Carbon absorption by S.S.

Let me apologize if this is too off topic for this group but I do read
you guys talking about carbon content, etc.
I build water cooled exhaust manifold systems for small 2 - 4 cyl sail
boat engines. The prevailing practice is to not build exhaust systems
from stainless steel. However, I believe that stainless would both
last longer and look better, the latter IS important, and am in doubt
as to the prevailing practice.
The operating conditions are as follows:
The actual manifold consists of the necessary exhaust passages welded
up from stainless pipe and encased inside a water jacket with engine
cooling water circulated through it. A short "mixing elbow" is
attached to the outlet of the water cooled manifold and sea water is
injected at this point to cool the exhaust gasses for passage through
a rubber hose to the hull outlet. Water and exhaust gas exiting the
hull outlet are relatively cool.
Estimated temperatures are: exhaust gas within the water cooled
manifold approximately 3-400 degrees F.
Gasses entering the mixing elbow - probably a little lower due to the
water cooled manifold.
Gasses and water exiting the elbow - warm to touch but can hold hand
on elbow below water injection point indefinitely. Gas and water
exiting outlet warm but not too hot to hold hand in exhaust stream.
Now my question. Since I believe that the prevailing "knowledge" is
based on the fact that stainless will absorb carbon at high
temperatures and become hard and consequently be inclined to crack or
break with vibration, at what temperature will stainless, say 316L,
start to absorb carbon from the exhaust gasses. In short, would a
stainless exhaust system absorb sufficient carbon over say, a 10 year
period to become brittle?
Your comments, or a pointer to existing data, will be highly
appreciated.
Bruce-in-Bangkok
(Note:remove underscores
from address for reply)
Reply to
Bruce in Bangkok
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yes it does last longer, but it requires a bit of training to do it right.
yes I have built them before, out of stainless steel.
Here is the problem. Stainless steel that will be exposed to hot salt water requires a better alloy than standard 304 (18-8). 316L is the most common marine stainless, but even it can't really hold up to this treatment for long. 317L would be the least I would choose. If you wanted one to really last then you would be be into Inconel 625 or 686.
All of these require the same treatment. You have to prep your pieces so all the edges are clean, with no plasma cut edges or any contaminants. Next you have to back purge all the welds with pure argon. Back purging simply removes air from the back of the weld seam so you don't get nickel or chromium oxides on the backs of the welds.
You can accomplish this with some small tubing, aluminum foil and aluminum foil air duct tape. You will need a source of argon, whether you are splitting the line to your TIG or running a second bottle.
If a weld is properly back purged and you penetrate all the way through the metal, then the weld should look just as shiny silver on both sides.
After welding you have to restore the corrosion resistance, by removing any stray iron particles in the surface with an acid of some type. Commercially available pickling pastes use nitric and hydrofluoric acids. They work very well, but are extremely TOXIC. Citric acid is very safe and if you add 5 amps of 24 volts DC it works very well. This is what I use for brewery repairs.
So to sum up you can make them from stainless steel as long as you follow the correct procedure.
Reply to
Ernie Leimkuhler
Thanks for the answer.
I agree with you about the alloy but only the mixing elbow has hot salt water in it. The main manifold is cooled with fresh water from the block's cooling system. Perhaps 304/316L for the main manifold and 317L for the elbow. Prep, shielding and cleaning I am familiar with.
A post from a different group commented on work hardening but I'm not so sure about that. The manifolds are built from sched 40 pipe and 4 - 6 mm plate and are subject to engine vibration only - which isn't a lot on a 20 - 50 H.P. motor and I don't *think* that will be a problem.
The problem I have is the "everyone says" comments and most owners listen to "everyone" before they listen to the guy doing the work.
Bruce-in-Bangkok (Note:remove underscores from address for reply)
Reply to
Bruce in Bangkok

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