Effect of GMAW Procedure on the Heat-Affected Zone (HAZ) Toughness of X80 (Grade 550) Linepipe

In view of the opportunities for new pipelines and a shortage of skilled labour, pipeline construction companies are exploring the use of new, more efficient, high-productivity welding procedures. However, in welding high strength steels, it is essential that the high strength and toughness be maintained in both the girth weld metal and the associated heat affected zone (HAZ) regions, particularly in strain-based designs in which significant longitudinal strains are anticipated. The measured toughness of the heat-affected zone (HAZ) is affected by base metal chemistry, welding procedure and the accuracy of locating the pre-test machined notch at the targeted zone, among other factors. In this study, the welding of X80 (Grade 550) linepipe by single torch, dual torch and tandem gas metal arc welding (GMAW) techniques has been examined. Both Charpy and CTOD testing have been employed to assess the toughness of the HAZ. It will also be demonstrated, that the measured CTOD value is dependent on the percentage sampled of the near-fusion line HAZ. Proper selection of the welding procedure to maintain a relatively fine grain size can significantly enhance the measured CTOD values.