Jerry wrote>

Don't forget that SS 253MA is available.  It will operate without
damage at 2000 degrees.  This is 300 degrees hotter than 321.    I
would build through the first bend with  .059 253MA and then switch to
a thinner 321 for the rest of the system.  Jerry

Mark wrote>
Jerry, this is an oversimplification. No metal can operate in a thermal
cycling, vibrating, acidic environment to 2000°F without damage. You are
looking for an acceptable life. What the spec sheet shows is that the
material has better capablities than 321 under the controlled conditions of
the test (whatever they were). In the environment of a IC engine exhaust
header, material science is still quite empirical.

An example, the large commercial turbofan combustion has a feature which was
eroding, limiting life between overhauls to about 20000 hours. A material
change was made to turbine blade material, being the latest/greatest in high
temp erosion resistance. The revised combustor feature is eroding at about
twice the rate. You can bet the manufacturer's engineers had all the
material specs, design experience with the materials, CFD and FEM models of
the stresses and environment, and likely did a combustor rig test and
possibly slotted the redesign into a full engine test.

What I am saying is, be conservative. 321 and inconel have known
characteristics for this environment. 253MA may be an improvment, but taking
a pound out of the header was a bad deal if it cracks. The material may be
"better" but what about the welds?  Broken Exhaust manifolds are so far the
#1 danger in homebuilt 13B installations.

Regards,
Mark

 
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