Study On Testing Methods About Cold Cracking Susceptibility Of Pipeline Girth Weld

In pipeline system, weld joints defects are the weak links of the pipeline structure.Cold cracks are the most common kind of defects in pipeline welding process. On theeffect of welding thermal cycle, the weld and heat affected zone may produce coldcracks, because of the changing of microstructure and properties, the function ofstress and the effect of diffusion hydrogen. Especially with the pipeline strength gradeimproving, the problem of cold cracking in girth welding process becomes so seriousthat it can threat the operation safety of the pipeline and becomes the most seriousdefects which must be avoided during welding. The mechanism of cold cracking inhigh strength steel is that the steel hardened becomes brittle due to the invasion andinducing of hydrogen and then cracks in the effect of restraint stress. So we analyzethe cold crack sensitivity of girth weld by specimens which can have differentrestraint intensity in the same condition such as the same material (the samemicrostructure) and the same welding parameters (the same heat input).Based on theinvestigation on domestic and foreign cold cracking test methods and combining theactual pipeline condition in our country, we have given the test method of coldcracking sensitivity about pipeline girth welds on the basis of Lehigh test. We studiedthe relationship between the slot length of Lehigh test and the diameter and wallthickness of pipeline by calculating the restraint intensity of different conditions usingfinite element analysis. And we finally have given the relationship model. After theverification of actual welding test, we optimized the preheat temperature and thewelding standard of pipeline girth welding to provide technical support for highstrength grade pipeline steel engineering.