Thin-plate structural parts generally refer to structural parts welded by steel plates (including stainless steel plates, galvanized plates, and white iron sheets) with a thickness of not more than 4 mm. Controlling and correcting the welding deformation of thin-plate structural parts requires superb technology. The following is a discussion on the consensus we have reached, which is limited to the level and is for reference only.
A.Causes of Welding Deformation
(1) Factors affecting welding thermal deformation
1. Welding process method. Different welding methods will produce different temperature fields and different thermal deformations. Generally speaking, automatic welding is more concentrated in heating than manual welding, the heating area is narrower, and the deformation is smaller. CO2 gas shielded welding wire is thin, the current density is large, the heating is concentrated, and the deformation is small.
2. Welding parameters. Namely welding current, arc voltage and welding speed. The greater the line energy, the greater the welding deformation. The welding deformation increases with the increase of welding current and arc voltage, and decreases with the increase of welding speed. Among the three parameters, the effect of arc voltage is obvious, so the deformation of automatic welding with low voltage, high speed and high current density is small.
3. The number of welds and the size of the section. The larger the number of welds, the larger the section size and the larger the welding deformation.
4. Construction method. The temperature field of continuous welding and intermittent welding is different, and the resulting thermal deformation is also different. Usually continuous welding deformation is larger, intermittent welding deformation is the smallest.
5. Thermophysical properties of materials. Different materials have different thermal conductivity, specific heat and expansion coefficient, resulting in different thermal deformation and welding deformation.
(2) Factors affecting the rigidity coefficient of welded components
1 Size and shape of components. As the rigidity of the component increases, the welding deformation is smaller.
2 tire clamp application. The use of tire clamps increases the rigidity of the components, thereby reducing welding deformation.
3 Assembly welding procedure. The assembly welding procedure can cause the change of rigidity and the position of the center of gravity of the component in different assembly stages, which has a great influence on the welding deformation of the control component.
Generally speaking, under the condition of small restraint, the welding deformation of welded components is large, and vice versa, the deformation is small.
B. Types of Welding Deformation of Thin Plate Structure
The welding deformation of any steel structure can be divided into overall deformation and local deformation.
The overall deformation is the change in the size or shape of the entire component after welding, including longitudinal and transverse shrinkage (shortening of the overall size), bending deformation (middle arch, sag) and twisting deformation.
Local deformation refers to the deformation of the local area of the component after welding, including angular deformation and wave deformation.
C. Principles and methods for controlling welding deformation of thin-plate structure
Thermal deformation during welding and the rigidity of welded components during welding are the two main factors that affect the residual deformation of welding. Based on these two main factors, it can be considered that welding residual deformation is unavoidable, that is, complete elimination of welding deformation is unlikely.
To control the residual deformation of welding, measures must be taken from the design of thin-plate structural parts and the construction process at the same time.
In the design of thin-plate structural parts, in addition to meeting the strength and performance of the components, it must also meet the requirements of minimum welding deformation and minimum labor-hours in the manufacture of components. Therefore, it is particularly important to optimize the layout of the plate joints. The layout of the plate joints in the design drawings often does not consider the manufacturability carefully, which is easy to cause welding deformation.
Welding process is one of the important processes in steel structure construction. A reasonable welding process is an effective method to reduce welding deformation and stress concentration.
In order to control the welding deformation of the components, effective measures should be taken as much as possible, such as: dividing the components into several small parts and component segments, so that the welding deformation is dispersed on each component, which is convenient for the control and correction of the deformation of the components;
Make the arrangement of the welds of each component symmetrical with the neutral axis of the segmented section of the component, or close to the neutral axis of the section, to avoid distortion and excessive bending deformation after welding;
For each major weld, select the smallest weld leg size and short weld as possible;
Avoid excessive concentration and cross-arrangement of welds;
Use wide and long steel plates as much as possible or structures that reduce the number of welds, etc.
The process methods for controlling the welding deformation of thin-plate structural parts are as follows:
(1) Assemble the components without compulsion of assembly stress;
(2) Adopt automatic welding and other gas shielded welding processes: such as the most advanced Ar+CO2 mixed gas MAG shielded welding.
(3) Reasonable selection of welding specification parameters and assembly welding sequence. Reduce the wire supply, reduce the current, voltage, and change the polarity (usually DC reverse polarity → DC positive polarity). Weld the short seam first and then the long seam, and take the step-by-step de-welding process from the inside to the outside.
(4) Use the rigid fixation method and the anti-deformation method as reasonably as possible.
D. Correction of Welding Deformation of Thin Plate Structural Parts
During the construction of the steel structure, although measures are taken to control the welding deformation in its component design and construction process, due to the characteristics of the welding process and the complexity of the construction process, the welding deformation is still inevitable. Welding distortions required by the design must be corrected.
The correction process is limited to correcting the local deformation of the welded components, such as angular deformation, bending deformation, wave deformation, etc. For the overall deformation of the component structure, such as longitudinal and transverse shrinkage (total size shortening), it can only be pre-placed by blanking or assembly. allowance to compensate.
Using the mechanical straightening method to correct the steel structure is easy to cause cold work hardening of the metal, and consumes a certain amount of plastic reserves of the material, and can only be used for materials with good plasticity. In actual production, special large-scale hydraulic presses, friction Press Correction.
The steel structure is corrected by the flame straightening method. After the straightening and cooling, the metal of the welded component obtains irreversible compressive plastic deformation, so that the deformation of the entire welded component is corrected.
The flame straightening method also consumes a part of the plasticity of the material, and should be used with caution for brittle materials or materials with poor plasticity. It is necessary to properly control the temperature of the flame heating. If the temperature is too high, the mechanical properties of the material will be reduced, and if the temperature is too low, the correction efficiency will be reduced.
Since the cooling rate has no effect on the correction effect, the method of spraying water while heating is often used during the construction process, which not only improves the work efficiency, but also improves the correction effect.
