抄録
In the previous paper, it was showed that 40mm thick 9% Ni steels and their welded joints had sufficiently higher fracture toughness than the required toughness values evaluated under the postulated defect and the stress conditions. Actually, they have been applied to an LNG tank of 140, 000kl in capacity.
As the demand for larger LNG tanks is still increasing, heavier thick 9% Ni steels are required. This paper describes work carried out using brittle fracture initiation and arrest tests for 50mm and 55mm thick 9% Ni steels and their GTAW joints. Moreover, ductile fracture resistance (R curve) of the weld metal with 70% Ni alloy welding materials was also investigated, because a crack in the weld HAZ tends to deviate into the weld metal in case of 9% Ni steel.
It has been shown that the fracture initiation and arrest toughness of 50 and 55mm thick plates is satisfactory as compared with the 40mm thick plates and that the R curve of the GTAW weld metal is remarkably high. From these results and the safety assessments for applicability to LNG tanks, it has been concluded that these steel plates can be used with high safety margin.
