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Welding of Coated Steels
Contact nowGalvanized steel is widely used and is becoming increasingly important. Manufacturers of many items such as truck bodies, buses, and automobiles are increasingly concerned with the effects of corrosion particularly when chemicals are used on roads for ice control. Galvanized metal is also used in many appliances such as household washing machines and driers and in many industrial products such as air conditioning housings, processing tanks, etc. Other uses for galvanized products are for high tension electrical transmission towers, highway sign standards, and protective items.
There are two basic methods of galvanizing steel. One is by coating sheet metal and the other is by hot dipping the individual item. The coated sheet metal is produced by the continuous hot dip process, but traffic control devices, high-tension transmission tower parts, etc., are made by dipping each item. The continuous hot dip or zinc coated sheet comes in eight classes based on the thickness of the zinc coating.
Welding of zinc-coated steel can be done, but specific precautions should be taken. When galvanized steel is arc welded the heat of the welding arc vaporizes the zinc coating in the weld area. This is because the boiling point of zinc (1600°F, 871°C) is below the melting point of steel (2800°F, 1538°C). The zinc volatilizes and leaves the base metal adjacent to the weld. The extent to which the coating is disturbed depends on the heat input of the arc and the heat loss from the base metal. The disturbed area is greater with the slower welding speed processes such as oxyacetylene welding or gas tungsten arc welding.
When galvanized sheet is resistance welded, the welding heat causes less disturbance of the zinc coating than the arc processes. The resistance to corrosion or rather the protection by the zinc is not disturbed since the zinc forced from the spot weld will solidify adjacent to the spot weld and protect the weld nugget. Resistance welding of galvanized steel is, however, more of a problem than arc welding.
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