I think that's called a firkin. Basically, it's a stainless cask - perfect for 'real ales' and genuine draught beers. It's probably only 9 gallons fluid capacity, and mine are double-walled - so it would make for a poor kettle.
I'd be much more tempted to use them for serving vessels, or fermentors.
Would be good for fermenting. Build a little rack to hold them on the sides and just put a stopped in the bung.
That's what I would have done with mine, if I had the space for racks of sideways kegs. I THINK the guy I sold mine to was going to do that. But I didn't hear back from him.