Duplex stainless steels (DSS) are widely used in offshore applications. These steels offer high strength and toughness as well as excellent corrosion resistance. However, DSS are susceptible to hydrogen embrittlement, i.e., mechanical degradation of steel material due to presence of atomic hydrogen in its microstructure. Currently, there are no standardised guidelines for evaluating resistance to cracking of DSSs in such applications, in a quantitative manner and against any established acceptance criterion.
The main objectives of this project are to develop an in-depth insight into the cracking behaviour of DSSs exposed to hydrogen and provide new, quantitative methods for assessing and designing against HISC, as well as providing data to enable the production of more reliable assets.
DSSs are widely used in oil and gas subsea applications, such as pipelines, manifolds, and risers. In these environments, DSS components are often connected to ferritic steel components, which necessitate the application of cathodic protection (CP) as means for corrosion prevention. Whilst being successful in preventing degradation of the ferritic parts, CP can generate hydrogen at the bare metal surface of the subsea structures. The generated hydrogen enters the material through absorption at the metal surface and causes embrittlement.
The cracking of this embrittled material is known as hydrogen-induced stress cracking (HISC) and is recognised as a major cause of failures of DSS components in-service. DSS are characterised by a dual-phase microstructure comprising approximately equal volume fractions of ferrite and austenite. The presence of two phases in DSSs challenges the hydrogen embrittlement study since ferrite and austenite exhibit different hydrogen-related properties. Ferrite has a high diffusion rate and a low solubility while the opposite is true for austenite.
A series of experimental and modelling tasks will be undertaken to characterise HISC in DSSs:
- Advanced microstructural characterisation of DDS parent and weld metal.
- Modelling of test specimen geometries, and numerical calculation of stress field at the stress raisers to be incorporated in the specimens.
- Mechanical and environmental testing of specimens of different sizes and notch acuity.
- Post characterisation of tested specimens using metallography and fractography.