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· Classification of SAW fluxes · SAW fluxes can be classified based on their application, chemical composition, manufacturing method, physical properties, and particle structure. In our country, classification is currently mainly based on manufacturing methods and chemical composition. Classified by application: Fluxes can be divided into fluxes for submerged arc welding of carbon steel, fluxes for submerged arc welding of alloy steel, fluxes for submerged arc welding of stainless steel, fluxes for submerged arc welding of copper and copper alloys, and fluxes for submerged arc cladding of stainless steel and nickel-based alloys, depending on the type of steel suitable for welding ； Fluxes are classified according to the applicable wire diameter into fluxes for fine wires (Φ1.6~Φ2.5mm) in submerged arc welding and fluxes for thick wires in submerged arc welding ； Based on the welding position, they can be divided into flat welding fluxes, submerged arc welding fluxes, and force-formed welding fluxes ； Based on specific applications, they can be classified into high-speed submerged arc welding fluxes, narrow-gap submerged arc welding fluxes, multi-wire submerged arc welding fluxes, and strip electrode surfacing welding fluxes, among others. Classified by chemical composition: Submerged arc welding fluxes can be divided into acidic fluxes and basic fluxes based on the ratio of acidic oxides to basic oxides in their composition. The higher the basicity of the flux, the higher the diffusion rate of alloying elements, and the greater the purity of the weld metal; as a result, the notched impact toughness also increases. Based on the SiO2 content in the flux, it can be divided into low-silicon flux and high-silicon flux. Fluxes with a SiO2 mass fraction of less than 35% are called low-silica fluxes ； Those with a SiO2 mass fraction greater than 40% are called high-silica agents. Based on the manganese content in the flux, it can be divided into manganese-free flux and manganese-containing flux. Fluxes with a manganese mass fraction of less than 1% are considered manganese-free fluxes, while those with a manganese content higher than this value are considered manganese-containing fluxes. Classified by the manufacturing method of flux: Based on the manufacturing method, fluxes can be divided into three main categories: fused fluxes, sintered fluxes, and bonded fluxes. Melting flux is produced by uniformly mixing raw materials in accordance with specific formulas, then melting them in a furnace; subsequent steps of water cooling for granulation, drying, and screening result in the finished flux ； Both sintered fluxes and bonded fluxes belong to the category of non-fusing fluxes; in both cases, the raw material powders are mixed together in accordance with specific formulas, a binder is added to form a wet mixture, which is then dried, crushed, and screened. The difference is that sintered flux is dried at temperatures of 400–1000°C, whereas bonded flux is dried at a lower temperature of 350–400°C. Melting fluxes have a uniform composition, high particle strength, low water absorption, and are easy to store; they are the most widely used type of flux in domestic production. Their drawback is that deoxidizers and ferroalloys cannot be added to them, due to the severe burnout that occurs during the melting process. Since non-fusion welding does not involve high-temperature melting during the manufacturing process, deoxidizers and ferroalloys added to the flux are hardly lost. A large amount of alloying elements can be transferred to the weld through the flux, compensating for the loss of these elements due to welding. It is often used for welding high-alloy steels or for surfacing applications. Classified by the physical properties of the flux: Based on how the viscosity of the flux in its molten state changes with temperature, fluxes can be divided into long-slag fluxes and short-slag fluxes. Slag whose viscosity increases sharply as the temperature decreases is called short slag, while slag whose viscosity changes slowly with temperature is called long slag. Short- slag fluxes have better weldability, facilitating slag removal and weld formation, whereas long- slag fluxes have the opposite effect. Classification by flux particle structure: Based on the structure of the flux particles, it can be divided into glassy flux and pumice-like flux. The glassy flux particles are transparent and colored, while the pumice-like flux particles are opaque and porous. The bulk density of glassy flux is higher than 1.4 g·cm-3, whereas that of pumice-like flux is less than 1 g·cm-3; therefore, glassy flux can better isolate the welding area from air intrusion. Classified by the deoxidizers and alloying agents added to the flux: Depending on the deoxidizers and alloying agents included in the flux, it can be further divided into neutral fluxes, active fluxes, and alloyed fluxes. 