Microstructure Transformation and Mechanical Properties of X80 Large Diameter Thick Wall Induction Heating Bend for Low Temperature Service

Abstract

Abstract For the requirement of pipeline station construction project, Grade X80 Longitudinally Submerged Arc Welded (LSAW) induction bend pipe 1422 mm in diameter and wall thickness greater than 25 mm have been developed for pipeline station service applications at −45 °C. The mother pipe of the bends was welded by Ni-Cr-Cu-Mo-Nb-V micro-alloyed Thermo Mechanical Control Process (TMCP) steel plates. After the heat cycle of the bent pipe manufacturing, the microstructure of the base material of the bent pipe consisted of lath bainite ferrite (LBF) and granular bainite (GB). Therefore, it can obtain high strength and excellent low temperature toughness, which can meet the requirements of the project. On the other hand, the welding of the longitudinal seam-welds of the bend mother pipe uses a typical multi-wire two-pass submerged arc welding (SAW) process, which has a large amount of welding heat input. This results in a coarse columnar weld structure with a large amount of fine acicular ferrite so that the seam weld still has a good low temperature impact toughness. However, after the thermal cycling of the bend, the acicular ferrite in the microstructure of the weld metal was greatly reduced, and the grain size was unevenly distributed, which caused the low temperature toughness of the weld metal to deteriorate significantly. In order to solve this problem, the Gleeble3500 thermal simulation test machine was used to test the phase transition critical point Ac3 of the base material and the seam weld metal of the mother pipe. In order to optimize the induction bend process parameters, the influence of heating temperature, cooling rate and tempering temperature on microstructure and mechanical properties were examined. In addition, on the basis of the existing welding process, the welding wire and flux for pipe-making seam-welding were improved, and the pipe-making welding process of the bent mother pipe was improved.