Critical Hardness of Hydrogen Stress Cracking for Circumferential Weld Joint of Buried Pipeline

If the coating of the outer surface of a buried pipeline is damaged by some cause and its cathodic protection is excessive, an electrochemical reaction can produce hydrogen. In order to maintain the load transfer function of a pipeline circumferential weld joint in a hydrogen environment as well, the critical hardness at which a constant strength level is ensured during tensile load was clarified by Slow Strain Rate Testing (SSRT). In cases where the hardness was greater than 320HV, the maximum tensile stress was seen to fall sharply in the hydrogen environment, while in cases where hardness was 320HV or lower, the maximum tensile stress fell approximately only 10% regardless of the welding method. The maximum tensile stress fell sharply when the major material of the pipe was material with a relatively hard structure called lath martensite or bainite. The contribution of upper bainite to the fall of the maximum tensile stress was about 1/3.5 of that of lath martensite. From the results of measurement by EBSD, the greater the material's local misorientation, which is a value corresponding to dislocation density, the more conspicuous the fall of the maximum tensile stress under the impact of the hydrogen. It is assumed that the dislocation that exists in steel is an important cause of hydrogen embrittlement, so the empirical tendency-the harder the material, the higher its hydrogen embrittlement susceptibility-has been revealed to be caused by the fact that the harder the material, the greater its internal dislocation. Judging from the above results, if the hardness is 320HV or lower, a constant strength level is maintained under a hydrogen environment regardless of the welding method or the metallic structure.