1. Neutral flux. A neutral flux is one that, after welding, does not cause any significant changes in the chemical composition of the deposited metal compared to that of the welding wire. Neutral flux is used for multi-pass welding and is particularly suitable for welding base metals with a thickness greater than 25 mm. The precautions for welding with neutral flux are as follows: (1) Since neutral flux contains no deoxidizer or only a small amount of it, the deoxidizer must be provided by the welding wire during the welding process. When performing single-pass welding or welding on base metals with severe oxidation, pores and weld cracks may occur. (2) When the arc voltage changes, neutral flux can maintain the stability of the chemical composition of the molten metal. Certain neutral fluxes are reduced in the arc zone; the oxygen released combines with the carbon in the welding wire, reducing the carbon content in the deposited metal. Some neutral fluxes contain silicates, which are reduced to manganese and silicon in the high-temperature zone of the arc. Even when the arc voltage varies significantly, the chemical composition of the deposited metal remains quite stable. (3) When parameters such as penetration depth, heat input, and number of weld passes change, mechanical properties such as tensile strength and impact toughness also change. (4) Although flux is neutral with regard to manganese and silicon, it may not be neutral with regard to active alloying elements, the most notable of which is chromium. Certain neutral fluxes can reduce the chromium content in the weld metal (compared to that in the wire). At this time, the chromium content in the welding wire should be slightly higher than that in the deposited metal. 2. Active flux. Active flux refers to a flux to which small amounts of manganese and silicon deoxidizers are added. It improves stomatal resistance and crack resistance. Active flux is mainly used for single-pass welding, especially on oxidized base materials. Precautions for welding with active flux are as follows: (1) Due to the presence of deoxidizers, the manganese and silicon contents in the deposited metal will vary with changes in arc voltage. Manganese and silicon will increase the strength of the deposited metal, while reducing its impact toughness. Therefore, when performing multi-pass welding with active flux, the arc voltage must be strictly controlled. (2) Among active fluxes, those that are more reactive possess stronger antioxidant properties, which can cause various problems during multi-pass welding. 3. Alloy flux. Alloy flux refers to a flux that uses carbon steel wire, with the deposited metal being alloy steel. A higher amount of alloying elements is added to the fluxes for use in transition alloys; most alloy fluxes are bonding fluxes and sintering fluxes. 02· Basic requirements for flux properties · In submerged arc welding, the flux plays a decisive role in the quality and mechanical properties of the weld; therefore, the following various requirements are imposed on the properties of the flux: (1) To ensure that the weld metal has the desired chemical composition and mechanical properties. (2) Ensure stable arc combustion and thorough welding metallurgical reactions. (3) Ensure that no cracks or pores are formed in the weld metal. (4) Ensure good weld formation. (5) Ensure good slag removal properties of the weld slag. (6) Ensure minimal release of harmful gases during the welding process. To meet this requirement, the flux should have appropriate components and alkalinity, so as to enable effective transfer of alloying elements, desulfurization, dephosphorization, and complete degassing. An appropriate amount of alkaline earth metals (sodium, potassium, and calcium) can be added to the flux to improve the stability of arc combustion. However, the presence of fluorine in the flux is detrimental to arc stability, and fluorine may also precipitate as harmful gases such as hydrogen fluoride and silicon fluoride. However, calcium fluoride in the flux plays a key role in preventing the formation of pores; therefore, the content of calcium fluoride in the flux should be properly controlled. An increase in the manganese oxide content in the flux enhances the desulfurization effect and improves the crack resistance of the weld. The deslagging ability of flux depends mainly on the difference in thermal expansion coefficients between the slag and the metal, as well as the chemical bonding force between the slag shell and the weld surface. Therefore, the components of the flux should ensure a significant difference between the coefficients of thermal expansion of the slag and the metal, while minimizing their chemical bonding force.