How It Works: What is a flux in welding and what are the ingredients of fluxes used in the welding of Carbon Steels?

Flux is a mixture of various minerals, chemicals, and alloying materials that primarily protect the molten weld metal from contamination by the oxygen and nitrogen and other contaminants in the atmosphere. The addition of certain chemicals and alloys also help to control arc stability and mechanical properties. 

Flux is used in the following arc welding processes Shielded Metal Arc Welding (SMAW), Flux Cored Arc Welding (FCAW) and Submerged Arc Welding (SAW). Let’s look at these welding processes and how flux is added and used in shielding the welding zone and in alloying.

Shielded Metal Arc Welding (SMAW) uses a solid core wire for the electrode material. To add the flux to the bare electrode we mix all the ingredients such as sodium and potassium silicate to the dry mix of materials and alloys to create a binder that is then added to the bare electrode by extrusion to make the SMAW coated electrode as shown in Figure 1

Figure 1, Image of extruding the flux on the bare electrode

As stated earlier, the main function of the flux coating is to protect the molten metal from contamination from the atmosphere by forming a shielding gas and slag to cover the molten metal as seen in Figure 2

Figure 2, A covered electrode SMAW

 Other functions of the flux are to;

• Ease of arc striking assist, arc stability and ionization 

• Control of bead shape

• Influence Penetration

• Add alloying elements to the weld

• Control the hydrogen to reduce the chance of induced cold cracking

• Aid in slag removal

• Control mechanical properties of the weld 

In principle there are three differing categories of flux coatings used with SMAW electrodes: 

• cellulosic based fluxes (e.g. E4211) 

• rutile-based fluxes (e.g. E4913) and 

• basic fluxes (e.g. E4918). 

Of the above, only the basic flux types, (E XX18, EXX28, E XX16) are classified as low hydrogen.

Flux Cored Arc Welding (FCAW) uses an outer metal sheath and a core containing a flux and alloying components. The process of adding the flux to the wire is by using a metal strip and passing it through a set or rollers which form a U shape. The flux compound is then added into the strip and it is closed by another U shape roll to make the final flux cored wires as seen inFigure 3

Figure 3, Image of manufacturing flux cored wire

There are three basic manufacturing types of fluxed wires, the butt, folded or overlap methods Figure 4

Figure 4, Typical cross section of flux cored wires

As with shielded metal arc welding electrodes the flux in the flux cored wire is made up of materials and alloys to protect the molten metal from contamination from the atmosphere by creating a shielding gas and a molten slag to cover and protect the weld Figure 5

Figure 5, Flux cored arc welding process

The difference with flux cored welding wires is the are two classes, those that require an external shielding gas (FCAW-GS) to help with protecting the molten metal and, those that do not and are classed as self-shielding flux cored wires (FCAW-SS). These self-shielding wires contain more complex elements to help shield the molten metal.

The fluxes for the gas shielded wires are made up of two main types, 1) rutile or titania types and 2) lime or basic types. In principle the basic fluxes are selected when improved weld metal properties are required.

Submerged Arc Welding (SAW) uses two separate consumables, the solid wire electrode, and the flux. The wire is fed from a coil through the feeder into the weld and covered by the flux which is fed on top of the joint to be welded through a hopper.  The arc is created under the granular flux hence the name submerged arc welding. Some of the flux is melted to create the slag which covers the weld pool and protects the molten metal from the contamination from the atmosphere while the remainder of unmelted flux can be recovered and reused. The submerged arc welding process can be seen in Figure 6

Figure 6, Submerged arc welding process

The solid welding wire electrodes are classed by base metal compositions and the fluxes are made by dry mixing carefully proportioned quantities of materials such as silica sand, metal oxides and amounts of halide salts. These materials are melted together at 1,500 to 1,700 degrees Celsius and then the molten material is chilled to cool. The product, when cooled, is ground and screened to certain particle sizes that forms the granular flux for welding.

Basic fluxes for SAW are made from elements such as Calcium, magnesium, sodium, potassium and manganese oxides, calcium carbonate and calcium fluoride whilst silica, and alumina are the constituents of acid-based fluxes. 

Submerged arc fluxes can be measured by their basicity index which is commonly used to describe the metallurgical behavior of a welding flux. The basicity index is a ratio between basic and acid compounds (oxides and fluorides) of which the flux is composed.

Welding fluxes can be divided into three groups:

1. Acid fluxes with a basicity index of <0.9

2. Neutral fluxes with a basicity index of 0.9-1.2

3. Basic fluxes with a basicity index of > 1.2

Basicity has significant influence on weld metal properties, particularly on toughness. Increasing basicity brings down the oxygen content and hence the inclusion level in the weld metal and thus increases the toughness.

Fluxes and their behaviour are a complex science and the above is meant to provide an introduction only to the types of flux and how they are categorized between the welding processes that use them.