Hydrogen-Induced Stress Cracking of Swaged Super Duplex Stainless Steel Subsea Components

Abstract

A recent subsea failure of two subsea connectors made of UNS S32760, a 25 wt% Cr super duplex stainless steel, led to an extensive root cause failure analysis. The components showed a single longitudinal crack along a swaged section, which arrested toward its thicker end. The brittle nature of the fracture surface, calcareous deposits on the component, and exposure to cathodic protection suggested hydrogeninduced stress cracking-a form of environmentally assisted cracking-as a plausible failure mechanism. Thus, the three causative factors promoting hydrogen-induced stress cracking, namely, a susceptible microstructure, a hydrogen bearing environment, as well as sufficiently high applied and residual stresses in the material were the focus of this investigation. This work details the material characterization work and presents a possible failure mechanism. The results showed that the failure developed from a combination of factors, typical for hydrogeninduced stress cracking. The measured hydrogen content in parts of the material exceeded 40 ppm, more than an order of magnitude higher than what is normally expected in super duplex stainless steels. Additionally, a highly anisotropic, coarse microstructure was observed, which in combination with the introduced cold-work from the swaging process and potential stress raisers from design and machining could have facilitated crack initiation, ultimately leading to the failure of the component. This hypothesis was reinforced by the presence of secondary cracks along the main, brittle fracture surface. Furthermore, mechanical testing results showed a detrimental effect on the material's properties due to the presence of residual hydrogen and the swaging operation.

Hydrogen-Induced Micro-Strain Evolution in Super Duplex Stainless Steel—Correlative High-Energy X-Ray Diffraction, Electron Backscattered Diffraction, and Digital Image Correlation

“…Grain boundaries in DSS may not be susceptible sites to crack formation since grain boundaries, particularly interphase boundaries, may act as strong trap sites for hydrogen, reducing the resistance to HE. Therefore, finely-grained microstructures typically outperform coarse ones, explained by trapping and elongated diffusion path due to a larger tortuosity (Woollin and Gregori, 2004;Chai et al, 2009;Bahrami and Woollin, 2010;Kivisäkk, 2010;Kivisäkk and Ciurea, 2011;Sofia Hazarabedian et al, 2019). Plastic deformation typically preludes crack formation, and therefore intensified deformation in the microstructure can be understood as the most incipient degradation stage in DSS.…”

“…The cracking of either phase in the duplex microstructure -not observed in this workwould imply the degradation of the other phase because the latter phase would have been insufficient in providing support against cracking. Typically, cracks initiate in ferrite grains in coarse duplex microstructures (>30 µm austenite spacing) (Byrne et al, 2016;Sofia Hazarabedian et al, 2019). The ferrite is thus considered as the more susceptible phase (Byrne et al, 2016;Sofia Hazarabedian et al, 2019).…”

“…Typically, cracks initiate in ferrite grains in coarse duplex microstructures (>30 µm austenite spacing) (Byrne et al, 2016;Sofia Hazarabedian et al, 2019). The ferrite is thus considered as the more susceptible phase (Byrne et al, 2016;Sofia Hazarabedian et al, 2019). Ferritic lattices become rapidly brittle during hydrogen uptake (Chai et al, 2009;Bak et al, 2016;Venezuela et al, 2016;Örnek et al, 2018).…”

A review on pitting corrosion and environmentally assisted cracking on duplex stainless steel

“…Not all circumstances make the ferrite more prone to chloride-induced stress corrosion cracking. For example, Örnek found that the austenite in 2,205 duplex stainless steel suffers from chloride-induced stress corrosion cracking in the drop test [115] . .…”

Corrosion-resistant alloy testing and selection for oil and gas production

“…The effect of Cr 2 N precipitate and the resulting depletion of Cr and Mo, on the localised corrosion resistance of duplex stainless are relatively well established [91][92][93]. Cr 2 N precipitation in the welds has also been implicated in HE/HISC [94], where Cr depletion mechanism cannot account for the increased HE susceptibility. It is possible that coherent alpha prime precipitation that was not detected by these authors contributed to increased HE susceptibility.…”