A bit off topic

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)
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Bruce in Bangkok wrote:

Can you get your parts investment cast?
I see anything of this sort, built up out of weldements, cracking like a SOB, in pretty short order, between the work hardening, and the stress risers from innacessable bits of weld joint.
Stainless will quite happily work harden, and crack, without the carbon. I'd bet a nickel that the vibrations are the culprit, not any carbon the parts might be exposed to. I sorta have my doubts that the carbon is playing much of a part, but I am not a mettalurgist, nor do I play one on TV. :-) But in a little sailboat diesel, the broken parts would be covered in the stuff right well enough!
Cheers Trevor Jones
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When Trevor Jones put fingers to keys it was 12/4/07 10:49 PM...

...
There was a thread about this on rec.crafts.metalworking a year or two ago. Probably worth the search. I don't remember the sort of equipment in question (not marine), but I recall the upshot being that SS was dramatically unsatisfactory for that exhaust system. I think the poster was blaming the carbon. But yeah, some SS alloys will work harden if you sneeze too hard on them.
--

Carl West
http://prospecthillforge.com : The Blacksmithing Classroom
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wrote:

I hadn't thought about work hardening but some stainless will certainly work harden.
I had thought carbon because years ago, in the A.F., I worked on aircraft and we were always taught NOT to make marks on the thin stainless exhaust system with a graphite pencil - well what do the instructors know, so I tried it. Made a nice heavy "U" shaped mark on the end of the exhaust where if it did fail wouldn't endanger anything and Lo! Two flights later a nice "U" shaped of exhaust was missing.
And the metalworking reference. I will search for that.
Thanks all.
Bruce-in-Bangkok (Note:remove underscores from address for reply)
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Bruce in Bangkok wrote:

Yup! Seen the pencil trick, on jet exhausts.
If your boat engine gets that hot, and holds the temperature that long, you have other problems, I think. Stainless. at red heat, turns to shit in an awful hurry, when it gets hit by oxygen.
As you said, you made some of heavy stock. I think that was part or all of your success. The trick question is to figure whether the reliability was as a result of having more mass to prevent movement from the vibrations, or if you simply had enough material that it had not corroded through "yet".
Cheers Trevor Jones
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wrote:

My question was originally prompted by the fact that no part of the exhaust gets hotter then you could touch - very quickly, like your mother used to test how hot the flatiron was before the days of thermostats. And that portion is only an inch or so long between the exit of the water cooled engine manifold and the point where the sea water is injected.
On my own engine the water cooled engine manifold is cast aluminum and the injection elbow is schedule 40 stainless and it is holding up after 10 years of use.
I also did some looking on Metalworking and found a lot of stuff about making exhaust pipes for cars, trucks and motorcycles. All reporting good luck. Of course they just used "stainless tubing" with no mention what the alloy was. I also found at least one S.S. alloy designed specifically for aircraft exhaust systems. And finally Ernie, over on the Welding site recommended (I think) 716L.
So, what I think I'm going to do is make the water cooled manifold from 316L, which I can get locally, since that certainly will be moe corrosion resistant then aluminum and then make the mixing elbow from 716, if I can get it, or simply thicker material.
Anyway, Thanks to all for the information.
Bruce-in-Bangkok (Note:remove underscores from address for reply)
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Bruce in Bangkok wrote:

If you can touch it, even quickly, without a sizzle, then I doubt that carbon absorbsion has anything to do with the issues.

