Murphy,
Stainless steel (304) transmits heat at about 1/3 the rate of mild steel. That is really one of the big reasons we use it. But, there are other reasons.
If you wrap normal, mild steel headers with an insulation, the heat in the pipes will rise dramatically. The higher the temperature, the faster the rate of chemical reactions--like rust. In many biological instances a mere increase of 10 degrees will double the rate of a reaction. Rust is not only caused by water, but is exacerbated by several substances, like salt for instance. The by products of combustion are particularly nasty in their corrosive effects--especially at exhaust temperatures which are around 1200 degrees. That is the reason why so many of you have seen the inside of headers "rotted out" while the outside seems perfectly normal.
I have yet to see a set of our stainless steel headers crack. I would be very surprised if they did as stainless is specifically used in these high temperature situations to resist such problems. Today, most every car on the market has stainless exhaust to comply with the Federal Emission rules that a car must pass emissions for 100,000 miles with no failures. Even with the less than optimal alloys the automotive industry uses (they'd sell their mother to save a penny on material costs) fatigue is not a problem with a stainless exhaust system. In fact, it is the material of choice for our vehicles.
David
