Effect of Weld Porosity on Super Duplex Stainless Steel Umbilical Tubes Under Hydrogen Induced Stress Cracking Exposure Condition

Abstract

Super Duplex Stainless Steel (SDSS) is the preferred material utilized in steel tube umbilicals, due to its excellent corrosion resistance and high mechanical strength. SDSS is susceptible to Hydrogen Induced Stress Cracking (HISC) under the influence of a Cathodic Protection (CP) system, especially the welded zone. Porosity (relevant rounded indication) is one of the most common weld flaws that impacts the SDSS weld reject rate based on historical acceptance criteria. A study has been carried out investigating the influence of weld porosity on 25% Cr (SDSS) butt welded tube, under the effect of CP potential by using a Slow Strain Rate Testing (SSRT) technique. The study focused on the correlation between the level of porosity and the material mechanical performance when exposed to a simulated CP system in seawater at an elevated temperature. The impact of HISC was evaluated through an assessment of various tube welds including welds with different levels of porosity quantified by number, size and location of pores, when exposed directly to natural CP system in seawater for 4 and 8 weeks. Post hydrogen pre-charging, the samples were subjected immediately to a modified Slow Strain Rate Test (SSRT). After hydrogen charging and SSRT the resulting fracture morphology was examined using a scanning electron microscope (SEM), and the failure mode was assessed in relation to porosity locations. Experimental results indicate that all hydrogen pre-charged samples with or without pores decrease in ductility after exposure to the CP system which was further confirmed by the fractography analysis. It was observed that susceptibility of SDSS to HISC is broadly similar for welds with porosity or without porosity and brittle phenomena is more pronounced at the exposed surfaces of the samples. The study has demonstrated that the butt weld of SDSS umbilical tube with a controlled porosity level in line with ASME design codes does not show an increased susceptibility to HISC under CP system when compared to welds with no porosity.