SHIELDED METAL ARC WELDING ELECTRODES

SMAW electrodes are solid (or cast) wire rods covered with a thick flux coating. The thickness and composition of the flux coating determines the electrodes operating characteristics and the mechanical and chemical properties of the deposited weld.

During welding, the flux coating dissolves and produces a gas and slag material. The gas-and-slag shield protects the molten weld metal fro contamination. When the weld deposit has cooled, the slag is removed.

ELECTRODE SELECTION

It is most important to select the proper electrode for each welding job. The quality, appearance, and economy of the weld will depend upon correctly selecting the most suitable electrode. There are a number of factors that must be considered in the choice of electrode:

1. Mechanical Properties. It is essential to know not only the kind of metal being welded (mild steel, cast iron, etc) but also its mechanical properties. The properties of each electrode are indicated by the identification numbers of the AWS electrode classification system. Select the electrode with mechanical properties most closely matching those of the base metals. The mechanical properties should also be of those that impart the highest ductility and impact resistance to the weld.
2. Chemical Properties. The electrode should have approximately the same chemical composition as the metal being welded. The chemical properties listed for an electrode is analyses (expressed in percentages) of the different alloying elements contained in the electrodes wire or core. These are nominal chemical analyses and will differ slightly among the same electrode classification among different electrode manufacturers.
3. Welding current. The electrode selected should be the one that most closely matches the type of power source being used. The type of welding current to be used with a particular electrode is also indicated by the AWS electrode identification numbers. As a general rule, the welder should select the maximum current and the maximum electrode diameter) that can be used with the thickness of the metal being welded. Here, consideration should be given to whether of not the metal has been preheated. Preheated metals require less current than those that have not been preheated.
4. Welding position. The AWS electrode identification numbers also indicate the welding position for which the electrode is designed. Not all electrodes are designed for every use in every welding position. The welder must match the electrode with the welding position being used.
5. Thickness of the metal. The thicker the metal, the greater the current required to produce a suitable weld. An increase in the amount of current required a corresponding increase in electrode diameter. The welder should match closely, the welding current being used to the electrode diameter recommended by the manufacturer.
6. Joint design. The design of the joint (and fitup) determines the degree of arc penetration (deep, medium, light, etc) which is specified by the AWS electrode identification numbers. The welder should select an electrode that gives the required arc penetration.
7. Welding passes. The number of passes is also determined by the type of electrode selected. Multiple passes require more current than a single pass.
8. Joint position. The position in which joints are welded, especially on multipass, is one of the important considerations that affect the choice of electrodes. For flat and horizontal joints, the so-called hot electrodes should be used. Electrodes used for vertical and overhead work, of course, must produce deposits that will stay in place and not fall out of the joint while in molten condition. Deposits of this type usually require an electrode no larger than 3/16” in diameter.
9. Working conditions. Be aware of the working conditions and select an electrode accordingly. Such factors as high temperature, low temperature, corrosive atmosphere and impact loading are important in electrode selection.

The 2 most common steel used is Alloy Steel and Stainless Steel and only these 2 types are discussed here.

SMAW ALLOY STEEL ELECTRODES.

The metal core of these electrodes is made of alloy steel instead of low carbon steel. The electrode coating is similar to the low hydrogen type. Some may also contain iron powder. They are designed for welding high strength alloy steel and can deposit welds with a tensile strength in excess of 100,000 psi. Common applications include the welding of high temperature, high pressure piping, carbon moly pipng used in high pressure, high temperature steam service, and plates or casting with a molybdenum content of approximately 0.50percent.

Electrodes in the E70XX series (E7010, E7011, E7013, E7015, E7016, E7020, E7025, E7026 and E7030) are commonly referred to as low-allow steel electrodes.

The E8010,-11,-13, E9010,-11,-13 and E10010,-11,-13 electrodes also belongs to this group.

SMAW STAINLESS STEEL ELECTRODES.

Stainless Steel electrodes are available with either lime or titanium coatings. The first is used only with DC electrodes positive (DCEP), the second can be used with both AC and DC electrode positive (DCEP) current.

The lime coated electrodes produces flat or slightly convex fillet welds. The slag covers the entire weld, spatter is at a minimum, and the impurities are fluxed from the weld metal. The titania coated electrode produces slightly concave type welds with a smother and more stable arc than that found with the lime-coated type.

The numbering identifications systems for stainless steel electrode differs from the one used for mild steel and low alloy electrodes. The prefixed E indicates an arc welding electrode. The 3-digit number following the prefixed letter indicates the types of stainless steel (eg 304, 310, 316, etc). Two more digits follow and are separated from the first three by a hypen. These last two digits indicate the coating, current polarity, and welding position of each electrode.