Fundamentals Of Hydrogen Induced Stress Cracking In Duplex Stainless Steels

Duplex stainless steels (DSSs) are widely used by oil and gas industry due to their good corrosion and mechanical properties, which made them suitable for subsea applications. In this environment the use of cathodic protection (CP), to prevent corrosion, generates hydrogen that can be absorbed at the metallic surfaces. Unfortunately, while this technique is useful and intended to prevent structural carbon-steel components from seawater corrosion, it can introduce atomic hydrogen to the DSS components (attached to the structural carbon steels), which are known to be susceptible to hydrogen embrittlement via hydrogen induced stress cracking (HISC). This phenomenon is recognised to occur once a critical level of stress and hydrogen concentrations are met in a susceptible microstructure. To investigate the fundamentals of this mechanism, a multifaceted, comparative programme of microstructural characterisation, investigation of low temperature creep (LTC) and environmental testing was conducted on two DSSs of UNS S31803 grade: a rolled and seam welded pipe component and a hot-isostatically pressed (HIPed) can. Strain distribution was observed during LTC using digital image correlation technique; the relevance of fracture toughness testing methods to assess DSSs against HISC was investigated with the determination of microstructural factors influencing the resistance to HISC. Furthermore, the influence of residual stresses (induced by welding) was investigated on HISC thresholds, through tensile testing of specimens extracted from a pipe-to-flange welded component retrieved from 12 years of operations. The measurement of residual stresses was carried out by the neutron diffraction technique. The microstructural homogeneity and directionality of the HIPed and wrought materials, respectively, were found to have a major influence on the accommodation of stresses during LTC and resistance to HISC. Environmental testing showed the superior performance of the HIPed material. This testing programme also pointed out the complexity of interpreting the data obtained using the conventional fracture-toughness-based testing methodologies, with a fatigue pre-crack notch, to assess DSSs against HISC, particularly in terms of crack initiation. The presence of micro and macro-residual stresses was determined in the small-scale specimens extracted from the pipe-to-weld component; however; the macro-residual stress state did not explain the relatively better HISC performance of the cross-weld specimens, compared to that in the parent materials.