Reasons and preventive measures for local deformation in welding
1.1 Causes
The rigidity of the processed parts is small or uneven, and there is shrinkage and deformation after welding; The uneven arrangement of welds in the processed parts results in uneven shrinkage, with larger shrinkage and deformation in areas with more welds; Improper operation by processing personnel, failure to symmetrically layer, segment, and intermittently weld, inconsistent welding current, speed, and direction, resulting in inconsistent deformation of the processed parts; Excessive biting during welding can cause stress concentration and excessive deformation; Uneven welding placement can cause deformation when stress concentration is released.
1.2 Preventive measures
When designing, try to evenly distribute the stiffness and welds of each part of the workpiece, and symmetrically set welds to reduce cross and dense welds; Develop a reasonable welding sequence to reduce deformation. If the main weld seam is welded first and then the secondary weld seam, the symmetrical weld seam is welded first and then the asymmetrical weld seam, the weld seam with large shrinkage is welded first and then the weld seam with small shrinkage is welded, and the butt weld seam is welded first and then the fillet weld seam is welded; For workpieces with large dimensions and multiple welds, segmented, layered, and intermittent welding should be used, and the current, speed, and direction should be controlled to be consistent; When manually welding longer weld seams, the intermittent welding method should be used in sections, with welding from the middle of the workpiece to both ends. During welding, personnel should be symmetrically dispersed to avoid deformation caused by heat concentration; For large workpieces with asymmetric shapes, after welding and correcting the deformation of small components, assembly welding should be carried out to reduce overall deformation; The workpiece should be frequently flipped during welding to counteract the deformation; For components that are prone to angular deformation after welding, pre deformation treatment should be carried out before welding, such as butt welding of V-shaped steel plates. The interface should be appropriately raised before welding to make it flat after welding; By using external welding reinforcement to increase the rigidity of the workpiece and limit welding deformation, the position of the reinforcement should be set on the opposite side of the shrinkage stress.
1.3 Processing methods
For deformed workpieces, if the deformation is not significant, fire baking correction can be used. If there is significant deformation, use the method of baking while using a jack to correct it.
Reasons and preventive measures for welding cracks in steel structures
2.1 Hot Cracks
Hot cracking refers to cracks that occur at high temperatures, also known as high-temperature cracks or crystalline cracks. It usually occurs inside the weld seam and may also appear in the heat affected zone. Its manifestations include longitudinal cracks, transverse cracks, root cracks, arc pit cracks, and heat affected zone cracks. The reason for its occurrence is due to the segregation phenomenon in the welding pool during the crystallization process. Low melting point eutectic and impurities exist in the form of liquid interlayers during the crystallization process, forming segregation. After solidification, the strength is also low. When the welding stress is large enough, the liquid interlayers or newly solidified solid metals will be pulled apart to form cracks. In addition, if there are low melting point eutectics and impurities on the grain boundaries of the base material, they will also be pulled apart when the welding tensile stress is large enough. In short, the occurrence of hot cracks is the result of the combined action of metallurgical and mechanical factors. The preventive measures for its causes are as follows:
Limit the content of easily segregated elements and harmful impurities in the base metal and welding materials (including welding rods, welding wires, fluxes, and shielding gases), especially controlling the content of sulfur and phosphorus and reducing carbon content. Generally, the sulfur content in steel used for welding should not exceed 0.045%, and the phosphorus content should not exceed 0.055%; In addition, the higher the carbon content in steel, the worse the welding performance. Generally, when the carbon content in the weld is controlled below 0.10%, the sensitivity to hot cracks can be greatly reduced; Adjust the chemical composition of the weld metal, improve the weld microstructure, refine the weld particle size to enhance its plasticity, reduce or disperse the degree of segregation, and control the harmful effects of low melting point common products; Using alkaline welding rods or fluxes to reduce the impurity content in the weld and improve the degree of segregation during crystallization; Properly increasing the shape factor of the weld seam and using multi-layer and multi pass welding methods can avoid centerline segregation and prevent centerline cracks; Adopting a reasonable welding sequence and direction, using smaller welding lines, overall preheating and hammering methods, filling the arc pit during arc extinguishing and other process measures
2.2 Cold Cracks
