Causes and control methods of welding deformation of steel structures

Types of deformation

linear deformation

1. Longitudinal deformation: caused by longitudinal shrinkage of the weld;

2. Lateral deformation: caused by the lateral shrinkage of the weld;

Angular deformation

The upper layer of the fillet weld has a large amount of welding and a large amount of shrinkage. Therefore, the angular deformation is mainly caused by the uneven lateral shrinkage of the weld in its height direction.

Bending deformation

For T-shaped sections, the shrinkage of the weld seam has an eccentricity with respect to the center of gravity, thus causing the section to bend upward, so the bending deformation is caused by the longitudinal shrinkage of the eccentric weld seam.

torsional deformation

During the welding process of steel structures, some special structural forms may experience wavy or spiral deformation, which is called torsional deformation, and its causes are complex.

Factors affecting welding deformation

The main cause of welding deformation is due to the local uneven heating of the weldment during the welding process, as well as the subsequent uneven cooling and the structure itself or external rigid restraint, through factors such as force, temperature and structure, thereby causing Uneven shrinkage deformation occurs in the welded joint area.

1) Material factors: Mainly due to the physical properties of the material itself, especially the thermal expansion coefficient, yield limit and elastic modulus of the material. The larger the expansion coefficient, the greater the welding deformation of the material. As the elastic modulus increases, the welding deformation will decrease, while a larger yield limit will cause higher residual stress and increase the deformation. The expansion coefficient of stainless steel is greater than that of carbon steel, so the welding deformation tendency of stainless steel is greater than that of carbon steel for two materials of the same thickness.

2) Structural factors: The design of the welded structure has the most critical impact on welding deformation. The general principle is that as the degree of restraint increases, the welding residual stress increases and the welding deformation decreases accordingly.

1. Structural stiffness is the ability of a structure to resist tensile and bending deformation. It mainly depends on the cross-sectional shape and size of the structure.

2. When the steel structure is not very rigid, if the welds are arranged symmetrically in the structure and the welding procedures are reasonable, only linear shrinkage will occur; when the welds are arranged asymmetrically, bending deformation will also occur; the center of gravity of the weld section and the joint When the center of gravity of the section is at the same position, as long as the welding procedure is reasonable, only linear shortening will occur; when the center of gravity of the weld section deviates from the center of gravity of the joint section, angular deformation will also occur.

3) Process factors: The main influencing factors are welding method, welding heat input (current and voltage), component positioning or fixing method, welding sequence, and the use of welding fixtures. The biggest impact is the welding sequence.

1. Large welding current, thick electrode diameter and slow welding speed will cause large welding deformation; 2. The deformation of automatic welding is small, but when welding thick steel plates, the welding deformation of automatic welding is slightly larger than that of manual welding;

2. Multi-layer welding When the welding seam of the first layer shrinks the most, the shrinkage of the second and third layer of welding seams is 20% and 5~10% of the first layer respectively. The more layers, the greater the welding deformation;

3. Intermittent The shrinkage of welds is smaller than that of continuous welds;

4. The transverse shrinkage of butt welds is 2 to 4 times greater than the longitudinal shrinkage;

5. Improper welding sequence or failure to weld partial components first, and then assembly and welding are easy to occur. Large welding deformation.

6. Welding methods such as submerged arc automatic welding, manual arc welding and CO2 gas shielded welding generate different amounts of heat and cause different deformations.

Control of welding deformation

design measures

Reasonable selection of welding size and form

While ensuring the structural bearing capacity, use smaller weld sizes as much as possible to reduce the impact of welding heat input on material properties.

Reasonable selection of weld length and quantity

Whenever possible, profiles and stamping parts should be used; where there are many and dense welds, a cast-welded joint structure can be used to reduce the number of welds. In addition, appropriately increasing the thickness of the wall plate to reduce the number of ribs, or using a profiled structure instead of a rib structure can prevent the structural deformation of the thin plate.

Reasonably arrange the welding seam position

Arrange the welds to be symmetrical to the neutral axis of the section as much as possible, or make the welds close to the neutral axis, which has a good effect on reducing the deflection deformation of beams and columns.

Process measures

anti-deformation method

Using anti-deformation to control welding deformation is the most commonly used welding method. During assembly, based on process testing and construction experience, appropriate pre-deformation of the components in the opposite direction to the welding deformation is performed to control the welding deformation. This method needs to be tested in advance. According to the design requirements of the weld, a steel plate with the same material and specifications is selected to make a test piece for welding in advance, so that the weld form and weld leg height meet the design requirements. After the welding is completed, it is measured after cooling to the ambient temperature. For the deformation of the wing plate, the measured value is used as a parameter to suppress the deformation. The press presses out the deformation value on the center line of the wing plate, so that both ends of the wing plate are in an upturned state in advance to offset the welding deformation. Once welded, it will be just flat. This method requires a hydraulic press of corresponding tonnage.

margin method

When blanking, the actual length or width of the part should be appropriately larger than the design size to compensate for the shrinkage of the weldment. This method is suitable for preventing shrinkage and deformation of the weldment. When placing the assembly table, allow for shrinkage. Generally, 5mm is allowed when the length of the bending member is not greater than 24m, and 8mm is allowed when the length is greater than 24m.

Rigid fixation method

During welding, a fixture is installed on the platform or on the overlapping components to increase the rigidity before welding. In this way, the shrinkage deformation of heating and cooling during welding is limited by the external force of the fixed fixture. However, this method is only suitable for plasticity. Better low carbon structural steel and low alloy structural steel are not suitable for medium carbon steel and steel with worse weldability.

① Fix the weldment on the rigid platform (suitable for rigid fixation when splicing thin plates).

② Combine the weldments into a more rigid or symmetrical structure (suitable for the control of structures such as T-beams).

