Recently, interest in shale gas has increased all over the world according to increasing skepticism about the safety of nuclear power and continuing supply instability of major oil-producing country.
South Korea has almost no natural gas reserves. Related industries have been operating around the gas transport ((PNG/LNG industry) and introduction. The LNG demand of countries like Australia, Taiwan and Japan has increased by expanding the shale gas development in the United States. In addition, demand for LNG carriers for transporting natural gas has increased significantly during the decades.
A number of pipelines are installed the LNG carriers. The production processes of the pipeline demand much working time because it is welded by welders. As a result, the work efficiency is reduced and an avoiding phenomenon of related industries occurs because of realization as 3D industry and lack of human resource of welder. Automated welding is emerging as a solution of these problems.
Pipe welding techniques are especially applied in various industries such as land plants, sea plants, construction and heavy industry. Mainly applied welding processes are at pipe welding SMAW, GMAW, GTAW, etc. Welding quality is determined by the welders at manual welding because it is possible to apply at all of welding positions. So automated welding using orbital welding machines or robotic welding machines has been researched to improve welding quality at pipe welding of shipbuilding industries.
Automated welding has been carried out about 40-45% at the main industry such as shipbuilding and automation industry. Orbital welding equipment that welds along the pipeline is developed with effort of automated welding on pipe joints. Orbital welding system is welding head is tracking through the reil installed along the pipeline. However, this system is time-consuming because the rail must be removed if the pipeline is changed.
The formation of stable back bead is very important at pipe welding of ships or industrial plants. However, direction of gravity, surface tension and arc force having effects on melt pool changes according to each section at pipe welding. And welding at all of the welding position is difficult because back bead becomes the concave face when welding is done between overhead position and vertical-up position among the welding position.
Therefore, a PC-based four-axis control welding system is manufactured in order to complement the disadvantages of these orbital welding. The four-axis control welding system is possible to create database of various welding parameters, thickness and diameter of pipe and head speed. Also, when base metal changes, welding is performed with optimal welding condition by using suitable data. Automated welding process can improve productivity through improving the work efficiency and getting the uniform welding quality without skilled welders.
In this paper, it is attempted to derive the optimum TIG welding conditions for applying the four-axis control welding system. Used materials are STS316L and STS304L that maintain great strength and toughness at low temperatures and have corrosion resistance on sea water. Grain boundary sensitization phenomenon by Cr depletion layer is easy to occur in stainless steel for pipes of LNG carriers. Therefore, it is attempted to derive the optimum TIG welding conditions for the purpose of suppression of defects and failure of the weld. To summarize the details for this is as follows.
The effect of welding current and welding speed is evaluated on bead welding of stainless steel. And the characteristics on impact of filler metal are investigated by changing feeding speed of filler metal.
During the butt welding, the influence of the gap between the butt surfaces act as a variable. The characteristics are investigated by changing the distance of the gap after selecting the optimum condition at welding beads. Also, presence or absence of defects is evaluated through the hardness measurement and microstructure observation of the weld and heat-affected zone.
Based on experiments of bead welding and butt welding, gabs and groove shapes are removed for control of overlap at pipe welding. Butt welding of pipe is performed. The effect of welding current and feeding speed of filler metal is evaluated through welding. Welding conditions is also derived by changing the position and angle of the touch and found though slope control. The microstructure of weld is analyzed in overlap and the central part of pipe welding.