Causes of welding deformation
Local and uneven heating of the weldment during the welding process is the cause of welding stress and deformation. During welding, the weld seam and the metal in the heated area near the weld seam expand. Since the surrounding cooler metal prevents this expansion, compressive stress and plastic shrinkage deformation occur in the welding area, resulting in different degrees of lateral and longitudinal shrinkage. Due to the shrinkage in these two directions, various deformations of the welded structure are caused.
Factors Affecting Deformation of Welded Structures
1) The influence of the cross-sectional area of the weld: the cross-sectional area of the weld refers to the metal area within the range of the fusion line. The larger the weld area, the greater the plastic deformation caused by shrinkage during cooling.
2) The influence of welding heat input: In general, when the heat input is large, the range of the high-temperature heating zone is large, and the cooling rate is slow, which increases the plastic deformation zone of the joint, regardless of longitudinal, transverse or angular deformation. Influence. But in surface surfacing, when the heat input increases to a certain extent, because the temperature of the entire plate thickness approaches, even if the heat input continues to increase, the angular deformation will no longer increase, but will decrease.
3) The influence of preheating and interlayer temperature of the workpiece: the higher the preheating temperature and interlayer temperature, the equivalent to the increase of heat input, which slows down the cooling rate and increases the shrinkage deformation.
4) Influence of welding method: Among the several methods commonly used in building steel structure welding, except for electroslag welding, submerged arc welding has a large heat input, and under other conditions such as the same weld area, the shrinkage deformation is relatively large. The heat input of manual arc welding is centered, and the shrinkage deformation is smaller than that of submerged arc welding. The heat input of CO2 gas shielded welding is small, and the shrinkage deformation response is also small.
5) Influence of weld position on deformation: due to the asymmetry of weld position in the structure, the asymmetry of weld position will cause various deformations.
6) The influence of structural rigidity on welding deformation: the rigidity of the structure mainly depends on the shape of the structure and its cross-sectional size. The structure with less rigidity has large welding deformation; the structure with high rigidity has small deformation after welding.
7) The influence of assembly and welding specifications on welding deformation: due to the different assembly methods adopted, it also affects the deformation of the structure. After the overall assembly is completed and then welded, the deformation is generally smaller than that of welding while assembling.
Measures to prevent welding deformation
1. Thermal adjustment method
Reducing the width of the welding heat-affected zone and reducing the degree of uneven heating can reduce welding deformation.
1) Use high-energy welding methods, such as carbon dioxide gas shielded welding instead of electrode arc welding.
2) Multi-layer welding instead of single-layer welding.
3) Replace large diameter electrodes with small diameter electrodes.
4) Use small current fast non-swing welding instead of high current slow swing welding.
2. Rigid fixation method
Generally, workpieces with high rigidity have small deformation after welding. If the rigidity of the workpiece can be increased before welding, the deformation of the workpiece after welding can be reduced. This measure to prevent deformation is called rigid fixation. Ways to increase rigidity include fixtures, supports, use of special molds, temporary fixing of workpiece points on a rigid platform, and use of pressed iron.
3. Forced cooling method
Adopting forced cooling to reduce the width of the heated area can achieve the purpose of reducing welding deformation.
1) Immerse the workpiece around the weld in water.
2) Use a copper block to increase the heat loss of the workpiece.
4. Preheating method before welding
For materials with poor weldability, such as medium carbon steel and cast iron, preheating is usually used to reduce welding deformation.
5. Control sequence method
For the same welding structure, if different welding sequences are used, the post-weld deformation will be different. Adopting a symmetrical welding sequence can effectively reduce welding deformation, as shown in Figure 10-24.
(2) Welding sequence of long welds When welding long welds, welding sequences such as symmetrical welding, gradual back welding, segmental and gradual back welding, and skip welding should be adopted.
(3) Weld the welds with large shrinkage first because the shrinkage of butt welds is larger than that of fillet welds. If there are both butt welds and fillet welds in a structure, the butt welds should be welded first, and then welded Fillet welds.