Effects of C, Nb and Ni on the toughness of heat affected zone of welded joints in steels of storage tanks for low temperature use

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.