What are the problems in the welding of steel frame structure?
Reasons and preventive measures of local deformation in welding of steel frame structure
1. Cause
The rigidity of the workpiece is small or uneven, and the shrinkage and degeneration are inconsistent after welding.
The welding seam of the workpiece itself is unevenly arranged, resulting in uneven shrinkage, and the parts with many welds have large shrinkage and large deformation.
Improper operation of the processing personnel, unsymmetrical layering, segmenting, intermittent welding, inconsistent welding current, speed, and direction, resulting in inconsistent deformation of the workpiece.
When the steel structure is welded, the bite is too large, causing welding stress concentration and excessive deformation.
The welding of the steel frame structure is not flat, and the deformation is caused when the stress concentration is released.
2. Precaution
When designing the steel structure of the building, try to make the rigidity and welding seam of each part of the workpiece evenly arranged, and set the welding seam symmetrically to reduce the intersection and dense welding seam.
Make a reasonable welding sequence to reduce deformation. For example, weld the main weld first and then weld the secondary weld, and weld the symmetrical part of the weld first and then weld the asymmetrical weld. Weld the weld with large shrinkage first, then weld the weld with small shrinkage, weld the butt weld first, and then weld the fillet weld.
For workpieces with large size welds, segmented, layered, intermittent welding is used, and the current, speed and direction are controlled to be consistent.
When manually welding long welds, the intermittent welding method should be used in sections, and the welding should be removed from the middle of the workpiece to both ends. During welding, the personnel should be distributed symmetrically to avoid deformation due to heat concentration.
If the shape of the large workpiece is asymmetrical, the small parts should be assembled and welded after the deformation is corrected, so as to reduce the overall deformation.
The workpiece should be flipped frequently during welding so that the deformations cancel each other out.
For parts that are prone to angular deformation after welding, pre-deformation treatment should be carried out before welding, such as V-groove butt of steel plates. Before welding, the interface should be properly elevated, so that it can be flattened after welding; the rigidity of the workpiece is increased by external welding reinforcement to limit the welding deformation, and the position of the reinforcement should be set on the opposite side of the shrinkage stress.
3. Approach
For the deformed workpiece, if the deformation is not large, it can be corrected by fire roasting. If the deformation is large, use the method of using a jack to correct it while baking.
Reasons and Preventive Measures of Welding Cracks in Steel Frame Structure
1. Hot crack
Hot cracks refer to cracks generated at high temperatures, also known as high temperature cracks or crystalline cracks. Usually occurs inside the weld, and may sometimes appear in the heat-affected zone. The manifestations are: longitudinal cracks, transverse cracks, root cracks, crater cracks and heat-affected zone cracks.
The reason for this is that there is segregation in the welding pool during the crystallization process. The low melting point eutectic and impurities exist in the form of liquid interlayer during the crystallization process to form segregation, and the strength is also low after solidification. When the welding stress is large enough, the liquid interlayer or the solid metal that has just solidified will be pulled apart to form cracks.
In addition, if there are low-melting eutectic and impurities on the grain boundaries of the base metal, they will also be pulled apart when the welding tensile stress is large enough. In short, the generation of hot cracks is the result of the combined action of metallurgical and mechanical factors. For its causes, the preventive measures are as follows:
Limit the content of easily segregated elements and harmful impurities in the base metal and welding materials (including electrodes, wires, fluxes and shielding gases). In particular, the content of sulfur and phosphorus should be controlled and the carbon content should be reduced. Generally, the content of sulfur in the steel used for welding should not be greater than 0.045%, and the content of phosphorus should not be greater than 0.055%.
In addition, the farther the carbon content of the steel is, the worse the welding performance is. Generally, when the carbon content in the weld is controlled below 0.10%, the thermal crack sensitivity can be greatly reduced.
Adjust the chemical composition of the weld metal, improve the weld structure, and refine the weld grain to improve its plasticity, reduce or disperse the degree of segregation, and control the harmful effects of low melting point co-products.
Use alkaline electrodes or fluxes to reduce the inclusion of impurities in the weld and improve the degree of segregation during crystallization.
Appropriately increase the shape factor of the weld, and use a multi-layer multi-pass welding method to avoid centerline segregation and prevent centerline cracks.
