What is the introduction of flux?
Flux has a very broad definition, including molten salt, organic matter, active gas, metal vapor, etc., that is, excluding the base metal and solder, it generally refers to the third type of all substances used to reduce the interfacial tension between the base metal and the solder.
Classification
There are many ways to classify fluxes, including classification according to use, manufacturing method, chemical composition, welding metallurgical properties, etc., and also classification according to the pH and particle size of the flux. No matter which classification method is used, it only reflects the characteristics of the flux from a certain aspect and cannot include all the characteristics of the flux. The editor of Zhongyuan Welding Materials Welding Rod Recycling Center said that the commonly used classification methods are as follows: According to the addition of deoxidizer and alloying agent to the flux, it can be divided into neutral flux, active flux and alloy flux, which are also commonly used abroad in ASME standards. classification method. [1] 1. Neutral flux Neutral flux refers to a flux in which the chemical composition of the deposited metal and the chemical composition of the welding wire do not change significantly after welding. Neutral flux is used for multi-pass welding, especially suitable for welding thickness greater than 25mm. parent material. Neutral flux has the following characteristics: a. The flux basically does not contain SiO2, MnO, FeO and other oxides. b. The flux has basically no oxidizing effect on the weld metal. c. When welding heavily oxidized base metal, pores and weld bead cracks will occur. 2. Active flux Active flux refers to a flux that adds a small amount of Mn and Si deoxidizers. It can improve the resistance to pores and cracks. Active flux has the following characteristics: a. Because it contains a deoxidizer, Mn and Si in the deposited metal will change with changes in arc voltage. The increase in Mn and Si will increase the strength of the deposited metal and reduce the impact toughness. Therefore, the arc voltage should be strictly controlled during multi-pass welding. b. Active flux has strong anti-porosity ability. 3. Alloy flux: More alloy components are added to alloy flux, which are used for transition alloy elements. Most alloy fluxes are sintered fluxes. Alloy flux is mainly used for welding low alloy steel and wear-resistant surfacing. 4. Smelting flux Smelting flux is to mix various mineral raw materials according to a given ratio, heat it to above 1300 degrees, melt and stir evenly, then release it from the furnace, and then quickly cool it in water to granulate it. Then it is dried, crushed, sieved, and packaged for use. Domestic smelting flux brands are represented by "HJ". The first digit after it indicates the content of MnO, the second digit indicates the content of SiO2 and CaF2, and the third digit indicates different brands of the same type of flux. 5. The sintering flux is mixed according to the given proportion and then dry-mixed, then the binder (water glass) is added for wet mixing, then granulated, then sent to the drying furnace for solidification and drying, and finally sintered at about 500 degrees. The brand of domestic sintered flux is represented by "SJ", the first digit after it indicates the slag system, and the second and third digits indicate different brands of the same slag system flux.
Element
Flux is composed of minerals such as marble, quartz, fluorite and chemical substances such as titanium dioxide and cellulose. Flux is mainly used in submerged arc welding and electroslag welding. When used to weld various steels and non-ferrous metals, they must be used in reasonable conjunction with the corresponding welding wires to obtain satisfactory welds.
The function of flux:
1. Remove oxides from the welding surface, reduce the melting point and surface tension of the solder, and reach the brazing temperature as quickly as possible.
2. Protect the weld metal from harmful gases in the surrounding atmosphere when it is in liquid state.
3. Make the liquid solder flow at a suitable flow rate to fill the solder joint.
The role of flux in submerged arc welding:
1.
Mechanical protection: The flux melts into surface slag under the action of the arc, protecting the weld metal from the intrusion of gases in the surrounding atmosphere into the molten pool when it is in a liquid state, thereby preventing pore inclusions in the weld.
2.
Transfer necessary metal elements to the molten pool.
3.
