stainless exhaust

[T&C]

The most common high performance material is 321 S.S. as Stainless steels
go.  If welded properly it has no tendency towards cracking at the weld.
Just look at how much you have to bend it before it will break, this is seen
in the delta between the Yield and the Tensile strength, usually quoted at
around 30ksi yield and 70ksi or so tensile.  This means that the metal will
give at 30ksi stress and will not break until you exceed 70ksi.  The real
danger with stainless is in cold working it as mentioned below this would be
more of a design flaw then the wrong material.

-----Original Message-----
From: quattro-admin at audifans.com [mailto:quattro-admin at audifans.com]On
Behalf Of Jörgen Karlsson
Sent: Sunday, August 05, 2001 9:05 AM
To: Quattro at Audifans. Com
Subject: RE: stainless exhaust


Hi,

I have had some bad luck with factory made stainless steel exhausts on other
cars then my Audi, internals in mild steel and welds that has cracked and
rusted away.

But a real stainless steel exhaust will last forever, it is usually made of
304 to 316 steel. We have a few shops here in Gothenburg that do custom
exhausts in stainless steel. At least two of them make their own mufflers
too, that is almost the only way to keep the price low. I guess that there
are hundreds of shops in the US that make similar exhausts.

Then to exhaust headers, I agree that stainless steel has some properties
that are not suited for headers. First of all it grow with temperature at
about twice the rate of mild steel and it is easier to make it crack then
mild steel. This is probably because you use much thinner material, the
wrong welding material and the fact that it grow more then mild steel.

But if we compare it with mild steel, mild steel in a header quickly get a
glow skin. This glow skin cracks and is blown away by the exhaust flow. This
is only a problem on a turbo car, the turbine does not like foreign objects
that hits the turbine wings when they rotate at 100000-250000rpm. It slowly
abrades the turbine wings.

The lower grade stainless (Ex. 304) steel also produce glow skin, but this
is at temperatures around 900-1100ºC (1650-2000ºF). The glow skin is also
very much thinner then that produced by mild steel. 316SS is better then 304
and I think that there is a grade called 310 or 312 that is made to handle
high temperatures even better.

Using the right welding material is very important, the right welding
temperature is also important. If the weld is overheated the properties of
the material can change, if this is combined with the wrong welding material
the result can be very brittle. Since the most common grades of stainless
steel don't harden with temperature the wrong welding material is almost
needed to make a very bad weld.

An other down side with these grades of stainless steel is that it can work
harden, if an header is long and take much of the exhaust systems weight it
will harden with time. This will cause cracks, but that can not be blamed on
the material.

Jörgen Karlsson
Gothenburg, Sweden.