Steel structure is more and more widely used in energy, transportation, metallurgy, machinery, chemical industry, electricity, construction, and infrastructure due to its advantages including lightweight, short construction period, strong adaptability, beautiful appearance, and convenient maintenance. At the same time, the use of thick steel plates is increasing in steel structure. A large number of steel structure projects use thick steel plates, which promotes the development of thick steel plate welding technology and also enriches the scope of construction steel.
Thick Plate Welding
When welding thick plates and ultra-thick plates, the amount of deposited metal of the filler welding material is large, the welding time is long, the total heat input is high, and the welding seam is high, the welding residual stress is large, and the post-welding stress and deformation are large. During the welding process, hot cracks and cold cracks are prone to occur. Before the thick plate is welded, the plate temperature is relatively low. At the beginning of welding, the temperature of the arc will be increased to 1250～1300℃. When the plate temperature changes suddenly, the temperature distribution of the thick plate is uneven, the weld metal becomes brittle, and the tendency to produce cold cracks increases. In order to avoid such problems, the thick plate must be heated before welding. In the actual manufacturing process, the welding process should be controlled to prevent welding cracks.
Positioned welding is the most problematic part of the thick plate construction. When the thick plate is positioned welded, the temperature will be quickly cooled by the surrounding "cooling medium", which will cause local excessive stress concentration, and result in cracks and damage to the material. The solution is to increase the preheating temperature, the length of the tack weld, and the size of the welding foot when the thick plate is positioned welding.
Multilayer and Multi-pass Welding
In the process of thick plate welding, an important process principle is a multilayer and multi-pass welding. A single-pass cannot fill the groove in the section As the groove of the thick plate weld is large. The consequence of wide-pass welding is that the base metal’s restraint stress to weld line is pretty large, and the weld strength is relatively weak, which is prone to appear weld crack or delayed crack. The advantage of multilayer and multi-pass welding is that the previous weld is a "preheating" process for the next one, and the latter weld is equivalent to a "post-heat treatment" process for the previous one. The stress distribution state during the welding process is effectively improved, which helps to ensure the welding quality.
Inspection During Welding
Thick plate welding is different from medium and thin plates. It takes several hours or even dozens of hours to complete a component’s welding. Therefore, it is particularly important to strengthen the intermediate inspection of the welding process. In order to find out problems timely, construction and inspection should be carried out simultaneously.
After the thick plate’s butt welding, the welding seam and the local base material within the range of 100-150mm on both sides should be heated immediately, and heated by infrared electric heating plate. After heating the temperature to 250～350℃, use asbestos to spread the cover for heat preservation for 2～6h, and then cooled by air. Such post-heat treatment can prevent cracks from appearing in the weld line and the heat-affected zone.
Control of Welding Deformation and Welding Stress of Thick Plate
In the welding process, the deformation after thick plates’ butt welding is mainly angular deformation. In order to control the deformation in actual production, a part of the weld bead on the front side is often welded firstly, then turn over the part and weld the weld channel bead on the reverse side after the carbon is cleaned out of the root, then turned over the part again and so on. Generally speaking, the parts could be turned over after three to five welding processes until all the welds on the front are fully welded.
In addition, set up tire mold fixtures to constrain the components to control deformation. This method is generally suitable for special-shaped thick plate structures. Due to the singular shape, cross-section, and cross-sectional dimensions of the thick plate structure, it is difficult to guarantee the dimensional accuracy in the free state. Therefore, it is necessary to make a tire mold fixture according to the shape of the component, then assemble, locate, and weld the component in a fixed state to control the welding deformation.
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