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Welding Electrode
An Electrode is a metal wire coated with a similar composition to the metal that has to be weld.
Types of Welding Electrode
Electrodes are mainly of two types according to their materials. These are:
- Carbon electrodes.
- Metal electrodes.
Carbon electrodes
These electrodes are used for carbon arc welding. They are approximately 30 cm long and vary in diameter from 2 mm to 13 mm, and are made of carbon or graphite.
Graphite electrodes are harder and more brittle. They withstand higher current densities, lasting longer than electrodes made of carbon; however, the arc column is harder to control.
Carbon electrodes are softer and do not carry much current as of the BSI graphite electrode, but the arc column is easier to control. In general, with the carbon electrode, higher-current density adds to arc stability, and a low-current density means longer life.
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| Welding Electrodes Types |
Although both the carbon and graphite electrodes are considered non-consumable, the disintegration of the electrode occurs at a slow rate during the process of welding, as a result of vaporization and oxidation.
Carbon arc electrodes are easier to manipulate if a point is ground 18 TO 25 mm on the electrode. This point resembles the point on a lead pencil angling back approximately 18 to 25 mm. The tip should be approximately 1.5mm in diameter.
Metal electrodes
There are three types of metal electrodes, which are :
- Bare metal electrodes
- Fluxed metal electrodes
- Heavily coated metal electrodes
Bare metal electrodes: As consumable electrodes, bare metal electrodes have limited use for welding of wrought iron and mild steel. Straight polarity is generally recommended.
The bare metal electrode is extensively used in the form of wire mounted on a spool in the process of welding. During welding, the arc end of the electrode wire is covered by an inert gas shield.
As non-consumable electrodes, tungsten electrodes are used in TIG welding and atomic hydrogen arc welding. Copper electrodes are used for various types of resistance welding processes (spot, projection, and seam).
Fluxed metal electrodes: As consumable electrodes, they produce improved welds as compared to bare metal electrodes. They carry a light coating flux. The flux assists both in eliminating undesirable oxides and in preventing their formation.
Heavily coated metal electrodes: They are used as consumable electrodes in shielded metal arc welding. The heavily coated arc electrodes are by far the most important ones used in all types of commercial welding. Over 95% of the total manual welding that is being done today is with coated electrodes. The effect of heavy coatings on electrodes is to provide a gas shield around the arc to eliminate the formation of undesirable oxides and nitrides in the weld metal.
In addition, this gas shield so produced covers the weld metal with a protective slag coating which prevents oxidation of the metal during cooling. Welds made from rods of this type have superior physical characteristics. Recommendations of the manufacturers of electrodes should be followed in the selection of an electrode for a given job.
Heavy coatings permit the use of larger welding rods, higher current, and greater welding speeds.
Functions of Coatings on Electrode
The coatings given on the electrodes perform the following functions. These functions are not common to all coated electrodes, since the coating put on a given electrode is largely decided by the kind of welding it has to perform.
- Providing a protective atmosphere.
- Providing slag of suitable qualities that may protect the molten metal.
- Stabilizing the arc.
- Facilitating overhead and odd position welding.
- Adding alloying elements to the weld metal.
- The refining grain size of the weld metal.
- Reducing spatter of weld metal.
- Improving deposition efficiency.
- Removing impurities and oxides.
- Influencing the depth of arc penetration.
- Controlling the cooling rate (generally slowing down) of the weld.
- Influencing the shape of the bead.
Principal constituents of the coating
Some of the principal constituents of electrode coatings are as follows:
- Slag forming constituents Si 0, Fe O, and MnO2.
- Deoxidizing constituents: Graphite, wood floor, aluminum powder.
- Binding constituents: Sodium silicate, potassium silicate, and asbestos fibers.
- Arc improving constituents: CaO, MgO, N, O, and TiO,
- Alloying constituents: Al, Cb, Ce, Co, Cr, Mn, Mo, Ni, W, and Zr.
Electrode size
They are generally manufactured in standard lengths 250 mm, 300 mm, 350 mm, and 450 mm. Similarly, the standard sizes of electrode diameters manufactured in India are 1.6 mm, 2 mm, 2.5 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm and 9 mm.
Classification of electrodes
Electrodes are specified/classified (or designated) accordingly! IS.815_1956 and IS: 814–1963. According to this system, a coated metal electrode is specified by six digits with a prefix letter M, which indicates suitable for metal arc welding. These six digits in the specification/ designation indicate as given below:
- First digit: It may be any number from 1 to 8. Each number stands for a particular type of coating provided on the electrode.
- Second digit: It also consists of any number from 1 to 6. Each number represents a particular weld position (s) for which electrode is suitable.
- Third digit: It consists of any number from 0 to 7. It denotes a particular current condition suitable for the electrode.
- Fourth digit: It may carry any number from 1 to 8. Each number represents a particular tensile strength of the weld metal in kg/cm2.
- Fifth digit: It may carry any number from 1 to 5, each number representing a specific percentage elongation of deposited weld metal. Impact strength of the weld metal.
- Sixth digit: It may carry any number from 1 to 5, each number representing a specific value of minimum impact strength of the weld metal.
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