③ Use welding fixtures to increase the rigidity and restraint of the structure.

④Use temporary supports to increase structural restraint.

Choose a reasonable assembly and welding sequence

The platform for steel structure production and assembly should have a standard horizontal surface. The stiffness of the platform should ensure that the components do not lose temperature or sink under the pressure of their own weight, so as to ensure the straightness of the components. Small structures can be assembled in one go, fixed with tack welding, and then completed in one go in a suitable welding sequence.

① For large and complex welded structures, as long as conditions permit, divide it into several components with simple structures, weld them separately, and then perform final assembly. The bases at the ends of the trusses and roof trusses, and the skylight frame support plates of the roof trusses should be welded into components in advance, and then assembled to the roof trusses and trusses after correction. The welding sequence of the roof trusses and trusses is: first weld the outer sides of the upper and lower chord connecting plates. seam, then weld the inner seam of the upper and lower chord connecting plates, then weld the connecting plate and web plate welds, and finally weld the web rod, the pad between the upper chord and the lower chord. After one side of the truss is completely welded, turn it over and perform welding on the other side. The welding sequence is the same. When manual welding, an even number of welders should be used to weld symmetrically from the middle of the upper and lower strings to both ends at the same time. When assembling, in order to prevent excessive stress and deformation of components during the assembly process, the specifications or shapes of different types of parts should comply with the prescribed size and template requirements. It is not advisable to use large external force to force the assembly during assembly to prevent After welding, excessive restraint stress occurs in the components and deformation occurs.

②The weld being welded should be close to the neutral axis of the structural section.

③ For structures with asymmetrically arranged welds, the side with fewer welds should be welded first during assembly and welding.

④For structures with symmetrical cross-section arrangement, the assembly and welding sequence is first to assemble the whole and then weld. The diagonal welding method should be used to balance the deformation during welding. At the same time, a flip frame or rotating mold should be used to form a boat-shaped position weld. Otherwise, an even number of welders should use flat welding and overhead welding respectively, and weld from the middle to both ends.

⑤ When welding long welds (more than 1m), the direction and sequence shown in Figure 12 can be used to weld to reduce shrinkage deformation after welding.

Welding process measures

During welding construction, appropriate welding current, speed, direction, and sequence should be selected to reduce deformation. When welding metal components, the short ones should be welded first, and then the long ones; the vertical ones should be welded first, and then the flat ones; butt joints should be welded first, and then the overlap joints should be welded from the middle to both sides, and from the inside to the outside. Concentrated welds should use jump welding, and long welds should use segmented step-back welding and symmetrical welding.

Welding deformation correction method

When the degree of bending and twisting deformation of components exceeds the current steel structure specifications and design requirements, they must be corrected. The methods include: mechanical correction method, flame correction method and hybrid correction method. During construction, it can be reasonably selected according to the actual situation. When correcting, the following principles should be followed: overall first, then partial; first main, then secondary; first lower, then upper; first main parts, then auxiliary parts.

mechanical correction method

Mechanical correction method uses mechanical force to correct welding deformation. Special correction machines, or straightening machines, presses, jacks and various small machines are often used to press and correct the deformation of components. When correcting, place the deformed part of the component between the two supports, and slowly apply force on the protruding part of the component to correct it.

flame correction method

The principle of using flame correction is the same as that of welding deformation, but it is used in the opposite way. By inputting heat to the metal, the metal reaches a plastic state, thereby causing deformation. After the component is locally heated, it relies on the difference in expansion and contraction of the heating zone. , causing the component to deform in a predetermined direction to achieve the purpose of correction. When using flames to heat correction components, be sure to keep the components in a free state. Some components with a large self-weight must be lifted off the platform with a spreader after heating to prevent the friction generated by their own weight from hindering deformation and affecting the correction effect. Using the flame correction method, the lateral bending and camber of a 20m-long steel column can be corrected within 6mm, and the deflection under the wing plate can be controlled within 2mm, which is far lower than the specification requirements. However, with flame correction, it is difficult to quantitatively determine the heating location, heating temperature, time, area length, etc. in actual construction. It mainly relies on accumulated experience.

Using flame correction is convenient and quick, but you must pay attention to several basic essentials. First of all, the heating temperature must be controlled well, generally between 650 and 850°C; the heating temperature must be controlled under different environments and temperatures; when the deformation of components with large deformation cannot be completely eliminated by one heating, the original heating should be staggered. The hot spot is heated and corrected for the second time; a reasonable correction sequence is adopted, first correcting the unevenness and inclination of the wing plate, and then correcting the lateral bending and arching; during the correction process, the correction situation should be checked frequently with a ruler, thin steel wire, level, etc. , to prevent overcorrection and new deformations.

Hammering method

The hammering method can not only eliminate the residual stress of the welded joint, but can also be used to extend the metal in the weld and the compression plastic deformation area around it to eliminate the welding deformation. The hammering method is often used to correct the plate structure that is not too thick, but this method The disadvantages of this method are high labor intensity and poor surface quality.

Strong electromagnetic pulse correction method (electromagnetic hammer method)

This method uses the electromagnetic field impact force formed by strong electromagnetic pulse to produce deformation on the weldment that is opposite to the residual deformation to achieve the purpose of correction. Its working principle is that the high-voltage capacitor discharges through an electromagnetic hammer composed of a disc-shaped coil, and a strong pulsed electromagnetic field is induced between the coil and the work to form a relatively uniform pressure pulse for correction. The advantage of using this method for correction is that there will be no impact damage marks on the surface of the workpiece such as those caused by hammering, and the impact energy can be controlled. However, this method can only be used for thin-walled welded components of aluminum, copper and other materials with high conductivity. .

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