Quote:
Originally posted by sammyg2
The mild steel gets "cooked" which causes the carbon to boil out of the poors of the metal. This makes it brittle and increases the tendency to crack.
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Sammy,
When mild steel gets that hot, it forms iron oxide on it's surface (otherwise known as rust but in this case, due to the high temperature that it's formed at, it isn't red rust). The big issue, though, is that mild steel doesn't have any structural integrity at that temperature so the engine vibrations cause it to fatigue crack rather easily.
Stainless steel is preferred in elevated temperature service due to it's resistance to form oxides and it's somewhat better strength. From a strength standpoint titanium is even better if you can afford it.