Theoretical Prediction of Thermal Cycles and Hardness of HAZ due to Twin Wire Submerged Arc Welding

Abstract

Twin wire submerged arc welding (SAW) is widely used in the joining of oil or gas pipelines because of its high productivity. However, when it is applied to join the high strength steel pipe, the heat affected zone (HAZ), especially the coarse grain zone, becomes very brittle. To investigate the strength and toughness of HAZ, thermal cycles in HAZ due to twin wire SAW must be studied. In present studies, theoretical equations of twin wire welding thermal cycles for both a thick plate and a thin plate were developed. The critical thicknesses for both a thick plate and a thin plate were determined. The welding thermal cycles for a medium thickness plate were predicted by linear interpolation using results of both a thick plate and a thin plate. Through the theoretical equations, thermal cycles of twin wire SAW with actual welding conditions used in pipe fabrication were predicted and verified by both FEM and experiment. A good agreement among theoretical results, FEM and experimental results was achieved. Furthermore, the microstructures and Vicker's hardness of HAZ was predicted based on CCT diagram and thermal cycles calculated by the proposed theoretical equations. Predicted microstructures and hardness were compared with the experimental ones. A good agreement was also obtained.