The influence of various metal elements on welding quality

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The welding wire contains 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: Image Silicon (Si) Silicon is the most commonly used deoxidizing element in welding wires; it prevents iron from oxidizing and can reduce FeO in the molten pool. However, when silicon is used for deoxidation alone, the resulting SiO2 has a high melting point (about 1710°C), and its particles are small, making it difficult for them to rise from the molten pool; this can lead to slag inclusion in the weld metal. Image: Manganese (Mn). The role of manganese is similar to that of 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, in addition to its deoxidizing function, allows it to combine with sulfur to form manganese sulfide (MnS), which is then removed (desulfurization). This helps to reduce the tendency for heat cracks caused by sulfur. Since it is difficult to remove the deoxidation products using silicon and manganese alone for deoxidation. Therefore, a combined deoxidation using silicon and manganese is commonly employed at present, allowing the resulting SiO2 and MnO to combine into a silicate (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 float to the surface, thereby achieving an effective deoxidization effect. Manganese is also an important alloying element in steel, as well as a key element for 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 ； It becomes very brittle when the Mn content exceeds 3% ； When the Mn content is 0.6–1.8%, the weld metal exhibits high strength and toughness. Image: Sulfur (S). In steel, sulfur often exists in the form of iron sulfide and is distributed in a network pattern at the grain boundaries, thereby significantly reducing the toughness of the steel. The eutectic temperature of iron combined with iron sulfide is low (985°C); therefore, during hot working, since the starting temperature for processing is usually between 1150 and 1200°C, the iron and iron sulfide eutectic has already melted, which leads 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 advanced high-quality steel lies in the levels of sulfur and phosphorus contained in them. 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 is distributed in granular form 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. Image: Phosphorus (P) – Phosphorus in steel can be completely dissolved within the ferrite structure. 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 plasticity and toughness. The effect is particularly severe at low temperatures, which is known as the cold-setting tendency of phosphorus. Therefore, it is detrimental to welding and increases the susceptibility of steel to cracking. As an impurity, the phosphorus content in steel also needs to be limited. Image: Chromium (Cr) – Chromium can increase the strength and hardness of steel, with only a minor reduction in its plasticity 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, such as 12CrMo, 15CrMo, 5CrMo, etc. Steel always contains 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. Image: Aluminum (AI). 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. Additionally, 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, welding wires containing aluminum tend to cause spatter, and an excessive amount of aluminum 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 too high. 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. Image: Titanium (Ti). Titanium is also a strong deoxidizing element; it can react with nitrogen to form TiN, thereby enabling nitrogen fixation and enhancing 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. Image: Molybdenum (Mo). In alloy steels, molybdenum enhances the strength and hardness of the steel, refines the grain structure, prevents temper brittleness and the tendency to overheat, and improves high-temperature strength, creep strength, and fatigue strength. When the molybdenum content is less than 0.6%, it increases plasticity, reduces the tendency to form cracks, and boosts 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. Image: Vanadium (V). Vanadium can increase the strength of steel, refine grains, reduce the tendency for grain growth, and enhance 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 possess high-temperature stability, thereby enhancing 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 would otherwise be unfavorable into favorable conditions.