Toughness on the welded joint, in special on the heat affected zone, is one of the most difficult requirements to be complied, when it is necessary to weld thick carbon steel pipes, with wall thickness over than 16 mm. Basically, it is defined by the final microstructure. In the weld metal it is the result of the combination between the hardenability and the cooling thermal cycle, related with welding heat input. In the area around the weld bead, called heat affected zone, it is the result of the welding heat input action on the base metal. It is possible to get a better microstructure in the heat affected zone, more suitable to toughness requirement, if heat input is decreased. Considering the weld metal, there is an appropriate range of heat input, which results in better toughness behavior. In order to get appropriate heat input for thick pipe welding, it was developed the submerged arc process with 4-wire tandem technique for pipe welding and the development benefits are shown on this paper.

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