Relationship between solidification sequence and toughness of carbon steel weld metal

This work aimed to investigate the segregation sequence of impurity elements in the weld metal of carbon steel with various solidification sequences, such as δ single-phase, hypo-peritectic, hyper-peritectic, and γ-single-phase solidification. Moreover, the relationship between the solidification sequence and toughness of the weld metal of carbon steel was studied. The solidification sequence was controlled by the nickel content of the base metals. Various amounts of sulfur and phosphorous were added to the base metals as impurity elements, and the segregation amount of the impurity elements in the weld metals increased in the order of δ-single-phase, hypo-peritectic, hyper-peritectic, and γ-single-phase. The amount of sulfur was higher than that of phosphorous in each solidification sequence. In δ-single-phase solidification, sulfur segregated during solidification and relaxed during cooling after solidification. In contrast, in γ-single-phase solidification, sulfur continued to segregate during and after solidification. The segregated sulfur was in the grain boundaries of the prior austenite at low temperatures. Charpy impact test results revealed that the ductile-brittle transition temperature (DBTT) increased, and the upper shelf energy decreased with an increase in the nickel content, specifically, only for the specimen of γ-single-phase solidification with S addition. In particular, the tendency in the specimens containing sulfur was more significant compared to the specimens containing phosphorous. Moreover, the intergranular fracture surface was only observed in the γ-single-phase solidification specimen, and sulfur was concentrated on the surface. Therefore, the superposition of segregation during and after solidification induced intergranular embrittlement and deterioration of toughness.