What are the characteristics of submerged arc welding flux?

Submerged-arc welding fluxes are generally classified into two categories: fused fluxes (such as HJ350 and HJ431) and sintered fluxes (such as SJ101 and SJ201). Currently, some users of submerged-arc welding fluxes differentiate between the applications of sintered fluxes and fused fluxes. In recent years, sintered fluxes have accounted for 80% of the market share, while the use of fused fluxes has been declining. In fact, each type has its own advantages and disadvantages; when used appropriately, they can help avoid many unnecessary complications.

The primary function of submerged-arc welding flux is to form a slag that shields the weld pool from atmospheric contamination and controls the chemical composition of the weld metal, thereby ensuring the mechanical properties of the weld and preventing defects such as porosity, cracks, and slag inclusions. Depending on the requirements of the welding process, the flux must also exhibit good arc stability and produce a slag with appropriate density, viscosity, and melting point, so as to facilitate effective weld formation. Furthermore, the solidified slag layer that forms after welding possesses excellent slag-removal characteristics.

Currently, welding fluxes in China are primarily classified according to their manufacturing method and chemical composition. Based on the manufacturing method, they can be divided into fused fluxes, submerged-arc fluxes, and bonded fluxes.

Fusion flux involves mixing the raw materials according to a specified formulation in a melting furnace, melting them, and then cooling with water, granulating, and drying. The resulting product is screened and packaged. Submerged-arc welding fluxes and bonded welding fluxes are non-melted fluxes, prepared by wet mixing, drying, pulverizing, and sieving the raw materials according to a formulated recipe. The difference is that sintered fluxes are dried (sintered) at 400–1000°C, whereas conventional fluxes are dried at 350–400°C.

As known to manufacturers of submerged-arc welding fluxes, molten fluxes are characterized by uniform composition, high particle strength, low moisture absorption, and easy storage, making them widely used in China today. However, a drawback is that deoxidizers and ferroalloys cannot be added to the flux, as they suffer substantial burn-off during the smelting process.

The characteristics of submerged-arc welding flux are as follows: during each high-temperature melting cycle, deoxidizers and ferroalloys can be added to the flux, enabling a substantial transfer of alloying elements into the weld metal to compensate for the burn-off of alloying elements from the welding wire. It is commonly used for welding high-alloy steels or for surfacing applications. In addition, submerged-arc welding flux exhibits excellent slag-removal properties, making it particularly suitable for narrow-gap submerged-arc welding of thick workpieces.

According to manufacturers of submerged-arc welding fluxes, the submerged-arc process relies on the arc as the heat source, while the flux primarily serves to provide mechanical protection and to participate in metallurgical reactions. In electroslag welding, the slag pool itself acts as the heat source. In addition to protecting the molten pool and facilitating metallurgical processes, the flux’s main function is to generate heat—specifically, resistive heating within the slag pool. The term “welding material,” also referred to as solder, has a broad definition that encompasses molten salts, organic compounds, active gases, metal vapors, and other substances. In other words, beyond the base metal and the welding material itself, it includes three types of substances designed to reduce the interfacial tension between the base metal and the welding material. These substances work to remove oxides from the weld surface, lower the melting point and surface tension, and enable the weld to reach the required temperature more quickly; they also protect the liquid weld metal from harmful gases in the surrounding atmosphere, and ensure that the liquid welding material flows at an appropriate rate to fully fill the joint.