HYDROGEN EMBRITTLEMENT OF DUPLEX STAINLESS STEELS TESTING OF DIFFERENT PRODUCT FORMS AT LOW TEMPERATURE

摘要

During the last 10 years, failures have occurred in duplex stainless steels in subsea applications. When the duplex stainless steels are cathodic protected, the hydrogen concentration on the surface increases significantly and when combined with stress, failures and cracks can occur due to hydrogen embrittlement. This study is a part of a larger test program covering hydrogen embrittlement in different duplex stainless steel grades and different product forms. So far, it has focused on UNS S32750. Materials have been tested using a constant load with dead weight, a potential of -1050 mV (SCE) and a low temperature, 4°C. The different product forms included in the test program were small and large diameter bar, extruded and welded tubes. In addition to investigate the different product forms, the objective was also to further investigate how the phase size in different product forms affects susceptibility to hydrogen embrittlement. Different loads were applied, relating to the material’s yield strength (Rp0.2) at 4°C. The results showed that the small diameter bar achieved a load of at least 120% of the Rp0.2(4°C). The large diameter bar managed 84% but failed at 93%. The extruded tube achieved at least 130% and the welded tube 120%. No crack initiations were observed, except in the failed specimens of large diameter bar. The difference in austenite spacing between the small bar and large diameter bar is a good indication of the test results achieved. The small bar material had an austenite spacing of 15 μm and the larger bar 32-51 μm. The fracture surface indicated brittle behavior and transgranular cleavage in the ferrite phase, which is observed typically in Hydrogen Induced Stress Cracking (HISC). It was also clear that the cracks are arrested in the ferrite/austenite boundaries and how important the microstructure and austenite spacing are to the susceptibility to HISC and its mechanism. Further studies are in progress for UNS S32205, UNS S32906 and newer super duplex stainless steel and they will be investigated using the same methodology.