Sulfur is one of the harmful impurities in the weld metal. Sulfur is most harmful when it exists in the form of FeS. It is distributed in the form of flakes or chains at grain boundaries in the form of low melting point eutectic Fe + FeS (melting point 985 ° C) or FeS + FeO. As a result, the tendency to thermal cracking is increased, while impact toughness and corrosion resistance are also reduced. At the same time, sulfur can also promote segregation of the weld and cause lamellar tearing when welding thick plates. Phosphorus is also a harmful impurity in the welds of most steels. Liquid steel can dissolve more phosphorus, while in solid steel, the solubility of phosphorus is only a few thousandths. Phosphorus mainly exists in the form of Fe2P and Fe3P in steel. They form low melting eutectics with iron and nickel, such as Fe3P + Fe, Ni3P + Fe. When the molten pool is rapidly solidified, these low-melting eutectics segregate on the grain boundaries, weakening the bonding force between the grains, and promoting the generation of thermal cracks. Phosphorus will also increase the cold brittleness of the weld, that is, reduce the impact toughness and increase the brittle transition temperature.
There are two main methods for controlling sulfur and phosphorus harmful elements in the weld. The first is to limit the content of sulfur and phosphorus in the welding material. The second is desulfurization and dephosphorization by metallurgical methods. The desulfurization reaction of the weld is as follows:
[FeS] + [Mn] = (MnS) + [Fe]
[FeS] + (CaO) = (CaS) + [FeO]
[FeS] + (MnO) = (MnS) + [FeO]
[FeS] + (MgO) = (MgS) + [FeO]
Manganese is a commonly used desulfurizing agent. The reaction product MnS is practically insoluble in molten steel, and most of it enters the molten slag, and a small amount of residue remains in the weld to form sulfide or oxide inclusions. However, because of the higher melting point of MnS, the inclusions are dispersed in a dot shape, so the harm is less. The temperature reduction process is conducive to the desulfurization, but only by increasing the Mn content can a better effect be achieved. In addition to the Mn element, basic oxides such as MnO, CaO, MgO, etc. in the slag can also be desulfurized. The generated MnS, CaS, and MgS do not dissolve in the molten steel and enter the slag. Because MnO and CaO are only basic oxides, they are contained in alkaline slag. Therefore, the desulfurization ability of alkaline slag is stronger than that of acid slag, and Has high crack resistance.
The dephosphorization of the weld is performed in two steps: the first step is to oxidize phosphorus to P2O5; the second step is to form a stable compound with the basic oxide in the slag into the slag, and increase FeO and CaO in the slag The content is beneficial to reduce phosphorus in the weld. The dephosphorization ability of acid slag is worse than that of alkaline slag. The specific reaction equation is as follows:
2 ［Fe3P］ +5 ［FeO］ = P2O5 + 11 ［Fe］
P2O5 + 3 （CaO） = {(CaO) 3 · P2O5}
P2O5 + 4 （CaO） = {(CaO) 4 · P2O5}
