heavy wall mild steel or so i am told is good! 304 stainless is crap as it cracks apparently, 321 stainless is the way to go if you want to use it! steam pipe is the best or so i'm told think its cast from memory & u can buy it in bends or something, have seen 3mm thickness & 4mm thickness & have been told not necessary to hpc it, but if it was mine i would definitely hpc extreme coat it (black stuff)!