What is welding flux?
Welding flux is a granular welding material that, when melted during welding, forms slag and gas. This substance plays a crucial role in protecting the molten metal and aiding in metallurgical treatment.
Flux composition
The welding flux is composed of a mixture of minerals, including marble, quartz, fluorite, and others, as well as chemicals such as titanium dioxide and cellulose.
The primary application of welding flux is in submerged arc welding and electroslag welding.
Related reading: Types of welding
When welding various steels and non-ferrous metals, it is important to use the welding flux with the appropriate welding wire to achieve a satisfactory result.
Types of welding flux
There are several ways to classify welding flux, including its usage, method of manufacture, chemical composition, metallurgical properties during welding, and the pH and particle size of the flux.
Regardless of the classification method used, it only highlights certain aspects of the welding flux and does not fully encompass all its characteristics.
Common classification methods include:
1. Neutral welding flux
A neutral welding flux is one that does not significantly alter the chemical composition of the deposited metal or the welding wire after welding.
This type of flux is often used for multi-pass welding, particularly when the base metal has a thickness greater than 25 mm.
The characteristics of a neutral welding flux are as follows:
a. The flux contains little to no oxides, such as SiO2, MnO, and FeO.
b. The flux does not cause oxidation in the weld metal.
c. Welding a seriously oxidized base metal may result in porosity and cracks in the weld bead.
2. Active welding flux
An active welding flux is one that contains a small amount of deoxidizing agents, such as Mn and Si. This type of flux can improve resistance to porosity and cracking.
The following are the characteristics of an active welding flux:
a. The presence of deoxidizers such as Mn and Si can cause changes in the chemical composition of the deposited metal as the arc voltage fluctuates. An increase in Mn and Si can increase the strength of the deposited metal but decrease its impact toughness. Hence, it is important to closely control the arc voltage during multi-pass welding.
b. The active welding flux has a strong ability to prevent porosity.
3. Alloy welding flux
An alloy welding flux contains additional alloy components that serve as transition elements. Most alloy welding fluxes are sintered.
This type of flux is primarily used for welding low-alloy steel and for wear-resistant surfacing.
4. Melting welding flux
Melting welding flux is produced by combining raw materials of various minerals in a specific proportion, heating them to over 1300 ℃, melting and thoroughly mixing them, and then cooling them in water to form granules.
The process continues with drying, crushing, sieving, and packaging for use.
In China, a common brand of melting welding flux is “HJ”. The first digit after the “HJ” designation indicates the content of MnO, the second digit represents the content of SiO2 and CaF2, and the third digit distinguishes between different brands of the same type of welding flux.
5. Sintered welding flux
After proportioning the ingredients, dry mixing is performed and a binder (water glass) is added for wet mixing. The mixture is then granulated.
Next, it is sent to a drying oven for curing and drying, and finally sintered at approximately 500 degrees.
In China, a common brand of sintered welding flux is represented by “SJ”. The first digit after the “SJ” designation indicates the slag system, while the second and third digits distinguish between different brands of the same type of slag system flux.
