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Author: Subject: Thermite casting
Saerynide
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[*] posted on 1-7-2011 at 09:19
Thermite casting


So I was wondering, has anyone tried or know anything about casting iron with thermite?

I've ran the thermite reaction before with no problem - the molten iron falling on to dry sand and giving a pretty lump of shiny bluish steel... but what would be a suitable mold material without worrying about a steam explosion? Sand/fireclay and oil?

I'm mostly interested in casting small objects, about the size of large coins, so I think it should be relatively easy if a suitable mold material is found.

I've alloyed and casted silver ingots once using someone else's propane torch, but I don't have one to borrow anymore and it wouldnt be able do iron anyway...

This would be a rather expensive way to cast... but I'd like to try out of curiousity and symbolism of merely having cast said objects this way.

[Edited on 7/1/2011 by Saerynide]

[Edited on 7/1/2011 by Saerynide]




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The WiZard is In
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[*] posted on 1-7-2011 at 10:21


Quote: Originally posted by Saerynide  
So I was wondering, has anyone tried or know anything about casting iron with thermite?


A/ Google sand casting.

B/ Gas Torch and Thermit Welding
Ethan Viall
McGraw-Hill 1921

I own an original copy you can DL/read yours at
Google.com/books


http://tinyurl.com/3q8bcyk

Page 379 & ff.
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Mr. Wizard
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[*] posted on 1-7-2011 at 12:01


I have used commercially available thermite products, which I will not name, to weld ground wires to grounding rods. The container for the reaction and the mold to form the welded portion were made of a graphite block. They also made a 'one use' device that was not reusable. The thermite did contain a lot of copper, and was rather quick. A key to a successful weld was to use a thin barrier of a metal disk between the reaction chamber and the mold section. The purpose of the metal barrier was to allow time for the more dense metal to accumulate under the molten oxide layer. The molten pure metal would finally burn through the separator and flow into the chamber where the wire and the ground rod were contained. The heat from the molten metal would bond the rod to a heavy wire with a corrosion proof connection. If the mold was worn, the material damp or moist, or if the wrong kind of metal disk was used, the whole process was ruined. The disk was crucial, and without it alumina and metal would flow into the mold and form a real mess.

If you are casting small items you might think about making a mold from graphite, but I think sand with a binder is more often used commercially.

It may be strictly correct to say 'iron casting', as it is iron, but in common terms what you would be getting is more of a steel. Cast iron in common English is an alloy containing large quantities of carbon and silicon. Of course your thermite product could have a custom alloy of your own choice.
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Saerynide
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[*] posted on 1-7-2011 at 22:45


Thanks for the great information! I was wondering about that last night... how to keep the slag from going into the mold first? At first, I was thinking maybe some kind of grate to skim off the floating slag as everything flowed along, but it might lose too much heat in the process... and there isnt quite the luxury of a controlled pour from a crucible :P

I remember when very small amts of thermite was used, all that comes out is slag or some mixed useless mess, but when I ran about 130 grams, I didn't have a problem with the slag mixing with the metal. I cant remember which came out first, but when everything cooled, I had a little geode of porous blackish grey material, and when broken with my hands, revealed a round lump of shiny steel inside (I still have it). I used the 2 flower pot method. Of course, this method as is wouldnt be quite good enough for casting...

I imagine the reaction chamber must have some sort of cone shaped bottom where the disk would be placed so the molten material would have time to collect and phase-separate? Any idea what the disk might be made of? Copper? Nickel? Steel?

I previously thought about graphite, but I think I'll have to stick to sand, since I want to cast (almost exact) copies of objects I already have...

BTW, was this commercial kit expensive? I wouldnt mind buying and using it if it's already preoptimized for the job. Most of the commerical products I see are on a rather large scale for rail repair, but I guess I can still look to them for engineering inspiration - scaling down is much easier than scaling up :P

[Edited on 7/2/2011 by Saerynide]




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watson.fawkes
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[*] posted on 2-7-2011 at 05:53


Quote: Originally posted by Saerynide  
BTW, was this commercial kit expensive? I wouldnt mind buying and using it if it's already preoptimized for the job.
The Wikipedia page exothermic welding lists the major brand names.
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The WiZard is In
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[*] posted on 2-7-2011 at 06:13


Quote: Originally posted by Saerynide  

I imagine the reaction chamber must have some sort of cone shaped bottom where the disk would be placed so the molten material would have time to collect and phase-separate? Any idea what the disk might be made of? Copper? Nickel? Steel?


Steel. Try a small ceramic flower pot, just be set up so that if
it breaks you don't set your house a light.

Use the flower pot as a mold and make a Papier-mâché
thermit holder good for one use.
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argyrium
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[*] posted on 2-7-2011 at 11:52


Quote: Originally posted by The WiZard is In  
Quote: Originally posted by Saerynide  

I imagine the reaction chamber must have some sort of cone shaped bottom where the disk would be placed so the molten material would have time to collect and phase-separate? Any idea what the disk might be made of? Copper? Nickel? Steel?


Steel. Try a small ceramic flower pot, just be set up so that if
it breaks you don't set your house a light.

Use the flower pot as a mold and make a Papier-mâché
thermit holder good for one use.


It may also be a really good idea to dry the flower pot out well in an oven before doing this. Less likely to crack from steam. Unused/new pots (no salts) are also a good idea. Clays can hang onto H2O quite tenaciously.
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