Types of coated electrodes-low hydrogen electrodes, Mechanical Engineering

Low Hydrogen Electrodes

Low hydrogen electrodes are so called because their covering is low in hydrogen bearing compounds and only traces of hydrogen or moisture are present in the arc atmosphere.

The low hydrogen electrodes were developed for welding hardenable high tensile high carbon alloy steels to eliminate the phenomenon known as "underbead cracking". This effect was often present when heavy sections of high tensile steel were welded with conventional high cellulose, rutile or iron - oxide type electrodes. These under bead cracks occur in the base metal just under the weld metal and are caused by the absorption of hydrogen into the  weld metal from the arc atmospheres. This absorbed hydrogen, then migrates into the base metal and collects in the highly restrained and fast cooled martenstic area causing cracking. Under bead cracking results only when both of the two conditions are present that is, first hydrogen is present in the arc atmosphere and martensite forms during cooling of the heat affected zone.

Although, these cracks do not normally occur in mild (low carbon) steels, they may occur whenever an ordinary electrode is used on high tensile steels. The elimination of hydrogen with its consequent under bead cracking permits the welding of "difficult - to - weld" steels with less preheat than that necessary with other types of electrodes, thus improving welding conditions.

The arc of the low hydrogen electrode is not as harsh or as deeply gouging as the arc of some other electrodes, such as E 6010, but penetration is sufficient for most welding jobs. Some welders may have difficulty when first using these electrodes because of porosity and slag Inclusion. However, high quality radiographically sound weld can be made with proper welding techniques. The currents (amperes) used with these electrodes generally are higher than those recommended for the more conventional electrodes of the same diameter.

As short an arc as possible should be used in all welding positions for best results. A long arc and "whipping" will generally result in porosity and trapped slag. Porosity may also be produced by moisture pick up and too long exposure to high humidity air. Porosity may also result unless the arc is struck ahead of the puddle each time a weld is started.

The deposited metal has a high resistance to hot and cold cracking; accordingly these electrodes are particularly suitable for the welding of heavy work pieces and very rigid mild steel constructions. They are also recommended for welding low alloy steel and steel of which the carbon and sulphur contents are higher than those of easily weldable mild steel. The mechanical properties of the deposited metal are excellent, the notch toughness value is high, at temperature below 0 °C as well.

The crack sensitivity of  weld and transition zone is extremely limited and less than that for all the types previously discussed. The deposition rate of the electrode approximates to that of the acidic type of electrode. Owing to the high solidification rate of the pool, which permits use of high welding currents for welding in difficult positions the low hydrogen electrode is the fastest type for these positions.

The weld metal of the low hydrogen electrodes sets rapidly. The slag, on the other hand, does not solidify quickly. The slag is not shut in by the weld metal.

When welding is done, attention must be paid to the necessity of having a very short arc and to the somewhat different technique for linking beads. The bridging of large openings, the penetration possibilities, the small number of layers and as a result the smaller deformations are factors highly appreciated in practice.  They are attributable to the fore mentioned solidification rates of the pool and the slag. For vertical position welding, the welder prefers an electrode as ease as slag flows out of the weld pool. The low hydrogen electrode has this property to a large extent. A new development in the field of low hydrogen electrodes is the zircon basic type which excels as regards the smooth appearance of the weld.

Posted Date: 9/14/2012 5:26:35 AM | Location : United States

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