Welding of Coated Steels

概要:

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 is below the melting point of steel. The zinc volatilizes and leaves the base metal adjacent to the weld.
When galvanized sheet is resistance welded, the welding heat causes less disturbance of the zinc coating than the arc processes. Resistance welding of galvanized steel is, however, more of a problem than arc welding.

There are many types of coated steels and some of them are welded. For example, tern plate steel, which is tin-lead coated, and aluminized steel. Steels coated with tin, cadmium, copper, and lead may be soldered.

Galvanized 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.

Weld Quality

The low boiling temperature of the zinc of the coating causes it to volatilize in the heat produced by an arc or by an oxyacetylene flame. The zinc in the gaseous state may become entrapped in the molten weld metal as it solidifies. If this occurs there will be porosity in the weld metal and if sufficient zinc is available it will cause large voids in the surface of the deposit.

The presence of the zinc in stressed welds can cause cracking and it may also cause delayed cracking due to stress corrosion. To eliminate this, the weld joint must be designed to allow the zinc vapor to completely escape from the joint. Fixturing, backing straps, etc., should be arranged to allow for the zinc to completely escape. Other ways to avoid zinc entrapment in weld metal is to use sufficient heat input when making the weld.

It is also important to secure complete and full penetration of the joint. The ultimate precaution would be to remove the zinc from the area to be welded.

When welding on galvanized steel or any coated steel, particularly those with coatings that produce noxious fumes, positive ventilation must be provided. Positive ventilation involves the use of a suction hose at the weld area. When using the gas metal arc process or the flux-cored arc process, the suction type gun nozzles should be used. Welding on zinc or other coated steels should never be done in confined areas.

For corrosion resistance of the weld it is sometimes advisable to use a corrosion-resistant weld metal. This can be done by using a bronze deposit such as a copper-zinc alloy, or a stainless steel electrode. In any case, when arc or oxyacetylene welding is used the area adjacent to the weld will lose the protective zinc coating which must be repaired.

Arc Welding

When using covered electrodes, the electrode selection should be based on the thickness of metal and the position that will be used when welding galvanized steel. The E-XX12 or 13 will be used for welding thinner material, the E-XX10 or 11 will be used for welding galvanized pipe and for welding hot-dipped galvanized parts of heavier thickness. The low-hydrogen electrodes can also be used on heavier thicknesses.

The welding technique should utilize slow travel speed to permit degassing of the molten metal. The electrode should point forward to force the zinc vapor ahead of the arc. The quality of welds will be equal to those of bare metal, assuming the weldability of the steel is equal.

The gas metal arc welding process is becoming more widely used for joining galvanized steel. For the thinner gauges the fine-wire short-circuiting method is recommended. In this case, the technique would be similar to that used for bare metal. The shielding gas can be 100% CO2 or the 75% argon and 25% CO2 mixture. The selection is dependent primarily on the material thickness and position of welding.

For certain applications, the argon-oxygen mixture is used. The amount of spatter produced when welding galvanized steel is slightly greater than when welding bare steel. The flux-cored arc welding process can be used as easily as gas metal arc welding for galvanized steel. It is recommended for the heavy gauges and on hot-dipped galvanized parts. The highly deoxidized type of welding electrode should be used.

The gas tungsten arc welding process is not popular for welding galvanized steel, since it is one of the slower welding processes and does cause a larger area of zinc adjacent to the weld to be destroyed. In addition, the volatilized zinc is apt to contaminate the tungsten electrode and require frequent redressing of the electrode.

In an effort to overcome this, extra high gas flow rates are sometimes used, which can be expensive. Other techniques may be used as well. If a filler rod is used it may be of either the highly deoxidized steel type or of the bronze type previously mentioned. In this case the arc is played on the filler rod and zinc contamination of the tungsten electrode is avoided.

The carbon arc welding process has been widely used for welding galvanized steel. Both the single carbon torch and twin carbon torch can be used. The twin carbon torch is used as a source of heat much the same as the oxyacetylene flame; however, when the single carbon is used the carbon can be played on the filler rod and extremely high rates of speed can be accomplished. Normally in this situation the filler rod, Type RBCuZn-A (60% Cu-40% Zn). By directing the arc on the filler rod it melts and sufficient heat is produced in the base metal for fusion but not sufficient to destroy the zinc coating. This process and technique is widely used in the sheet metal duct work industry.

Torch Brazing

The oxyacetylene torch is also widely used for brazing galvanized steel. The technique is similar to that mentioned with the carbon arc. The torch is directed toward the filler rod which melts and then fills the weld joint. A generous quantity of brazing flux is used and this helps reduce the zinc loss adjacent to the weld.

Repairing the Zinc Coating

The area adjacent to the arc or gas weld will be free of zinc because of the high temperature of the weld. To produce a corrosion-resistant joint, the zinc must be replaced in this area and on the weld itself unless the nonferrous filler metal was used.

There are several ways of replacing the zinc. One is by the use of zinc base paste sticks sometimes called zinc sticks or galvanized sticks sold under different proprietary names. These sticks are wiped on the heated bare metal. With practice a very good coating can be placed which will blend with the original zinc coating. This coating will be thicker than the original coating, however.

Another way of replacing the depleted zinc coating is by means of flame spraying using a zinc spray filler material. This is a faster method and is used if there is sufficient zinc coating to be replaced. The coating should be two to two-and-one-half times as thick as the original coating for proper corrosion protection.

Other Coated Metals

One other coated metal that is often welded is known as tern plate. This is sheet steel hot dipped with a coating of a lead-tin alloy. The tern alloy is specified in thicknesses based on the weight of tern coating per square foot of sheet metal. Tern plate is often used for making gasoline tanks for automobiles. It is thus welded most often by the resistance welding process. If it is arc welded or oxyacetylene welded the tern plating is destroyed adjacent to the weld and it must be replaced. This can be done similar to soldering.

Aluminized steel is also widely used in the automotive industry particularly for exhaust mufflers. In this case, a high silicon-aluminum alloy is coated to both sides of the sheet steel by the hot dip method. There are two common weights of coating, the regular is 0.40 ounces per square foot and the lightweight coating is 0.25 ounces per square foot based on coating both sides of the sheet steel. Here again, if an arc or gas weld is made on aluminum-coated steel the aluminum coating is destroyed. It is relatively difficult to replace the aluminum coating and, therefore, painting is most often used.

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