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

由于奥氏体不锈钢和镍基合金具有良好的耐腐蚀性能，因此被广泛应用于压水堆一次回路内部。然而，众所周知，当材料暴露在高温含水环境下时，材料的加工会对材料的应力腐蚀开裂（SCC）的敏感性产生强烈影响。从历史上看，部件是通过传统的制造路线制造的，比如锻造和焊接；然而，由于降低了加工成本、更灵活的制造和更短的交货时间，人们希望通过增材制造生产接近净形状的部件。然而，目前对这种工艺生产的材料的冶金质量对材料性能的影响的认识不足。因此，对现代近净成形制造技术（如激光粉末床熔合）制造之间的关系有一个基本的了解是至关重要的，这样就可以判断当前制造实践的变化所造成的潜在退化。这反过来又要求对影响/控制环境退化的各种潜在机制及其与最终效应的联系有科学的了解。SCC是最隐蔽的材料降解形式之一，其在制造部件中的启动行为是主要的技术挑战。尽管不锈钢、镍基合金在轻水反应堆环境中的SCC性能已经得到了广泛的研究，但使用近净形技术生产的组件的SCC数据还不可用。该项目的总体目标是表征通过激光粉末床熔合生产的增材制造（AM）不锈钢的微观结构，并比较三种条件下材料的机械性能（拉伸强度、断裂韧性）和环境辅助开裂（EAC）的敏感性：锻造、AM和热处理。第二个目标是发展对工作中的加工-微观结构-机械性能关系的理解，从而提出工艺变更以优化材料性能。

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.