Galvanised Fasteners Hot Dip Spun
Galvanised Bolts and Nuts
Hot Dip Spun Zinc Galvanising process
Fastener Hot Dip Spun Galvanising pre-treatment consists of four separate stages. The Steel Fastener is immersed in a degreasing solution which removes light oil contamination; Pickling acid strips the fasteners steel surface back to clean base steel, a flux solution is applied to the steel fastener to assist with the galvanizing reaction.
Fasteners are placed in baskets to be galvanized then immersed in molten zinc at a temperature of around 450°C until the temperature of the work is the same as the zinc. During this process, the molten zinc reacts with the surface of the steel to form a series of zinc/iron alloys.
Fasteners are removed from the zinc, the zinc on its surface will begin to solidify and is rapidly removed from the bath before the zinc on the work solidifies completely. The basket is then placed in a centrifuge and spun for several seconds, this removes excess zinc from the surface of the fastener and prevents the items sticking together to ensure a smooth finish. Work is then transferred to a quench tank where it is cooled to allow handling. Nuts, bolts and washers 8mm and above can be Hot Dip Spun Galvanized, in practice, it is normal for bolts and screws to be galvanized.
Nuts should be galvanized as blanks and then tapped oversize, the threads then lightly oiled.
When assembled, the nut thread is protected by contact with the coating on the bolt. Even after many years of service, galvanized nuts can readily be unfastened even though the threads have never been galvanized. There is some tendency for hot dip galvanizing to be thicker in thread roots. Fasteners hot dip galvanized at a high temperature of around 550°C tend to take on a uniform, dull grey.
Galvanizing Fastener Strength Considerations
Galvanising bolts up to class 8.8 USA grade 5 is normal practice and fasteners can be galvanized without difficulty. Grade 10.9 bolts may require shot blast-cleaning as an alternative treatment prior to hot-dip galvanizing and there may be some tempering back. Grade 12.9 bolts and higher strength fasteners should not be galvanized.
Galvanised Thread Galling
Galvanised fasteners can suffer with 'Thread lock-up' normally called 'Galling' it occurs in the threads of galvanized fasteners where there is a coating build up in the thread roots; this can be overcome with lubrication ensuring the correct clamping force is developed. Beeswax has been found to be a most effective lubricant, Molybdenum disulphides, or tallow, have also been specified for this purpose.
Galvanising Fasteners Europe
There have been incidents of European undersized galvanised bolts being supplied as a standard thread for use with UK overtapped nuts. It is crucial to recognise that the continental European practice is to use an undersized thread bolt that when galvanised is the same thread dimensions as an uncoated bolt it will then easily mated with a standard thread nut; the reverse is the practice in the UK in that the nut is retapped to fit the Galvanised bolt.
Hot dip spun galvanizing H.D.S.Galvanising as it is also called is a molten zinc coating on threaded fasteners and is superior to electro zinc plating; the cost per year of rust-free life prove that hot-dip galvanizing is by far the most economical coating compared to zinc electroplating.
Galvanizing has for over 100 years been seen as the most environmentally friendly fastener finishing process that prevents corrosion. Galvanizing is sustainable and produces little waste, zinc that does not form a coat on the bolt or nut remains in the galvanizing bath for reuse.
Zinc is not a ferrous metal its properties make it recyclable without loss of chemical or physical properties.
Galvanized bolts and nuts can be assembled or installed, and can also be removed, re-galvanized and re-used. As part of an end of life recycling process zinc can be easily removed in the scrap steel production process.
Galvanized fasteners offer:
- A permanently bonded tough coating
- Easy to clean surface
- Long maintenance-free life
- Lowest overall cost compared to other coatings
hot dip spun zinc galvanising, HDSG, h.d.s.g, Galvanizing
Galvanised fasteners, galvanized bolts