Cold cracking generally refers to the phenomenon where the temperature of a weld seam drops to the martensitic transformation temperature range (below 300-200 ℃) during the cooling process. It can occur immediately after welding or a longer period of time after welding, hence it is also known as delayed cracking. There are three basic conditions for its formation: the formation of a hardened structure in the welded joint; The existence and concentration of diffused hydrogen; There is a significant welding tensile stress. The main preventive measures include:
Choose reasonable welding specifications and line energy to improve the microstructure of the weld and heat affected zone, such as preheating before welding, controlling interlayer temperature, slow cooling or post heating after welding, etc., to accelerate the escape of hydrogen molecules; Using alkaline welding rods or fluxes to reduce the diffusion oxygen content in the weld seam; Welding rods and fluxes should be strictly dried according to the prescribed requirements before use (low hydrogen welding rods should be kept at 300 ℃~350 ℃ for 1 hour; acidic welding rods should be kept at 100 ℃~150 ℃ for 1 hour; flux should be kept at 200 ℃~250 ℃ for 2 hours), and the groove and welding wire should be carefully cleaned to remove dirt such as oil, moisture, and rust, in order to reduce the source of hydrogen; Timely heat treatment after welding One is to conduct annealing treatment to eliminate internal stress, temper the quenched structure, and improve its toughness; The second is to carry out hydrogen elimination treatment to allow hydrogen to fully escape from the welded joint; Improve steel quality and reduce layered inclusions in steel; Take various process measures that can reduce welding stress.
Related issues in welding inspection of steel structures
3.1 Differences and connections between weld grade, inspection grade, and evaluation grade
The welds that require internal quality inspection are classified into first and second levels according to their quality grades, known as first level welds and second level welds, which are the weld grades.
The inspection level refers to the accuracy achieved by the inspection and testing, that is, the degree of precision of the detection results obtained by combining the testing instrument and the testing method. Ultrasonic testing adopts the GB/T11345-1989 standard, which is divided into three levels of A, B, and C according to the detection level from low to high. Radiographic testing adopts the GB/T3323-1987 standard, which is divided into three levels of A, AB, and B according to the detection level from low to high. They respectively specify the detection method, detection surface, detection range, and allowable defect equivalent (dB value) of manual ultrasonic testing, as well as the sensitivity to be achieved by radiographic testing (the relationship between radiographic thickness and image quality indicator).
The evaluation level refers to the internal quality level of the weld seam determined by the flaw detection personnel based on the defect measurement according to the standard after detecting the defect. Specifically, ultrasonic testing refers to measuring the length of defects with wave heights between the measuring line and the scrap line (Zone II), and then grading the defects according to Table 6 of the standard GB/T11345-1989; Radiographic inspection refers to measuring the length and size of defect indications on the film, and comprehensively rating them according to Table 6, Table 7, Table 9, and Table 10 of the standard GB/T3323-1987 (see 16.1-16.4 of this standard). This is a requirement that every inspector must be proficient in.
3.2 Handling, re exploration, and expansion of defects exceeding the standard
GB 50205 "Code for Acceptance of Construction Quality of Steel Structures" only specifies the testing methods, testing ratios, and qualification levels, without clear requirements for defect handling. According to JG 181 "Technical Specification for Welding of Steel Structures in Buildings" and other industry welding inspection standards and specifications, the following measures can be taken for detected defects:
Defects that are not allowed to be detected must be repaired. After repair, the weld seam is considered qualified only after passing the same testing method.
For welds that require spot checks and inspections, if no defects are found, an expanded inspection should be conducted in an area of 10% of the entire weld length at both ends of the inspected area and not less than 200mm (when length is allowed).
a. If no excessive defects are found in the expanded inspection area, the weld seam should be considered qualified. b. If excessive defects are found in the expanded inspection area, the entire weld seam shall be inspected.
For weld seams that require spot checks during on-site installation, if defects are found that are not allowed, expansion inspection shall be carried out according to the following principles:;
a. Add two welds of the same type welded by the same welder for inspection. If no excessive defects are found in these two expanded welds, the batch of welds should be considered qualified. b. If excessive defects are found in these two expanded weld seams, then each weld seam containing excessive defects shall be sampled again according to the above principles. c. If no excessive defects are found in the re sampled welds, the batch of welds should be considered qualified. d. If any defects exceeding the standard are still found in the welds sampled again, the welder shall conduct a full inspection of the type of weld they welded. Meanwhile, it is possible to negotiate an appropriate increase in the proportion of other weld seam inspections.