This study focuses on the characteristics of the toughness of a base plate and weld heat affected zone (HAZ), because its importance is becoming bigger in terms of a structural integrity. Several factors, such as reheating temperature, reduction per pass in the recrystallization region, and finish cooling temperature were varied to understand their effects on the base plate toughness. In order to examine how alloy elements can affect the toughness of the weld HAZ, a simulated HAZ technique was utilized and their effects were investigated by carrying out CTOD and Charpy impact tests. In addition, thick steel plates of 76.2 mm in thickness were produced and the toughness of the base plate and weld HAZ was evaluated to make sure if the factors which were proven to be effective on small scale specimens have an identical influence on the thick steel plates.
Industries for the production of oil and gas in the offshore region have been demanding thick steel plates with high strength and excellent toughness, because offshore environment where offshore structures operate is becoming severer, for example, getting deeper and colder. High strength is required for offshore structures which operate on the deep sea and toughness properties are considered as the most important requirement for structures operating on the cold seas, such as Sakhalin and Alaska. As the offshore structures become bigger, steel plates used should be thicker, stronger and tougher. Generally, the increase in strength and thickness has a negative effect on the toughness of steel plates. The microstructure of higher strength steel is likely to have more brittle phases, making brittle crack initiation much easier, compared to that of lower strength steel.