Study on fracture behavior analysis of X70 steel girth weld with toughness scatter

This study conducting a comparative analysis of the microstructure and mechanical properties of various girth welds on X70 pipeline steel. The toughness scatter about flux-core arc welding of X70 girth weld joint was studied, and the factors contributing to it were explored. Through Charpy impact test, optical microscope and scanning electron microscope, the causes of different properties of welds on API 5L X70 pipeline steel and the scatter mechanism of girth weld impact toughness were explored. Results indicate that wider weld pools correlate with higher heat inputs, which contribute to the formation of a coarse columnar crystal structure and facilitates the precipitation of Martensite-Austenite (M-A) constituent. These M-A constituents, along with the proportion of different microstructures, have a direct impact on the impact toughness of the welds. As the volume fraction of M-A constituents and the proportion of coarse lath bainite grains increase, the impact toughness decreases. The initiation and propagation of crack at the massive M-A constituent, and the fine granular M-A constituent has a certain hinder effect on the crack. The impact toughness scatter is mainly caused by the high proportion of coarse lath bainite grain, and the uneven distribution of M-A constituent.