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This post was last edited by huanghong on 2021-6-20 09:47. Copper-coated welding wire for CO2 gas shielded welding is a highly efficient, energy-saving, and material-saving welding material; it produces aesthetically pleasing welds and is suitable for welding low-carbon steel and low-alloy steel. CO2 welding wires have a low carbon content, usually below 0.1%, and also contain alloying elements such as Si, Mn, S, P, Cr, Al, Ti, Mo, and V. The impact of these alloying elements on welding properties is explained below one by one: 01 What is the impact of silicon (Si) on weldability? Silicon is the most commonly used deoxidizing element in welding wires; it prevents iron from reacting with oxygen and can reduce FeO in the molten pool. However, when silicon is used alone for deoxidation, the resulting SiO2 has a high melting point (about 1710°C), and its particles are very small, making it difficult to float out of the molten pool; this easily leads to slag inclusions in the weld metal. 02 What is the effect of manganese (Mn) on weldability? Manganese has a similar role to silicon, but its deoxidizing ability is slightly weaker than that of silicon. When deoxidation is carried out using manganese alone, the resulting MnO has a high density (15.11 g/cm3) and does not easily float to the surface of the molten pool. The presence of manganese in the welding wire, apart from its deoxidizing effect, also allows it to react with sulfur to form manganese sulfide (MnS), which is then removed (desulfurization). This helps to reduce the tendency for hot cracking caused by sulfur. Using silicon and manganese individually for deoxidation makes it difficult to remove the deoxidation products. Therefore, a combined deoxidation using silicon and manganese is currently commonly employed, allowing the resulting SiO2 and MnO to combine into silicates (MnO.SiO2). MnO.SiO2 has a low melting point (about 1270°C) and a low density (about 3.6 g/cm3); it can aggregate into large slag masses in the molten pool and rise to the surface, thereby achieving an effective deoxidation effect. Manganese is also an important alloying element in steel, as well as a key element that affects hardenability; it has a significant impact on the toughness of the weld metal. When the Mn content is < 0.05%, the toughness of the weld metal is very high ； When the Mn content exceeds 3%, it becomes very brittle ； When the Mn content is 0.6–1.8%, the weld metal exhibits high strength and toughness. 03 What is the impact of sulfur (S) on weldability? Sulfur in steel usually exists in the form of iron sulfide and is distributed in a network pattern along the grain boundaries, thereby significantly reducing the toughness of the steel. The eutectic temperature of iron and iron sulfide is relatively low (985°C). Therefore, during hot working, since the starting temperature for processing is generally 1150–1200°C, the iron-iron sulfide eutectic has already melted, leading to cracking during processing. This phenomenon is known as “thermal brittleness due to sulfur”. This property of sulfur causes hot cracks in steel during welding. Therefore, the sulfur content in steel is generally strictly controlled. The main difference between ordinary carbon steel, high-quality carbon steel, and superior quality steel lies in the amounts of sulfur and phosphorus they contain. As mentioned earlier, manganese has a desulfurization effect, as it can form manganese sulfide (MnS) with sulfur, which has a high melting point of 1600°C and appears in granular form distributed within the crystal grains. During hot processing, manganese sulfide possesses sufficient plasticity, thereby eliminating the harmful effects of sulfur. Therefore, it is beneficial to maintain a certain manganese content in steel. 04 What effect does the phosphorus (P) element have on weldability? Phosphorus in steel can be completely dissolved in the ferrite. Its strengthening effect on steel is second only to that of carbon; it increases the strength and hardness of steel. Phosphorus improves the corrosion resistance of steel, but it significantly reduces its ductility and toughness. Especially at low temperatures, the effect is more severe; this is known as the cold brittleness tendency of phosphorus. Therefore, it is detrimental to welding and increases the crack sensitivity of steel. As an impurity, the phosphorus content in steel also needs to be limited. 05 What effect does the chromium (Cr) element have on weldability? Chromium can increase the strength and hardness of steel, with only a minor decrease in ductility and toughness. Chromium possesses strong corrosion and acid resistance, which is why austenitic stainless steels generally contain a high amount of chromium (over 13%). Chromium also possesses strong antioxidant properties and heat resistance. Therefore, chromium is also widely used in heat-resistant steels; steels such as 12CrMo, 15CrMo, and 5CrMo all contain a certain amount of chromium. Chromium is an important constituent element of austenitic steel as well as an element that promotes ferritization; in alloy steels, it enhances the oxidation resistance and mechanical properties at high temperatures. In austenitic stainless steels, when the total amount of chromium and nickel is 40% and Cr/Ni = 1, there is a tendency for thermal cracking ； When Cr/Ni = 2.7, there is no tendency for thermal cracking. Therefore, in 18-8 type steel, when the Cr/Ni ratio is around 2.2–2.3, chromium tends to form carbides within the alloy steel, which reduces its thermal conductivity. It also leads to the formation of chromium oxide, making welding more difficult. 06 What is the impact of aluminum (Al) on weldability? Aluminum is one of the strong deoxidizing elements; therefore, using aluminum as a deoxidizer not only reduces the formation of FeO but also facilitates its reduction. This helps to effectively suppress the chemical reactions that produce CO gas in the molten pool, thereby improving resistance to CO-induced pores. Furthermore, aluminum can also combine with nitrogen to carry out nitrogen fixation, thereby reducing nitrogen pores as well. However, when aluminum is used for deoxidation, the resulting Al2O3 has a very high melting point (around 2050°C); it exists in a solid state within the molten pool, which can easily lead to slag inclusion in the weld. At the same time, aluminum-containing welding wires tend to cause spatter, and too high an aluminum content can reduce the heat-cracking resistance of the weld metal; therefore, the aluminum content in welding wires must be strictly controlled and should not be excessive. If the aluminum content in the welding wire is properly controlled, the hardness, yield strength, and tensile strength of the weld metal all increase slightly. 07 What is the impact of titanium (Ti) on weldability? Titanium is also a strong deoxidizing element; it can react with nitrogen to form TiN, thereby enabling nitrogen fixation and improving the resistance of the weld metal to nitrogen pores. If the contents of Ti and B (boron) in the weld microstructure are appropriate, the weld microstructure can be refined. 08 What is the effect of molybdenum (Mo) on weldability? Molybdenum in alloy steel can increase the strength and hardness of the steel, refine the grain structure, prevent temper brittleness and the tendency to overheat, and enhance high-temperature strength, creep strength, and fatigue strength. When the molybdenum content is less than 0.6%, it can improve plasticity, reduce the tendency to form cracks, and enhance impact toughness. Molybdenum has a tendency to promote graphitization. Therefore, common molybdenum-containing heat-resistant steels such as 16Mo, 12CrMo, and 15CrMo have a molybdenum content of around 0.5%. When the molybdenum content in alloy steel is between 0.6% and 1.0%, molybdenum reduces the plasticity and toughness of the alloy steel, while increasing its tendency to harden upon quenching. 09 What is the effect of vanadium (V) on weldability? Vanadium can increase the strength of steel, refine grains, reduce the tendency for grain growth, and improve hardenability. Vanadium is a strong carbide-forming element, and the carbides it forms remain stable below 650°C. It has age-hardening properties. Vanadium carbides exhibit high-temperature stability, thereby improving the high-temperature hardness of steel. Vanadium can alter the distribution of carbides in steel, but it tends to form refractory oxides, which increases the difficulties associated with gas welding and cutting. In ordinary welds, when the vanadium content is around 0.11%, it can play a role in nitrogen fixation, turning what is unfavorable into favorable.