Five must to know welding electrode coatings
In welding, flux always has a vital role in a number of ways and that is why it is frequently utilized in electrode outer coatings. Sometimes it is present in core of the filler wires (as in case of FCAW wires and flux cored TIG rods). First you need to know that what are the primary functions of flux in a welding process?
Flux helps;
To remove impurities from the molten metal.
In slag formation which can be easily removed from the weld.
To protect the weld deposit from air contamination.
To add alloy materials in the weldment.
To add an additional filler metal to the weldment.
Moreover care is required for handling of coated electrodes, even a slight blow or poor handling can lead to the damage. An electrode with damage coating may cause severe weld defects like porosity, undercuts, lack of penetration etc. Listed below are the types of coating that are normally applied in commercial/industrial welding operations.
1. Cellulose electrodes
Such coatings are made of cellulose which includes organic materials (primarily wood flour is used). They are thin and assist in easy removal of slag and are most suitable for positional welding (best for vertical). On heating, these type are dissociated into hydrogen and carbon dioxide which in turn serve as shielding gases. Normally direct current power supply is employed but addition of some stabilizers can also facilitate the use of alternating current for cellulosic electrodes.
2. Rutile electrodes
As we know rutile is the mineral of titanium and this type electrode contains titanium dioxide in coating. The presence of titanium dioxide, gives an acidic slag and other contents of coating provides gas shielding of hydrogen, oxides of nitrogen and carbon for sound protection. These type electrodes are used for low carbon steels in all positions and produce sound weld with sound mechanical properties. Some addition of cellulose in rutile electrodes, provides an additional supply of gas shield and are known as medium rutile electrodes. Sometimes minor addition of cellulose in rutile gives further improvement in performance, such a combination is called heavy rutile electrodes.
3. Iron oxide electrodes
Such type of coating contains metallic oxides including iron oxide, silicates and manganese oxide which produce acidic slag. Both direct and alternating current can be employed on these electrodes. Due to high oxygen content these electrodes may lead to a low strength weld deposit. For this addition of some deoxidizers in iron oxide electrodes, assists in producing a controlled deoxidized weldment with best mechanical properties and also due to which the slag can be removed easily. Such addition makes the iron oxide electrodes suitable for positional welding.
4. Basic electrodes or Hydrogen controlled electrodes
Basic electrode contains calcium carbonate, magnesium carbonate, calcium fluoride and other minerals along with water. These electrodes are required to store in dry condition and properly bake before use. With the formation of basic slag, sound welds with sound mechanical properties are achieved. Gas shield includes carbon dioxide with low hydrogen and oxygen content. The control of hydrogen provides protection from cracking and makes these electrode suitable for high strength steels, low alloy steels and low carbon steels.
5. Iron Powder electrodes
Iron powder is added in all types of coating to increase the efficiency of electrodes. Iron powder addition increases the deposition rate. It reduces the voltage and enables the cellulosic electrodes to cope with alternating current. Moreover it controls the flow of slag that is quite beneficial in positional welding.
