The occurrence of deformation induced ferrite transition (DIFT) during back heating assisted friction stir welding pipeline steel: The influence on the toughness of welded joint

A pipeline steel was friction stir welded without and with preheat at 150 degrees C and 300 degrees C via back-heating plate. The microstructural evolution of the welded joints was studied and related to impact toughness. At conventional welding without preheating, bainite and martensite formed in the welded joint and semicircular ferrite phase band formed on the top surface of the welded joint because of the occurrence of deformation induced ferrite transition (DIFT). The semicircular band ferrite phase in the center and particularly at the advancing side embedded into the welded joint because of the spiral downward movement of the material. While, the shape of ferrite phase at the retreating side was flat on the top surface of the welded joint. The ferrite phase band split the integrity of the welded joint with embedding into the top surface. The crack easily propagated along the ferrite phase, leading to its dramatic deflection at the advancing side of welded joint during impact test, which eventually increased impact toughness greatly. But the crack propagated straightly at the center and retreating side and particularly at the retreating side, and the impact toughness was quite low with the existence of brittle martensite phase. With preheating to 150 degrees C and 300 degrees C, only bainite phase formed in welded joints, and the DIFT gradually weakened with the decrease of frictional force between the stirring tool and the top surface of the welded joint. As a result, the formation of semicircular ferrite phase band weakened, and only a little of it formed on the top surface of 300 degrees C preheating welded joint. Finally, the dramatic deflection of the crack at the advancing side occurred at 150 degrees C preheating welded joint, but disappeared at 300 degrees C preheating welded joint. Without the formation of brittle martensite phase, all positions in the stir zone of 150 degrees C and 300 degrees C preheating welded joints showed excellent impact toughness. However, as high preheating temperature led the coarsening of bainite lath, the impact toughness of 300 degrees C preheating welded joint was slightly lowered than that at 150 degrees C. Hence, medium preheating temperature is recommended in back heating assisted friction stir welding pipeline steel.