Conditions necessary for change in structure 4

[Tobiman]

I'd like to get views as to what other people think can cause a change in the structure of a metal after undergoing accidental overheating.How to find out if the metals structure has changed, and what it has changed to.

 

[MikeMet]

Very generic question, but here goes.

Yes for most metals you can change the microstructure with over heating. However, the temperature to cause change can be radically different for different alloys, time to cause change is different etc. So there are lot of variables that need to be determined.

You can determine if change has occurred by doing metallography, either insitu or taking a sample and doing it the lab. Of course you have to have good idea of what the original microstructure was before overheating. You can also do mechanical testing, such as impact, hardness or tensile testing. Different microstructures can have different properties. But as above, you have to have a good idea of what the un-overheated properties would be for comparison.

Hope that helps.

 

[EdStainless]

Often metallography alone won't tell you the damage. Tensile strength, impact properties, and corrosion testing are often needed to fully evaluate the extent of the damage.
Often by the time you can see it in the structure the damage is so bad that the material is unsuitable for use.

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Plymouth Tube

 

[Wrenchbender]

If the accidental overheat exceeded the last processing temperature (e.g. tempering, aging), a significant change in microstructure probably occurred. Have to appreciate that rate processes are exponential functions and things might happen suddenly. Time at temperature may also be a factor, depending. How do you know for sure if there has been a change in structure? As others have said above. Might also do some coupon tests.

Also the quench rate could effect the result. (e.g. Was the overheat localized such that the adjacent material 'sucked out' the heat rapidly; or did the entire bulk section cool slowly?)

 

[PAN]

Please consider API 579-1/ASME FFS-1 Part 11 (Assessment of Fire Damage)...