[paul.fr]

What do they use to cut or salvage metal, underwater? Is it gas, or something like magnesium?

[Karen W.]

I thought it was a gas underwater torch..welder.. I know he has referred to it as "Gas Tungston Arc Welding!"  but I am not sure.. How it works exactly.. I know the gas mixture is mixed in a container surrounding the the piece being welded under the water, but that is for underwater Dry welding.. there is a "wet underwater welding" which he does also, but I am clueless how they do it! My Nieces Husband does that in the military.. thats his job..

[another_someone]

I thought it was a thermic lance (iron/oxygen, where the gas tanks being the oxygen).

[another_someone]

http://www.krl.com.au/whatisalance.htm

A Thermic Lance is capable of cutting through :

• All ferrous and non-ferrous metals (recommended)
• Refractory materials such as linings and bricks (recommended)
• Underwater cutting of metal tracks, bridge supports (recommended)
  
• Concrete or rock structures (mainly for piercing holes only)

[Karen W.]

Oh you mean the tool is called a thermic Lance?

[another_someone]

Oh you mean the tool is called a thermic Lance?

Yep - iron rods burnt in pure oxygen.

[Karen W.]

That sounds right to me... ok would that be the same as welding rods they use for regular arc welding ...? Or are these just for underwater welding!

[another_someone]

That sounds right to me... ok would that be the same as welding rods they use for regular arc welding ...? Or are these just for underwater welding!

No, arc welding just uses a massive electric current - no flame.

I would rather suspect that using large electric currents underwater may not be healthy.

[lyner]

The thermite process involves powdered aluminium mixed with powdered iron oxide. The aluminium is more reactive than iron and there is a violent displacement reaction, producing iron and aluminium oxide. You start it off with magnesium to get the initial temperature high enough.
It is not used much these days but I have seen rails welded together this way. It is a 'low tech' method and was favoured by bank robbers because they didn't need to carry and store gas bottles. The thermite lance had the mixture stuffed down the middle of an iron tube and the tube gradually melted as it cut its way through the safe door.
I have done the School demo of thermite - very hot with lots of smoke and a lump of hot iron falls to the bottom of the water - filled beaker; they love it.
Gas welding underwater is not a problem as long as you blow a big enough bubble around the flame to avoid heat loss.

[turnipsock]

is it not just an oxy-acetaline torch? you mix oxgen and acetaline and ignite it...it burns.

[lyner]

is it not just an oxy-acetaline torch? you mix oxgen and acetaline and ignite it...it burns.

Google "Thermite". It'll tell you all about it.
I love doing the demo.