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

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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.