Welding galvanized steel
Galvanized steels are welded easily and satisfactorily by all commonly practised welding techniques.
Closer control of welding conditions than for uncoated steel is usually necessary but procedures are simple and well established. This section details procedures for all suitable welding techniques for galvanized steel including GMA (gas metal arc), carbon arc, GTA (gas tungsten arc), manual arc, and oxyacetylene welding.
Where galvanized steel is to be welded, adequate ventilation must be provided consistent with the Safe Work Australia Model Code of Practice for Welding Processes. Grinding of edges prior to welding may be permitted to reduce zinc oxide fumes formed during welding and eliminate weld porosity which can sometimes occur. All uncoated weld areas must have the corrosion protection reinstated.
Work sponsored by International Lead Zinc Research Organization, New York and carried out by E N Gregory of the Welding Institute, Cambridge, England, has been used in recommendations on GMA welding and manual metal arc welding. Recommendations are based on Australian practice and terminology. Information has also been supplied by Liquid Air Australia Limited and Welding Industries of Australia and reviewed in 2018 by technical staff of Weld Australia.
GMA welding galvanized steel
GMA (gas metal arc) welding, also known as CO2 or MIG welding, is a versatile semi-automatic welding process which is convenient and easy to use. It is particularly suited to the welding of thinner materials.
Welding galvanized steel vaporizes the zinc near the arc (zinc boils before steel melts). The zinc oxidises in the air to a fine white powder.
In the GMA welding of galvanized steel, the presence of the zinc coating has no effect on weld properties although some weld spatter is produced. Arc stability is excellent and is not affected by the galvanized coating, although some reduction in welding speed is required.
The GMA welding process
The weld takes place in a protective gas shield. A small diameter consumable wire electrode of 0.8 mm to 1.6 mm is fed automatically to the weld torch. The high current density resulting from the small diameter of the wire is in the region of 200 A/mm2.
The constant voltage type power sources employed offer instantaneous self-adjustment of the arc so that the arc length remains constant even when the operator varies the distance between the electrode and the work piece – power sources are designed to increase welding current as the arc length shortens and the wire burns off at a higher rate to maintain the original arc length. When the arc is lengthened, current is reduced and the wire is consumed at a lower rate, again maintaining the original pre-set arc length. Welding parameters provide for two different types of metal transfer in GMA welding:
- Spray transfer, in which globules of metal are detached magnetically from the wire and propelled across the arc. This is the high current/high voltage form of the process which is used in the flat position on plate thick enough to prevent burn-through.
- Short circuiting transfer sometimes known as ‘dip transfer’, in which lower currents and voltages are used. The end of the wire dips into the molten weld pool while a globule of metal is being transferred. Short circuiting transfer occurs about 100 times per second producing a characteristic buzzing sound. The process is used for welding thin sheet and for positional welding of all thicknesses.
Galvanized steels are welded easily and satisfactorily by all commonly practised welding techniques.
Shielding gas for GMA welding galvanized steel
Galvanized steel is welded satisfactorily using the GMA process and pure carbon dioxide shielding gas which provides excellent weld penetration, but considerable weld spatter. The use of a spatter release compound may be worthwhile.
Alternatively, the more expensive Ar – CO2 or Ar – CO2 – O2 mixes provide adequate weld penetration, a superior weld bead, and far less spatter. A 92% Ar – 5% CO2 – 3% O2 mixture has been found to provide excellent results on galvanized sheet up to 3.0 mm thickness.
Welding conditions
GMA welding speeds should be lower than on uncoated steel as specified in the weld conditions tables, to allow the galvanized coating to burn off at the front of the weld pool. The reduction in speed is related to the thickness of the coating, the joint type and the welding position, and is generally of the order of 10% to 20%.
Fillet welds in steel with thicker galvanized coatings may be welded more readily if the current is increased by 10 A. The increased heat input helps to burn away the extra zinc at the front of the weld pool.
Penetration of the weld in galvanized steel is less than for uncoated steel so that slightly wider gaps must be provided for butt welds. A slight side to side movement of the welding torch helps to achieve consistent penetration when making butt welds in the flat position.
Effect of welding positions in GMA welding galvanized steel
To achieve complete penetration in the overhead position on sheet with heavy coatings of 85 µm, weld current should be increased by 10 A and voltage by 1V.
