Application of new technology in steel structure welding 5 Welding technology of cast steel and cast steel joints Due to its good processing performance and complex and diverse architectural modeling, the cast steel joints have been gradually promoted in some large-span space tubular truss steel structures, especially in the processing of complex intersections. The cast steel joints are uniquely endowed by nature. The advantage, however, due to the high carbon equivalent of cast steel, the content of impurities, especially S and P, is difficult to control, and the as-cast microstructure is coarse, resulting in poor weldability of the cast steel, and the welding process is very demanding. It is to reduce the residual stress and prevent the occurrence of welding cracks. At present, cast steel joints have been successfully used in some steel structure projects, such as the giant truss cast steel support of the Guangzhou Convention and Exhibition Center, the national stadium (Bird's Nest) truss column cast steel joints, especially the Tianjin Economic and Technological Development Zone sculpture, creating steel castings. The most extensive welding plate is the landmark building built for the 20th anniversary of the Tianjin Development Zone. It is one of the key projects of Tianjin Development Zone in 2004. The design adopts a straight curved curved panel composed of double spirals. The main body has an elevation of 50 meters. The main body of the sculpture is divided into 7 sections and assembled on site. The cast steel material is GS-20Mn5N (DIN7182), and the thickness of the plate is gradually thinner from bottom to top. 150mm, the thinnest 20mm, the thickness change is gradual treatment, the total steel consumption is 350 tons. The project has formed a whole set of new technology in the design, installation and welding of cast steel special structure, which fills the domestic super thick plate. The blank of the cast steel structure indicates that China has reached a new level in the welding of steel structure cast steel joints. 6 low temperature welding technology At present, the winter construction of steel structures is becoming more and more common. Because the welding environment has a great influence on the welding quality of steel structures, the winter negative temperature welding technology is especially important for the control of welding quality. "Technical Regulations for Welding of Steel Structures in Buildings" (JGJ81-2002) stipulates that when the ambient temperature of the welding operation area is lower than 0 degrees Celsius, appropriate measures shall be formulated according to steel and welding consumables; and the Japanese Architectural Society's "Steel Structure Engineering" (JASS6) regulations The minimum soldering temperature is minus 5 degrees Celsius. (1) The influence of ambient temperature on welding has the following points: 1 The welded joint has increased cooling rate and the cold crack sensitivity is also increased; 2 The preheating effect is deteriorated, and the same heat source can be used in the low temperature environment, and the preheating effect should not be achieved at the same time; 3 the effect of welding residual stress is intensified; 4 Ambient temperature has an adverse effect on the operation of the welder. (2) Low temperature welding measures} 1 Preferred welding consumables: In low temperature environment, low-hydrogen or ultra-low-hydrogen welding consumables should be selected as much as possible, and baking and thermal insulation measures should be strictly applied to the welding consumables; 2 Pre-weld protection: cover the welding area to make the welding area form a relatively closed space, reduce the loss of heat. If the protective shed is unconditionally set, other effective measures should be taken to protect the welding area; during gas shielded welding, welding Gas cylinders should also take corresponding measures to keep warm; 3 Preheating and interlayer temperature: The preheating temperature in the low temperature environment should be slightly higher than the welding preheating temperature at normal temperature, and the heating area is greater than or equal to twice the thickness of the steel plate in the component welding zone and not less than 100 mm. Base metal, the temperature between the soldering layers is not lower than the preheating temperature or the minimum temperature specified by the standard (JGJ81-2002) is 20 °C (both high values); 4 Increase the heat input during tack welding: appropriately increase the heat input of the tack weld, increase the cross section and length of the weld, and use the same preheating conditions as the formal weld, do not arc on the base metal outside the groove, and let go The arc crater must be filled during the arc, which can effectively reduce the shrinkage crack caused by the positioning welding; 5 Use a reasonable welding process: use narrow swing, multi-layer multi-pass welding, and strictly control the interlayer temperature; 6 After welding, heat and heat preservation: After welding, the welded joints are post-heat-heat-treated, which can effectively control t8/5, which is beneficial to the diffusion of hydrogen gas, preventing cold cracks caused by excessively fast cooling speed, and appropriate post-heating. At the temperature, the preheating temperature can also be appropriately lowered. In short, before the low-temperature welding of steel structures, the welding process qualification test must be done according to the actual situation. If necessary, the low-temperature weldability test should be carried out for specific steel grades, and a suitable welding procedure guide should be made to guide the actual welding. In the low temperature environment, the adverse effects on the welder's operation should also be given enough attention. The general ambient temperature should not be lower than -15 °C. In recent years, low-temperature welding technology has been widely used in domestic and especially northern construction steel structures, from ordinary low-carbon steel (Q235) to low-alloy high-strength steel (such as Q345, Q420), and has accumulated considerable experience. Greatly enriched the welding technology of steel structures in China. 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