Steel Pipe is widely used in all walks of life, the advantage of using Steel Pipe is the use of air in the formation of dense oxide protective layer of metal zinc to protect the internal steel structure. In the case of welding, scratches, due to the presence of Zn-Fe primary battery, relatively active zinc plating part can be used as sacrificial anode, delay the corrosion of steel, corrosion resistance is good. However, due to the presence of galvanized layer, it is easy to produce cracks, pores and slag in welding, and it is difficult to obtain good welding quality.
Steel Pipe is generally plated in a low-carbon steel layer of zinc, galvanized layer is generally 20um thick. Zinc has a melting point of 419 ° C and a boiling point of about 908 ° C. In welding, zinc melts into a liquid floating on the surface of the bath or at the root of the weld. Zinc in the iron has a large solid solubility, zinc liquid will penetrate deep along the grain boundary weld metal, low melting point zinc to form a "liquid metal embrittlement." At the same time, zinc and iron can form intermetallic brittle compounds, such as Fe3Zn10, FeZn10 and so on. These brittle phases reduce the plasticity of the weld metal and produce cracks under tensile stress. If the fillet welds, especially the fillet welds of the T-joints, are most prone to penetrating cracks. Steel Pipe welding, the groove surface and the edge of the zinc layer, under the arc heat, resulting in oxidation, melting, evaporation and evaporation of white dust and steam, easily lead to weld pores. ZnO, which is formed by oxidation, has a higher melting point of about 1800 ° C or more. If the parameters are too small during the welding process, ZnO
Slag, while Zn as a deoxidizer to produce FeO-MnO or FeO-MnO-SiO2 low melting point oxide slag. If you choose the welding specification is not appropriate, improper handling practices, it is easy to weld the edge of the galvanized layer melting and even expand the melting area, it is possible to damage the galvanized layer, especially in the elongated arc and large swing operation, Wide melting area, destruction of galvanized layer is more serious. At the same time, due to the evaporation of zinc, volatile a lot of white smoke, the human body has a stimulus, damage, therefore, choose to produce a low amount of dust and welding methods, materials must also consider the factors.
Steel Pipe welding methods are more common in Steel Pipe structure commonly used gas welding, manual arc welding, CO2 gas shielded welding, submerged arc welding, TIG welding and other methods.
Gas welding used in the past, commonly used in galvanized pipe welding, due to gas heat input is not concentrated, prone to defects, poor mechanical properties of the weld and other shortcomings, the current basic phase out in the installation industry. Gas welding on the galvanized layer damage.
CO2 gas shielded welding of Steel Pipe welding performance is good, when the use of appropriate welding specifications and match the protection of gas, welding materials, access to high-quality welded joints. The method is less used in engineering practice.
TIG welding arc energy concentration, less damage to the galvanized layer, and easy to form a good single-sided welding double-sided forming joints, is worth using a welding method, but the welding speed, slower, cost expensive.
Manual arc welding is the most commonly used in the pipeline installation of a welding method. In the case of the correct choice of electrodes, such as J421, J422, J423 and other titanium oxide and titanium calcium electrode welding, because these electrode coating contains a lot of rutile and ilmenite, the electrode melting rate is relatively large, relative Increased melting speed. If not in the swing conditions, but can destroy the melting edge of the bath and other galvanized layer, generally not to expand the melting area, can reduce the zinc liquid on the weld metal penetration; in the use of the correct method of operation and welding materials, Get the mechanical properties of the joint is better, there is no defect in the welding quality. As a result of manual arc welding relative to the TIG welding cheap, fast, with the operation of skilled welders, the use of manual arc welding process.
Steel Pipe pre-welding preparation and the general low-carbon steel is the same, need to pay attention to is to carefully handle the groove size and the nearby galvanized layer. For the penetration, the groove size should be appropriate, generally 60 ~ 65 °, to leave a certain gap, usually 1.5 ~ 2.5mm; in order to reduce the penetration of zinc on the weld, before welding, the groove can be galvanized Layer after cleaning and then welding. In the actual work, the use of a concentrated slope, not leaving blunt edge technology for centralized control, two welding process, reducing the possibility of not penetrating.
The electrode should be selected according to the base material of the galvanized pipe. Generally, the low carbon steel is considered to be easy to operate, and J422 is more common. Welding method: welding the first layer of welding in the weld, try to make the zinc layer melting and vaporization, evaporation and escape the weld, can greatly reduce the liquid zinc to stay in the weld. In the welding of the weld, the same layer in the first layer as much as possible so that the zinc layer melting and vaporization, evaporation and escape the weld, the method is the first part of the electrode forward out about 5 ~ 7mm or so, when the zinc layer Melt and then return to the original position to continue to apply welding. After welding to clean up the weld in time, and brushing zinc-rich primer, good anti-corrosion measures.