To promote a smooth and straight surface of the weld, the melting point of the flux should be 10-30°C lower than the melting point of the solder for good shape. Under special circumstances, the melting point of the flux can be higher than that of the solder. If the melting point of the flux is too lower than that of the solder, it will melt prematurely and the flux components will lose their activity when the solder melts due to evaporation and interaction with the base material. The choice of flux usually depends on the properties of the oxide film. For alkaline oxide films such as oxides of Fe, Ni, Cu, etc., acidic flux containing boric anhydride (B2O3) is often used. For acidic oxide films, for example, for cast iron oxide films containing high SiO2, alkaline Na2CO3 is often used. The flux produces fusible Na2SiO3 and enters the slag. Some fluoride gases are also commonly used as fluxes. They react uniformly and leave no residue after welding. BF3 is often mixed with N2 to braze stainless steel at high temperatures. The flux used for brazing below 450°C is soft solder. There are two types of soft solder. One is water-soluble, which is usually composed of a single hydrochloride and phosphate or an aqueous solution of Soger salt. It has high activity and corrosion resistance. It is highly resistant and needs to be cleaned after welding. The other is a water-insoluble organic flux, usually based on rosin or artificial resin, with organic acids, organic amines or their salts of HCl or HBr added to improve film removal ability and activity.
Flux control
1. Flux drying and heat preservation control. Before using the flux, first bake it according to the specifications of the flux instructions. This drying specification is obtained based on testing and process inspection control, and is a correct data with quality assurance. This is an enterprise standard, and different enterprises The required specifications are also different. Secondly, the flux drying temperature and holding time recommended by JB4709-2000 <<Steel Pressure Vessel Welding Regulations>> are recommended. Generally, when the flux is dried, the stacking height does not exceed 5cm. The welding material library often uses more instead of less in terms of the number of dryings at one time, and uses thicker instead of thin in terms of stacking thickness. This should be strictly managed to ensure the drying quality of the flux. Avoid stacking too thickly and extend the drying time to ensure the flux is thoroughly baked. [2] 2. On-site management and recovery and disposal control of flux. The welding area should be cleaned. Do not mix debris into the flux. The flux including the flux pad must be distributed according to regulations. It is best to wait for use at around 50℃ and prepare it in time. Recycling of flux to avoid contamination; the flux used continuously for many times should be sieved through 8-mesh and 40-mesh sieves to remove impurities and fine powder, and mixed with three times the amount of new flux before use. It must be dried at 250-350℃ and kept warm for 2 hours before use. After drying, it must be stored in an insulated box at 100-150℃ for reuse next time. Storage in the open air is prohibited. If the site is complex or the relative environmental humidity is high, the control site must be managed in a timely manner to keep it clean, carry out necessary tests on the moisture resistance of the flux and mechanical mixtures, control the moisture absorption rate and mechanical inclusions, and avoid piles and fluxes. mixed. [2]3 Flux particle size and distribution require that the flux has certain particle size requirements. The particle size must be appropriate so that the flux has a certain air permeability. The welding process does not reveal continuous arc light to avoid air contamination of the molten pool and the formation of pores. Flux is generally divided into two types, one with a normal particle size of 2.5-0.45mm (8-40 mesh), and the other with a fine particle size of 1.43-0.28mm (10-60 mesh). The fine powder smaller than the specified particle size is generally not more than 5%, and the coarse powder larger than the specified particle size is generally larger than 2%. The flux particle size distribution must be detected, tested and controlled to determine the welding current used. [1-2] 4. Control of flux particle size and stacking height. A flux layer that is too thin or too thick will cause pits, spots and pores on the surface of the weld, forming an uneven weld bead shape. The thickness of the flux layer must be strictly controlled. Within the range of 25-40mm. When using sintered flux, due to its low density, the flux stacking height is 20%-50% higher than that of smelting flux. The larger the diameter of the welding wire, the higher the welding current, and the thickness of the flux layer will also increase accordingly; due to irregularities in the welding process and unfair handling of fine powder flux, intermittent uneven pits will appear on the surface of the weld. The appearance quality is affected and the shell thickness is partially weakened.
