This paper describes an extensive study on the effects of C, Nb and Ni on the toughness of heat affected zone of welded joints in low Ni (1.5~3.5%) steels for LPG storage tanks and 9%Ni steel for LNG storage tanks. The toughness of heat affected zone of welded joints is mainly studied with use of the specimens orenared by thermal cycle simulation. The decrease in C content to 0.01 % improves the toughness of fusion line of welded joint of the 2.5% Ni steel, while it deteriorates the toughness of 9%Ni steel. In both steels, the fraction of island martensite in the welded joint decreases, resulting in the improvement of toughness. However, coarsening of microstructure, which lowers toughness, is more extensive for the 9%Ni steel. The difference in microstructure coarsening of both steels seems to be a cause for the difference in the toughness. The addition of 0.03%Nb improves the toughness of the 0.01~0.05%C~2.5%Ni steel through the refining of microstructure, but does not improve the toughness of the 0.01~0.06%C~9%Ni steel. The increase in Ni content from 1.5% to 2.5% improves the toughness of the 0.01%C steel, while a further increase of Ni content to 3.5% lowers toughness. This comes from the intensive formation of island martensite. These results are applied to the production of 0.01%C~2.5%Ni and 0.06%C~9%Ni steels with good toughness of fusion line of welded joints for LPG and LNG storage tanks, respectively.
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