Exhaust elbow

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NormanS

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I am well aware that exhaust elbows deteriorate. It's not really surprising when you have hot exhaust gases mixing with hot salt water in cast iron. With the boat now ashore for the winter, and having run the engine up to temperature, and then run antifreeze through the exhaust, and taken out the pump impeller, I thought I'd better take off the exhaust elbow to have a look at it. It's two years since I last checked it. The elbow is OK, but the connector which feeds the hot water into the elbow, fell apart in my hands. Much better to find this now in November than mid season.

The engine is a Volvo MD22 (Perkins Prima). There was an article in the Summer edition of PBO, about the making of a stainless steel exhaust elbow, which is quite feasible, but I have read of problems with weld integrity of stainless used for this purpose. Does anyone here have any long term knowledge about the pros and cons?

Thanks in anticipation.
 
Not a direct answer to your question I'm afraid, but as nobody's yet responded I'll just mention that Beta now recommend aluminium elbows for their small engine series (at least). Very much thicker than the stainless steel ones, and I'm told they should last longer.
 
NormanS said:
The engine is a Volvo MD22 (Perkins Prima). There was an article in the Summer edition of PBO, about the making of a stainless steel exhaust elbow, which is quite feasible, but I have read of problems with weld integrity of stainless used for this purpose. Does anyone here have any long term knowledge about the pros and cons?
Click to expand...

I fitted an American made stainless elbow to my 1GM10 and have now done two seasons with it. There were a lot of sucked teeth and sharply indrawn breaths here about it, but as far as I can see it hasn't fallen apart yet. I plan to take it off some time this winter and give it a good hard stare. It costs around 40% of what Yanmar charge for a mild steel one ...
 
JumbleDuck said:
I fitted an American made stainless elbow to my 1GM10 and have now done two seasons with it. There were a lot of sucked teeth and sharply indrawn breaths here about it, but as far as I can see it hasn't fallen apart yet. I plan to take it off some time this winter and give it a good hard stare. It costs around 40% of what Yanmar charge for a mild steel one ...
Click to expand...

My somewhat ancient 3gm30 Yanmar has a bronze exhaust elbow as far as I can assess. Why would anybody make one in MS???

Graeme
 
pagoda said:
My somewhat ancient 3gm30 Yanmar has a bronze exhaust elbow as far as I can assess. Why would anybody make one in MS???
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Beats the heck out of me, and Yanmar want almost two hundred quid for a mild steel one. The additional costs of using stainless would be lost in the noise - but then they wouldn't sell as many, would they ...
 
I have changed both my high rise exhaust elbows to stainless steel ones constructed from 3mm ? wall thickness chemical resistant s/s ... better than 316 at elevated temperatures .... I don't remember the propriety name sorry. I had them made up of tube section by a professional welder. Really doesn't cost much. New construction went from the exhaust manifold to the hose.

They've been in use for a couple of years ..... maybe 400 hrs .... very happy with them after all the hassle I had with the Yanmar stock units.
 
NormanS said:
The engine is a Volvo MD22 (Perkins Prima). There was an article in the Summer edition of PBO, about the making of a stainless steel exhaust elbow, which is quite feasible, but I have read of problems with weld integrity of stainless used for this purpose. Does anyone here have any long term knowledge about the pros and cons?

Thanks in anticipation.
Click to expand...
O.K. get the popcorn out.....Stainless Steel is fine provided you use the correct grade and welding procedure to avoid "weld decay" or more properly called Chromium Carbide Precipitation.

Chromium Carbide Precipitation leads to Intergranular corrosion which is a form of relatively rapid and localised corrosion associated with the defective microstructure resulting from the carbide precipitation. When austenitic steels have been exposed for a period of time in the range of approximately 425 to 850°C, or when the steel has been heated to higher temperatures and allowed to cool through that temperature range at a relatively slow rate (such as occurs after welding or air cooling after annealing), the chromium and carbon in the steel combine to form chromium carbide particles along the grain boundaries throughout the steel. Formation of these carbide particles in the grain boundaries depletes the surrounding metal of chromium and reduces its corrosion resistance, allowing the steel to corrode preferentially along the grain boundaries. Steel in this condition is said to be "sensitised".

