Galvanized steel is widely used in various industries, due to its use in the process of application of air composition to form a dense oxide coating, A good protective film is formed on the metal zinc to protect the steel structure in the work, ensuring the reasonable and effective application of the engineering quality. In the case of welding, scratch, due to the presence of the primary battery, the corresponding active galvanizing management control, By sacrificing the performance of the anode, the rust of steel can be delayed, and the dead corner of the structure can meet the needs of the design, so as to ensure the complete and effective control measures and modes of application. In the end market of the West to East Gas Transmission Project, the use of a large number of galvanized pipes put forward higher requirements for the welding and quality of galvanized pipe. The traditional galvanized pipe thread connection is easy to leak, resulting in a large amount of maintenance work, and the installation of open pipe is used in the installation, which seriously restricts and affects the appearance of the architectural engineering form and aesthetic requirements. Under the influence of this trend and background, seeking new welding methods and welding modes is the main application measures and methods of galvanized pipe construction. Because of the existence of zinc plating layer, it is easy to produce cracks, blowhole and slag in welding, which makes it difficult to guarantee its quality in the welding process, and become the main defects and constraints affecting welding technology.
Welding characteristics of galvanized steel
Galvanized pipe is a new type of mixed material and action mode composed of a layer of zinc outside of low carbon steel, the thickness of the galvanized layer is generally about 20um, Because zinc has a high strength melting point of 419 °C and boiling point of 908 °C, so in the flux process is easy to form a variety of influence and constraints, become the basic premise of affecting the quality of galvanized pipe. In welding, zinc melts and floats as a liquid on the surface of the pool or at the root of the weld. And zinc in the welding time easy to produce a larger solid solubility, easy to penetrate into the weld metal, Thus forming a liquid metal state, this state of metal is a kind of easy to catalyze the metal and way, so it is easy to be affected by various factors and the impact of cracks and fracture phenomenon. These brittle phases reduce the plasticity of the weld metal, and are prone to produce defects such as fillet welds in installation and use, which cause serious influence and restriction factors on the construction quality.
When galvanized steel is welded, the zinc layer on the groove surface and edge is treated under the heat action of an electric arc, It can produce oxidation, melting and even evaporation, so that the treatment mode and treatment mode in the use of the process of easy to cause porosity in the weld.
If the selection of welding specifications are not appropriate, improper operation, it is easy to use suitable welding gaps and edges exist in the various zinc melting points for strict processing, To ensure that a variety of possible damage to the zinc coating, so that the various ways of welding and management control factors for comprehensive analysis and management, to ensure the operation of its arc welding under the welding. At the same time, due to the evaporation of zinc, a large number of white smoke, It can stimulate and hurt the human body, but also produce less dust in the welding process, so it must be considered in welding process and management control method.
Requirements for Welding Galvanized Pipe in City Gas Application
With the constant acceleration of the current social development, in the life of urban residents, the amount of gas is increasing. How to ensure that the urban residents in the life of the reasonable control of gas has become the focus of the current municipal gas supply engineering to explore and explore the form is the main basis of municipal engineering construction. As the main pipeline form of natural gas supply and transportation, the welding quality of galvanized pipe is the main method and measure to affect the application of natural gases. In galvanized steel welding method more, generally is to use common gas welding and arc welding as the main welding process, With the development of science and technology, carbon dioxide gas shielded welding, automatic submerged arc welding and other new welding methods have been applied in pipeline welding, and become the main means and methods of welding.
Gas welding used to be used in the welding of galvanized pipe, due to the welding in the process of welding is becoming more and more perfect, its mechanical properties in welding seam is too poor, its shortcomings and the management analysis of the application.
The welding performance of CO _ 2 gas shielded welding is good for galvanized steel. When suitable welding specification and matching shielding gas and welding material are adopted, high quality welding joint can be obtained. This method is seldom used in engineering practice.
Tungsten Argon Arc Welding (TIG) is a kind of welding method that is worthy to be used because of its concentrated arc energy, less damage to zinc coating and easier formation of good one-side welding and double-side forming joints.
Manual arc welding is the most popular welding method in pipeline installation. In the case of correct power and power electrode, such as titanium oxide and calcium titanium-type electrodes J421, J422 and J423, Because of the large amount of rutile and ilmenite in the coating, the melting rate of the electrode is large and the melting speed is relatively increased. If the zinc coating such as the front of the molten pool can be damaged under the condition of no swing, the melting zone will not be enlarged and the penetration of liquid zinc to the weld metal will be reduced. The welding quality with good mechanical properties and no defects can be obtained by using correct operation method and welding material.
According to the reality of the current construction team, because the manual arc welding is cheaper and faster than TIG welding, the manual electric arc welding process is adopted in the case of skilled welders.
Welding process control
The preparation before welding of galvanized steel is the same as that of ordinary mild steel. It is important to take care of the groove size and the galvanizing coating nearby. For penetration, groove size should be appropriate, generally 60 ~ 65 °, to leave a certain gap, generally 1.5 ~ 2.5 mm; In order to reduce the penetration of zinc on the weld, before welding, the groove can be removed after the zinc coating welding. In the actual supervision work, the centralized beveling technology is adopted, and the two-layer welding technology is used for centralized control, which reduces the possibility of incomplete penetration.
Welding rod should be selected according to the base material of galvanized pipe. Generally, J422 is more common in low carbon steel because of easy operation.
Welding technique: when welding the first layer of multi-layer welding, try to make the zinc layer melt and vaporize, evaporate and escape the weld, which can greatly reduce the liquid zinc in the weld. When welding fillet welds, the zinc layer is melted and vaporized and evaporated in the first layer as much as possible to escape the weld, The method is to move the end of the electrode forward about 5 ~ 7 mm, and then return to the original position to continue welding when the zinc layer is melted. When horizontal welding and vertical welding, if the use of short slag electrode such as J427, the tendency to undercut will be small; If the back and forth transportation strip technology is adopted, the welding quality without defects can be obtained.