Hanging and Handling
Facilities exist to galvanize components of virtually any size and shape, depending on handling equipment and layout of the galvanizing plant.
Most articles to be hot dip galvanized will be suspended from a jig and/or overhead crane using wires, chains, brackets or hooks while being processed.
The maximum size and weight that a particular galvanizer can process should always be checked at the design stage.
A directory listing the dimensions of all galvanizing baths operated by GAA members is available on the website: www.gaa.com.au
Adequate hanging points should be provided, e.g. suspension holes or lugs, taking into consideration article size and the lifting capacity of equipment.
For long, straight sections, 2 lifting lugs are preferred to avoid wire or chain marks.
Where possible, articles are hung on a 45° angle (approximately) to ensure efficient drainage of pre-treatment solutions and molten zinc. This avoids rough surfaces and lets the air escape from the highest point, preventing explosions.
Long items will often be withdrawn from the bath at a shallow angle to avoid the lower submerged end from touching the bottom of the kettle. A shallow withdrawal angle causes the zinc to flow off at a slower rate leading to a heavier zinc layer on the top surface and greater quantities of ash trapped on the bottom surface of the steel article.
Small items such as fasteners, nuts and brackets may be placed into baskets rather than hung. See 'Centrifuging process' for more information.
Items longer or deeper than the bath size may be galvanized by using a double-dipping method. See 'Double dipping process' for more information. In these cases, material handling considerations will impact on cost. A better method may be to use bolted connections or modules for assembling post galvanizing.
The location of the vent and drain holes shall be determined by the shape of the fabrication and the angle at which it is suspended for galvanizing, as well as the enclosed volume of zinc in the fabrication when draining. A good rule of thumb for the designer is to think of items being lowered into and lifted from the galvanizing bath at an approximate 45° angle as discussed.
- Holes should be placed as close to corners and/or connections as practical.
- Holes must be located as close to the high and low points of hollow sections as possible to prevent air locks, entrapment of pre-treatment chemicals and zinc puddling.
- Holes should be orientated in the same plane as the fabrication.
- Holes should not be located in the centre of end plates and connections.
- Holes should be diagonally opposed where possible.
Dimensions of holes shall be determined by the trapped volume of air in the fabrication and the surface area of the steel in the vented area. Each square metre of steel surface produces approximately 200g of zinc ash, which must be able to escape through the holes.
- Minimum hole size is ø10mm
- Hole diameters should be at least the same size as the steel thickness.
- Having bigger holes (where feasible) is always better for the galvanizing outcome.
Refer to 'Hollow Sections' for applicable hole size charts.
Refer to 'Hollow Vessels' for applicable hole size chart.
Size & Weight
i. Centrifuge process
Small items are placed into a basket to be dipped and centrifuged. The size of baskets, centrifuges and other equipment will vary, just like general galvanizing baths.
Typically this process involves all the same stages as the general galvanizing process with the added centrifuging (or spinning) stage that occurs after withdrawal from the molten zinc. The centrifuging (or spinning) removes the excess zinc from the small articles, including from any threads or holes.
The coating thickness and mass requirements differ from other batch galvanized pieces due to the spinning process removing excess zinc.
Note: Not all galvanizers have centrifuge facilities and not all small items will be galvanized via the centrifuge process.
ii. Double dipping process
Double dipping is a term used to describe the process of galvanizing an item which is longer, wider or deeper than the relevant available bath dimensions. In this procedure, the item is lowered into the bath so that half or more of its ‘over dimension’ is immersed in the molten zinc.
When the galvanized coating has been achieved on the immersed section, the item is withdrawn from the bath and adjusted in handling so that the ungalvanized portion can be immersed in the molten zinc.
In the double dipping procedure an overlap of zinc coating will occur and this will normally have to be addressed in the case of visually obvious structural elements, in particular any requirements for architecturally exposed structural steelwork should be identified prior to order. In addition, double dipping increases the possibility of distortion (dimensional instability) of fabricated items. Guidance in these cases should be sought from the galvanizer.
Distortion (Dimensional Stability)
When steel sections or fabrications are immersed in molten zinc, their temperature is raised to that of the molten zinc, which is typically 450°C. The rate at which the steel reaches this temperature across its entire surface will depend on:
- the thickness of the individual sections making up the item,
- the total mass of the item,
- the dimension of the item, and
- speed of immersion.
At galvanizing temperatures, there is no change to structural steel's metallurgical microstructure and the process is not hot enough to have any heat treating effects on the mechanical properties of most structural steels after galvanizing.
However, at galvanizing temperatures, the yield strength of steel is temporarily lowered by approximately 50%. If any attached steel is not at the same temperature and any stresses exist, the weaker area will be subject to movement by the stronger area. There is a responsibility on the designer, the fabricator and the galvanizer to co-operate in ensuring distortion risks are minimised or eliminated.
Basic design rules for avoiding distortion
- Maximise the uniformity of heat transfer into and out of the steel.
- Ensure venting and draining is adequate. This will allow the article to be immersed in and withdrawn from the molten zinc as quickly as possible.
- Minimise section thickness variations wherever possible in the fabrication.
- Minimise the effect of stresses while the article is in the molten zinc.
- Use symmetrically rolled sections in preference to angle or channel frames. I-beams are preferred to angles or channels.
- Ensure assembly and welding techniques minimise stresses in components making up the article.
- If cutting plate to size, ensure all sides are cut using the same technique. Guillotine is the preferred cutting technique.
- Bend members to the largest acceptable radii to minimize local stress concentration
- Accurately pre-form members of an assembly so it is not necessary to force, spring or bend them into position during joining.
- Continuously weld joints using balanced welding techniques to reduce uneven thermal stresses.
- Staggered welding techniques to produce a structural weld are acceptable.
- For staggered welding of material 4mm or less, weld centres should be closer than 100mm.
- Avoid designs that require double dipping. It is preferable to build assemblies and sub-assemblies in suitable modules allowing for quick immersion and galvanized in a single dip so the entire article can expand and contract uniformly.
- Ensure the structural design of the item is sufficient to support its own weight at 50% of the steel's specified yield strength.
- Avoid using large areas of thin (under 8mm), unbraced flat plate.
- Use temporary bracing or reinforcing on thin-walled and asymmetrical designs.
Risk of distortion for various items
Low risk: All hot rolled structural sections, fabrications containing angles, channels and universal hot rolled sections, tube and RHS sections and fabrications, ribbed or corrugated plate sections, grating, and heavy plate (over 16mm).
Medium risk: Light section roll formed products, long light walled conduit and tubing, fabrications containing asymmetrical weldments or steel of significantly different thickness, medium plate (8-16mm), and some double dipped items.
High risk: Thin sheet and plate (under 8mm depending on shape, area and bracing), floor-plate, deep plate web girders, platforms containing floor-plate, long channel sections with multiple weldments (cleats) on one side of web.
For more information visit Galvanizers Association Of Australia to download the PDF.