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COATED ELECTRODES

Types of electrodes

When selecting an electrode, the first rule is to select one which produces a weld metal quality equal to or better than that of the base material and, when necessary, is approved for the material in question. Welding position and type of joint are other factors, which influence the choice of electrode, as different electrodes have different properties in different welding positions and types of joint. The most common types of electrodes are: 1. The Organic type (Cellulose) 2. The Rutile type

3. The Acid type

4. The Basic type (Low Hydrogen) Organic electrodes contain large quantities of organic substances such as cellulose. The metal transfer is referred to as explosion arc and the electrodes are well suited for vertical down welding.

Rutile electrodes contain large quantities of the mineral rutile (up to 50 %) or components derived from Titanium Oxide. Rutile electrodes can also contain cellulose. The rutile type of electrode has especially good welding properties both with AC and DC. The organic-rutile electrode is usually the cold welding type, characterized by a spray arc globular transfer, which is an advantage when welding in different positions. This type of electrode is when alloyed well suited for re- and hard surfacing because of its shallow penetrations and high weld buildup. Big opening between plates can easily be bridged using this type of electrode. The rate of welding is not particularly high, but the deposit is of good quality and slag is easily removed.

Unalloyed rutile electrodes are not normally recommended for welding steel with nominal tensile strength exceeding 440 Mpa. The impact values are low because of oxygen level in the weld metal. Rutile electrodes are relatively insensitive to moisture.

Acid electrodes produce an Iron Oxide / Manganese Oxide / Silica type of slag, the metallurgical character which is an acid. The coating contains oxides of the low pH value hence the term acid.

Acid electrodes provide good fusion, a high rate of welding and are equally suitable for AC and DC. The arc is stable and slag is easily removable, even if it is the first bead in a Vgroove weld. Alloyed acid electrodes are suitable for welding steel with a nominal tensile strength of up to 440 Mpa.

3.01

Basic electrodes

Basic electrodes are often referred to as Low Hydrogen electrodes. After special heat treatment the coating has a very low hydrogen content, hence the name.

Basic electrodes with low moisture absorption (LMA) have a lower initial moisture content and the speed of remoisturing is much lower than of normal basic electrodes. Unalloyed basic electrodes give moderate welding speed in the flat position but are faster than other types when welding vertically upwards. The reason for this is that basic electrodes can be deposited at a higher current in the vertical position than other types of electrode. In addition, the amount of weld metal deposited per electrode is greater than that of

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