Influence of plastic strain on sour service resistance of line pipe

The influence of plastic strain combined with sour service on the resistance of heavy-wall SAWL large-diameter pipes of grade X65 and X70 to HIC (Hydrogen Induced Cracking) and SSC (sulfide stress cracking) has been investigated. Plastic strain was applied on longitudinal test specimens from base metal and weld of an X65 pipe by four-point bending. SSC four-point bend tests were performed on 15 mm thick specimens at an additional elastic stress of 80 % of the specified minimum yield strength of the base material. Tests were carried out at dedicated fit-for-purpose conditions (with the term 'fitness-for-purpose' as defined in ISO 15156-2) in the presence of 20 kPa H2S at various pH levels. For comparison in-situ straining was also considered. For that purpose SSR (slow-strain rate) tests were performed at 100 kPa and 20 kPa H2S partial pressure in order to determine the in-situ strain limits of the material. The influence of pre-straining and aging was investigated on X65 and X70 pipes. The materials were pre-strained to 2 % and aged for 1 hour. HIC and SSC four-point bend tests have been performed at 100 kPa H2S. The test results are discussed and compared to non-strained material.