Welds in the vertical downwards position may require a speed reduction of 25% to 30% by comparison with uncoated steel, depending on joint type and coating thickness, to prevent rising zinc vapour from interfering with arc stability.
Butt welds in the overhead and horizontal vertical positions require little reduction in speed because the zinc vapour rises away from the weld area.
Appearance of GMA welds in galvanized steel
Surface appearance of GMA welds in galvanized steel is satisfactory although a certain amount of weld spatter is generated, regardless of whether CO2 shielding gas or an Ar – CO2 mixture is used.
Minor coating damage will occur and repairs to the weld area to restore the corrosion protection should be carried out.
Adhesion of weld spatter to the gun nozzle, and to the work piece with resulting marring can be prevented by application before welding of an aerosol spray petroleum base or silicone base spatter release compound available from welding consumables suppliers. Any adhering spatter particles can then easily be brushed off. Silicone-based compounds may interfere with paintability and preparation of the area for painting will be required.
Spatter may also build up in the nozzle of the torch interrupting the flow of shielding gas, in extreme cases causing weld porosity and erratic feeding of filler wire. The application of a spatter release compound to the welding torch nozzle reduces the adherence of spatter particles and with the help of a small wire which can be rubbed inside the nozzle.
GMA braze welding
An extension of the GMA process, GMA braze welding utilises a filler metal with a lower melting point than the parent metal. The joint relies neither on capillary action nor on intentional melting of the parent metal. Shielding gases of Ar – O2 type are the most suitable, the low oxygen level being sufficient to permit excellent edge wash and a flat weld without causing surface oxidation. The low heat input minimises damage to the coating on the underside of the parent plate, enables the corrosion resistant bronze filler to cover any of the coating damaged by the arc, and minimises the level of distortion when welding sheet metal components.
Manual metal arc welding galvanized steel
Manual metal arc welding is recommended only for galvanized steel of 1.6 mm thickness or thicker, as difficulty may occur with burning through on light gauges. GMA, GTA, or carbon arc welding are recommended for sheet lighter than 1.6 mm.
In general, manual metal arc welding procedure for galvanized steel sheet is the same as for uncoated steel although the following points should be noted:
- The welding electrode should be applied a little more slowly than usual with a whipping action which moves the electrode forward along the seam in the direction of progression and then back into the molten pool. All volatilisation of the galvanized coating should be complete before bead progress, after which welding is the same as for uncoated steel.
- A short arc length is recommended for welding in all positions to give better control of the weld pool and to prevent either intermittent excess penetration or undercutting.
- Slightly wider gaps up to 2.5 mm are required in butt joints in order to give complete penetration.
- Grinding of edges prior to welding will satisfactorily reduce fuming from the galvanized coating. Welding schedules will then be the same for uncoated steel.
- To restore the corrosion protection repairs to the coating must be carried out.
Electrodes for manual metal arc welding galvanized steel
In general, electrodes to Australian Standard AS/NZS 4855 (ISO 2560) classifications E4312 and E4313 are recommended as suitable for all positions. In butt and tee-joint welds in the flat and horizontal-vertical positions the E4918 basic coated electrode is highly suitable, giving fast, easy welding, improved bead shape, and easier slag removal.
With metal recovery rates of between 110% and 130%, both rutile and basic coated iron powder electrodes perform satisfactorily on galvanized steel, giving a good weld profile with freedom from undercutting, and easy slag removal.
In butt joints in plate with vee edge preparation, an electrode should be chosen which limits the tendency to produce a peaky or convex deposit run since this can cause slag entrapment which will not be removed by subsequent weld runs.
Undercutting in fillet welds is reduced if rutile coated electrodes with a less fluid slag are used since these produce a concave weld profile. Electrodes with very fluid slags tend to produce concave weld profiles with more prevalent undercutting, which is difficult for the welder to rectify.
Different brands of electrodes complying with the same specification may behave differently when used in welding galvanized steel and it may be advisable to carry out simple procedure tests before commencing production welding.
Physical properties of arc welds in galvanized steel
Extensive tensile, bend, radiographic and fatigue testing at the Welding Institute, Cambridge, Engalnd for International Lead Zinc Research Organisation has shown the properties of sound GMA welds and manual metal arc welds in galvanized steel to be equivalent to those of sound welds in uncoated steel. Test welds were made without removing the galvanized coating from edges to be welded.
