This study performed welding on various welding positions in the flat, overhead, and vertical down positions using a pulsed gas metal arc welding (P-GMAW) process. Despite the same amount of heat input on various welding positions, the welding characteristics varied depending on the welding positions. The effect of gravity on the welding process determined the different formation of the weld bead, and the influence of molten pool behavior on various welding positions changed the microstructure in the weld metal. The current and voltage signals were synchronized with a high-speed camera using a data acquisition (DAQ) system. To induce the one pulse one drop (OPOD) process of metal transfer, the shielding gas was used 95% Ar+5% CO2. The microstructure of the weld metal was analyzed in relation to molten pool behavior.

The parent metal have shown higher corrosion than the weld zone under severe Arctic corrosion conditions.

One of the most important issues associated with liquefied natural gas (LNG) storage tanks, such as LNG carrier cargo tanks and land LNG tanks, is their structural integrity. In order to ensure the operating life and safety of LNG storage tanks used under operating conditions such as thermal and cyclic loadings, the securing of safety evaluations for fatigue performance is considered to be of particular importance. There have been various efforts to reduce the production costs of LNG storage tanks, such as the optimum selection of materials and the development of new low temperature materials. This, the motivation of this study is to evaluate new material candidates for LNG storage tanks. This study begins with a comprehensive review of the characteristics of low temperature alloys such as SUS 304L, Invar, A5083 and 9% Ni steel that are widely used for LNG storage tanks. Then, the fatigue characteristics of a newly developed low temperature material, 7% nickel steel are investigated. Finally, the fatigue performance of 7% nickel steel is compared with that of 9% nickel steel.

There is growing demand to reduce air pollution from shipping, and it is expected that liquefied natural gas (LNG) will be widely adopted as a marine fuel in the near future. To design and construct LNG-fueled vessels, the design of the LNG fuel tanks and evaluation of the performance of joints welded using low temperature materials should be conducted in advance. This study focuses on the applicability of 9 wt% nickel alloy steel as a low temperature material for LNG fuel tanks. The rule scantling process and FE analysis for the initial design of an IMO type C independent tank were examined. Charpy V-notch impact tests were performed on the welded joints of Shielded Metal Arc Welding (SMAW), Submerged Arc Welding (SAW) and Flux Cored Arc Welding (FCAW) processes at cryogenic temperature. The weld metal exhibited the lowest impact absorbed energy following FCAW and SMAW while the fusion line showed the lowest impact absorbed energy in SAW. However, all of the welding processes satisfied the requirements of BV and DNVGL for impact test. In addition, the correlation between the groove angle and weld penetration shape in FCAW was investigated considering the productivity of the LNG fuel tank construction.

In terms of steel alloying elements, generally, nickel is used as an austenite stabilizing element to increase the toughness of steel. The low temperature materials, such as nickel alloy steels with a nickel content of 3.5% to 9%, stainless steel and Invar, show excellent toughness at low (173 K) and cryogenic (108 K) temperatures. In particular, in the shipbuilding industry, it is mainly used for liquefied ethane and Liquefied Natural Gas (LNG) carriers, and research on low-temperature steels are attracting attention again as regulations on environmental issues are strengthened in recent years. Therefore, in this study, fatigue and fracture performances of nickel alloy steel containing 9% or less among nickel alloy steels are evaluated. Moreover, we assess the Fatigue Ductile to Brittle Transition (FDBT) of nickel alloy steels based on crack tip opening displacement (CTOD). In order to discuss the fatigue and fracture performances of nickel alloy steels, microstructure analysis carried out. As a result, CTOD and Fatigue Crack Growth Rate (FCGR) of nickel alloy steels increases as nickel contents increase. In addition, FDBT of 9% nickel alloy steel is the lowest compared to other nickel alloy steels.