Embrittlement factors during the fracture of ERW welded joints of low-carbon low-alloy steel pipes according to the results of Charpy impact tests

Abstract

In order to determine the influence of various embrittlement factors on the fracture of welded joints of a pipe made of low-carbon low-alloy steel obtained by electric resistance welding (ERW), we performed multiple Charpy impact tests. The fracture surfaces were analyzed by using macro- and microfractographic techniques. The microstructure was characterized by means of optical microscopy, scanning electron microscopy (SEM) and electron backscattered diffraction in SEM. It was discovered that the formation of large bainitic grains in the ferrite-bainite microstructure formed in the core of the ERW pipe wall, as well as the presence of globular 2–7 μm oxysulfide nonmetallic inclusions, exert a significant embrittlement effect on the base metal of the ERW pipe joints. The formation of a predominantly ferritic coarse-grained microstructure with a 1.4-fold increase in grain size and a 2-fold increase in the content of {001} crystallographic clusters parallel to the fracture surface, as well as the formation of elongated deformed oxysulfide inclusions inherited from the base metal in the zone of the fusion line of the ERW pipe joint are responsible for the decrease in the level of toughness and the appearance of brittle fractures with low values of Charpy impact toughness in the fusion line zone.