The presence of any weld porosity due to volatilisation of the galvanized coating during welding has no effect on joint properties except in loss of fatigue strength which can be avoided.
Different brands of electrodes complying with the same specification may behave differently when used in welding galvanized steel and it may be advisable to carry out simple procedure tests before commencing production welding.
GTA brazing Galvanized steel
GTA (gas tungsten arc) process, also known as argon arc, provides an excellent heat source for braze welding.
In GTA brazing, the weld area is shielded from the atmosphere by a protective flow of inert argon gas. A non-consumable tungsten electrode is employed with a separate ‘Cusilman’ (96% Cu, 3% Si, 1% Mn) filler wire, as used for carbon arc welding. The argon barrier prevents oxidation of the electrode or the weld pool and welds of excellent appearance result. The process allows continuous welding at very high speeds, particularly with mechanised arrangements.
In the GTA brazing of galvanized steel the arc should be played on the filler wire rather than on the weld area to prevent undue coating damage.
The following variations in welding technique are also recommended to minimise contamination of the tungsten electrode by traces of zinc oxide fume:
- Hold the weld torch at a 70° angle rather than the 80° angle normally used for uncoated steel.
- Increase shielding gas flow from 6 to 12 l/min to flush zinc oxide fume from the electrode area.
Corrosion resistance of GTA brazed joints made in galvanized steel is excellent. During the welding operation the corrosion resistant brazed metal tends to wet and flow out over the small area from which the galvanized coating has been volatilised, so ‘healing’ the coating.
GTA welding is recommended only as a heat source for brazing galvanized steel, not as a fusion welding technique. When used for fusion welding the tungsten electrode is fouled rapidly by zinc oxide fume.
Oxyacetylene welding galvanized steel
Oxyacetylene welding galvanized steel sheet either with or without a filler rod is generally carried out on the lighter gauges. Because zinc volatilises at about 900 °C while steel melts at about 1500 °C, the necessary welding temperature usually results in coating damage and the need for subsequent treatment of damage areas.
Brazing
Coating damage may be overcome by adopting brazing techniques. Brazing employs much lower temperatures (900 °C), producing very little coating damage in the area adjacent to the weld. The weld metal itself is corrosion resistant and tends to wet and cover all bare steel in the weld area so that joints are normally acceptable without further treatment.
The suggested filler rod is a copper-zinc-silicon alloy, such as Austral Tobin Bronze (63% Cu, 37% Zn, 0.3% Si, 0.15% Sn). Prior to brazing, the edges of components should be painted for about 6 mm back with a flux such as Comweld Copper and Brass Flux or Liquid Air 130 Flux.
The lowest practical heat input is desirable and flame adjustment must be oxidising, as this helps to reduce local loss of zinc in the weld zone. Butt welds are preferred to lap joints and the gap in such welds should be equal to half the thickness of the sheet.
Welding galvanized steel reinforcement
In order to volatilise the zinc coating and so achieve adequate weld penetration, both tack welds and load-bearing welds in galvanized steel reinforcement require greater heat input than similar welds in uncoated steel reinforcement. Manual metal arc, GMA and torch welding processes are all suitable techniques, as detailed in AS/NZS 1554 Part 3. In the case of GMA welding, the use of pure CO2 shielding gas will help weld penetration.
Butt splice welds
In general, welds are made without changes to standard operating parameters other than reduced welding speed to achieve greater heat input. To achieve sound welds, all cracked or damaged areas on bar ends must be removed by sawing or grinding. To provide access for welding at least one bar end must be bevelled.
Lap splice welds
Welds are made satisfactorily using the welding processes listed above. A reduction in welding speed and an increase in heat input will help to volatilise the zinc coating and achieve adequate weld penetration.
For manual metal arc welding, the use of electrodes of a size and type which facilitate volatilisation of the zinc coating will minimise the possibility of weld porosity and liquid metal embrittlement. Cellulose-coated electrodes have given good results. Procedure testing may be helpful.
Alternatively, the galvanized coating may be removed prior to welding by using an oxy-fuel gas flame, or by grit blasting or grinding.
