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

"Cheshire Steve"  wrote in message 
news:U66dnXwpSq22vODVnZ2dnUVZ8qjinZ2d@bt.com...
>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

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

Cheshire Steve Wrote: 
> 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
----------------- 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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> 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.
>

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

Cheshire Steve Wrote: 
> > 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.
> >
> 
> 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

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 4500°F
(2482°C). 

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.~~~~~"

~1510ºC (2750ºF) - 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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> "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 4500°F
> (2482°C).
>

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