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Submerged arc welding flux is one of the important factors that ensure welding quality and determine the properties of the weld. Choosing the right flux can yield welds that meet technical requirements at the lowest production costs. In addition to considering the process characteristics of submerged arc welding, such as high dilution rates and high heat input, it is also necessary to fully understand the impact of flux types on the composition of the weld. In submerged arc welding, the final chemical composition of the weld is the result of the interaction between the base material, the welding wire, and the flux; therefore, the flux used in submerged arc welding must be compatible with the type of steel being welded as well as the welding wire. 01 Selection of flux for submerged arc welding of carbon steel (1) When using boiling steel wires such as H08A and H08MnA for welding, it is necessary to use fluxes with high manganese and high silicon content, such as those in the HJ43x series, in order to ensure that the weld metal obtains the necessary silicomanganese alloying through metallurgical reactions, thus forming a dense weld metal with sufficient strength and toughness. (2) When welding thick plates that require high toughness at the welds, medium-manganese and medium-silicon fluxes (such as HJ350, SJ301, etc.) and H10Mn2 high-manganese welding wire should be selected. By directly infiltrating manganese into the weld metal from the welding wire, and enabling an appropriate amount of silicon to be incorporated into the weld metal through the reduction reaction of SiO2 in the flux, weld metal with high impact toughness can be obtained. (3) For the single-sided welding process of thick and medium plate butt joints at high currents without groove preparation, a welding flux with high oxidizing properties, such as one containing manganese and silicon, should be used in combination with low-carbon welding wire of type H08A or H08MnA, in order to minimize the carbon content in the weld metal and improve its crack resistance. (4) For welding joints with significant rust on the workpiece surface, SJ501 flux with strong rust resistance should be selected, and welding wire of the appropriate grade should be chosen according to the strength requirements. (5) For high-speed submerged arc welding of thin plates, SJ501 sintered flux should be used along with wire of the corresponding strength grade. In this case, there are generally no special requirements regarding the strength and toughness of the joint; the main consideration is to ensure good formation and fusion of the weld at high welding speeds. 02 Selection of flux for submerged arc welding of low-alloy steel: When welding low-alloy steel, due to its high strength, the material is sensitive to hardening; as a result, there is a higher tendency for cold cracks or hydrogen-induced delayed cracks to occur in the heat-affected zone and the weld metal. Although the thermal cycle of submerged arc welding helps prevent the formation of cold cracks, in the welding of thick plates, high residual stresses in the weld seam, coupled with the gradual accumulation of hydrogen within it, still make hydrogen-induced delayed cracking likely to occur. Therefore, when welding low-alloy steel by submerged arc welding, low-hydrogen fluxes of the HJ25x series with a higher basicity should be selected first. Since these fluxes are all low-manganese and medium-silicon fluxes, their effect in reducing and alloying Si and Mn during the welding metallurgical reaction is not significant. Therefore, it is necessary to use alloy welding wires with a moderate silicon content, such as H08MnMo, H08Mn2Mo, and H08CrMoA. Secondly, to ensure that the strength and toughness of the joint are not lower than those of the base material, high-alkalinity fluxes with a weak silicomanganese reduction reaction should be used, such as HJ250 and SJ101 fluxes. Welds made with these fluxes have higher purity of the weld metal and fewer non-metallic inclusions, which helps to maintain the toughness of the joint. When performing multi-layer, multi-pass welding on thick plates of low-alloy steel, fluxes with good deoxidizing properties should be used. Since fluxes with high alkalinity have poor deoxidizing capabilities, whereas sintered fluxes with high alkalinity exhibit good deoxidizing properties, sintered fluxes are generally preferred for such applications. 03 Selection of flux for submerged arc welding of stainless steel: In submerged arc welding of stainless steel, the main task of the flux is to prevent excessive loss of alloying elements; therefore, a flux with low oxidizing properties should be chosen first. The commonly used melting flux for submerged arc welding of stainless steel in our country is HJ260, a low-manganese, high-silicon, medium-fluoride type flux. Since this flux still has a certain degree of oxidizing property, it is necessary to use chromium-nickel steel wire with a high content of chromium and nickel. Flux types HJ150 and HJ172 can also be used for submerged arc welding of stainless steel. Although this type of welding has a lower oxidizing tendency and results in less loss of alloying elements, its weldability is moderate and its slag removal performance is poor; therefore, it is rarely used in multi-layer, multi-pass welding processes for thick stainless steel plates. SJ103, a fluor-alkali type sintering flux, not only ensures that the weld metal contains sufficient amounts of Cr, Mo, and Ni alloys, but also features good processability, excellent slag removal, and attractive weld morphology. 04 Selection of melting fluxes: At present, most of the fluxes produced in China are melting fluxes, with over 30 different types available. Among them, HJ431 accounts for around 80% of the total production of melting fluxes. Low-carbon steel welded structures commonly use H08A or H08MnA welding wires, and high-manganese, high-silicon fluxes such as HJ431 are generally selected. Through these fluxes, certain amounts of Si and Mn alloying elements can be introduced into the weld metal, thereby enabling the weld metal to possess excellent comprehensive mechanical properties. When using manganese-free, low-manganese, or medium-manganese fluxes, satisfactory results can also be achieved by using high-manganese welding wires (such as H08MnA) or certain alloy steel welding wires. When welding low-alloy steel structures, neutral or basic fluxes should be selected (such as HJ350, HJ250, etc.). Especially when welding low-alloy steels with high weld strength grades and good low-temperature toughness, fluxes with a higher basicity must be selected. 05 Selection of Sintering Fluxes