Abstract
A project has been initiated to investigate the performance of longitudinally submerged-arc welded large-diameter pipes in terms of resistance to Hydrogen Induced Cracking (HIC) under non-standard test conditions. Conventional HIC resistance testing, performed according to the standard test conditions given in NACE TM0284-2011 in solution A at 100 kPa (1 bar) H2S, not necessarily provides information about the HIC resistance of steel designed for sour service under more severe conditions. On the other hand, applying HIC test conditions based on mildly-sour operating conditions (known as "fit-for-purpose" testing) allows the qualification of sweet service steels up to the pH and H2S partial pressure related limits of the application. In view of the above, HIC resistance testing of two longitudinally submerged-arc welded (SAWL) large diameter pipes of grade X65 (one pipe intended for sweet service, another pipe for sour service) and of another X70 sour service pipe has been performed under conditions, where the partial pressure of H2S exceeds or falls below 100 kPa. The HIC performance has been evaluated by means of ultrasonic and metallographic techniques. For conditions with lower partial pressure of H2S the limits of HIC resistance of the X65 sweet service grade related to pH and partial pressure of H2S have been exemplarily established. A higher partial pressure of hydrogen sulfide up to 500 kPa and a longer test duration up to 6 months have been applied to the HIC resistant X65 and X70 materials as well as to the sweet service steel of grade X65 for comparison. The test results show a higher general corrosion attack, as expected, but still excellent HIC resistance even under the most severe test conditions, provided the steels are fully resistant under NACE TM0284 standard test conditions.