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Richard.
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I had a spell of them when I more involved with the track bikes and was welding cracked cases, rear sets, swing arms etc so I adapted a gas BBQ into a little pre heater. Made such a difference on the way that stuff welded up when it was heat soaked. I only took em to 60-70 degrees to heat out any moisture but it's a good thing if there is lots to do. I'm less involved now with em.
backpurge
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I have a garage full of bits to do for folk, mostly off Lambrettas which are generally more straight forward, good idea with the pre heater although most of the stuff doesn't get much thicker than 3mm, anything over 6mm and I switch over to Ar/He, either Universal or Heavy then I can go up to 12mm without any problems
Richard.
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The little pre heater was not so much to aid with welding current but more for drawing out any locked in moisture or contaminants. These castings I find have such variable weldability. Some weld lovely and clean some are a battle from the start. I only built it as I was getting loads of it at one point been the only guy on the track scene with an acdc and I'd just prep them up for welding then chuck em in on very low and leave em half hour to draw out what I could. That repair you've done was a difficult one and I've only done a couple where I've had to replace similar amounts of metal to that. In all honesty I don't believe mine looked as tidy as your repair either (I'll put that down to material ha ha) 
Richard.
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12mm is bl00dy impressive.
And to get that from a gas mix is even more so. Is that using a single phase machine?
backpurge
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Richard, argon is inert, high density and easy to ionise which makes it ideal as a base for most shielding gas mixtures and a great gas for TIG as you can weld virtually any material to a high standard with it, the drawback with argon is it transfers the least amount of heat across the arc of any gas, so where possible adding O2, CO2, He, H2, N2 to the gas will increase the arc/weld pool temperature.
Helium is inert but hard to ionise so with high He levels arc ignition can be difficult, I know of several companies where the arc is struck under argon, once established the gas automatically switches over to pure He to give them higher deposition rates and improved weld quality
In most cases a change from Ar to a mixed gas will improve weld quality and massively reduce production times
The idea behind this is, if you leave your amps the same and change the gas you can increase deposition rates and improve weld quality without increasing the heat input. This is the sort of stuff I spend most of my time doing and in a lot of cases we can halve people's welding times on a particular job just by changing the gas.
Helium is inert but hard to ionise so with high He levels arc ignition can be difficult, I know of several companies where the arc is struck under argon, once established the gas automatically switches over to pure He to give them higher deposition rates and improved weld quality
In most cases a change from Ar to a mixed gas will improve weld quality and massively reduce production times
The idea behind this is, if you leave your amps the same and change the gas you can increase deposition rates and improve weld quality without increasing the heat input. This is the sort of stuff I spend most of my time doing and in a lot of cases we can halve people's welding times on a particular job just by changing the gas.
