Longitudinal cracks on boiler reheater tubes

[aleemullah]

Hi Guys,

I work at a coal fired power plant as a maintenance engineer and lately we have been facing the issue of boiler reheater tubes cracking at bends. I have attached couple of pictures of last two events. One tube had the crack on the side of bent surface, while other on the outside of bent surface.

This appears to be lip fracture (fish mouth?). I have suggested to perform hardness and metallography of the affected tubes. Will that be helpful?

Any help will be appreciated in helping us solve the problem as this is causing forced outages at our plant.

Regards,
Aleem

 

[metengr]

What is the tube material and operating temperature? We may be able to assist in general observations based on collective experience (40+ years). A metallurgical lab analysis of at least one bend failure is necessary to establish a root cause of the failure. I would not have any other testing until this lab analysis is completed.

 

[aleemullah]

Hi Thanks for the quick response.

The material for these tubes are SA 213 TP347h and the operating temperature is around 540C

 

[metengr]

This could be strain induced precipitation hardening (SIPH) which results in brittle material in bends of 347H tube material which have not been solution annealed after forming. I have seen this before for tube bends which were supplied as-formed, and failed within several months - it is not good news. The failures tend to be brittle, thick-lipped failures on the extrados or along the neutral axis of the bend and occur early on in life. Remove one or more of these bends and have hardness testing performed at a reputable metallurgical lab.

Get these to a metallurgical lab for complete analysis. If this is not SIPH it could be cracking related to condensate trapped in bends during off-line conditions. Look at the ID if you can and evaluate the surface.

 

[aleemullah]

Thanks metengr for your assistance. After some more research I have come across similar failures associated with SIPH.

After we perform the metallurgical analysis, I will get back with the results and the suggestions provided by lab for any input from eng-tips.

Thanks again.
Aleem

 

[EdStainless]

Don't rule out more mundane causes. I have seen rows of pits on the inside of 321/347H where a scratch from cold draw catches water during a shut down.
It could be useful to look at the degree of sensitization/stabilization at various locations in the tube, variations in thermo-mechanical history could be part of this.

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P.E. Metallurgy, Plymouth Tube

 

[mrfailure]

Another mundane possible cause could be from overheating, particularly short term overheating where cause is actually an upstream leak or obstruction. Even stress corrosion cracking is possible (if, for example, there might be chlorides present within the steam). You really will need that examination by a metals lab as suggested above to know what is going on.

 

[replica]

May I know the media inside and outside the tube? Is the inside media steam and outside coal? What is the pressure? 540C is just above creep temperature range which is 530C and within sensitization range of 400 to 815C....347 eventhough it has carbide former (Ta and Columbium) in it to avoid sensitization, As far as I know it has to be supplied in stabilized condition to get full benefit of it and not in solution annealed state as in the later did not form meaningful carbide to avoid sensitization(formation of Cr23C6)

 

[EdStainless]

replica, Most people rely on these alloys stabilizing in service. This has two problems. The first is that if the initial exposure in the lower end of the sensitization range you will get grain boundary carbides and Cr depletion before the stabilizing effect can kick in. The second issue is that you get very non uniform size and distribution of carbides, which can reduce creep resistance. And then there is SIPH also.
Since this is H material there will always be some mix of sensitization and stabilization, but too much of hte first and you are screwed.
But at the same time nobody wants to pay for bending, solution anneal, and stabilization, even though that is the correct way to do it.

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P.E. Metallurgy, Plymouth Tube

 

[replica]

Edstainless....Agree with you..I doubt if the stabilization can take place during service as the normal operating temperature is not in the range of where the carbide former will form to avoid sensitization...The carbide former will form at high temperature....If the material is supplied in solution annealed state both Cr23C6 and TaC are in solution...I believe during heating to the operating temperature the Cr23C6 will form and TaC cannot form ...So this does not suit the purpose...meaning that the carbide former must utilize carbon before carbon can take Cr....

 

[EdStainless]

When we make tubing in these alloys (321/347) we are careful with the final anneal so that we leave some Ti/Ta-C in the structure. A very fine well dispersed carbides will not impact the properties, and it will help the alloy to stabilize more rapidly in service.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

These austenitic tube alloys have been around for years in boiler superheaters and reheaters and will become sensitized over time in elevated temperature service, regardless of stablization. In elevated steam temperature service, there is minimal concern regarding sensitization because you will get long life out of this material.