Duplex stainless steels (DSSs) are produced to have a microstructure ideally composed of 50% ferrite and 50% austenite. The combination of the two phases confers the high strength and the corrosion resistance of the alloys. Owing to these superior properties, DSS are widely used in subsea environments, such as pipelines, manifold components and risers for oil and gas exploration and production. In these environments, DSS parts are often connected to ferritic steel components, which necessitate the application of cathodic protection (CP) as a means of corrosion prevention. Whilst successful in preventing degradation of the ferritic parts, CP can generate hydrogen at the surfaces of the subsea structure, whereupon it can be absorbed into the alloy and cause embrittlement. Cracking of this embrittled material is known as hydrogen induced stress cracking (HISC) and is recognised as a major cause of catastrophic failures in service, such as within the Foinaven field in the North Sea.
Due to crystallographic structures of the two phases, ferrite (body centred cubic) and austenite (face centred cubic) show different behaviours towards hydrogen ingress and loading conditions, with hydrogen diffusion in ferrite found to be five orders of magnitude higher than that in austenite. From failure investigations and testing programmes carried out on DSS components, it is understood that manufacturing issues can determine microstructures (e.g. austenite spacing, nitride precipitates) and consequently the HISC resistance of duplex stainless steels. Hence, the resistance to HISC of a particular grade differs according to the manufacturing process employed, e.g. hot isostatic pressing, forging, casting, etc. However, because of the complexity of DSS microstructures and, despite clear environmental and economic consequences of HISC failures, the micro-mechanism of hydrogen embrittlement remains largely unknown.
The aim of this PhD project is to enable a step change in our understanding of HISC cracking mechanism using the state-of-the-art experimental methods. It is anticipated that the outcome of the project will have a wide application in optimising manufacturing processes to prevent catastrophic failures. Particularly, the project aims to study the effect and role of various microstructural parameters, as well as residual stress contents, associated with manufacturing and fabrication processes, on the HISC resistance of DSSs. The data obtained might subsequently be used to improve existing guidelines for design of duplex stainless steels against HISC. For this purpose, the project will test a range of parent and welded materials, and focus on key parameters which might affect the materials’ HISC resistance, including the effect of microstructure, low temperature creep properties and residual stresses.