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Basic Coated Low Hydrogen Electrodes for SMAW

Basic Coated Low Hydrogen Electrodes for SMAW

Coating of basic electrodes involves big amount of alkaline–earth metals carbonates, main of them are marble (CaCO3) and fluorspar (CaF2). In addition, there are small amounts of quartz sand and rutile. There is a usage of ferrotitanium, ferrosilicium, and ferromanganese and sometimes ferroaluminum as deoxidizers. Binders are liquid sodium silicate or mixed sodium-potassium silicates.

Shielding gases are provided by thermal decomposition of CaCO3 according to the following reaction: CaCO3→CaO+CO2 Strong deoxidizers (titanium, aluminum, silicon) start to react with carbon dioxide while the coating is melting, for example as in reaction  2CO2+Ti→2CO+TiO2 . Carbon dioxide that doesn’t react with deoxidizers dissociates with active oxygen gas evolution CO2→CO+1/2O2  , therefore the arc atmosphere is oxidative. At high temperatures Si, Mn and Ti coexist with dissolved oxygen in the metal.

At lower temperatures oxygen starts to react with deoxidizers thereby forming oxides. Usually most active elements are Ti and Si, they form acidic oxides TiO2 and SiO2 which have high affinity to basic slag. Basic slag contents high quantity of CaO. “Washing” of welded bath by slag causes formation of stable compounds like CaOTiO2 and CaOSiO2  from acidic oxides, cleaning by such a way metal from non-metallic inclusions. As a result with proper technological production and application oxygen content in metal 0.02-0.03%. Low oxygen content and as a result low content of oxides inclusions provide relatively high plasticity of weld metal.

Another important benefit of basic coated electrode is good resistance to crack formation. It is necessary to mention that high crack resistance is reached only when electrode with low moisture content in coating are used. Basic coated electrodes allow welding in all dimensional positions, mainly with direct current DC+. Relatively high purity of weld metal permits to use basic coated electrodes for welding of critical constructions. With additional alloying these electrodes can be used for welding of high strength steels, heat resistant steels and facing welding works. Electrodes with basic coating have also some deficiencies: low workability, sensitivity to porosity formation, special requirements for packing, storage, transportation and preparing to welding, sensitivity to contaminations, difficulties with AC. basic coated electrodes  

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