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 harden ability 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 behaviour. 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. Introduction Normally, micro-alloyed steel pipes are manufactured according to API 5L (1), issued by "American Petroleum Institute", which states the minimum requirements. Toughness is considered as a supplementary requirement by this standard, but it has been required by some technical specifications, at temperatures as low as -30°C, depending on the application characteristics. On the base metal, this characteristic depends on the steelmaking and rolling practices and the steel mills have been developed special ways to cast and roll the material, which are resulting in good final properties. On the welded joint, which includes the weld seam and the heat affected zone, toughness is basically the combination among base metal dilution, chemical composition of the consumables used and heat input applied by the welding process. Heat input also has special influence on the heat affected zone, which is the base metal with its microstructure modified by the welding heat. When wall thickness over 16 mm is being welded, the high heat input applied results in a toughness drop to a level which is not compatible with the balance of the joint. Normally, high heat input results in lower toughness. Aiming to get higher toughness performance on the welded joint, with thickness over than 16 mm, in special on the heat affected zone, submerged arc process with 4-wire tandem technique was developed. With this system, there was a heat input reduction by the welding speed increasing and, as result, it was possible to get higher levels of toughness, both on the heat affected zone and on the weld metal. This paper describes the benefits of the 4-wire system, with lower heat input, on the toughness performance of the welded joint. Heat input influence on toughness of heat affected zone Heat input is defined as the electrical energy supplied to a unit length of weld seam.(2) The welding energy, or heat input, causes modifications in the microstructure of the base metal, around the weld bead, in an area called as heat affected zone. This modified area, usually, has an extension up to 6 mm from de fusion line, depending on the heat intensity. In this area, due to the modifications in the microstructure, the toughness level, obtained on plates by rolling practices, is considerably reduced. The heat affected zone is divided in some regions, according to figure 1, which depend on the temperature reached during the welding. This temperature varies with the distance of the heat source.
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