Effect of Residual Stresses on the HIC Resistance of ERW Pipes for Line Pipe Applications

Four material grades for production of ERW pipes welded by high-frequency induction (HFI) have been selected for hydrogen induced cracking (HIC) tests. API 5L Grade B, X52 and X65 have been intentionally produced from non sour service material in order to ensure sufficient HIC damage during exposure in NACE TM0284 test solution A. The highest material strength included was grade X70 designed for sour service to investigate whether HIC cracks can be initiated solely by the presence of high residual stresses.

In addition to a substantial characterization of mechanical properties and residual stresses, a series of experiments under simulated residual stress to determine the HIC resistance of these pipe materials in NACE TM0284 test solution A has been carried out using the four-point-bend test setup. The crack area ratio (CAR) accounting for the extent of HIC cracking was as well correlated with basic alloying characteristics of the steels as with residual stresses. 

The results of the test series combined with supporting theoretical considerations and finite-element (FEM) modelling revealed that in the case of HFI pipes the pre-material suitability for sour service is of much higher importance than residual stresses induced by pipe forming. It is demonstrated that there is no negative impact of residual stresses on the HIC resistance.

Keywords: ERW pipes, sour service, hydrogen induced cracking, NACE TM0284, residual stress