Control Measures for Welding Cracks in Thick Plate Steel Structures
Thick-plate steel structural members are large, the steel plate is thick, the welding deposit is large, the joints are complex, the residual stress is large, and cracks are prone to occur, which affects the welding quality. This is the focus and difficulty of engineering quality control. The general measures in production are: 1. Thermal crack prevention measures: 1-1. Control the chemical composition of the weld. Reduce the content of elements that are easy to segregate, such as sulfur and phosphorus, in the base metal and welding materials that form low-melting eutectic. Reducing the carbon content in the steel can reduce the tendency of hot cracking; increasing the Mn content can improve the hot cracking resistance of the weld. 1-2. Control welding process parameters and conditions. Control the welding current and welding speed so that the width-to-depth ratio of the upper part of each weld bead section can reach 11 to 12, which can improve the thermal crack resistance, and control the shape of the welding pool at the same time. Longitudinal thermal cracks. Preheating before welding can reduce the tendency to form cracks; a reasonable welding sequence can make most welds welded with a small degree of restraint, reduce the tensile stress when the weld shrinks, and also reduce the tendency of hot cracks. 2. Measures to prevent cold cracks: 2-1. Carry out process qualification test before welding to determine process parameters. 2-2. Preheating before welding can prevent cracks and improve performance to a certain extent. 2-3. Alkaline low hydrogen electrode E5Q/E55 type is used for manual welding; CO: gas shielded welding can choose flux-cored wire; submerged arc welding can choose F5021-H08MnA electrode, and it is dried in strict accordance with the regulations. The drying temperature is 350—450℃, after drying at 100—Keep warm at 150cC for 1 to 2 hours, put it in a heat preservation cylinder and take it as needed. 2-4. Select a reasonable shape of the weld, and process the bevel of the parts strictly according to the drawings. 2-5. Remove rust and water within 30-50mm on both sides of the weld to reduce the source of hydrogen. 2-6. Cracks often appear on the first pass weld and weld root, so the length, height and interval of tack welding should also be specified accordingly. If the tack weld has to be carried out in the groove, the height of the weld should be less than 2/3 of the groove depth, and the length should be greater than 40mm. Preheat before tack welding if necessary. Check the tack welds before welding, and if there are cracks, they must be removed and re-welded. The length and spacing of tack welds should be determined by the thickness of the base metal and the length of the structure. 2-7. In order to reduce internal stress and prevent cracks during welding, forcible assembly should be avoided during assembly. 2-8. Appropriately increase the current and reduce the cooling rate, which helps to avoid the formation of hardened structures. 2-9. The temperature between layers should be controlled slightly higher than the preheating temperature. 2-10. Slow cooling should be carried out after low-temperature welding of Q345C. In order to slow down the cooling rate of the weld and the heat-affected zone and prevent the occurrence of cold cracks, the weldment should be kept warm in time, that is, the weldment after welding should be heated to 250 ~ 350 . , and use asbestos to keep warm for 2. Air cooling after 6h can reduce the hydrogen content in the weld and prevent cold cracks. The heating method and width are the same as the preheating before welding. Although the construction of thick-plate steel structures is difficult, as long as the control measures are appropriate, the proportion of cracks in the components will be greatly reduced, the quality of the project will be significantly improved, and the cracks in the welds will be avoided, which can effectively control the quality of industrial construction projects.