Burnthrough prediction in pipeline welding

Abstract

Welding of branch connections on gas pipelines at full line pressure is frequently an operational necessity. Weld parameters must be selected so that heat inputs are low enough to avoid burnthrough yet not so low that hydrogen-assisted cold cracking occurs. Current techniques rely on the observation that burnthrough does not occur if peak temperatures on the inner surface are kept below 980^°C. At these temperatures, rate-dependent flow is the dominant mechanism. The problem is one of creep rupture occurring at temperatures in excess of 980^°C with times of the order of seconds. Material constitutive models for the analysis of welding must include both rate-dependent and rate-independent plastic flow as well as the effects of phase transformations. Material properties at elevated temperatures are usually not available for pipeline steels and must be extrapolated from values at lower temperatures. An exploratory study using 3D thermal-mechanical finite element analysis of welding on pressurized vessels is presented and includes comparisons with experiment. The agreement is encouraging. The material failure does occur in appropriate locations but the predictions are generally overconservative. Estimated material properties, especially damage and rupture properties at high temperatures could be improved.