• Journal of Welding and Joining
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• 제27권5호
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• pp.74-80
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• 2009

The membrane of the LNG carriers consists of thin strips of INVAR(Fe-36%Ni) steel plates, and the junction between INVAR strips is fabricated by welding. Thousands of the raised edge joints, regularly spaced, are located around all the side of the tank corner near the transverse bulkhead, and TIG welding is manually made on the top of the raised edges. Since the thickness of all the laminated edge plies is extremely thin and the weld position is under a bad accessibility, highly skilled workers are required to perform welding relatively for a long welding time. An alternative scheme for the corner membrane fabrication is proposed in the study to improve the installation workability and thus productivity. The scheme replaces the welded edges with the preformed corrugation ones. A panel strip with regularly-spaced corrugations is installed at the corner instead of the individual flat strip of which edge is vertically raised to be welded with the adjacent strip. In the study, a series of the evaluation on the corrugated edge members was performed to assess the applicability to the real LNG carrier fabrication. Opening displacement at the raised edge was experimentally examined. Elastic stiffness regressed from the displacement was nearly same in both edge types. Edge displacement and local stresses were calculated under hydrostatic pressure and temperature change due to liquefied cargo. Fatigue test was performed on both corrugated and welded edge specimens consisting of two or five plies of invar strips. Fatigue strength of the corrugated specimens was not less than that of the welded specimens.

• Journal of Welding and Joining
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• 제34권1호
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• pp.41-46
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• 2016

With the recent increase in size of ships and offshore structures, there are more demand for thicker plates. As the thickness increases, it is known that fatigue life of the structures decrease. To improve the fatigue life, post weld treatments techniques, such as toe grinding, TIG dressing and hammer peening, are typically employed. However, these techniques require additional construction time and production cost. Therefore, it is of crucial interest steels with longer fatigue crack growth life compared to conventional steels. This study investigates fatigue crack growth rate (FCGR) behaviours of conventional EH36 steel and Ferrite-Bainite dual phase EH36 steel (F-B steel). F-B steel is known to have improved fatigue performance associated with the existence of two different phases. Ferrite-Bainite dual phase microstructures are obtained by special thermo mechanical control process (TMCP). FCGR behaviours are investigated by a series of constant stress-controlled FCGR tests. Considering all test conditions (ambient, low temperature, high stress ratio), it is shown that FCGR of F-B steel is slower than that of conventional EH36 steel. From the tensile tests and impact tests, F-B steel exhibits higher values of strength and impact energy leading to slower FCGR.

• Corrosion Science and Technology
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• 제13권2호
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• pp.70-80
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• 2014

Duplex stainless steels with nearly equal fraction of the ferrite(${\alpha}$) phase and austenite(${\gamma}$) phase have been increasingly used for various applications such as power plants, desalination facilities due to their high resistance to corrosion, good weldability, and excellent mechanical properties. Hyper duplex stainless steel (HDSS) is defined as the future duplex stainless steel with a pitting resistance equivalent (PRE=wt.%Cr+3.3(wt.%Mo+0.5wt.%W)+30wt.%N) of above 50. However, when HDSS is welded with gas tungsten arc (GTA), incorporation of nitrogen in the Ar shielding gas are very important because the volume fraction of ${\alpha}$-phase and ${\gamma}$-phase is changed and harmful secondary phases can be formed in the welded zone. In other words, the balance of corrosion resistance between two phases and reduction of $Cr_2N$ are the key points of this study. The primary results of this study are as follows. The addition of $N_2$ to the Ar shielding gas provides phase balance under weld-cooling conditions and increases the transformation temperature of the ${\alpha}$-phase to ${\gamma}$-phase, increasing the fraction of ${\gamma}$-phase as well as decreasing the precipitation of $Cr_2N$. In the anodic polarization test, the addition of nitrogen gas in the Ar shielding gas improved values of the electrochemical parameters, compared to the Pure Ar. Also, in the erosion-corrosion test, the HDSS welded with shielding gas containing $N_2$ decreased the weight loss, compared to HDSS welded with the Ar pure gas. This result showed the resistance of erosion-corrosion was increased due to increasing the fraction of ${\gamma}$-phase and the stability of passive film according to the addition $N_2$ gas to the Ar shielding gas. As a result, the addition of nitrogen gas to the shielding gas improved the resistance of erosion-corrosion.