[DHK]

Whoa... Hey -- be nice... Just THINK about this... What IS mild steel? It's iron with some chromium, molybdenum and some other traces in it. "Mild steel" is actually a catch-all for a wide range of alloys. What IS stainless? It's iron with more chromium (particularly, up to 18%), molybdenum and other traces in it. Stainless is "stainless" because it has enough chromium in it to form a chromium oxide layer on the outside -- that shiney surface you see. This gives it corrosion protection. I know that most people think of jet turbine blades and stuff when they hear the word stainless. But "stainless" is a catch-all too. Jet turbines aren't built from 304, 316 or 321... "Mild steel" has chromium in it too. But unlike "stainless" it has just enough to increase tensile strength. Stainless has more chromium to give it corrosion protection, but at the expense of tensile strength (in general -- I'm not talking about exotic alloys here). You wouldn't use stainless bolts as head bolts would you? They would stretch far too much. The point is, both mild steel and garden variety stainless are low carbon steels. Both are subject to work hardening and embrittlement at high temps. But anyone who's machined stainless will tell you -- it's easy to work harden it and "burn" it. Far easier than mild steels and not just because it's hard to cut... For a tube-manifold, I'd want something which remains ductile for durability. Stainless wouldn't be my choice -- and I have to say I've learned this the hard way. Now the rest of you can go learn it this way too if you like... It's a big classroom, and it's always open!