Contour Enthusiasts Group Archives Forums Modifications Duratec Performance Some interesting info on Header Wrap

Those are all high chromium and chromium nickel based stainless steels, not basic mild steel.

I don't know that we really can do any debate of hydroforming since none of us knows enough about it right? We also can't get that kind of part from the aftermarket so I'm guess the primary choices will be single walled stainless or mild steel, with mild steel being the predominate form of material we will use.
So are we all in trouble? I don't think so. I mean it won't last forever but it ought to be sufficient for most peoples purposes.

The big problem with mild steel is the diffusivity of the material at the grain boundaries causes cracking at elevated temperatures. Stainless also suffers with this but it has a higher service temp. It is a time-temperature based problem with some concern due to the atmosphere that the metal is subjected to.
I don't think we have enough information with our applications to honestly say what the lifespan of a mild steel part is without actual testing.

However, some speculation RE: MSDS headers; they have ceramic coating on the inside, this isolates the steel from the gasses you have flowing in the exhaust stream depending upon thickness and porrosity of the layer. Therefore the risk of failure is probably from intergranular diffusion, not really from oxidation. The diffusion of carbon and other alloying elements to the grain boundaries will make the mild steel brittle along those grain boundaries and softer in the middle of the grains where the carbon used to be, and likely to crack under stress. How long will be dependent upon the amount of time that the temperature stays AT or above the diffusivity point, or 2/3 the melting point of the material. Keep the amount of time at or above this point to a short period and the material ought to last quite a while. Eventually they will crack but based on what I've seen of temps in a production (non-race) car I would guess it would be in terms of years as long as they are coated internally; even though todays engines run higher temps than in years past. Of course no wrapping will let the heat conduct to the air easier and keep it cooler therefore will last longer.