焊超细晶组织演变及其强化机制具有重要意义。.本项目研究了RAFM钢搅拌摩擦焊成形、热输入、微观组织，揭示了不同焊接参数对晶粒尺寸、晶粒结构、晶界结构、位错结构以及沉淀相特征的影响规律；分析了搅拌头作用区域局部热场、应变场和应变速率等特征；以搅拌摩擦焊实验和模拟分析结果为依据对搅拌摩擦焊过程局部温度、变形量和应变速率对再结晶和固相转变行为的影响进行了阐述，获得了流变应力与局部温度、应变速率等参量，讨论了动态再结晶形核与长大、马氏体相变、沉淀相的溶解与在析出，以及位错、晶界、亚晶等亚结构变化等过程。开展了RAFM钢FSW接头及焊缝区的高温拉伸和蠕变试验，获得抗拉强度（417MPa）、600℃/200MPa条件下蠕变断裂时间1173h；讨论了原奥氏体晶粒尺寸、马氏体板条尺寸、对蠕变性能的影响规律；分析了M23C6形态、尺寸和长大速率对蠕变断裂行为的影响；研究了蠕变过程中晶粒长大、晶粒结构和晶界结构变化、位错运动和亚结构回复，获得了M23C6附近形成的Laves相及其粗化是导致蠕变孔洞形核的主要原因。研究了回火处理对RAFM钢FSW超细晶组织热稳定性和高温蠕变性能的影响规律与机制，发现FSW焊缝区经700℃/1h回火后，M23C6碳化物更加细小，并在晶内析出，这延缓了Laves相的形成和长大，使FSW焊缝区蠕变寿命较母材提高了约10%。.本项目所开展的RAFM钢搅拌摩擦焊超细晶演变行为与强化机制，阐述超细晶组织形成机制与演化规律，揭示超细晶组织蠕变机制及其关键影响因素，发展超细晶组织性能优化原理与方法，为发展核电低活化钢高品质焊接技术奠定基础，对低活化钢搅拌摩擦焊在核电领域发展和应用具有参考价值。