Covered Welding Electrodes
a. When molten metal is exposed to air, it absorbs oxygen and nitrogen, and becomes brittle or is otherwise adversely affected. A slag cover is needed to protect molten or solidifying weld metal from the atmosphere. This cover can be obtained from the electrode coating. The composition of the electrode coating determines its usability, as well as the composition of the deposited weld metal and the electrode specification. The formulation of electrode coatings is based on well-established principles of metallurgy, chemistry, and physics. The coating protects the metal from damage, stabilizes the arc, and improves the weld in other ways, which include:
(1) Smooth weld metal surface with even edges.(2) Minimum spatter adjacent to the weld.(3) A stable welding arc.
(4) Penetration control.
(5) A strong, tough coating.
(6) Easier slag removal.
(7) Improved deposition rate.
The metal-arc electrodes may be grouped and classified as bare or thinly coated electrodes, and shielded arc or heavy coated electrodes. The covered electrode is the most popular type of filler metal used in arc welding. The composition of the electrode covering determines the usability of the electrode, the composition of the deposited weld metal, and the specification of the electrode. The type of electrode used depends on the specific properties required in the weld deposited. These include corrosion resistance, ductility, high tensile strength, the type of base metal to be welded, the position of the weld (flat, horizontal, vertical, or overhead); and the type of current and polarity required.
b. Types of Electrodes
. The coatings of electrodes for welding mild and low alloy steels may have from 6 to 12 ingredients, which include cellulose to provide a gaseous shield with a reducing agent in which the gas shield surrounding the arc is produced by the disintegration of cellulose; metal carbonates to adjust the basicity of the slag and to provide a reducing atmosphere; titanium dioxide to help form a highly fluid, but quick-freezing slag and to provide ionization for the arc; ferromanganese and ferrosilicon to help deoxidize the molten weld metal and to supplement the manganese content and silicon content of the deposited weld metal; clays and gums to provide elasticity for extruding the plastic coating material and to help provide strength to the coating; calcium fluoride to provide shielding gas to protect the arc, adjust the basicity of the slag, and provide fluidity and solubility of the metal oxides; mineral silicates to provide slag and give strength to the electrode covering; alloying metals including nickel, molybdenum, and chromium to provide alloy content to the deposited weld metal; iron or manganese oxide to adjust the fluidity and properties of the slag and to help stabilize the arc; and iron powder to increase the productivity by providing extra metal to be deposited in the weld.
The principal types of electrode coatings
for mild steel and are described below.
(1) Cellulose-sodium (EXX10). Electrodes of this type cellulosic material in the form of wood flour or reprocessed low alloy electrodes have up to 30 percent paper. The gas shield contains carbon dioxide and hydrogen, which are reducing agents. These gases tend to produce a digging arc that provides deep penetration. The weld deposit is somewhat rough, and the spatter is at a higher level than other electrodes. It does provide extremely good mechanical properties, particularly after aging. This is one of the earliest types of electrodes developed, and is widely used for cross country pipe lines using the downhill welding technique. It is normally used with direct current with the electrode positive (reverse polarity).(2) Cellulose-potassium (EXX11). This electrode is very similar to the cellulose-sodium electrode, except more potassium is used than sodium. This provides ionization of the arc and makes the electrode suitable for welding with alternating current. The arc action, the penetration, and the weld results are very similar. In both E6010 and E6011 electrodes, small amounts of iron powder may be added. This assists in arc stabilization and will slightly increase the deposition rate.(3) Rutile-sodium (EXX12). When rutile or titanium dioxide content is relatively high with respect to the other components, the electrode will be especially appealing to the welder. Electrodes with this coating have a quiet arc, an easily controlled slag, and a low level of spatter. The weld deposit will have a smooth surface and the penetration will be less than with the cellulose electrode. The weld metal properties will be slightly lower than the cellulosic types. This type of electrode provides a fairly high rate of deposition. It has a relatively low arc voltage, and can be used with alternating current or with direct current with electrode negative (straight polarity).