Differences of Short - term Overheating and Long - term Overheating ( for Failure Analysis ) 1

[NuEng]

Greeting to all eng-tips members,


How do we know the differences of short-term overheating or long-term overheating ? what testing shall I do to examine the differences ? Would you mind to give the literature,too ?
I really appreciate any help you can provide. Have a good day.

 

[metengr]

Short term overheating is typically driven by a damage mechanism known as stress rupture. The result will be noticeable deformation near or at final failure in the form of distortion and ductile failure. Depending on the severity of exposure to short term overheat, one can observe radical microstructural changes.

Whereas, long term overheat results in less distortion, relatively speaking to short term overheat, and is driven by creep deformation. In most cases, creep damage occurs in the form of voids and micro-cracking which will be present near final failure or at final failure of the material.

For either damage mechanism confirmation is required using metallograhic examination.

The above are very broad based observations with ferrous materials.

 

[NuEng]

Short term overheating is typically driven by a damage mechanism known as stress rupture. The result will be noticeable deformation near or at final failure in the form of distortion and ductile failure. Depending on the severity of exposure to short term overheat, one can observe radical microstructural changes.

Whereas, long term overheat results in less distortion, relatively speaking to short term overheat, and is driven by creep deformation. In most cases, creep damage occurs in the form of voids and micro-cracking which will be present near final failure or at final failure of the material.

For either damage mechanism confirmation is required using metallograhic examination.

The above are very broad based observations with ferrous materials.

Thank you for your response,

So, we can say Long-term overheating as long as we found voids and micro cracking, sir metengr ? Do you have any literature so I can easily recognize the failure, sir ? Thank you, sir.

 

[metengr]

NuEng;
You can look up the terms I mentioned above on the Internet. This is what I tried to do so that you can follow up on your own.

So, we can say Long-term overheating as long as we found voids and micro cracking, sir metengr ?

Not necessarily, long term overheating can also result in subtle changes to the original microstructure which can degrade it with no obvious creep damage. This is why you need to look up this information. Failure mechanisms must be applied on a case by case basis with an understanding of exposure to service conditions.

Failure analysis requires unique expertise based on experience associated with various processes and evaluation of failures from those processes.

I can't tell you how many labs profess expertise in so-called failure analysis and don't know the difference between a boiler tube and exhaust pipe.

 

[blacksmith37]

Analysis will require metallography . as stated; However as a starting point long term creep failures show minimum distortion (" brittle"), while a short term overheat will show significant plastic deformation . Generally; This is for wrought carbon and low alloy , up to 9Cr. Test bars show the same characteristics.
I don't mean to get off subject ,but some unusual failures; In the dark ages, refinery furnace tubes would coke-up and turbine drills could plastic ally expand/thin a tube if not operated properly. And steam /air decoking could drastically overheat a tube. ( only for refinery tubes.)

 

[mrfailure]

You have to start with what these mechanisms mean. Short term overheating occurs because, as the name suggests, a condition exists that heats the boiler tube over a short period of time. In my experience, this usually occurs because of an upstream blockage or leak of water/steam flow, or a burner is impinging on the tube. Short term can happen anywhere in the boiler. Depending on tube temperature, creep can be present but lack of creep damage observed in cross-section usually means you have a short term overheat. Long term overheating occurs because of long-term degradation of the tube from temperature exposure. In ferritic tubing you will see degradation of the original pearlitic structure and creep void formation in the rupture lips. Long term overheating often happens immediately upstream of a transition to a higher-temperature material (i.e. boiler designers probably should have transitioned to the better material sooner). Also, long-term overheating only occurs in steam tubing, and not in the waterwalls. Rupture can look similar to short term overheating in the form of a fish-mouth rupture. Other forms of leakage can occur in both mechanisms though, including formation of a blister with a leak in the middle, or rupture that is so dramatic because of the energy released that it can cause transverse separation of the tube.

EPRI has great materials on this subject; you should take advantage if your organization is a member.

 

[Guest102023]

per metengr: Failure analysis requires unique expertise based on experience associated with various processes and evaluation of failures from those processes.
I could not agree more, and it is especially true for boilers.
Not having 'grown up' around steam, I have found root cause determination of failed tubes to be often difficult, even when the failure mechanism is fairly obvious. Being a quick study on the environment and process in which the failure occurred (i.e., context) is an essential attribute of a failure analyst.
.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"