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Date

12-04-2007

Posted by Bruce in Bangkok on December 4, 2007, 10:27 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 Trevor Jones on December 4, 2007, 10:49 pm

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Bruce in Bangkok wrote:

> 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)

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

Posted by Carl West on December 5, 2007, 1:06 am

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When Trevor Jones put fingers to keys it was 12/4/07 10:49 PM...

> Bruce in Bangkok wrote:

>> 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.

...

> 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

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
Reduce. Reuse. Recover. Refurbish. Repair. Repurpose. Recycle.

Posted by Bruce in Bangkok on December 5, 2007, 2:27 am

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wrote:

> When Trevor Jones put fingers to keys it was 12/4/07 10:49 PM...

>> Bruce in Bangkok wrote:

>>> 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.

>...

>> 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

>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.

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)

Posted by Trevor Jones on December 5, 2007, 8:22 am

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Bruce in Bangkok wrote:

> 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)

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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