Manual arc welding process
Manual arc welding is a manual welding method that uses the heat generated by the arc to melt the metal to be welded. Because it requires simple equipment and is flexible to operate, it can easily weld welds formed by different positions and joints in space. Therefore, it is currently widely used.
Before welding, the workpiece to be welded and the welding clamp are connected to the two poles of the electric welding machine respectively, and the welding rod is clamped with the welding clamp. During welding, the welding rod and the workpiece are instantly in contact to form a short circuit, and then they are separated by a certain distance (about 2-4mm) to ignite the arc.
The workpiece under the arc melts immediately to form a semi-oval molten pool. After the electrode coating melts, part of it becomes gas to surround the arc and isolate it from the air, thereby protecting the liquid metal from oxygen and nitrogen; part of it becomes slag, which is either sprayed into the molten pool alone or sprayed into the molten pool together with the liquid metal droplets generated by the melting of the welding core.
In the arc and the molten pool, the liquid metal, slag and arc gas will undergo certain physical and chemical changes, such as gas dissolving into the liquid metal, undergoing oxidation-reduction reactions, etc. The gas and slag in the molten pool float due to their light weight. When the arc is removed, the temperature drops, and the metal and slag will solidify one after another. In this way, the two pieces of metal are connected by the melted and crystallized weld metal. Since the slag shrinks differently from the metal, it will slip on the slag shell and the metal interface. The slag shell will fall off automatically or after being knocked off, revealing a metal weld with fish scales.
The main equipment for manual arc welding is the electric welder. The electric welder is a power source that generates arcs for welding components, and there are two types: AC and DC. At present, there are many types of electric welders produced in China, which can be divided into: AC electric welders and DC electric welders according to their structure.
There are two different connection methods for DC electric welders. When the electrode is connected to the negative electrode and the workpiece is connected to the positive electrode, it is the positive connection method; otherwise, it is the reverse connection method. Generally, when welding with alkaline low-hydrogen electrodes, in order to make the arc burn stably, it is required to use the DC reverse connection method; when welding thick steel plates with acid electrodes, the positive connection method is used, because the temperature of the anode part is higher than that of the cathode part, and the positive connection method can obtain a greater penetration depth; when welding thin steel plates and non-ferrous metals, the reverse connection method is used. When welding with AC, since the polarity changes alternately, there is no need to select the polarity connection method.
The welding material for manual welding is the welding rod, which consists of a steel core and a coating wrapped outside the steel core.
Steel core
The main function of the steel core (welding core) is to conduct electricity and form a deposited metal with a certain composition at the end of the electrode. The welding core can be made of various steels. The composition of the welding core directly affects the composition and properties of the deposited metal. Therefore, the welding core is required to minimize the content of harmful elements. In addition to limiting S and P, some welding rods have required the welding core to control As, Sb, Sn and other elements.
Fluorescent coating
The welding rod coating can also be called a coating. It is applied to the welding core mainly to facilitate welding operations and ensure that the deposited metal has a certain composition and properties. The welding rod coating can be made of hundreds of raw material powders such as oxides, carbonates, silicates, organic matter, fluorides, ferroalloys and chemical products, mixed according to a certain formula ratio. Various raw materials can be divided into the following categories according to their role in the welding rod coating:
1. Stabilizer Makes the welding rod easy to strike the arc and keeps the arc burning stably during the welding process. All easily ionized substances can stabilize the arc. Generally, alkali metal and alkaline earth metal compounds are used, such as potassium carbonate, sodium carbonate, marble, etc.
2. Slag-forming agent During welding, slag with certain physical and chemical properties can be formed, covering the surface of the molten metal, protecting the weld pool and improving the weld formation.
3. Deoxidizer Through the metallurgical chemical reaction during the welding process, the oxygen content in the weld metal is reduced and the mechanical properties of the weld are improved. The main deoxidizers are ferromanganese, ferrosilicon, ferrotitanium, etc.
4. Gas-forming agent Under the high temperature of the arc, it can decompose and release gas to protect the arc and the molten pool and prevent the intrusion of oxygen and nitrogen in the surrounding air.
5. Alloying agent It is used to compensate for the burning of alloy elements during welding and to transition alloy elements to the weld to ensure that the weld metal obtains the necessary chemical composition and properties.
6. Plasticizing lubricant Increase the plasticity, slip and fluidity of the coating powder during the electrode pressing process to improve the pressing quality of the electrode and reduce eccentricity.
7. Adhesive: It makes the coating powder have a certain viscosity during the pressure coating process, so that it can be firmly bonded to the welding core, and makes the welding rod coating have a certain strength after drying.