Quote:
Originally Posted by 86MLR
Due to the inherited metallurgy of 304 it tends to work harden from heat cycling.
321 is much better IRT coping with heat cycling, as is mild steel.
This is based on my experience with both turbo and NA cars with alot of track abuse.
There are other variables, poor welds or not correctly supported turbos, but, even with all these issues engineered out during fabrication you cannot get past how 304 work hardens through heat cycing.
From the header back 304 is good to go IMO, but for headers I won't use it.
Options may vary.
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The real issue with 304 Stainless in a header is inter-granular corrosion at elevated temperatures. This is a known and well documented issue. The high chromium content in 304 promotes chromium carbide precipitation at high temperature which depletes the chromium (the element that gives 304 it's anti-corrosion properties) and weakens the grain structure until failure.
Both 321 Stainless and 409 Stainless have a small amount of titanium added to the alloy to inhibit intergranular corrosion. 321 is more expensive because it also has nickel in addition to the titanium. Basically, if you want a header material that is pretty, will last longer and you're willing to pay the premium for it, get 321. Otherwise, 409 is a good way to go since it ultimately has a higher stiffness at high temps than 304 (yes, 304 is stiffer and stronger than 409 at lower temps). Naturally, mild steel doesn't have any chromium in it in the first place so there's no chromium carbide precipitation to happen...it'll just rust through since it has negligible corrosion resistance anyway so it has to have thicker walls to have the same life span as a 409 header.
All things being equal of course (good quality welds with the correct welding filler used, design considerations, etc. etc.).