Creep Cavities in Short Overheating 1

[replica]

Dear all,

Is it possible to see creep cavities in short overheating tube that caused stress rupture with fish mouth appearance? As far as I know creep is time depending so for short term overheating.....we cannot see cavities..Am I right?

 

[mrfailure]

It is quite possible to see voids or porosity at fairly low magnification in short term overheating, the result of plasticity during the rupture process. These are not creep voids, though they will look similar to creep. You can confirm by looking at microstructure and hardness of the rupture compared with base metal.

 

[replica]

mrfailure

Thank you for your comments..so Can I say that these voids are not the creep voids but coalescence of voids due to deformation at high temperature where the yield strength dropped as the temperature increased and cause rupture. Am I right? How do we actually distinguish between the failure due to short term over heating and creep because both has reduction in hardness due to softening? Is it by the appearance of the fracture where overheating exhibited fish mouth and thin fracture edge while creep with much thicker fracture edge?

 

[mrfailure]

I usually like to make a second mount in-line but immediately upstream of the rupture and look at the condition of the tube there, as well as 180 deg. away from the rupture. This will show you tube condition without the deformation. Look to see if the tube at this location exhibits wall thinning, and whether you have have significant ID scale growth as well. You can use this information to calculate tube metal temperature to see if long-term overheating was occurring. You also can look at this microstructure to see if there there is evidence of isolated creep voids on grain boundaries, and how much spheroidization had occurred during service from service temperature exposure. Creep or long-term overheat mechanisms generally do not occur in tubes carrying water (i.e. waterwall tubing) except in unusual circumstances, but short-term overheating is possible in superheater and reheater steam tubing. If you do determine short-term overheat, then you need to find out why and look for upstream leaks or blockages that might be the cause.

 

[replica]

mrfailure

Thank you very much for a useful information and suggestion. I really appreciate it.

 

[blacksmith37]

A "fish mouth" rupture appearance indicates short time ,high temperature exposure. Long term creep rupture will be longitudinal cracklike ruptures. As noted check in other locations to determine a temperature profile in the area.

 

[mrfailure]

With all due respect, I think blacksmith37's post is misleading. Creep rupture in superheater tubes also can and often do exhibit fishmouth rupture. You may macroscopically see the appearance of longitudinal cracking on the ID and OD surfaces which is really cracking in the oxide scale on both surfaces. Because of low pressure, long-term overheating of reheater tubes do not usually exhibit fishmouth rupture but you still need to see ID scale growth and look for evidence of microstructural degradation and creep immediately upstream of the rupture to characterize.

 

blacksmith37,
mrfailure is correct; there are exceptions to this general 'rule'.

I have found that boiler tube failure investigations can be quite tricky, especially if (like myself) you did not grow up around them.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."