The welding wire contains alloy elements such as Si, Mn, S, P, Cr, AI, Ti, Mo, and V. The effects of these alloy elements on welding performance are described below:
Silicon (Si)
Silicon is the most commonly used deoxidizing element in welding wires, which can prevent iron from oxidizing and can reduce FeO in the molten pool. However, using silicon alone for deoxidation, the resulting SiO2 has a high melting point (about 1710 ℃), and the particles of the product are small, making it difficult to float out of the molten pool, easily causing slag inclusions in the weld metal.
Manganese (Mn)
The effect of manganese is similar to that of silicon, but its deoxidation ability is slightly worse than that of silicon. When manganese is used alone for deoxidation, the density of MnO generated is relatively high (15.11g/cm3), and it is not easy to float out of the solution tank. "The manganese content in the welding wire, in addition to deoxidation, can also be combined with sulfur to form manganese sulfide (MnS), which is removed (desulfurized), thereby reducing the tendency for sulfur induced thermal cracking.". Due to the use of silicon and manganese alone for deoxidation, it is difficult to remove the deoxygenated products. Therefore, at present, silicon manganese combined deoxidation is mostly used to composite the generated SiO2 and MnO into silicate (MnO · SiO2). MnO · SiO2 has a low melting point (about 1270 ℃) and a low density (about 3.6 g/cm3), which can agglomerate into large pieces of molten slag and float out in the molten pool, achieving good deoxidation effect. Manganese is also an important alloying element in steel and an important hardenability element, which has a significant impact on the toughness of weld metal. When the Mn content is less than 0.05%, the toughness of the weld metal is very high; When the Mn content is>3%, it is very brittle; When the Mn content is 0.6~1.8%, the weld metal has high strength and toughness.
Sulfur (S)
Sulfur often exists in steel in the form of iron sulfide and is distributed in a network at grain boundaries, significantly reducing the toughness of steel. The eutectic temperature of iron plus iron sulfide is relatively low (985 ℃). Therefore, during hot processing, the starting temperature of processing is generally 1150~1200 ℃, and the eutectic of iron and iron sulfide has melted, resulting in cracking during processing. This phenomenon is called "sulfur thermal brittleness". 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 high-grade high-quality steel is the content of sulfur and phosphorus. As mentioned earlier, manganese has a desulfurization effect because it can form high melting point (1600 ℃) manganese sulfide (MnS) with sulfur, which is distributed in grains in a granular manner. During thermal processing, manganese sulfide has sufficient plasticity to eliminate the harmful effects of sulfur. Therefore, it is beneficial to maintain a certain manganese content in steel.
Phosphorus (P)
Phosphorus can be completely dissolved in ferrite in steel. "Its strengthening effect on steel is second only to carbon, increasing its strength and hardness. Phosphorus can improve the corrosion resistance of steel, while plasticity and toughness are significantly reduced.". Especially at low temperatures, the impact is more severe, which is known as the cold brittleness tendency of phosphorus. Therefore, it is unfavorable for welding and increases the crack sensitivity of steel. As an impurity, the content of phosphorus in steel should also be limited.
Chromium (Cr)
Chromium can improve the strength and hardness of steel, while the plasticity and toughness decrease slightly. Chromium has a strong ability to resist corrosion and acid, so austenitic stainless steel generally contains more chromium (over 13%). Chromium also has strong antioxidant and heat resistance. Therefore, chromium is also widely used in heat resistant steels, such as 12CrMo, 15CrMo, 5CrMo, and so on. Steel contains a certain amount of chromium [7]. Chromium is an important constituent and ferritized element of austenitic steel. It can improve the oxidation resistance and mechanical properties of alloy steels at high temperatures. In austenitic stainless steel, 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, when Cr/Ni=about 2.2 to 2.3 in general 18-8 type steel, chromium is prone to produce carbides in alloy steel, which leads to poor heat conductivity of alloy steel and easy to produce chromium oxide, making welding difficult.
Aluminum is one of the strong deoxidizing elements, so using aluminum as a deoxidizing agent can not only reduce the production of FeO, but also facilitate the reduction of FeO, effectively inhibiting the chemical reaction of CO gas generated in the molten pool, and improving the ability to resist CO pores. In addition, aluminum can also combine with nitrogen to play a nitrogen fixation role, so it can also reduce nitrogen pores. However, when aluminum is used for deoxidation, the resulting AI2O3 has a high melting point (about 2050 ℃) and is present in the molten pool as a solid state, easily causing slag inclusion in the weld. At the same time, welding wires containing aluminum are prone to splash, and excessive aluminum content can also reduce the thermal cracking resistance of the weld metal. Therefore, the aluminum content in the welding wire must be strictly controlled and should not be excessive. If the aluminum content in the welding wire is properly controlled, the hardness, yield point, and tensile strength of the weld metal are slightly improved.
Titanium is also a strong deoxygenation element, and can also react with nitriding to synthesize TiN for nitrogen fixation, improving the ability of weld metal to resist nitrogen porosity. If the content of Ti and B (boron) in the weld microstructure is appropriate, the weld microstructure can be refined.
Molybdenum in alloy steel can improve the strength and hardness of steel, refine grains, prevent temper brittleness and overheating tendencies, improve high-temperature strength, creep strength, and endurance strength. When the content of molybdenum is less than 0.6%, it can improve plasticity, reduce the tendency to produce cracks, and improve impact toughness. Molybdenum has a tendency to promote graphitization. Therefore, generally, the molybdenum content of heat resistant steels containing molybdenum, such as 16Mo, 12CrMo, and 15CrMo, is about 0.5%. When the content of molybdenum in alloy steel is between 0.6 and 1.0%, molybdenum will decrease the plasticity and toughness of alloy steel, increasing the quenching tendency of alloy steel.
Vanadium (V)
Vanadium can improve the strength of steel, refine grains, reduce the tendency of grain growth, and improve hardenability. Vanadium is a relatively strong carbide-forming element, and the carbides formed are stable below 650 ℃. It has an age hardening effect. The carbides of vanadium have high temperature stability, which can improve the high temperature hardness of steel. Vanadium can change the distribution of carbides in steel, but vanadium is prone to form refractory oxides, increasing the difficulty of gas welding and cutting. Generally, when the vanadium content in the weld seam is about 0.11%, it can play a nitrogen fixation role, turning unfavorable into favorable.
