Since both the wire and the flux will have an effect on the weld metal composition, and hence on the mechanical properties, the welding engineer is faced with choosing the appropriate wire/flux combination for the application. This article discusses some of the characteristics of wires and fluxes. The next article will review the specifications.
The welding wire is generally of a composition that matches that of the parent metal and wires are available for the welding of carbon and low and high alloy steels, stainless steels, nickel and copper/nickel alloys. In addition, submerged arc welding may be used for surfacing with corrosion or wear resistant coatings using both wires and flat strips. The wires may be solid or metal cored. Strips may be rolled or sintered.
Welding wires vary from 1.2mm ('thin' wire or twin wire submerged arc) to 6.4mm in diameter and are capable of carrying welding currents ranging from 150 to 1600amps. The wires for ferritic steels are generally copper coated to increase contact tip life, improve electrical conductivity and extend the shelf life. Stainless steel and nickel alloy wires are bright drawn and uncoated. The wire is supplied on reels weighing 10 to 50kg and can also be obtained in large pay-off packs weighing up to 500kg. The strip used for surfacing is supplied in 15 to 240mm widths but the thickness is a standard 0.5mm. As with the wire, strip is available in a range of coil weights.
Whilst the wire is relatively simple and is designed to match the parent metal composition and/or mechanical properties, the flux is far more complex. The functions of the flux are:
- to assist arc striking and stability
- to form a slag that will protect and shape the weld bead
- to form a gas shield to protect the molten filler metal being projected across the arc gap
- to react with the weld pool to provide clean high quality weld metal with the desired properties
- to deoxidise the weld pool
- provide deoxidants
- in some circumstances, to provide additional alloying elements into the weld pool
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.
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. Calcium, magnesium, sodium, potassium and manganese oxides, calcium carbonate and calcium fluoride are the basic constituents of a flux; silica and alumina the acid constituents. Acid fluxes have a basicity index of 0.5 to 0.8; neutral fluxes 0.8 to 1.2; basic fluxes 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.
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.
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.
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, pipework and offshore structures where either good high or low temperature properties are required.
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 been countered.
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 if the flux is not used within a specified (short) timeframe, the flux hoppers on the welding equipment should also be heated, to limit moisture pick-up during storage. 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.