exhaust manifold steel

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.

As for headers made of materials that are magnetically responsive, I
have ridden in a lot of homebuilts with exhaust headers made of muffler
shop steel...heck, many of the engines had cast iron and steel blocks,
and there was never a compass problem once standard compensation methods
were made when the compass was installed. TIG welded steel tube
fuselages present more of a compass deviation problem than the exhaust
stacks located a couple of feet away. Even with a certified type
aviation engine, if the mag compass deviation were a problem, there is
still a pretty good size chunk of iron in the crank and rods to attract
that needle. Hasn't been a problem about which I've heard a lot of
complaint.

Stealth Pilot wrote:

On Wed, 6 Aug 2008 23:21:26 -0400, "Peter Dohm"
wrote:

"Bob Kuykendall" wrote in message
...
On Aug 6, 10:01 am, wrote:

...Stainless is iron
with lots of chromium and nickel. Inconel, IIRC, is chromium and
copper, and Monel is chromium and nickel, no iron in either...

Actually, I think that Monel is an alloy of nickel and copper, with
trace amounts of other stuff including iron.

Thanks, Bob K.

----------new post-----------

I just "googled" and found that there about a zillion Inconel alloys, but
that the primary constituents are Nickel and Chromiun with some others
including Iron and is primarily for high temperature applications.

Monel is promarily Nickel and Copper plus additional constituents and is
primarily for corrosion resistance.

Both were developments of the same company.

Peter


inconel being particularly suited to aircraft exhausts because it is
more resistant to erosion of the metal by the hot exhaust gasses.

if you arent going to use inconel at least use 316 stainless for its
non magnetic qualities.

aircraft compasses are actually useful at times.
Stealth Pilot

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.