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Old 10-10-2003, 07:32 PM   #16
LyveWRX
Scooby Specialist
 
Member#: 19057
Join Date: May 2002
Chapter/Region: MWSOC
Location: The Lost Valley, Detroit, MI
Vehicle:
2002 WRX
& Blue Ford & '72 CB350

Default Ummm as far as metal goes....

I think that the WRC cars use mostly Ni-based super-alloy exausts. The cost is the problem, for sheet matl' (coil) 2x-5x over SS, they form better but also are really gummy in dies and machining.
But they hold much better than 50% UTS at T>2000F (almost to their melting points). (My data sheets are at work or I'd give some specs and actual strengths.) This would be a really good choice since the Anti-Lag basically turns the header/up-pipe into the combustion chamber of a jet engine. And thats what the Ni-(Co & Cr etc) based superalloys were developed for.

I woudn't want my header pipes to experience forces above the yield point at temp. Tensile strength is when failure occurs. Yeild is when things start to bend.

The Ti(and or V) additions to 321 (and 347) ss provide resistance to a phenomenon called sensitization. Sensitization is caused by Cr combining with the C in the steel. These solid Cr-carbide precipitates form along grain boundaries. This then can result in Stress Corrosion Cracking and Intergranular Corrosion. Also the mechanical properties change vastly as the C is removed from solution in the Fe.

SCC is accelerated by the presence of Cl.

Road Salt produces Cl ions in water.

However 304L stainless is much more common on the market (from what I hear I dont use that particular grade) and since the C content is so low sensitization is much lower thou not prevented totally.

As to the header faiures due to cracking they would likely have to be properly examined in place at the time of failure to really know for sure why. SCC does (IIRC) produce a charecteristic fracture surface, a good met-lab w/ sem could possibly tell you.


nick


( my choices would be:

A Ni based alloy, I'll look one up tomorrow.

347/321 SS (321 is marginally better)

Weight no object: grey cast iron bout a grade 50 or so w/ large well distributed graphite flakes.

316L

304L

304

316

409 (most stock exausts are made of this now)

Aluminized Carbon Steel

)
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