An Evaluation of the Effect of Residual Stresses on the Static Strength and Dynamic Toughness of Multi-Pass Girth Welding in Iranian Natural Gas Transmission Pipeline Using New Design Criteria

The tensile test in the weld and heat-affected zone of welded structures give global results due to the presence of various metallurgical microstructures in these regions. The best way to estimate the strength of material in these areas is converting the results of hardness test to yield strength (or ultimate strength) by empirical relationships. In this study, the hardness test was carried out in various areas of the girth welded steel pipe used in Iranian natural gas transportation projects. Then, dimensionless parameters Rya (the average residual stress over material yield strength), Rym (the maximum residual stress over material yield strength), Ru2 (the difference in residual stress over material ultimate strength) and Ru3 (the difference between maximum and minimum of three-dimensional residual stresses over the material ultimate strength) were presented to determine the actual mechanical behavior of structure for the first time. Hardness and tensile tests identify the minimum yield strength in the heat-affected zone adjacent to weld gap. The increase in hardness values from the root pass to the next passes is consistent with increase in the carbon content. Identification of maximum Rya criterion and critical mode of reduced static strength (reducing of stress design) in the center of weld gap on the outer surface of the pipe are consistent with tensile test results. The rupture zone in the tensile test with highest Rym criteria represents the effect of residual stresses on the static behavior of structure (in addition to the hydrostatic nature of residual stresses). The sharp decline in Ru2 criteria is consistent with the exponential reduction in impact strength perpendicular to the weld gap. It can be concluded that with regard to the effect of plane stresses (in thin-walled pipes) Ru2 criterion is more appropriate in assessing the impact strength of steel than Ru3.