Shipbuilding steel plates of heavier thickness, such as those over 50mm have been commonly used in large container ships because of the increased ship-size. The present paper focuses on the fracture toughness in welded joints of high strength shipbuilding steel plates of heavy thickness, and makes it clear that the fracture toughness of the welded joints decreases in the case of large mismatching of hardness in the base metal and the weld metal. The fracture toughness Kc is strongly dependent on the matching of hardness in the welded joints of high strength shipbuilding steel plates of heavy thickness. The estimated Kc, which is usually referred to as the hull structural toughness assessment, is calculated based on the results of CVN impact tests and is expected to be equivalent to the measured Kc. However, the measured Kc was seen to be much smaller than the estimated Kc, when the mismatching of hardness increased in the welded joint. The test results suggest that particular consideration must be given to mismatching of hardness for preventing brittle crack initiation in welded joints of high strength steels of heavy thickness.
