9% Ni 강 다층 용접열영향부의 취성파괴 저항성능에 미치는 LBZ 존재의 실질적 영향

In authors` previous study using the synthetic HAZ specimens, it was found that, like other structural steels, there were local brittle zones (LBZs) in the weld heat-affected zones (HAZs) of QLT (quenching, lamellarizing and tempering) - treated 9% Ni steels which are used for inner walls of domestic LNG (liquefied natural gas) storage tanks. In the continuing line of research, this study was performed to evaluate the influence of the LBZs on the brittle fracture resistance of the actual weldment of the steel. Crack tip opening displacement (CTOD) tests and compact crack arrest (CCA) tests were conducted to measure the crack initiation toughness and the crack arrest toughness respectively, and the results from both tests were compared each other. The results indicated that unlike CTOD test which showed the toughness decrease as approaching to fusion line, the crack arrest toughness values of the regions near fusion line, i.e., the regions containing LBZs, were higher than those of the other regions. It was due to the fact that the arrest toughness was ruled by the large fraction of fine-grained HAZs surrounding LBZs instead of LBZs themselves. Conclusively, by the comparison of the brittle crack arrest toughness, K_a, with brittle crack initiation toughness, K_c, calculated from CTOD test results, it was suggested that the LBZ was not a main risk factor affecting the fracture mechanical safety of the actual 9% Ni steel weldments.