Carbon absorption by S.S.

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Carbon absorption by S.S. Bruce in Bangkok 12-04-2007
Posted by Bruce in Bangkok on December 4, 2007, 10:43 pm
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 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)

Posted by Ernie Leimkuhler on December 5, 2007, 12:50 am
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> 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.
>

yes it does last longer, but it requires a bit of training to do it
right.

> 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.
>

yes I have built them before, out of stainless steel.


> 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)

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.

Posted by Bruce in Bangkok on December 5, 2007, 3:33 am
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On Tue, 04 Dec 2007 21:50:29 -0800, Ernie Leimkuhler

>
>> 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.
>>
>
>yes it does last longer, but it requires a bit of training to do it
>right.
>
>> 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.
>>
>
>yes I have built them before, out of stainless steel.
>
>
>> 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)
>
>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.

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)

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