Influence of Manufacturing Process and Resulting Microstructure on HISC Susceptibility of 25Cr Duplex Stainless Steel Pipe

Abstract

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.