Adopt reasonable welding sequence and direction, adopt smaller welding line energy, overall preheating and hammering method, and fill the arc crater when ending the arc.
2. Cold cracks
Cold cracks generally refer to the fact that the temperature of the welding seam drops to the martensitic transformation temperature range (below 300-200 °C) during the cooling process. It can appear immediately after welding, or it can occur for a long time after welding. It is also called delayed cracking. There are three basic conditions for its formation: the welded joint forms a hardened structure, the existence and concentration of diffusible hydrogen, and there is a large welding tensile stress. The main preventive measures are:
Select a reasonable welding specification and line energy to improve the microstructure of the weld and the heat affected zone, such as preheating before welding, controlling the temperature between layers, slow cooling or postheating after welding, etc. to accelerate the escape of hydrogen molecules.
Use alkaline electrodes or fluxes to reduce diffusive oxygen content in the weld.
The welding rod and flux should be dried in strict accordance with the specified requirements before use (low hydrogen electrode is kept at 300℃～350℃ for 1h; acid electrode is kept at 100℃～150℃ for 1h; flux is kept at 200℃～250℃ for 2h). Carefully clean the groove and welding wire to remove dirt such as oil, moisture and rust to reduce the source of hydrogen.
Heat treatment should be carried out in time after welding. One is to carry out annealing treatment to eliminate internal stress, temper the quenched structure and improve its toughness. The second is to carry out hydrogen elimination treatment, so that hydrogen can fully escape from the welded joint; improve the quality of steel and reduce the layered inclusions in the steel. Take various technological measures to reduce welding stress.
Related Problems in Welding Inspection of Steel Frame Structure
1. The difference and connection of weld grade, inspection grade and evaluation grade
Welds that require internal quality inspection are classified into first-level and second-level according to the quality level, called first-level welds and second-level welds, which are weld levels.
The inspection level of steel structure welds refers to the accuracy achieved by inspection and testing, that is, the accuracy of the inspection results obtained by the combination of inspection instruments and inspection methods. Ultrasonic flaw detection adopts GB/T11345-1989 standard and is divided into three levels: A, B and C according to the detection level from low to high. Radiographic flaw detection adopts GB/T3323-1987 standard and is divided into A, AB, B according to the detection level from low to high. three levels. They respectively stipulate the detection method, detection surface, detection range and allowable defect equivalent (dB value) of manual ultrasonic flaw detection, as well as the sensitivity (relationship between transillumination thickness and image quality meter) to be achieved by radiographic flaw detection.
The evaluation level refers to the internal quality level of the weld determined by the flaw detector after the flaw is detected according to the standard. Specifically, ultrasonic flaw detection refers to the defect grading according to the standard GB/T11345-1989 Table 6 after the wave height is measured between the length measurement line and the waste judgment line (zone II). Radiographic inspection refers to measuring the length and size of the defect indication on the negative film, which every inspector must be proficient in.
2. Excessive defect treatment and re-examination and expansion
The detected non-permissible defects must be repaired. After repairing, the welding grid will be considered as qualified after the inspection according to the same inspection method.
For welds that require random inspection, if no allowable defects are found, the inspection shall be expanded in areas of 10% of the length of the entire weld at both ends of the inspected area and not less than 200mm (when the length is allowable).
If no out-of-standard defects are found in the extended inspection area, the welded grid shall be considered. If defects exceeding the standard are found in the expanded inspection area, the welding seam shall be fully inspected.
For welds that require random inspection for on-site installation, if defects are found that are not allowed, the inspection shall be expanded according to the following principles.
a. Increase the inspection of two welds welded by the same welder of this type. If no defects beyond the standard are found in these two expanded inspection welds, the batch of welded seams should be considered.
b. If defects exceeding the standard are found in these two expanded inspection welds, each weld containing excessive defects shall be sampled for two more welds according to the above principles.
c. If no defect exceeding the standard is found in the welded seam of the re-sampling inspection, the batch of welded seam grids shall be considered.
d. If the welds that are sampled again are still found to have defects exceeding the standard, the welders of this type welded by the welder shall be fully inspected. At the same time, it can be negotiated to appropriately increase the inspection ratio of the remaining welds.