It should be noted that carbide precipitation depends upon carbon content, temperature and time at temperature. The most critical temperature range is around 700°C, at which 0.06% carbon steels will precipitate carbides in about 2 minutes, whereas 0.02% carbon steels are effectively immune from this problem.

It is possible to reclaim steel which suffers from carbide precipitation by heating it above 1000°C, followed by water quenching to retain the carbon and chromium in solution and so prevent the formation of carbides. Most structures which are welded or heated cannot be given this heat treatment and therefore special grades of steel have been designed to avoid this problem. These are the stabilised grades 321 (stabilised with titanium) and 347 (stabilised with niobium). Titanium and niobium each have much higher affinities for carbon than chromium and therefore titanium carbides, niobium carbides and tantalum carbides form instead of chromium carbides, leaving the chromium in solution and ensuring full corrosion resistance.

Another method used to overcome intergranular corrosion is to use the extra low carbon grades such as Grades 316L and 304L; these have extremely low carbon levels (generally less than 0.03%) and are therefore considerably more resistant to the precipitation of carbide.

OK so far so good - keep awake at the back there.......

So select a suitable grade of SS and minimise time in the sensitising zone and finally if paranoid perform a solution anneal at 1000°C after completion of all welding. finished product should be extremely resistant to sea water (Chloride) corrosion.

Obtaining small quantities of the more "exotic" stainless Steel alloys may prove problematic and cost an arm and a leg so for Joe average the choice would be 316L which, while not ideal, should give good service life (>20 years) if properly welded (no preheat, 316L filler wire, keep the interpass temperature as low as possible (~40°C), small beads (low heat input) etc.
 
cliff said:
Chromium Carbide Precipitation leads to Intergranular corrosion .... and are therefore considerably more resistant to the precipitation of carbide.
Click to expand...

I presume that's taken from http://www.azom.com/article.aspx?r=48.357066285649&svr=6&lang=en_us&articleID=1177, which goes on to say

Many environments do not cause intergranular corrosion in sensitised austenitic stainless steels, for example, glacial acetic acid at room temperature, alkaline salt solution such as sodium carbonate, potable water and most inland bodies of fresh water. For such environments, it would not be necessary to be concerned about sensitisation. There is also generally no problem in light gauge steel since it usually cools very quickly following welding or other exposure to high temperatures.​

which suggests to me that there may not be a problem with the fairly thin material used to make exhaust elbows. I suppose it will depend on how slowly the flange/tube joint cools, so I'll look at that area with particular interest when I have the elbow off.
 
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JumbleDuck said:
I presume that's taken from http://www.azom.com/article.aspx?r=48.357066285649&svr=6&lang=en_us&articleID=1177, which goes on to say

Many environments do not cause intergranular corrosion in sensitised austenitic stainless steels, for example, glacial acetic acid at room temperature, alkaline salt solution such as sodium carbonate, potable water and most inland bodies of fresh water. For such environments, it would not be necessary to be concerned about sensitisation. There is also generally no problem in light gauge steel since it usually cools very quickly following welding or other exposure to high temperatures.​

which suggests to me that there may not be a problem with the fairly thin material used to make exhaust elbows. I suppose it will depend on how slowly the flange/tube joint cools, so I'll look at that area with particular interest when I have the elbow off.
Click to expand...
Yep, couldn't be arsed typing it out when someone else had done a reasonable job of explaining the situation.
As for your perception of thin wall material being no problem forget it - even 22g sheet can suffer from CP Ref: urinals fitted to one of HM Subs - corroded to hell round the welds - wrong material - 316 NOT 316L.
Over the years I have come across numerous examples of thin wall fabrications failing due to carbide precipitation (Carbon too high and too high a heat input from welding)
If you chose to ignore my advice to use 316L and small passes of 316L filler wire (low heat input) and keeping the interpass temp very low etc. then I suggest you have a solution anneal, followed by a water quench, carried out on the elbow to drive any carbides formed back into solid solution and relieve any residual stresses from forming or welding.
Personally I would use Cunifer 30 if I could get my hands on some 2½" heavy wall tube, or failing that Cunifer 10 (both available but in 6m lengths)
 
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