Views:194 Author:Site Editor Publish Time: 2020-05-22 Origin:Site
Galvanized steel tube for sale is widely used in the production. The advantage of using galvanized steel tube is to protect the internal steel structure by using the metal zinc which can form a dense oxide protective layer in the air. In the case of being welded and scratched, due to the existence of Zn Fe primary battery, the relatively active galvanized part can be used as a sacrificial anode, which can delay the corrosion of steel and has good corrosion resistance. However, due to the existence of zinc coating, it is easy to produce cracks, pores and slag inclusions in the welding process, so it is difficult to obtain good welding quality.
Generally, galvanized steel is coated with a layer of zinc outside the low carbon steel, and the galvanized layer is generally 20um thick. The melting point of zinc is 419 ° C and the boiling point is about 908 ° C. In welding, zinc melts into liquid and floats on the surface of molten pool or at the root of weld. Zinc has a large solid solubility in iron. The liquid zinc will etch the weld metal along the grain boundary, and the low melting point zinc will form "liquid metal embrittlement". At the same time, zinc and iron can form intermetallic brittle compounds, such as fe3zn10, fezn10, etc. These brittle phases reduce the plasticity of weld metal and produce cracks under tensile stress. If the fillet weld is welded, especially the T-joint fillet weld, it is most likely to produce through cracks. During the welding of galvanized steel, the zinc layer on the groove surface and edge will oxidize, melt and evaporate under the action of arc heat, and even emit white smoke and steam, which will easily cause weld porosity. The melting point of ZnO formed by oxidation is higher than 1800 ° C. if the parameters are too small in the welding process, ZnO will be caused
At the same time, feo-mno or feo-mno-sio2 oxide slag with low melting point is produced because Zn becomes deoxidizer. If the welding specification is not appropriate and the operation method is not appropriate, it is easy to melt the zinc coating at the edge of the weld and expand the melting area, which may damage the zinc coating, especially in the case of lengthening the arc and large swing operation, widening the melting area and damaging the zinc coating are more serious. At the same time, due to the evaporation of zinc, a large number of white smoke is emitted, which has a stimulating and injurious effect on human body. Therefore, the selection of welding methods and materials with a low amount of smoke is also a factor to be considered.
There are many welding methods for high precision galvanized steel tube, such as gas welding, manual arc welding, CO2 gas shielded welding, submerged arc automatic welding, tungsten argon arc welding, etc.
In the past, gas welding was commonly used in the welding of galvanized pipes. Due to the lack of centralized heat input, defects and poor mechanical properties of welds, gas welding is now basically eliminated in the installation industry. Gas welding has great damage to zinc coating.
The welding performance of CO2 gas shielded welding for galvanized steel is good. When suitable welding specification and matching shielding gas and welding materials are used, high quality welding joints can be obtained. This method is seldom used in engineering practice.
TIG arc energy concentration, less damage to the zinc coating, and easy to form a good one-sided welding and two-sided forming joint, is a welding method worth using, but the welding speed, slower, more expensive.
At present, manual arc welding is the most common welding method in pipeline installation. In the case of correct selection of electrodes, such as j421, J422, j423 and other titanium oxide type and calcium titanium type electrodes, due to the large amount of rutile and ilmenite in the coating of these electrodes, the melting rate of the electrodes is large, which increases the melting rate relatively. Under the condition of not swinging, it can only destroy the zinc coating at the front of the molten pool, which will not expand the melting area, so it can reduce the penetration of zinc liquid to the weld metal; under the condition of correct operation methods and welding materials, it can obtain the welding quality with good mechanical properties and no defects of the joint. Due to the low price and fast speed of manual arc welding compared with TIG welding, manual arc welding process is adopted under the condition of skilled welders.
The preparation of galvanized steel before welding is the same as that of ordinary low-carbon steel. It is necessary to carefully handle the groove size and nearby galvanized layer. In order to achieve penetration, the groove size shall be appropriate, generally 60 ~ 65 °, with a certain gap, generally 1.5 ~ 2.5mm; in order to reduce the penetration of zinc into the weld, the galvanized layer in the groove can be removed before welding. In the practical work, the centralized beveling, no blunt edge technology for centralized control, two-layer welding process, reducing the possibility of incomplete penetration.
The electrode shall be selected according to the base material of galvanized pipe. Generally, J422 is widely used for low carbon steel due to its operability. Welding method: when welding the first layer of multi-layer welding, try to melt the zinc layer and make it vaporize and evaporate to escape from the weld, which can greatly reduce the liquid zinc left in the weld. During fillet welding, the zinc layer shall be melted and vaporized and evaporated in the first layer as much as possible to escape from the weld. The method is to move the end of the electrode forward about 5-7mm. When the zinc layer is melted, it shall return to the original position and continue to weld forward. After welding, the welds shall be cleaned in time, zinc rich primer shall be applied, and anti-corrosion measures shall be taken.
Welding quality assurance measures
Control from five aspects: human, material, machine, method and environment.
① Human factor is the key point of welding control. Therefore, before welding, welders with skilled skills and welder certificate shall be selected for necessary technical training and disclosure. It is not allowed to change at will to ensure the relative stability of the welder for welding the pipeline.
② Control of welding materials: ensure that the purchased welding materials are from regular channels, with quality assurance certificate and conformity certificate, and meet the process requirements; strictly control the recycling of electrode head to ensure the flow direction and consumption; strictly bake the welding materials according to the process, and distribute the consumption within half a day at a time.
③ Welding machine: the welding machine must have reliable performance and meet the process requirements; the welding machine must have a qualified ammeter and voltmeter to ensure the correct implementation of the welding process. The welding cable shall not be too long, and the welding parameters shall be adjusted when it is too long.
④ Welding process method: ensure the strict implementation of special operation method of galvanized pipe, conduct groove inspection before welding, control of welding process parameters and operation methods, inspect appearance quality after welding, and add NDT after welding if necessary. Control the level of welding and the amount of welding materials for each pass.
⑤ Welding environment control: ensure that the temperature, humidity and wind speed during welding meet the process requirements.
During the construction of hot-dip galvanized pipe, the correct welding process shall be adopted, the inspection and acceptance shall be carried out in strict accordance with the specifications, and the anti-corrosion treatment (zinc rich paint) shall be done timely after the welding is completed. It is feasible in the construction process of open and closed air conditioning system, which can improve the construction speed and the firmness of the pipe connection. Therefore, the hot-dip galvanized pipe can be welded and connected under the construction conditions and on the premise of relevant protection and anti-corrosion measures.