Welding Galvanized Steel

Many commonly practiced welding and cutting techniques can be used on galvanized steel (see American Welding Societys (AWS) specification D-19.0, Welding Zinc Coated Steel). Welding on galvanized steel can be necessary if the final structure is too large to be dipped in a galvanizing bath or for structures that must be welded in the field.

Welding galvanized steel should always be done in well-ventilated locations to minimize fume inhalation. The AWS publication, AWS/ANSI Z49:1, Safety and Cutting in Welding, covers all aspects of welding safety and health. However, galvanized steel can be welded without removing the zinc coating if special procedures are followed.

Listed below are abbreviated procedures for welding galvanized steel using the most common welding techniques.

.Gas Metal Arc Welding

.Shielded Metal Arc Welding

.Oxyacetylene Welding

.Stud Welding

.Welding Rebar

.Friction Welding

.Resistance Welding of Zinc-coated Steel

Gas metal arc welding, also known as Metal-Inert Gas (MIG) welding, is a versatile semi-automatic welding technique particularly suited for the welding of thinner materials (<1/2 [13 mm] thick).

Welding speeds for GMAW are typically slower for galvanized surfaces. These reduced speeds allow longer time for the zinc to burn off at the front of the weld pool. Increasing the current supplied to the welding electrode may provide sufficient means to burn off zinc coatings of greater thickness.

Weld penetration depth is decreased when welding galvanized steel. When performing butt-welds, larger gaps must be provided. Consistent penetration is achieved by employing a side-to-side motion of the welding torch when butt-welding in the flat position. Spatter increases when welding galvanized steel using CO2 shielding gas. The formation of spatter particles is directly proportional to the thickness of the zinc coating. Therefore, spatter formation is greater for hot-dip galvanized steel than continuously galvanized (sheet) steel.

Spatter particles can adhere to the steel surface causing an unsightly appearance. Applying a silicon-, petroleum- or graphite-based spatter release compound before welding can reduce spatter adherence. These compounds allow spatter particles to be easily brushed off after welding.

Increasing the heat, reducing welding speed, and using an argon-CO2 shielding gas when GMA welding can give a more stable arc and produce smoother weld deposits with minimum spatter and zinc loss.

The most common arc welding process is shielded metal arc (SMAW) welding. SMAW welding is a process in which flux-covered electrodes ranging from 9 to 18 inches (23 to 46 cm) in length, and 1/16 to 5/16 inches (1.6mm to 8.0 mm) in diameter, are used.

Weld depth penetration is decreased in the SMAW welding of galvanized steel, as is the case for GMAW welding, the root opening for butt-welds must be increased from that of uncoated steel surfaces. However, it is possible to obtain full weld depth penetration by altering the normal weld techniques for uncoated steel. If the angle of the electrode is reduced from the normal 70º to 30º , and the weld speed is reduced significantly, normal weld depths can be achieved by moving the electrode back and forth in line with the joint.

Spatter formation also increases using SMAW welding. Generally, spatter formation does not increase to an extent where anti-spatter compounds are required. Slower welding speeds allow for more of the zinc coating to burn off and reduce the amount of spatter formation. As for GMAW welding, it is not usually necessary to increase the current to the electrode to increase the amount of zinc burned off.

Reducing the angle of the electrode and reducing the weld travel speed will significantly increase the quality of SMAW weld on the zinc-coated surface. Steels with thicknesses greater than 1/2 (13 mm) are recommended to be welded with SMAW. The following considerations should be noted when applying SMAW on galvanized steel:

The welding electrode should be applied slower than normal, with a whipping action that moves the electrode slightly forward of the weld pool and then backward into the weld pool. This ensures that all of the zinc is burned off before the weld bead progresses. After volatilization, the welding is the same for uncoated steel.

Weaving and multiple weld beads should be avoided. Heat input into the joint should be kept to a minimum to avoid undue damage to the adjacent coating, while not sacrificing the heat required to burn off the zinc prior to bead progression. A short arc length is recommended for welding in all positions to give better control of the weld pool and to prevent either intermittent excessive penetration or undercutting. Weld penetration depth is decreased when welding galvanized steel. When performing butt-welds, larger gaps must be provided. Consistent penetration is achieved by employing a side-to-side motion of the welding torch when butt-welding in the flat position. Deviations from SMAW techniques for uncoated surfaces and zinc-coated surfaces occur due to the extra heat required for removal of the zinc coating. Inducing a whipping motion during welding allows for as much as possible of the zinc coating to be removed prior to weld formation. A more disturbed weld pool results, increasing the fluidity of the slag and spatter formation.

Resistance welding is generally used to join galvanized steel less than 1/4 inch thick and with a zinc coating lighter than 1 oz/ft2 (305 g/m 2 ). Coatings up to 1.5 oz/ft 2 (460 g/m 2 ) have been successfully welded, but electrode life is much shorter than with lighter coatings. On heavier coatings, it is necessary to replace or redress worn electrodes more frequently. Sheet materials can be resistance welded without removal and with little damage to the zinc coating. Most after-fabrication galvanized coatings are thicker than recommended for resistance welding and become impractical to do so.