Silicon (Si)
Silicon is the most commonly used deoxidizing element in welding wire. It can prevent iron from combining with oxygen and can reduce FeO in the molten pool. However, when silicon is used alone for deoxidation, the SiO2 generated has a high melting point (about 1710℃) and the particles of the product are small, which is difficult to float out of the molten pool and easily causes slag inclusion 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 large (15.11g/cm3), and it is not easy to float out of the molten pool. In addition to the deoxidation effect, manganese contained in the welding wire can also combine with sulfur to form manganese sulfide (MnS), which is removed (desulfurization), so the tendency of hot cracks caused by sulfur can be reduced. Since it is difficult to remove the deoxidation products when silicon and manganese are used alone for deoxidation. Therefore, silicon-manganese combined deoxidation is currently mostly used to make the generated SiO2 and MnO compound into silicate (MnO·SiO2). MnO·SiO2 has a low melting point (about 1270℃) and a low density (about 3.6g/cm3). It can condense into large pieces of slag in the molten pool and float out, achieving a good deoxidation effect. Manganese is also an important alloying element in steel and an important hardenability element. It has a great influence 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 greater than 3%, it is very brittle; when the Mn content = 0.6~1.8%, the weld metal has higher strength and toughness.
Sulfur (S)
Sulfur often exists in steel in the form of iron sulfide and is distributed in a network at the grain boundary, thereby significantly reducing the toughness of steel. The eutectic temperature of iron plus iron sulfide is relatively low (985℃). Therefore, during hot working, since the starting temperature of the working is generally 1150-1200℃, the eutectic of iron and iron sulfide has melted, which leads to cracking during working. This phenomenon is the so-called "hot brittleness of 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 high-quality steel lies in the amount of sulfur and phosphorus content. As mentioned earlier, manganese has a desulfurization effect, because manganese can form manganese sulfide (MnS) with a high melting point (1600℃) with sulfur, which is distributed in the grains in granular form. During hot working, manganese sulfide has sufficient plasticity, thus eliminating 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 the strength and hardness of steel. Phosphorus can improve the corrosion resistance of steel, while significantly reducing plasticity and toughness. The impact is more serious at low temperatures, which is called the cold kneeling tendency of phosphorus. Therefore, it is not conducive to welding and increases the crack sensitivity of steel. As an impurity, the content of phosphorus in steel should also be limited.
Welded steel pipe
