To confirm the safety performance of LNG storage tank, the change in fatigue crack growth rate and fracture toughness within X-grooved weld heat-affected zone (HAZ) of newly developed 9% Ni steel, which was SMAW welded, was investigated. These materials were produced by QT (quenching, tempering) heat treatment. The weld metal specimens were prepared by taking the same weld procedure applied in actual inner shell of LNG storage tank. All tests were performed in the temperature ranging from R.T. and -162℃. The fatigue crack growth behavior was carried out using CT specimen. Investigation has been carried out to study the influence of temperature and weld effect on fatigue crack growth behavior. Also, Fracture surfaces after tests were observe by scanning electron microscope (SEM).

Newly developed heavy thick plates of 9% Ni steel for large capacity of LNG tank were fabricated to conduct a fatigue crack growth test. The weld metal specimens were also fabricated by taking the same weld procedures which are applied to actual LNG storage tank inner shell. The effect of changes in load ratio, R, and test temperature on the fatigue crack growth rate has been investigated. Separate fatigue crack growth experiments were performed at load ratio of 0.1 and 0.5 at -162℃ and compared to the behavior at room temperature. The fatigue crack growth rates of weld metal were nearly the same as those of the base metal irrespective of load ratio change at room temperature. A decrease in temperature decreased the fatigue crack growth rates of base metal but in the case of weld metal only small scatters appeared in the fatigue crack growth rate compared with those of base metals. The fatigue crack growth rates were dominated by residual stress due to welding processes rather than temperature effects.