The technology of arc welding is introduced, and the analysis of welding metallurgical process.
Release time:
2022-07-23
【Summary Description】 Arc: A strong and long-lasting gas discharge phenomenon, positive and negative electrodes have a certain voltage, and the gas between the two electrodes should be in the state of ionization medium. When igniting a welding arc, the two electrodes (one pole is the workpiece, the other pole is filled with metal wire or welding rod) are usually connected to the power supply, short contact and rapid separation, short circuit occurs when the two poles contact each other, forming an arc. This is called contact arcing. After the arc is formed, as long as the power supply maintains a certain potential difference between the two poles, the arc can be maintained. Arc features: low voltage, high current, high temperature, high energy density, good mobility, etc. Generally, the voltage of 20~30V can maintain the stable combustion of the arc, and the current in the arc can be from tens of amperes to thousands of amperes. To meet the welding requirements of different workpieces, the temperature of the arc can reach more than 5000K, which can melt various metals. Arc composition: cathode area, anode area, arc column area three parts, Arc welding power source: The power source used for welding arc is called arc welding power source, which can be divided into four categories: AC arc welding power source, DC arc welding power source, pulse arc welding power source and inverter arc welding power source. DC positive connection: when the workpiece is connected to the anode and the welding rod is connected to the cathode, it is called DC positive connection. At this time, the workpiece is heated greatly and is suitable for welding thick and large workpieces; DC reverse connection: when the workpiece is connected to the cathode and the electrode is connected to the anode, it is called DC reverse connection. At this time, the workpiece is heated less and is suitable for welding thin and small workpieces. When using AC welding machine for welding, there is no positive or negative connection problem because the polarity of the two poles changes alternately. welding metallurgical process In the process of arc welding, the interaction of liquid metal, slag and gas is the process of metal re-smelting. However, due to the particularity of welding conditions, the welding chemical metallurgy process has different characteristics from the general smelting process. First of all, the welding metallurgy temperature is high, the phase boundary is large, and the reaction speed is fast. When air intrudes in the arc, the liquid metal will undergo strong oxidation and nitridation reactions, and a large amount of metal will evaporate. However, the water in the air and the hydrogen atoms decomposed from the oil, rust and water in the workpiece and Welding materials can dissolve into the liquid metal at the high temperature of the arc, resulting in reduced plasticity and toughness (hydrogen embrittlement) and cracks. Secondly, the welding pool is small and the cooling is fast, which makes it difficult for various metallurgical reactions to reach a balanced state, the chemical composition in the weld is uneven, and the gas and oxides in the molten pool are too late to surface, which is easy to form defects such as pores and slag inclusions, and even cracks. In order to ensure the quality of the weld, the following measures are usually taken in the arc welding process: (1) During the welding process, the molten metal is mechanically protected from the air. There are three protection methods: gas protection, slag protection and gas-slag joint protection. (2) metallurgical treatment of the welding pool, mainly by adding a certain amount of deoxidizer (mainly ferromanganese and ferrosilicon) and a certain amount of alloying elements in the Welding materials (welding rod, welding wire, flux), FeO in the molten pool is excluded in the welding process, and the burning loss of alloying elements is compensated.
Arc:A strong and persistent gas discharge in which a certain voltage is applied between the positive and negative electrodes and the gas medium between the two electrodes should be in an ionized state. When igniting a welding arc, the two electrodes (one pole is the workpiece, the other pole is filled with metal wire or welding rod) are usually connected to the power supply, short contact and rapid separation, short circuit occurs when the two poles contact each other, forming an arc. This is called contact arcing. After the arc is formed, as long as the power supply maintains a certain potential difference between the two poles, the arc can be maintained.
Arc features:Low voltage, high current, high temperature, high energy density, good mobility, etc. Generally, the voltage of 20~30V can maintain the stable combustion of the arc, and the current in the arc can be from tens of amperes to several thousand amperes to meet the welding requirements of different workpieces. The temperature of the arc can reach more than 5000K, which can melt various metals.
Arc composition: cathode area, anode area, arc column area three parts,
arc welding power supply:The power source used for welding arc is called arc welding power source, which can be divided into four categories: AC arc welding power source, DC arc welding power source, pulse arc welding power source and inverter arc welding power source.
DC positive:When the workpiece is connected to the anode and the electrode is connected to the cathode by DC welding machine, it is called DC positive connection. At this time, the workpiece is heated greatly and is suitable for welding thick and large workpieces;
DC reverse connection:When the workpiece is connected to the cathode and the electrode is connected to the anode, it is called DC reverse connection. At this time, the workpiece is heated less and is suitable for welding thin and small workpieces. When using AC welding machine for welding, there is no positive or negative connection problem because the polarity of the two poles changes alternately.
welding metallurgical process
In the process of arc welding, the interaction of liquid metal, slag and gas is the process of metal re-smelting. However, due to the particularity of welding conditions, the welding chemical metallurgy process has different characteristics from the general smelting process.
First of all, the welding metallurgy temperature is high, the phase boundary is large, and the reaction speed is fast. When air intrudes in the arc, the liquid metal will undergo strong oxidation and nitridation reactions, and a large amount of metal will evaporate. However, the water in the air and the hydrogen atoms decomposed from the oil, rust and water in the workpiece and Welding materials can dissolve into the liquid metal at the high temperature of the arc, resulting in reduced plasticity and toughness (hydrogen embrittlement) and cracks.
Secondly, the welding pool is small and the cooling is fast, which makes it difficult for various metallurgical reactions to reach a balanced state, the chemical composition in the weld is uneven, and the gas and oxides in the molten pool are too late to surface, which is easy to form defects such as pores and slag inclusions, and even cracks.
In order to ensure the quality of the weld, the following measures are usually taken in the arc welding process:
(1) During the welding process, the molten metal is mechanically protected from the air. There are three protection methods: gas protection, slag protection and gas-slag joint protection.
(2) metallurgical treatment of the welding pool, mainly by adding a certain amount of deoxidizer (mainly ferromanganese and ferrosilicon) and a certain amount of alloying elements in the Welding materials (welding rod, welding wire, flux), FeO in the molten pool is excluded in the welding process, and the burning loss of alloying elements is compensated.
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