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要经过复杂的工艺路线来优化其性能，这包括锻造和热处理，以形成屈服强度超过1500兆帕的调质马氏体组织。然而，仍然有一个主要问题是它们在使用中对环境辅助开裂(EAC)的敏感性，这与某些环境中的充氢效应有关，不仅是在阴极条件下，而且在阳极极化和局部腐蚀期间也是如此。这一重要问题，以及对标准加速实验室测试的缺乏信心，与对这些复杂材料所涉及的机制的了解不足有关。因此，本项目的目的是加强对暴露在水环境中的高强度马氏体时效钢的EAC的机理理解，并阐明微观结构对材料性能的作用，这些材料性能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在热处理过程中的微观结构演变。-了解微观结构特征对这些材料的局部腐蚀行为的作用，使用包括极化和循环伏安在内的电化学测试技术，还研究了扫描开尔文探针力显微镜(SKPFM)。-将这些材料的耐大气和局部腐蚀与氢脆敏感性相关联。还研究了实验室试验(慢应变速率拉伸试验和大气腐蚀试验)与服役条件之间的关系。

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.