Submerged arc welding Fluxes may be categorised in two ways:

by the method of manufacture (fused or agglomerated) or

by its activity (neutral, active or alloying).

Within these broad groupings the fluxes may be classified further by their constituents, silica, manganese oxide, calcium fluoride etc.

Flux classification based on Basicity Index

Perhaps the most convenient method of classifying, however, is by reference to the 'basicity index' (BI) of the flux.

The index is calculated by dividing the sum of the percentages of the basic constituents by the sum of the acid constituents.

1. Calcium,

2. magnesium,

3. sodium,

4. potassium and manganese oxides,

5. calcium carbonate and

6. calcium fluoride are the basic constituents of a flux;

7. silica and alumina the acid constituents.

The Basicity index used herein is that set forth by Tuliani et al in the publication Welding & metal fabrication, August 1969 pp 327-339 , which is essentially as follows

Acid fluxes have a basicity index of 0.5 to 0.8;

Neutral fluxes have a basicity index of 0.8 to 1.2;

Basic fluxes have a basicity index of 1.2 to 2.5 and highly basic fluxes 2.5 to 4.0.

The basicity of a flux has a major effect on the weld metal properties, most importantly the notch toughness. As a general rule the higher the basicity the higher the notch toughness.

Acid flux:

The acid fluxes contain substantial amounts of silica, silicates in the form of calcium and/or manganese silicate and manganese oxide. These fluxes react with the weld pool and will raise

both silicon and manganese content of the weld together with a high oxygen content. The result of this is that the toughness of the weld is poor but the fluxes will tolerate rusty surfaces, will detach easily and give a good weld appearance. They are especially useful for single pass high speed welding such as fillet welding of web to flange girder joints.

Neutral flux:

Neutral fluxes are designed to have little or no effect on the chemical analysis of the weld metal and therefore on the mechanical properties. They contain low silica, calcium silicate and alumina and do not add significant amounts of silicon and manganese to the weld.

Basic Flux:

The basic fluxes fill much the same role in submerged arc welding as basic coatings do in manual metal arc welding. They have a low silica content and are composed of varying amounts of calcium carbonate and/or fluoride, alumina, calcium, manganese and magnesium oxides and rutile.

This combination of compounds gives a clean, low sulphur, low oxygen weld metal with good to excellent notch toughness. As a general rule, the higher the basicity, the higher the toughness. The transfer of silicon and manganese into the weld metal is also limited. Such fluxes are preferred for the welding of high quality structural steels, pressure vessels, pipe work and offshore structures where either good high or low temperature properties are required.

Flux classification based on Manufacturing

1. Fused Fluxes

2. Agglomerated fluxes

Fused flux:

The fused fluxes are acid, neutral or slightly basic and are manufactured by mixing the constituents together, melting them in an electric furnace and crushing the solidified slag that is produced to give a flux with a glassy appearance.

These fluxes are homogeneous, resistant to moisture pick-up and mechanically strong so that they do not break down but maintain the required particle size. The high temperatures required by the melting operation mean that some constituents, particularly the de-oxidants present in the highly basic fluxes, decompose and are lost. This limits the range of applications of these fluxes to general structural work where sub-zero service temperatures will not be encountered.

Agglomerated fluxes:

The agglomerated fluxes may be neutral, basic or highly basic. They are made from a wet mix that is corned, dried and baked to achieve a low moisture content. This low temperature process means that strong deoxidants and ferro-alloys can be incorporated without being lost. The binders used in the corning process, however, are hygroscopic so that moisture pick-up

can be a problem on the shop floor. Baking of the flux prior to use may be necessary and the flux should be stored on the welding equipment in heated hoppers. The flux may also suffer mechanical damage during recirculation, breaking down to form a dust. Although a small particle size is capable of carrying a higher current, too many fines in the flux will give rise to gas being trapped between the slag and the weld pool. This will result in unsightly gas flats or pockmarking on the weld surface. To avoid this, the recirculating system should be equipped with filters to remove both large particles of detached slag and the fine dust.

Fluxes are supplied in bags, generally plastic, weighing from 25 to 40kg and in plastic drums of up to 250kg. Recently some suppliers have been packing the flux in hermetically sealed bags, aka vacuum packed electrodes. This method is useful in that the flux can be used straight from the bag with guaranteed low hydrogen levels and without the need to bake prior to use.

This article was written by Gene Mathers.