IIRC there was a post over on the wrec, tha specifically warned against 316, anywhere near the clorides in salt water. Beucoup corrosion potential when combined with the acids from the exhaust.
Cheers Trevor Jones
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wrote:

I got a message, maybe on metallurgy that was a retired Nuclear Engineer and he specifically talked about clorides. I also googled for cloride cracking and from what I read heat, stress and clorides will play hell with 300 series stainless. So... stainless, particularly the commonly available stuff, is probably not the best stuff to make water injection elbows with.
Have to draw back and think about stainless exhausts for a while before I build the next one, I guess.
Bruce-in-Bangkok (Note:remove underscores from address for reply)
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Bruce in Bangkok wrote:

Time to look at what has separated punters from their money since before there was technology.
Shiny covers, to fit OVER the stuff that corrodes. :-)
Shiny shit sells! So does anything titled "billet", or "Carbon Fibre".
Lesse "Brand New!!! Chromed Billet Carbon Fibre Covers!!! WONT Last Long!!!" (that's the truth in advertising bit. Should include that!)
:-)))) But I digress...
Cheers Trevor Jones
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When Bruce in Bangkok put fingers to keys it was 12/5/07 2:27 AM...

Never mind. That's the story I was remembering.
--

Carl West
http://prospecthillforge.com : The Blacksmithing Classroom
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Yep, me too. :) And the idea was shot-to-hell last time too. :/
Alvin in AZ
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wrote:

Mixing 316 and chlorides (read: saltwater) is verboten. I'm a retired nuclear engineer. 316 is the alloy of choice for nuclear plant piping on the reactor side. Even PPM of chlorides in contact with the metal can induce chloride stress corrosion and resulting cracking. Every procedure and operation in a plant is chloride-aware. Every chemical and material that comes in contact with SS must be analyzed for chloride content. Chlorinated solvents aren't allowed on the site.
We had a major problem at TVA's Sequoyah nulcear plant with chloride stress corrosion even before the plant was started up, because the SS piping wasn't adequately protected from chlorides while in storage during construction.
Can't help you with any other stainless alloy but I'm intimately and painfully familiar with 316.
I do know that any piping or equipment in the plant that was in contact with river water was made of either carbon steel or monel metal. Monel is something you might consider if you're going for bling. It polishes up to a unique and gorgeous luster. I still have some rings that I made out of condenser tubing.
Something else you might do is look at what the Navy uses for manifolding. They have cubic dollars to spend so presumably they choose the best material for the job.
John -- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com <-- best little blog on the net! Tellico Plains, Occupied TN Why the US is losing its competitivve edge:"It used to be that the USA was pretty good at producing stuff teenaged boys could lose a finger or two playing with."-James Niccol
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Not in any way to argue with your information, more with a sense of irony at what the average "yachtie" will believe, but 316L is the material of choice for underwater marine fittings like propeller shafts amongst the yachting community. I suspect that this is probably a matter of economics and 316L is the cheapest stuff that will last for a little while as from my own experience it is very susceptible to crevice corrosion.
However, if you have priced either grades of bronze suitable for continuous under salt water use or monel you can see why 316L might be recommended as a (it will last until it gets out of the shop)compromise. As you can see I'm a bit cynical about yachting gear - aluminum winch capstans mounted directly onto a stainless shaft and located in the bows where it is sure to come in contact with seawater.
In any event, your information is of value as I now believe that the fresh water cooled manifold is not a problem it is the mixing elbow where the salt water is injected that is most likely to fail. Now to solve that problem.
Thanks for the information it will be of great help.
Bruce-in-Bangkok (Note:remove underscores from address for reply)
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Why would we price it? :/ You got prices? Tell us. :) I'll find it interesting, if it's informative, rather than argumentative bullshit, that sound fair to you? :)
All I know about prices is non-magnetic stainless steel is selling for about the same as brass. Last time they were both about 75c a pound. :)
Alvin in AZ
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On Thu, 6 Dec 2007 21:24:00 +0000 (UTC), snipped-for-privacy@Example.com wrote:

Not too sure what you are trying to say here but you can't use brass around saltwater nor are the 300 series of stainless really the best to use either. Something like nickel aluminum bronze or monel (yes I know it is a form of SS) would probably be a better selection for a long lived solution.
Bruce-in-Bangkok (Note:remove underscores from address for reply)
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