There are various do’s-and-don’ts that should be observed by the detailer when it comes to welding of fabricated structures or components which are to be galvanized.
The first considerations relate to the residual stresses which may lead to distortion. (Refer to: Avoiding Distortion during Hot Dip Galvanizing)
The second relates to proper venting and the avoidance of entrapment of liquids particularly with overlapping joints.
Overlapping welded joints where the surfaces are in contact require special attention. Cleaning solutions used in the galvanizing process can become trapped between overlapping and contacting surfaces. Upon immersion into the galvanizing bath at temperatures of more than 450 °C this moisture is converted to superheated steam which may result in localized uncoated surfaces and pressure increases which could possibly rupture the fabrication.
Examples of overlapping joints include Plates welded to another plate or member and back-to-back angles or channels. Back-to-back joints are fraught with problems and should be avoided – for more information on this go to …….
The best way to mitigate this situation is to avoid welded overlapping joints altogether substituting them with a bolted assembly, but where this is not a practical solution, there are a number of options available to minimize these effects:
It’s important to consult with the responsible engineer and / or the galvanizer before fabricating any piece that will include overlapping surfaces. The galvanizer's experience may greatly assist in deciding between a completely seal-welded or intermittent welded joint.
Fully seal welding can prevent the ingress of the various cleaning solutions during the galvanizing process, However, seal-welding can inadvertently affect the structural behaviour of the welded components, and the use of seal-welding requires a variance to comply with AWS D1.1, Structural Welding Code – Steel
Consequently, seal welding should only be specified with approval from the responsible engineer!
Even with seal welding, the enclosed area between overlapping parts may still trap condensation - and any pin-holes in the welds may allow pre-treatment solutions to enter the enclosed area during the galvanizing process, so large seal welded surfaces will require additional venting in the form of a hole (or holes) drilled in one of the overlapping surfaces or alternatively, the use of intermittent welds.
The general guidelines for the placing of vent holes are:
- Overlapping surface areas less than 100 cm/2 generally do not require venting
- Overlapping surface areas between 100 and 400 cm/2 should be vented with a hole of approximately 2 x the plate thickness but not less than 10 mm.
- Overlapping surface areas above 400 cm/2 should be vented with a hole of approximately 2 x the plate thickness but not less than 12 mm.
- When the overlapped surface area reaches 2500 cm/2, vent holes shall be minimum Ø20mm and progressively placed every 2500 cm/2.
Alternatively, the use of intermittent welding can be considered, but is not preferred.
Intermittent welding requires that a gap of at least 2.5 mm (3/32 inch) is allowed between the overlapping surfaces - this is because the viscosity of the molten zinc will not allow it to enter and drain from a lesser gap, but will permit the ingress of the less viscous cleaning fluids. This will mean the possibility of uncoated areas in the immediate vicinity of the lap joint and may cause unsightly iron oxide to weep out of the joint later.
The general guidelines for the placing of intermittent welds are:
- Overlapping surface areas between 100 and 400 cm/2 should be provided with an intermittent gap of 25 mm.
- Overlapping surface areas above 400 cm/2 should be provided with an intermittent gap of 50 mm.
- When the overlapped surface area reaches 2500 cm/2, an intermittent gap of 100 mm should be provided and progressively placed every 2500 cm/2 thereafter
- 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 Steel after Galvanizing
AWS D-19.0 calls for welds of galvanized steel on areas free of zinc. Thus, for galvanized structural fabrications, the zinc coating should be removed at least 25 to 100 mm from either side of the intended weld zone and on both sides of the piece. The most common and preferred removal method is grinding back the zinc coating, but burning the zinc away or pushing it back from the weld area can also be effective.
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.