Lesson 3 - Covered Electrodes for Welding Mild Steels

3.4 SELECTING THE PROPER MILD STEEL COVERED ELECTRODE Many factors must be considered when selecting the proper electrode for a given application. Some items to be considered are: a) Type of Base Metal - Welding mild steels or low carbon steels (carbon content below 0.30%) with mild steel coated electrodes presents no problems as far as tensile strength is concerned since the tensile strength of the weld metal usually exceeds the tensile strength of the base metal. However, chemistry of the base metal is important. Welds made on free machining steels that have a relatively high sulfur content, will be porous unless welded with a low hydrogen type electrode such as E7018. Sometimes off analysis steels or mild steels of doubtful analysis are encountered. In this case, one of the low hydrogen types would be the best choice. b) Position of the Weld - Weld position will determine whether an all-position electrode or a flat and horizontal type electrode should be used. Higher welding currents, and therefore, higher deposition rates are possible when welding flat or horizontally. Whenever possible, the work should be positioned both for ease of welding and to attain the highest welding speed. c) Available Equipment - Electrode choice will depend on whether AC or DC welding machines are available. If both currents are available, consider these general facts. 1. For deepest penetration, use DC reverse polarity (Electrode Positive). 2. For lower penetration and higher deposition rate, use DC straight polarity (Electrode Negative). 3. For freedom of arc blow, use AC. d) Plate Thickness - When welding sheet metal, low penetration electrodes should be chosen. Heavier plate may demand an electrode with deep penetration. Very heavy plate may require a deep penetrating electrode for the initial or root pass, and a higher deposition type for succeeding passes. e) Fit-Up - Some electrodes are more suitable than others for bridging gaps between the members to be welded. This is termed "poor fit-up" and some electrode manu- facturers produce electrodes that are specially formulated for this purpose. f) Welding Costs - The major factors that affect welding costs are labor and overhead, deposition rate, efficiency of the electrode being used and the cost of the elec-

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