IMPROVED RESISTANCE TOWARDS HYDROGEN INDUCED STRESS CRACKING (HISC) OF HOT ISOSTATICALLY PRESSED (HIP) DUPLEX STAINLESS STEELS UNDER CATHODIC PROTECTION

摘要

Hot isostatically pressed (HIP) and forged duplex stainless steels are both candidate materials for subsea components. In the present study, the resistance towards hydrogen induced stress cracking (HISC) of these materials has been compared. Testing was performed by four-point bending of fatigue pre-cracked SENB (single edge notch bend) specimens polarised to -1050 mV vs. SCE in a 3.5 wt percent NaCl solution at 4 deg C. The specimens were loaded to different crack tip opening displacement (CTOD) values for a maximum duration of 30 days. Three HIP materials, one duplex steel (DSS, UNS S31803) and two super duplex steels (SDSS, UNS S32550 and UNS S32760) were investigated and compared to a forged SDSS material (UNS S32760). For the HIP materials no cracking was detected for specimens loaded at CTOD levels lower than 0.06 mm, while some slight cracking was observed for a CTOD of 0.08 mm. The CTOD threshold value for HISC thus appears to be in the range from 0.06 to 0.08 mm for these materials. The forged SDSS material on the other hand showed significant cracking at CTOD levels above 0.03 mm, while no cracking was observed at CTOD levels below 0.016 mm. The testing thus demonstrates the improved resistance towards HISC for the HIP materials.