Hydrogen Induced Stress Cracking (HISC) Resistance and Improvement Methods for Super Duplex Stainless Steels
The paper reviews the history of HISC failures of duplex and superduplex stainless steels when deployed subsea and subject to cathodic protection (CP) at potentials around 1V SCE. The test methods applied to investigate the problem and subsequently used to develop current design codes that deal with HISC are reviewed. Data from these investigations is compared with other testing using the same cast and batch of bar product, but where controlled shot peening is used to induce compressive residual stresses in the surface of the test samples. Parent pipe material and cross welded samples of seamless pipes were also tested. Peened material showed a 10 to 15% improvement in the threshold stress to initiate HISC. The paper also discussed the Advanced Forging Process (AFP) production route, recently developed to provide both increased notch toughness at low design temperatures and improved resistance to HISC in forgings used to make 10k weld neck and swivel ring flanges for subsea manifolds. Data is presented showing an increase in the threshold stress ratio (applied stress divided by the actual 0.2% proof strength) from 85% to 97.5% We also find a corresponding increase in impact toughness of AFP material, allowing use of the products at design temperatures down to minus 70degC. This is attributed to the dissolution of detrimental nitride precipitates within ferrite grains in the forgings and transforming these into advantageous intergranular reformed austenite. Key Words; superduplex, HISC, toughness, forgings, residual stress, shot peening,