SMAW process is used majorly in fabrication industry. Different grades of stick electrodes are available today in market for different material and as per the application.
We all know the function of the coating on covered electrode. Primary are shielding of the weld metal, stabilization of the arc and many others are alloying additions to weld metal, concentration of the arc stream, furnish slag for fluxing, characteristics for welding position, control of weld metal soundness, insulation of the core wire and specific mechanical properties to the weld metal.
The coating ingredients of covered electrode includes
- Alloying Elements and de-oxidizers- Alloying elements such as molybdenum, chromium, nickel, manganese and others impart specific mechanical properties to the weld metal. De-oxidizers such as Ferrosilicon, Ferro-chromium, Ferro-manganese are used.
- Binders- Soluble silicates such as sodium and potassium silicates, are used in the electrode coating as binders. Functions of binders are to form a plastic mass of coating material capable of being extruded and baked. The final baked coating should be hard so that it will maintain a crater and have sufficient strength so that it will not spall, crack or chip. Binders are also used to make coating non-flammable and avoid premature decomposition.
- Gas Formers- Common gas forming materials used are the carbohydrates, hydrates, and carbonates. Examples would be cellulose (such as wood flock), the carbonates of calcium and magnesium, and chemically combined water as is found in clay and mica. These materials evolve carbon dioxide (CO2), carbon monoxide (CO), and water vapor (H2O) at the high temperature of the welding arc. Free moisture is another gas-forming ingredient that is found particularly in cellulosic type electrodes and is a part of the formulation in amounts of 2%-3%. It has a marked influence on the arc and is a necessary ingredient in the E6010 type electrode.
- Arc Stabilizers- Air is not sufficiently conductive to maintain a stable arc, so it becomes necessary to add coating ingredients that will provide a conductive path for the flow of current. This is particularly true when welding with alternating current. Stabilizing materials are titanium compounds, potassium compounds, and calcium compounds. (potassium silicate, potassium oxalate, Zirconium carbonate, Lithium carbonate, Titania etc)
- Fluxes and Slag Formers- These ingredients are used primarily to give body to the slag and impart such properties as slag viscosity, surface tension, and melting point. Silica and Magnetite, Rutile , Titania, Potassium titanate, Absestos, Alumina, Silica floor, iron oxide, Fluorspar etc. are materials of this type.
- Plasticizers- Coatings are often very granular or sandy, and in order to successfully extrude these coatings, it is necessary to add lubricating materials, plasticizers, to make the coating flow smoothly under pressure. Sodium and potassium carbonates, Glycerine, China Clay, Talc, Mica are often used.
Reference: ESAB –arc welding technology manual.
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