Hydrogen-Induced Cracking of Laser Beam and Gas Metal Arc Welds on API X65 Steel

Abstract

Hydrogen-induced cracking (HIC) behavior was analyzed for gas metal arc (GMA) and laser beam (LB) welds on American Petroleum Institute (API) X65 steel. The GMA welds consisted of acicular ferrite (AF) with some widmanstatten ferrite (WF), while the LB welds had bainitic ferrite (BF) with some AF. The welds and heat affected zone (HAZ) of GMA exhibited a hardness of 220-250 HV, while those of the LB had a hardness of 230-290 HV. The LB welds and HAZ exceeded the hardness limit of 250 HV for pipeline steel, defined by the National Association of Corrosion Engineers (NACE) standard. Slow strain rate tests (SSRT) were performed in air and in-situ with hydrogen to observe HIC behavior. The ultimate tensile strength of the GMA welds decreased by 13%, while that of the LB welds decreased by 16% after hydrogen charging. Both welds showed a dimple fracture in the center and quasi-cleavage fracture along the edges. When austenite transforms to BF, it is known to grow along the directions of twins or the Kurdjumov-Sachs relation. These directions are strongly related to the coincidence site lattice, specifically Σ3 and Σ13b, and have good crack resistance. Because of this grain boundary characteristic, the LB welds with a BF microstructure showed good HIC behavior compared to the GMA welds. (Received May 7, 2019; Accepted June 12, 2019)