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Specifications for carbon dioxide shielded welding and operating procedure manuals: The CO2 gas used in carbon dioxide shielded welding is mostly an industrial by-product, which is compressed into a liquid state and bottled for supply. At room temperature, when the standard cylinder is full, the pressure is 5–7 MPa (50–70 kgf/cm2). It cannot be used below 1 MPa (10 psig). The purity of C02 gas used for welding is specified by general technical standards to be over 99%; if its purity turns out to be low during use, it should be purified. When using carbon dioxide gas shielded welding for welding low-carbon steel and low-alloy steel, in order to ensure that the welds possess high mechanical properties and to prevent the formation of pores, it is necessary to use alloy steel wires containing deoxidizing elements such as manganese and silicon; at the same time, the carbon content in these wires must also be controlled. Among them, H08Mn2SiA is used quite frequently, mainly for welding low-carbon steel and low-alloy steel ； H 04Mn 2SiTiA has a very low carbon content and contains 0.2%–0.4% titanium; it possesses strong resistance to porosity and is used in welds where high density is required. The standard parameters for carbon dioxide gas shielded welding include power supply polarity, wire diameter, arc voltage, welding current, gas flow rate, welding speed, wire extension length, and DC circuit inductance. (1) Power supply polarity: For welding ordinary materials using carbon dioxide gas shielded welding, reverse polarity DC is used ； For high-speed welding, surfacing, and casting iron repair welding, direct current positive polarity should be used. (II) Wire diameter: The wire diameter for carbon dioxide gas shielded welding can generally be selected from the table. (III) Arc voltage and welding current: For a wire of a certain diameter, in carbon dioxide gas shielded welding, when a lower arc voltage and a smaller welding current are used, the droplets formed by the melting of the wire connect the base material to the wire, creating a short-circuit condition known as short-circuit transfer. Most CO2 gas shielded welding processes use short-circuit transfer welding. When the arc voltage is high and the welding current is large, the droplets fall as small particles, a phenomenon known as particle transfer. ∮1. This transition is commonly used in the automatic welding of medium and thick plates with 6 or ∮2.0 mm welding wires. ∮Wires over 3mm are rarely used. ∮Wires with a diameter of 0.6–1.2 mm primarily use short-circuit transfer; as the wire diameter increases, the number of spatter particles also increases accordingly. When a 1.6 mm wire is used while still maintaining a short-circuit transition, spatter becomes very severe. Table of wire diameters for CO2 gas shielded welding (mm): Base metal thickness ≤4 >4 Wire diameter 0.5–1.2 1.0–1.6 Welding current and arc voltage are the key process parameters. To achieve good weld formation, reduce spatter, and minimize welding defects, the arc voltage and welding current must be matched to each other, with the welding current being adjusted by changing the wire feeding speed. The typical process parameters for minimizing spatter, as well as the range of process parameters used in actual production, are detailed in the table below. Process parameters for CO2 gas shielded welding: Wire diameter: 0.8, 1.2, 1.6. Typical process parameters: Arc voltage (V): 18, 19, 20; Welding current (A): 100–110, 120–130, 140–180. Process parameters used in production: Arc voltage (V): 18–24, 18–26, 20–28; Welding current (A): 60–160, 80–260, 160–310. When welding at low currents, if the arc voltage is too high, metal spatter will increase ； If the arc voltage is too low, the welding wire tends to penetrate into the molten pool, causing instability in the arc. During high-current welding, if the arc voltage is too high, metal spatter increases, and pores are likely to form ； If the voltage is too low, the arc becomes too short, resulting in poor weld formation. (IV) Gas flow rate: The flow rate of carbon dioxide gas is related to factors such as welding current, welding speed, wire extension length, and nozzle diameter. The gas flow rate should increase as the welding current increases, the welding speed increases, and the wire extension length increases. The typical flow rate of carbon dioxide gas ranges from 8 to 2,500 I. /minute. If the flow rate of carbon dioxide gas is too high, the oxidation of the gas at high temperatures will exacerbate the loss of alloying elements, weaken the deoxidizing and reducing effects of silicon and manganese, resulting in the formation of large amounts of silica and manganese oxide slag on the surface of the weld. This makes the weld prone to defects such as pores ； If the flow rate of carbon dioxide gas is too low, the stiffness of the gas flow layer is insufficient, resulting