Thermite process in action

I see some interesting pictures of the thermite process in use to weld the tramway sections on the steam railway through Portmadoc. See this
link http://www.isengard.co.uk/#News
I thought it was done in place, but this looks like it takes place in a pot above and then is tapped or simply pours into place. It gives me an interesting idea for some instant sand casting - the thermite cupola - anyone any thoughts ?
Steve
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Lot's of white smoke and a very hot fire the fire brigade can't put out... Accusations about letter bomb's and a large steel door going clang...
If I have anymore thoughts I'll let you know :)
Steve
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Cheshire Steve Wrote:

----------------- from the web -------------------- c. The superheated steel is contained in a crucible located immediately above the weld joint. The exothermic reaction is relatively slow and requires 20 to 30 seconds, regardless of the amount of chemicals involved.
The parts to be welded are aligned with a gap between them. The superheated steel runs into a mold which is built around the parts to be welded. Since it is almost twice as hot as the melting temperature of the base metal, melting occurs at the edges of the joint and alloys with the molten steel from the crucible.
Normal heat losses cause the mass of molten metal to solidify, coalescence occurs, and the weld is completed. If the parts to be welded are large, preheating within the mold cavity may be necessary to bring the pats to welding temperature and to dry out the mold. If the parts are small, preheating is often eliminated.
The thermit welding process is applied only in the automatic mode. Once the reaction is started, it continues until completion. ---------------------------------------------------------- My immediate thought is that at a temperature of some 2300C your standard sand is likely to be glass by the time you have finished.
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I am not sure where you get the 2300C from - but I can see lack of temperature control must make it quite an art !
In the pictures from Porthmadoc it looks more like normal casting temperature - if it were at 2300C it would be so hot it should be a bluey-white and would destroy any refractory lining of the crucible. I wonder what they use as a mould when welding rails - cement or clay ? Clearly they can get it to work.
I think the problem is that our chemistry master did this at school, and now nearly 40 years later I want to see it again.
Steve
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Cheshire Steve Wrote:

http://www.google.co.uk/search?sourceid=navclient&aq=t&ie=UTF-8&rls=WZPA,WZPA:2006-10,WZPA:en&q=themit+welding
"b. The heat for welding is obtained from an exothermic reaction or chemical change between iron oxide and aluminum. This reaction is shown by the following formula:
8A1 + 3fe304 = 9FE + 4A1203 + Heat
The temperature resulting from this reaction is approximately 4500F (2482C).
c.~~~~Since it is almost twice as hot as the melting temperature of the base metal, melting occurs at the edges of the joint and alloys with the molten steel from the crucible.~~~~~"
~1510C (2750F) - melting point of typical structural steel;
http://www.thermitwelding.demon.co.uk/profile.html shows the equipment in use and teh spares one can buy. One might deduce that one gets through a lot of refactory liners.
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I might be being dumb here, but if the equation is that simple, then why not add some iron (e.g. as iron filings) to control the temperature ? It might also make the reaction a little less violent, and save on refractory.
Steve
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