ngi protection of threads - ac -...
TRANSCRIPT
PROTECTION OF THREADS
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Outline:
Galvanising threaded components and assemblies with internal/external threads is common practice for galvanisers but can require some measures to protect those areas prior to galvanising and some effort after galvanising in order to make the threads serviceable.
Specifics:
There are many different items that have both internal and external threads that require galvanising; some of these include:
� Threaded Bolts/Rods � Cage Bolts � Sockets
� Bungs � Tie Rods/Bracing � Bolted Connections
There are two ways to process these items through the galvanising plant depending on the size of the item. The first way is through the centrifuge plant (commonly referred to as the spinner, see Figure 1.) which is used for smaller components up to 1200mm in length. The items are placed in a steel basket and submerged into the zinc, once ready they are raised out of the zinc and spun at a high revolution. The centrifugal force then pushes the excess zinc to the side of the basket helping to create a cleaner thread, although this may not guarantee a useable item and may still require some attention. Figure 1. Spinning Plant The second way is through the main plant and is used for items that will not fit in the centrifuge plant. These items include cage bolts, long bracing rods and assemblies fabricated with internal/external threads. External Threads (Male): The molten zinc in the galvanising bath is heated to approximately 450°C, as an item is withdrawn from the bath the zinc rapidly cools to below its melting point of 420°C and starts to set. The excess zinc in the grooves of the threads generally runs to the underside and solidifies, preventing the use of the threaded component. There are a number of ways to counter this issue, these include;
1. Heating the threads after galvanising with a gas torch and then wire brush cleaning the excess free zinc from the grooves. During the galvanising process the outer layers of steel metallurgically bond with the zinc to create a zinc-‐iron alloy. This zinc-‐iron alloy has a much higher melting point than the free zinc (650°C compared to 420°C). Care needs to be taken to not overheat the area to allow only the free zinc
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to be removed. By removing only the free zinc there is still a galvanised coating remaining on the threads.
2. Running a die over the threads. This will ensure a clean thread however will remove
all the zinc coating leaving the component un-‐galvanised.
3. Masking the area with a protective material. These materials include using high temperature tape, silicone, duct tape and masking paints. Each of these methods have their own pros and cons but all will leave the thread ungalvanised and will require some work to clean the thread. For a list of materials and case study completed by the American Galvanizers Association, please see the link below:
https://www.galvanizeit.org/images/uploads/pdfs/Masking_Materials_for_Preventing_Hot-‐Dip_Galvanizing,_Bernardo_Duran__Thomas_Langill,_Ph.D_(Galvanizing_Notes,_2010_Dec)_.pdf
Internal Threads (Female): Commonly found on pipework with sockets or bungs for troughs and tanks, internal threads will tend to clog through the galvanising process. Similar to external threads there are methods to fix this issue.
1. Masking the area with silicone is one of the most commonly used methods. By carefully smearing the silicon into the thread or filling the hole with silicon, the zinc will not bond with the steel leaving the thread uncoated. Ash will need to be removed from the thread prior to use.
2. Re-‐tapping the threads. This will ensure a clean thread however will also likely remove the zinc coating.
3. Leaving a bolt in the hole through the galvanising process. The bolt will then be sealed in place from the molten zinc and may require heating with a gas torch in
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order to free the zinc around the bolt before unscrewing. There will be no zinc coating on the thread using this process.
Example of threads on unprotected versus protected tie rods before and after galvanising:
Masking the internal and external threads with silicone on a pump base frame with adjustable feet: