Thin plate structural members generally refer to structural members made of steel plates (including stainless steel plate, galvanized plate and white iron sheet) with a thickness of no more than 4 mm. For example, the roller drill shed, cab and electric shovel cab produced by our factory belong to this category. The control and correction of welding deformation of thin plate structural parts need advanced technology. The following will discuss the control and correction of welding deformation of lower thin plate structural parts together with the quick screening and editing.
1、 Causes of welding deformation
Arc welding is an uneven and rapid heating and cooling process. During and after welding, the welding components will be deformed. The most fundamental factors affecting the welding deformation are the thermal deformation and the rigidity condition of the welding components. The thermal deformation in the welding process is constrained by the rigid condition of the component, and the compressive plastic deformation occurs, which results in the welding residual deformation.
（1） Factors affecting welding thermal deformation
Welding process method. Different welding methods will produce different temperature fields and different thermal deformation. Generally speaking, automatic welding is more concentrated than manual welding, with narrow heating zone and less deformation. CO2 gas shielded welding wire is thin, with high current density, concentrated heating and small deformation.
2. Welding parameters. Arc voltage, and welding speed. The larger the linear energy, the larger 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 arc voltage has an obvious effect, so the deformation of automatic welding with low voltage, high speed and high current density is small.
3. Number of welds and section size. 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 thermal deformation is also different. Generally, the deformation of continuous welding is large, and the deformation of intermittent welding is minimum.
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. With the increase of component rigidity, the welding deformation is smaller. 2. Application of tire clamp. The use of the jig increases the rigidity of the components, thus reducing the welding deformation.
3 assembly welding procedure. The assembly welding procedure can cause the change of the rigidity and the position of the center of gravity of the component in different assembly stages, and has a great influence on controlling the welding deformation of the component.
Generally speaking, the welding deformation of welded members is large under the condition of small constraint, and on the contrary, the deformation is small.
2、 Types of welding deformation of thin plate structure
Welding deformation of any steel structure can be divided into overall deformation and local deformation. Overall deformation refers to the change of the size or shape of the whole member after welding, including longitudinal and transverse shrinkage (total size reduction), bending deformation (middle arch, middle sag) and twisting deformation. Local deformation refers to the deformation in the local area of the component after welding, including angular deformation and wave deformation.
3、 Principle and method of controlling welding deformation of thin plate structure
The thermal deformation during welding and the rigidity condition of welding components during welding are two main factors that affect the welding residual deformation. According to these two main factors, it can be considered that welding residual deformation is inevitable, that is, it is impossible to completely eliminate welding deformation. To control welding residual deformation, measures must be taken at the same time from two aspects of thin plate structure design and construction technology.
In the design of thin plate structural parts, in addition to meeting the strength and service performance of components, it must also meet the requirements of minimum welding deformation and minimum labor time consumption in component manufacturing. Therefore, it is particularly important to optimize the plate seam layout. The plate seam layout in the design drawings often does not give full consideration to the process, which is easy to cause welding deformation.
Welding technology is one of the important technologies in steel structure construction. Reasonable welding process is an effective method to reduce welding deformation and stress concentration.
In order to control the welding deformation of components, effective measures should be taken as much as possible, such as: dividing the components into several small parts and component sections, so that the welding deformation is dispersed on each component, so as to facilitate the control and correction of component deformation; Make the arrangement of the welds of each component symmetrical with or close to the neutral axis of the sectional section of the component to avoid distortion and excessive bending deformation after welding; For each main weld, the weld leg size and short weld shall be selected as much as possible; Avoid excessive concentration and cross arrangement of welds; Wide and long steel plates or structural forms that can reduce the number of welds shall be used as much as possible.
The process methods for controlling welding deformation of thin plate structural parts include:
（1） Assembly of components without assembly stress;
（2） Automatic welding and other gas shielded welding processes are adopted; Such as the most advanced
Ar + CO2 mixed gas MAG shielded welding.
（3） Reasonably select welding specification parameters and assembly welding sequence. Reduce welding wire supply
Dosing: reduce the current and voltage and change the polarity (usually DC reverse polarity → DC positive polarity). The short weld shall be welded first and then the long weld shall be welded. The back welding shall be carried out in sections from the inside to the outside.
（4） The rigid fixation method and the anti deformation method shall be reasonably used as far as possible.
4、 Correction of welding deformation of thin plate structure
In the construction process of steel structure, although measures are taken to control welding deformation in the design and construction process of its components, welding deformation is still inevitable due to the characteristics of the welding process and the complexity of the construction process. Therefore, the welding deformation exceeding the design requirements must be corrected.
The correction process is only limited to correcting the local deformation of 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 reduction), it can only be compensated by the pre setting allowance during blanking or assembly.
The use of mechanical correction method to correct the steel structure is easy to cause metal cold work hardening, and a certain amount of plastic reserves of materials are consumed, which can only be used for materials with good plasticity. In actual production, special large-scale hydraulic press and friction press may be used in the process of mechanical correction.
The flame correction method is used to correct the steel structure. After the correction and cooling, the part of the metal of the welded member will obtain irreversible compressive plastic deformation, and the deformation of the whole welded member will be corrected. The flame correction method also consumes part of the plasticity of the material, and the brittle material or the material with poor plasticity should be used with caution. The temperature of flame heating shall be properly controlled. If the temperature is too high, the mechanical property of the material will be reduced, and if the temperature is too low, the correction efficiency will be reduced. Since the cooling speed has no effect on the correction effect, the method of water cooling while heating is adopted in the construction process, which not only improves the work efficiency but also improves the correction effect.
To sum up, welding deformation is inevitable in the process of steel structure manufacturing. Only effective methods and measures can be taken to control the welding deformation and correct the welding deformation exceeding the tolerance requirements, so as to meet both the quality requirements of steel structure and the economic requirements.