Based on alloy development and manufacturer independent supply experience of super duplex steel over a 30 year period the paper considers some of the less well known but recurring problems and the methods used to ameliorate them.

In 2018 the apparent failure of two subsea jumper end-fittings was detected by remotely operated vehicle (ROV) inspection prompting their retrieval from the seabed for analysis. The components were manufactured from UNS S32760 a 25 wt% Cr super duplex stainless steel (SDSS) and connected to the flexible pipe by swaging. The end fittings were exposed to the cathodic protection system presenting extensive accumulation of calcareous deposits upon inspection. A single longitudinal crack was observed on each end-fitting propagating from end-to-end of the ferrule i.e. the swaged section of the part. The visual appearance of the fracture surface suggested at first glance cleavage fracture and thus the possibility of hydrogen induced stress cracking (HISC).This paper summarizes the results of an extensive investigation aimed at elucidating the root-cause of the failure. Given the suspicion of a hydrogen embrittlement phenomenon the scope of the work was designed to address the three HISC causative factors namely a susceptible alloy microstructure a critical tensile stress level and the presence of atomic hydrogen. The experimental techniques included scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) electron backscatter diffraction (EBSD) analysis mechanical and corrosion examinations as well as slow strain rate testing (SSRT).

Due to its attractive combination of strength corrosion resistance and cost 25% Cr Duplex Stainless Steel Pipe is used extensively in subsea production systems. Pipes are made by different production methods. The various production methods affect the microstructure and the mechanical properties of the final product. Components used subsea are externally exposed to cathodic protection. Experiences have shown that 25Cr duplex stainless steel is vulnerable to hydrogen induced stress cracking (HISC). The assumption is that the resulting microstructure affects the resistance. This is reflected in the DNVGL-RP-F112 design guideline which uses austenite spacing to determine a design factor. In this paper the HISC susceptibility of 25Cr duplex stainless-steel pipes produced through hot extrusion with- and without subsequent cold drawing forging and centrifugal casting have been examined. Two different test methods have been used; i) Stepwise (slow) load increase and ii) Slow Strain Rate Testing. Samples pre-charged with hydrogen and samples without hydrogen were included in the test program. Pre-charged samples were also polarised cathodically during testing under stress.The microstructure was characterised including measurements of austenite spacing. After testing the samples were examined in optical microscope for secondary cracks. In addition the fracture surfaces were examined in scanning electron microscope for characterisation of fracture morphology. Reduction in area were calculated for all samples. Finally hydrogen content in selected samples were measured with a melt extraction technique.The tests revealed that 25Cr duplex stainless steel from the different production methods included in the test showed various degree of HISC and that the effect was dependant on the production method and resulting microstructure. Hot extruded material with no cold deformation showed the highest HISC resistance while centrifugal cast material seemed to be more exposed to HISC than the other methods. The fracture surfaces of all hydrogen charged test materials showed features indicating a reduction in ductility due to HISC as well as both ductile and brittle fracture characteristics across the surfaces. The fracture surfaces for the reference specimens showed ductile fracture characteristics. The hydrogen content in the charged samples were in the range 50-80 wppm.The ranking of production methods was as follows: hot extruded pipes > hot extruded pipes with subsequent cold drawing > forged pipes >centrifugal cast pipes.The two test methods – stepwise load increase and SSRT – gave consistent test results.