With the rapid development of high-rise buildings, steel structures are widely used in residential buildings. Steel structure is also used in space long-span steel structure, light steel structure and steel-concrete composite structure and other building structures. As a new type of building structure, steel structure has a huge impact on the housing construction in our country. Together with the concrete structure, steel structure has become an indispensable part of the building structure. Control the quality of steel structure fabrication and installation, and promote the steel structure to meet the needs of modern architecture.
1、 Causes of welding deformation in steel structure processing
1.1 effect of welding stress
In the welding process, steel structure parts are heated and cooled rapidly by high temperature welding of welding materials. In this process, the welding stress of steel structure parts will spread rapidly with the change of temperature. In the diffusion process, due to the influence of various factors, it will not be distributed on the steel structure. In the welding process, the welding deformation and deformation degree of steel structure determine the service life of rail components and affect the production cost of rail components.
1.2 influence of welding process
Welding process is an important factor to determine the welding deformation size. In the welding process, the same material and equipment, under the operation of different workers, will produce different degrees of welding deformation. It can be seen that workers' experience in welding process has a great influence on welding deformation and welding deformation. The application degree of welding technology, such as preheating time of rail components, can only be determined by considering a series of complex factors, such as the temperature and illumination of a day; The contact area between welding material catalyst and rail components in the process of TS welding and the fixing process of steel structure members, the impact degree of stress on steel structure is different in the welding process, which will affect the welding deformation.
1.3 impact on welding materials
The influence of welding material properties on welding deformation is also an important factor. Welding process is a kind of technology of heating steel structure parts rapidly. The transfer degree of thermal stress in the heating process is the key factor affecting the welding deformation. The non-standard preheating temperature of welding materials and the thermal conductivity of welding materials directly determine the degree of welding deformation
2、 Control method of steel structure processing and welding
2.1 factors to be considered in welding
(1) Reasonable optimization of welding process. There are three manufacturing processes in the welding process of steel structure parts, which are preheating, processing and correction in turn. Among them, the common preheating methods are rigid fixation method, pre stretching method and reverse deformation method. Different methods should be adopted according to different steel structures. For example, the deformation of large steel structure parts in the welding process has a great impact on the overall product. The reverse deformation must be used during preheating. The deformation amount is calculated in advance according to the material characteristics, and the reverse deformation compensation is conducted before preheating. The best choice to prevent welding deformation is to eliminate the deformation in the welding process; the steel structure parts sensitive to thermal stress induction are generally determined by the size of the steel structure parts, and the pre stretching method is suitable for large steel structures. Aluminum parts; rigid fixation method is suitable for small steel structures. In the welding process, most of the methods are focused on how to reduce the welding stress, such as preheating each welding material before welding. The time and degree of preheating need to be considered in combination with the temperature of the day and different welding materials. In order to achieve uniform stress transfer in the welding process.
2.2 introduction of welding method
At present, the commonly used welding methods are: electrode arc welding, whose principle is to manually operate the welding electrode; submerged arc welding, whose principle is to use the arc to burn under the flux layer; the principle of carbon dioxide gas shielded welding is to use carbon dioxide gas shielded welding. The principle of MIG / MAG welding is mining. An arc welding method with inert gas as shielding gas and welding wire as melting electrode is introduced. The principle of plasma arc welding is to obtain high energy density plasma arc through the restriction of water-cooled nozzle on the arc.
3、 Technical measures to control and solve welding deformation of steel structure
3.1 in the steel structure welding construction, the key process is to control the welding deformation, which is the rigid deformation or plastic deformation caused by the non-linear temperature change in the welding process. The welding deformation will seriously affect the fatigue strength and toughness of the welded joint and reduce its corrosion resistance. In order to avoid welding deformation, the following measures should be taken: (1) reasonable arrangement of welding groove to reduce welding deformation; (2) reasonable arrangement of welding process to avoid stress concentration; (3) corresponding anti deformation measures should be taken in fillet weld welding, such as synchronous symmetrical welding technology for symmetrical components; (4) The welding stress can be reduced by selecting suitable welding materials and adopting proper welding process.
When using low temperature welding technology to weld steel structure, due to the different preheating temperature, the welding strength of different parts is different, especially in the welding of thick plate, the lower temperature will greatly affect the strength index of steel plate, which is because the hardness of steel is very low under low temperature conditions, which is very easy to produce embrittlement. From the microscopic point of view, because the low temperature will accelerate the cooling rate of the weld, a brittle structure called martensite is produced between the weld and the heat affected zone. Therefore, when using low temperature welding, we should pay special attention to the influence of ambient temperature on welding.
3.2 select the correct welding process form, material and process
In the selection of welding joint groove form, in order to make the arc fully penetrated to the root of the weld, it is necessary to open a groove on the joint. When selecting the shape of the groove, the groove form that is easy to process should be selected, and the weld can be completely welded, so as to reduce the welding cost and reduce the possibility of deformation after welding.
In the selection of welding materials, the specific chemical composition, physical and mechanical properties, joint form and other factors should be fully considered. If the welding materials are not selected correctly, it is easy to cause brittle fracture in the transition zone of the weld. Therefore, in ultra-low temperature welding, low hydrogen alkaline or titanium calcium type electrode should be selected as far as possible under the premise of ensuring the design strength, because its yield strength is lower and impact toughness is better; at the same time, preheat the electrode to improve the fatigue strength of the electrode.