Electrodes are designed by metallurgists and welding engineers who specify the composition of coated electrodes to achieve various results in weld metal and the finished weld. Manufacturers of electrodes have developed arc welding electrodes to weld ferrous

alloys such as cast iron, rolled steel, chrome steel, nickel chrome steel, manganese steel, and non-ferrous alloys such as bronze, brass, copper, or nickel alloys. They have also developed electrodes for welding materials in the pure state, such as aluminum. The

ingredients of the electrode coatings can be selected to provide shielding gas, flux, and slag during welding. See ELECTRODE.

Historical Background

In the early years of welding, strips of sheet, bare steel rod or wire (sometimes baling wire) were used as consumable electrodes to provide the filler material for metal arc welding. These early electrodes produced an unstable arc which was difficult to initiate, and resulted in welds that were porous and brittle. Better welds seemed to result when the wire and rod were slightly coated with a film of rust, or when the wire or

rod had a light lime coating remaining after lime had been used as a lubricant in the drawing process. It was soon determined that the bare electrode must be either

sul-coated, which is a special rust-coated finish produced by spraying the wire with water before the last drawing, or lime coated. The light lime coating assisted in keeping the arc steady by producing a vapor which would conduct the current, but it did not significantly improve weld quality. Many materials were used in experiments to find substances that could be added to improve weld results. When the lime coated or sul-coated electrode was wrapped in newspaper, a gaseous shield for the arc was formed which improved the weld, apparently the result of the cellulose in the paper. Among the other materials used

experimentally to form cellulosic coatings were sawdust, cotton, wood flour, wheat flour and rice flour.

The carbon content of welding rods for arc welding steel changed the characteristics of the weld. Carbon steel electrodes were manufactured in two grades. A

higher carbon content was supplied for arc welding and a lower carbon content for gas welding. Today, electrodes containing from 0.13% to 0.18% carbon are widely used for welding mild steel.

Coatings

In general, there are two types of electrodes for welding ferrous alloys: medium or semicoated, and heavily coated electrodes.

Medium or semicoated electrodes are made by dipping the core wire material in a liquid flux, and withdrawing it to allow the flux coating to dry. A coating of this type usually represents only 1% to 2% of the weight of the electrode.

The heavily coated electrodes are dipped several times to obtain the desired thickness, or they are passed through an extrusion press, the generally accepted practice, where a coating is applied uniformly by extrusion. Specifications require that the extruded coating be concentric with the electrode; a coating which is 3% or more off center is unsatisfactory.

Core Wire Material

The most suitable core material for steel electrodes is a high-grade rimmed steel. Killed or semi-killed steels do not function as well. A typical specification for a widely used type of electrode follows:

Carbon                        .13 to .18%

Manganese                  .40 to .60%

Silicon                        0.06%

Sulphur                       0.04%

Phosphorus                 0.04%

The tensile strength of the metal, and to some extent, the smoothness and soundness of the deposit are affected by the carbon and manganese content. Minimum sulfur content is important; it should be as far below 0.04% as reasonably possible.

Composition of Coatings

Electrode coatings usually contain substances such as silicon, calcium, barium, and magnesium. Water glass, a solution of sodium silicate, is usually used as a binder, but various gums, glues and lacquers are also used. They are classed as cellulosic and mineral.

Principal stabilizers for both the cellulosic and mineral coatings are titanium dioxide, feldspar and calcium carbonate. Ferromanganese also appears in both types of coatings as a deoxidizer used for porosity control. It also tends to balance the manganese burned out in arc transfer. In some coatings, alloy ingredients such as molybdenum, which has good transfer characteristics, are added.

Cellulosic. This coating contains such ingredients as sodium silicate, ferromanganese, titanium dioxide and alpha and beta cellulose. This type of coating is sometimes referred to as the high ignition loss type, because a considerable portion of the coating burns away to form a gas in the arc.

The cellulosic coating is a high-quality coating used for all-position electrodes; however, it is suitable only for DCEP. Arc action produces a forceful spray weld metal transfer with deep penetration.

Mineral. A mineral electrode coating usually consists of metallic oxides and silicates. This type of coating produces an abundance of slag which provides ample coverage and complete shielding. Electrodes with mineral coatings are usually confined to welding

in the horizontal and flat position. These electrodes produce high quality weld metal using either ac or dc.

The mineral ingredients in this coating form gases around the molten and vaporized material from the core iron as they pass through the arc, protecting them

from the atmosphere as they form a molten slag covering the weld metal.

Rutile (titanium dioxide) is an important substance in mineral-coated electrodes. The rutile coated electrodes have a coating of moderate thickness so that globular transfer and a rapid rate of solidification occurs. This feature adapts well to joints with a relatively poor fit-up. In addition to an abundance of rutile, these electrode coatings include some ferromanganese, feldspar, and sodium silicate. These general purpose, all-position electrodes operate on ac or DCEN.

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