For welding wire containing Si, Mn, S, P, Cr, Al, Ti, Mo, V and other alloying elements. The influence of these alloying elements on the welding performance is described below:
Silicon (Si)
Silicon is the most commonly used deoxidizing element in welding wire, it can prevent iron from combining with oxidation, and can reduce FeO in the molten pool. However, if silicon deoxidation is used alone, the resulting SiO2 has a high melting point (about 1710°C), and the resulting particles are small, making it difficult to float out of the molten pool, which can easily cause 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. Using manganese deoxidation alone, the generated MnO has a higher density (15.11g/cm3), and it is not easy to float out of the molten pool. The manganese contained in the welding wire, in addition to deoxidation, can also combine with sulfur to form manganese sulfide (MnS), and be removed (desulfurization), so it can reduce the tendency of hot cracks caused by sulfur. Since silicon and manganese are used alone for deoxidation, it is difficult to remove the deoxidized products. Therefore, silicon-manganese joint deoxidation is mostly used at present, so that the generated SiO2 and MnO can be composited into silicate (MnO·SiO2). MnO·SiO2 has a low melting point (about 1270°C) and a low density (about 3.6g/cm3), and can condense into large pieces of slag and float out in the molten pool to achieve a good deoxidation effect. Manganese is also an important alloying element in steel and an important hardenability element, which has a great influence on the toughness of the 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 more than 3%, it is very brittle; when the Mn content is 0.6-1.8%, the weld metal has a higher strength and toughness.
Sulfur (S)
Sulfur often exists in the form of iron sulfide in steel, and is distributed in the grain boundary in the form of a network, thus significantly reducing the toughness of steel. The eutectic temperature of iron plus iron sulfide is low (985°C). Therefore, during hot working, since the processing start temperature is generally 1150-1200°C, and the eutectic of iron and iron sulfide has been melted, resulting in cracking during processing, This phenomenon is the so-called “hot embrittlement of sulfur”. This property of sulfur causes the steel to develop hot cracks during welding. Therefore, the content of sulfur 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 amount of sulfur and phosphorus. As mentioned earlier, manganese has a desulfurization effect, because manganese can form manganese sulfide (MnS) with a high melting point (1600 ° C) with sulfur, which is distributed in the grain in granular form. During hot working, manganese sulfide has sufficient plasticity, thus eliminating the harmful effect of sulfur. Therefore, it is beneficial to maintain a certain amount of manganese in steel.
Phosphorus (P)
Phosphorus can be completely dissolved in ferrite in steel. Its strengthening effect on steel is second only to carbon, which increases the strength and hardness of steel. Phosphorus can improve the corrosion resistance of steel, while plasticity and toughness are significantly reduced. Especially at low temperatures, the impact is more serious, which is called the cold kneeling tendency of phosphorus. Therefore, it is unfavorable to 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 increase the strength and hardness of steel without reducing the plasticity and toughness. Chromium has strong corrosion resistance and acid resistance, so austenitic stainless steel generally contains more chromium (more than 13%). Chromium also has strong oxidation resistance and heat resistance. Therefore, chromium is also widely used in heat-resistant steel, such as 12CrMo, 15CrMo 5CrMo and so on. Steel contains a certain amount of chromium [7]. Chromium is an important constituent element of austenitic steel and a ferritizing element, which can improve the oxidation resistance and mechanical properties at high temperature in alloy steel. In austenitic stainless steel, when the total amount of chromium and nickel is 40%, when Cr/Ni = 1, there is a tendency of hot cracking; when Cr/Ni = 2.7, there is no tendency of hot cracking. Therefore, when Cr/Ni = 2.2 to 2.3 in general 18-8 steel, chromium is easy to produce carbides in alloy steel, which makes the heat conduction of alloy steel worse, and chromium oxide is easy to produce, which makes welding difficult.
Aluminum (AI)
Aluminum is one of the strong deoxidizing elements, so using aluminum as a deoxidizing agent can not only produce less FeO, but also easily reduce FeO, effectively inhibit the chemical reaction of CO gas generated in the molten pool, and improve the ability to resist CO pores. In addition, aluminum can also combine with nitrogen to fix nitrogen, so it can also reduce nitrogen pores. However, with aluminum deoxidation, the resulting Al2O3 has a high melting point (about 2050 ° C), and exists in the molten pool in a solid state, which is likely to cause slag inclusion in the weld. At the same time, the welding wire containing aluminum is easy to cause spatter, and the high aluminum content will also reduce the thermal cracking resistance of the weld metal, so the aluminum content in the welding wire must be strictly controlled and should not be too much. If the aluminum content in the welding wire is properly controlled, the hardness, yield point and tensile strength of the weld metal will be slightly improved.
Titanium (Ti)
Titanium is also a strong deoxidizing element, and can also synthesize TiN with nitrogen to fix nitrogen and improve the ability of weld metal to resist nitrogen pores. If the content of Ti and B (boron) in the weld structure is appropriate, the weld structure can be refined.
Molybdenum (Mo)
Molybdenum in alloy steel can improve the strength and hardness of steel, refine grains, prevent temper brittleness and overheating tendency, improve high temperature strength, creep strength and durable strength, and when molybdenum content is less than 0.6%, it can improve plasticity, Reduces tendency to crack and improves impact toughness. Molybdenum tends to promote graphitization. Therefore, the general molybdenum-containing heat-resistant steel such as 16Mo, 12CrMo, 15CrMo, etc. contains about 0.5% molybdenum. When the content of molybdenum in alloy steel is 0.6-1.0%, molybdenum will reduce the plasticity and toughness of alloy steel and increase the quenching tendency of alloy steel.
Vanadium (V)
Vanadium can increase 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 formed carbides are stable below 650 °C. Time hardening effect. Vanadium carbides 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 easy to form refractory oxides, which increases the difficulty of gas welding and gas cutting. Generally, when the vanadium content in the weld seam is about 0.11%, it can play a role in nitrogen fixation, turning disadvantageous into favorable.
Post time: Mar-22-2023
