L51707 Factors Affecting Heat Affected Zone Root Strains in Pipeline Girth Welds and Repairs

Abstract

A previous study on hydrogen cracking in the heat affected zone of pipeline girth welds and repairs found that large strains in the HAZ transverse to the weld played a major role in causing cracking. Large transverse strains were believed to arise from bending of the weld due to the thermal contraction of the weld around the pipe circumference. Large root strains were simulated in the laboratory using a bend test which enabled the effect of material composition and strength to be studied. In the bend test, the specimen deflection at the onset of cracking, indicated by a drop in load, was used as a measure of susceptibility to cracking. The study showed that increasing the base metal strength for the same HAZ (achieved experimentally by heat treating the same base metal) resulted in a decrease in the critical deflection. This suggested that considerable strain concentration was occuring in the HAZ which increased as the base metal strength increased. Although the study succeeded in quantifying the effects of base metal composition and strength on the sensitivity to cracking, the program did not attempt to address the factors that influence the degree of strain applied to the weld. Specifically, the study only considered a single weld metal strength, and only one pipe wall thickness was addressed. Thus it was not clear whether the move to higher strength pipes welded with higher strength electrodes or a change in the dimensions of the pipe might increase the exposure to cracking risks. Furthermore, if bend tests are used to screen materials, the question is raised as to whether the acceptance level of critical deflection should be changed for higher strength materials. Heat affected zone cracking was observed in both complete circumferential welds (tie-in welds) and in part-circumferential welds (repairs). Various procedural details, such as heat input and length of repair, could influence the root strain and might warrant specific controls to minimize risk of cracking. This study assessed the effects of weld metal strength, pipe thickness, pipe diameter, heat input, and weld (repair) length on root strains in girth welds. A literature review was conducted and simple analysis methods were applied to identify areas with a high risk for cracking. The results show that high tensile stresses in the axial direction on the inside surface of the pipe result from the radial contraction of the weld and consequent bending of the pipe. Welding procedures with high heat inputs and few weld passes tend to have the greatest effect on stress. Multipass welds decrease the tensile stress, which becomes compressive after a certain thickness. Base metal and weld metal strength were shown to significantly impact strain in the heat-affect zone.