Effect of second thermal cycle on the microstructure of the coarse grain heat-affected zone of in-service welded pipeline steel

Welding thermal simulation, metallographic analysis and transmission electron microscope (TEM) were used to study the metallurgical microstructure and fine structure of the coarse grain heat-affected zone (CGHAZ) of in-service welding and the zones that subject to second thermal cycle at different peak temperatures. The results show that the main metallurgical microstructures of the CGHAZ of in-service welding are granular bainite and lath bainite. After second thermal cycle, the types of mierostructure have no change, but the shape and amount of each kind of microstructure and the grain size of original austenite are different when the peak temperature of second thermal cycle is different. Observing by TEM, the fine structures of each HAZ are ferrite laths and M-A constituents that distribute on or between ferrite laths. The M-A constituents in the CGHAZ of in-service welding are main strip-shaped. After second thermal cycle, the M-A constituents in the intercritically reheated CGHAZ are refined and lower bainite that has typical midrib morphology produces in the zone.