exhaust manifold steel

One of the problems seen most with stainless headers is difficulty in
getting proper support. I was talking about erosion from the flame, but
many stainless headers fail from vibration cracking because the effort
to save weight leads to tubes that are too thin to withstand the
vibration. The extreme effort of that weight savings prevents the
addition of braces and other reinforcement to keep them together.

Many times cracks are repaired incorrectly without the proper back
purging to protect the inside while TIG welding the cracks. The inside
contacts the atmosphere while in a molten state. The solidified weld
looks good on the outside, but the internal surface looks grainy visible
to the naked eye and obvious to the touch. Under the magnifying glass
the texture is sharp mountain peaks with deep sharply Vee'd valleys,
These present a notch effect leading to another crack in shorter order
than the first.

Also the chemistry of the metal is changed (those sharp peaks are
crystal oxides and carbides of the component elements of the stainless)
making it much less resistant to the erosive exhaust gas than the proper
alloy of stainless is.

Peter wrote:

"Bruce A. Frank" wrote

Actually though the 3xx series stainless can develop magnetic response,
sometimes very strong, 302, 303 and 304 grades in particular, from cold
working they can be annealed at about 800 degrees C which eliminates the
response. 310 and 316 remain virtually response free from cold working.

321 is extremely heat resistant and therefore can be used in thinner
sections to save weight. In the fully annealed condition it is free of
any magnetic response. Unless the engine is some wildly modified racing
setup dumping three foot flames out of the exhaust stacks, very thin 321
will likely make it through the TBO of the engine. 316 will also, but
would need to be just a bit thicker walled tube to begin with.

The more exotic alloys monel, inconel, hastaloy and so on, are not
necessary for normal installations. Ceramic coated mild steel headers
are some of the smoothest flowing and will also last to most engine's
TBO.

This is interesting. I fly a TB20. Socata used to make exhausts out of
stainless but they didnt last all that long - not engine TBO for sure.
I don't know the alloy though.

In 2000, for the GT range, they changed over to Inconol and I
understand the exhausts last 'for ever'. Now, the problem has shifted
onto the pipe couplings etc. These are very expensive, of the order of
$150 each, and they contain an inconol gas sealing strip.