Bill Freeman wrote:

Paul,

The austenitic stainless steels are the 300 series,
and are non-magnetic.  They do not harden by  heat treat.
The 400 series is the martensitic stainless steels, which
DO heat treat, and are magnetic.  Martensite is the main phase
of an iron alloy that is formed when heat treating.
Chromium carbide precipitation on the grain boundaries is
why you can't weld 300 series stainless steels without
either re-heat-treating the whole unit (and losing any
beneficial cold-work hardening present) or with special
versions of the alloy to prevent the chromium carbide
precip on the grain boundaries.  Chromium carbide
corrodes easily and the result of welding 304 is a
dramatic "knife-edge" corrosion attack on either side of the
weld, at the appropriate temperature location.  Remember
when welding, the pool is molten and the solid sheet
is ambient, so you have a location with every temperature
in between, guarenteeing a location for chromium carbide.

The solution is to use either 316L or 321 alloy, which use
exotic additions (columbium, if my memory serves) to
scavenge the carbon and prevent the problem.  SO --
if the primary problem is chromium carbides, 321 should be
OK.   IMHO, there is more to it than this and the SS
exh life will be dramatically shortened above 1600F.  Of course,
std A/C exhausts are 321.

Inconel (again, as my memory serves) is a primarily nickel
based alloy, rather than primarily iron based like the stainless
steels.  This is the real difference in hot corrosion.

If you can get a nicely done Inconel exh for a reasonable price,
it has to be more heat-resistant than the SS321 model.

Bill

 
The Aircraft Rotary Engine Newsletter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org