This study presents a numerical index to quantify the influence of non-ideal joint geometry on the defect susceptibility of the root pass for circumferential pipeline welds. The combination of manufacturing tolerances on component dimensions and thermal distortion during fabrication is known to result in deviations from ideal joint geometry in virtually all welded joints. Although not problematic in many joint geometries, under the unique conditions present during field tie-in welds on pipelines, deviations such as a high-low offset at the inner pipe wall have been correlated with an increased occurrence of root weld defects such as cold cracking.

During the deposition of the root weld pass, significant plastic strains are produced in the hoop direction of the pipe as a result of the constrained thermal expansion and shrinkage in the weld fusion zone and surrounding material. After the material has cooled to a uniform temperature, this plastic strain results in a residual tensile hoop stress which will tend to produce local bending across the weld joint. In joints with close to ideal geometry, such as those used for procedure qualification, the material typically has enough capacity to accommodate this bending and the probability of defect occurrence is relatively low. In production welds, the presence of non-ideal joint geometry will create a stress/strain concentration at the weld root which has the potential to produce more severe conditions for defect formation than those present during qualification.

Quantifying the effect of non-ideal geometry, necessitates an improved understanding of the interdependence of welding parameters, joint geometry, and the resulting residual stress and strain distribution in circumferential joints. To enable the application of this understanding in industrial practice, it is desirable to simplify the complex multi-physics of residual stress development into a minimal expression which considers only parameters which may be known or estimated prior to welding. This work utilizes a combination of analytical and numerical methods to identify the dominant material properties and procedural variables that drive defect susceptibility. The resulting parameters are synthesized in the form of a quantitative index that allows industrial practitioners to characterize the defect susceptibility of any given welding procedure and joint preparation. In addition, this index represents an enhanced understanding of the interaction between joint geometry and welding residual stresses which is essential to establish improved guidelines for acceptable levels of geometric deviations as well as to guide the development of new and more robust pipeline welding procedures.


A Quantitative Index to Assess the Influence of Joint Fit-Up on Pipeline Weld Root Discontinuities

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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