According to the current shipbuilding trends for mass transportation, the size of container ships was getting bigger. Therefore, it is required for shipbuilding companies to increase the welding efficiency. To enhance the welding efficiency, the use of the high heat input welding is essential. For the high heat input welding, it is also necessary that the impact toughness of the weld HAZ should not be damaged by grain growth in HAZ. To be able to weld the full thickness of 50mm plate by single pass electro gas arc welding, the required weld heat input is about 50kJ/mm. Therefore, the thick steel plate should be welded by ultra high heat input welding to meet these requirements. This paper describes the developed steels for use in ultra high heat input welding and its design concept. To confirm the stability of weld joint, various tests were done for the steel plate such as impact toughness at low temperatures and hot cracking susceptibilities at the high heat input. Target was 355MPa yield strength and impact toughness below -20oC. The developed steel shows sufficiently high strength and high impact toughness at -40oC by the 55.0kJ/mm heat input.
Container ships have been growing bigger and bigger accompanied with thicker hull plate. Thicker hull plate requires more welding passes that takes longer time to fabricate associated with high cost. The steel plates produced by using thermomechanical control process have been used for high heat input welding up to about 10 ~ 15kJ/mm. In addition to these high heat input welding, shipbuilding companies consider the higher heat input for single pass welding for the full thickness of 50mm. In that case, heat input will be as high as about 50kJ/mm when the welding is done by electro gas arc welding