It is the key and difficult point of engineering quality control that the steel structure with thick plate has large components, thick steel plate, large amount of welded metal, complex joints, large residual stress and easy to crack, which affect the welding quality.How to Prevent Cracks:
1, thermal crack prevention measures
1) Control the chemical composition of the weld.Reduce the content of low melting point eutectic elements, such as sulfur and phosphorus, which are easy to segregate in base metal and welding material.The tendency of hot cracking can be reduced by reducing the content of carbon in steel.Increasing the content of Mn can improve the thermal cracking resistance of the weld.
2) Control welding process parameters and conditions.The thermal cracking resistance can be improved by controlling the welding current and welding speed to make the ratio of width to depth at the upper part of each weld pass reach 1.1 ~ 1.2. Meanwhile, the shape of welding pool can be controlled to avoid the formation of sharp long weld pool and the formation of longitudinal thermal cracks on the weld surface.Preheating before welding can reduce the tendency of forming cracks.Reasonable welding sequence can make most of the welds to be welded with a small degree of confinement, reduce the tensile stress during weld shrinkage, and also reduce the tendency of hot crack.
2, cold crack prevention measures
1) Carry out process evaluation test before welding to determine process parameters.
2) Preheating before welding can prevent cracks and improve performance to a certain extent.
3) Manual welding using alkaline low hydrogen electrode E5Q/E55 type;CO: Flux-cored wire can be used for gas shielded welding;F5021 - H08MNA welding electrode can be used for submerged arc welding, and drying in strict accordance with the provisions.The drying temperature is 350-450 ℃. After drying, it is kept for 1 ~ 2h at the temperature of 100-150cc. Then, it is put in the insulation cylinder for use and take as needed.
4) Choose a reasonable weld shape and process the parts groove strictly according to the drawing.
5) Clean up rust and water on both sides of the weld within the range of 30 ~ 50mm to reduce the source of hydrogen.
6) Cracks tend to appear in the head weld and welding root, so the positioning welding length, welding foot height and spacing should also be made corresponding provisions.If the positioning weld has to be carried out in the groove, the weld height should be less than 2/3 of the groove depth, and the length should be greater than 40mm.Preheat before location welding if necessary.Check the location weld before welding. If there is a crack, it must be cleaned and rewelded.The length and spacing of location welding shall depend on the thickness of base metal and the length of structure.
7) In order to reduce the internal stress and prevent cracks during welding, forced assembly should be avoided during assembly.
8) Increase the current appropriately to reduce the cooling rate, which helps to avoid the formation of hardened tissue.
9) Control the temperature between layers, which should be slightly higher than the preheating temperature.
Although the construction of thick plate steel structure is difficult, but as long as the control measures are appropriate, the proportion of component cracks will be greatly reduced, the engineering quality will be significantly improved, the weld cracks will be avoided, and industrial buildings can be effectively controlled