in poor protection of the molten pool and droplets; this also makes it easy for defects such as pores to appear in the weld. (5) Welding speed: As the welding speed increases, the width of the weld seam, the bead height, and the penetration depth all decrease accordingly. If the welding speed is too fast, the protective effect of the gas is compromised, and the cooling rate of the weld increases as well. This reduces the plasticity of the weld and results in poor weld shape. Conversely, if the welding speed is too slow, the weld width increases significantly; the heat in the molten pool becomes concentrated, making defects such as burn-through more likely to occur. (VI) Wire protrusion length refers to the length of the welding wire that extends beyond the electrode tip during welding. An increased length of the wire protruding from the welding device raises its resistance, which in turn speeds up the melting rate of the wire. When the protruding length is too great, the wire overheats and melts in sections, resulting in an unstable welding process, severe metal spattering, poor weld quality, and a reduced protective effect of gases over the molten pool ； Conversely, when the length of the welding wire extending outward is too short, the welding current increases and the distance between the nozzle and the workpiece is reduced, causing the nozzle to overheat. This leads to metal spatter sticking to or blocking the nozzle, thereby affecting the flow of air. Generally, in wire CO2 gas shielded welding, the length of the wire extending outside the weld area is 8 to 14 mm ； Coarse wire CO2 gas shielded welding, with the wire extension length of 10–20 mm. (7) Inductance in the DC circuit: In a welding circuit, an appropriate inductance is generally connected in series to stabilize the welding arc and reduce spatter. When the inductance value is too high, the growth rate of the short-circuit current is too slow, which leads to large metal splashes and the wire breaking into segments, resulting in arc extinction or making it difficult to initiate an arc ； When the inductance value is too low, the growth rate of the short-circuit current becomes too fast, resulting in fine metal splashes that cause uneven weld edges and poor weld formation. Furthermore, the coiled welding cable acts as an additional inductance; therefore, once the welding process is stabilized, it should not be altered casually. The operating techniques for semi-automatic carbon dioxide gas shielded welding are similar to those of shielded metal arc welding, and it is easier to master than shielded metal arc welding. The following points should be noted regarding the operating procedures for semi-automatic carbon dioxide gas shielded welding: 1. Due to the low no-load voltage of the flat-characteristic power supply, as well as the use of a solid wire, it is difficult to establish a stable arc ignition point when starting the arc, and spattering of the wire tends to occur. Furthermore, due to the low initial welding temperature of the workpiece, defects are likely to occur at the arc initiation point. The short-circuit arc initiation method is generally used ； Before starting the arc, the end of the welding wire must be cut off, as the spherical tip formed by melting can cause spattering when restarting the arc ； When starting the arc, choose the right position and use the reverse arc-starting method. 2. Excessive rapid arc termination can easily lead to cracks and pores at the weld pool; the arc termination process must be more precise than in shielded metal arc welding. Pause briefly at the weld pool, then slowly raise the welding torch, ensuring that the first layer of weld at the joint overlaps by 20–50 mm in thickness. 3. For butt welding and fillet welding, the welding torch should be tilted slightly, using a leftward welding technique; this allows the groove to be seen clearly, making it easier to avoid miswelding. Both the left welding method and the right welding method can be used when performing fillet welding. 4. Vertical welding and overhead welding. There are two welding methods for vertical welding: one involves welding from top to bottom, which is fast, easy to operate, and results in a smooth and attractive weld seam ； However, the penetration depth is low and the joint strength is poor, making it suitable for welds where strength requirements are not applicable. Another method involves welding from bottom to top; this results in a greater weld depth and a higher reinforced surface, but the outer shape is rough. For overhead welding, a fine welding wire, low current, low voltage, and short-circuit transition should be used to maintain stability during the welding process ； The C02 gas flow rate should be slightly higher than that during flat and vertical welding ； When the pool temperature rises and the molten iron tends to flow downward, the welding torch can be moved back and forth to ensure a smooth weld shape.