The effect of microstructure on the localised corrosion and environmentally assisted cracking of ultra-high strength stainless steels

超高强度马氏体时效钢需要经过复杂的工艺路线来优化其性能，包括锻造和热处理，以形成屈服强度超过1500 MPa的淬火和回火马氏体显微组织。然而，仍然有一个主要的问题，他们的敏感性环境辅助开裂（EAC）在服务中，这是有关的氢充电在某些环境中的影响，不一定在阴极条件下，但也在阳极极化和局部腐蚀。这一重要问题，以及对标准加速实验室测试缺乏信心，因此，本项目的目的是加强对高强度马氏体时效钢暴露于水环境中的EAC的机理理解，并阐明微观结构对材料性能的作用，可用于未来合金的优化（尽管本项目不包括新合金的开发）。该项目的主要目标是：-通过常规（光学、硬度、X射线衍射（XRD））和先进的表征技术（包括扫描电子显微镜（SEM）、聚焦离子束（FIB）、透射电子显微镜（TEM）），评估超高强度沉淀硬化不锈钢的微观结构演变，作为热处理的函数。- 为了了解这些材料的局部腐蚀行为的微观结构特征的作用，使用电化学测试技术，包括极化和循环伏安法，扫描开尔文探针力显微镜（SKPFM）也进行了研究。将这些材料的耐大气腐蚀性和局部腐蚀性与氢脆敏感性相关联。实验室测试（慢应变速率拉伸试验和大气腐蚀试验）和在服务条件之间的相关性也进行了研究。

Ultra-high strength Maraging steels are subject to a complex processing route to optimise their performance, which involves forging, and heat treatment, to develop a quenched and tempered martensitic microstructure with yield strengths over 1500 MPa. However, there is still a major concern regarding their susceptibility to environmentally assisted cracking (EAC) in-service, which is related to the effects of hydrogen charging in certain environments not necessarily under cathodic conditions but also during anodic polarization and localized corrosion. This important issue, and a lack of confidence in standard accelerated laboratory testing, is related to a poor understanding of the mechanisms involved in these complex materials.Therefore the aim of this project is to enhance the mechanistic understanding of EAC of high strength Maraging steels exposed to aqueous environments and to elucidate the role of microstructure on the materials performance that could be used for the optimization of future alloys (although the development of new alloys is not covered in this project). The mechanistic understanding could also be exploited for the development of industrial testing conditions which would be meaningful and representative of the in-service conditions.The key objectives of the project are to:- evaluate the microstructural evolution of ultra-high strength precipitation hardened stainless steels, as a function of heat treatment, via conventional (optical, hardness, X-ray diffraction (XRD)), and advanced) characterization techniques which include scanning electron microscopes (SEM), focused ion beam (FIB), transmission electron microscope (TEM). - understand the role of microstructural features on the localized corrosion behaviour of these materials, using electrochemical testing techniques including polarization and cyclic voltammetry, scanning kelvin probe force miscroscope (SKPFM) is also investigated.- Correlate the resistance of these materials to atmospheric and localized corrosion to hydrogen embrittlement susceptibility. The correlation between laboratory testing (slow strain rate tensile testing and atmospheric corrosion testing) and in-service conditions is also investigated.