TP 347 after long thermal exposure replica

[Liuk]

Hello,

The replica examination shows some cavities (acc. to focus changing) along grain boundaries. Electropolishing and Kalling's reagent was used in this case.

I have some doubts about cavities formation reason. What is your opinion? Thank you in advance!

 

They are not cavities but artefacts; possibly bubbles in the acetate or stains from inadequate cleaning. Use only acetone for the final rinse.

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

 

[Liuk]

Maybe I wasn't clear, but I did mean that these "features" are below pipe surface (I can see some bottom when I change focus. Silicon rubber was used for replica. I can say exactly it isn't remaining from cleaning (sometimes there are small drops of alcohol on examined surface, but it is clear seen). We don't perform electropolishing and I don't know exactly: if any influence like this is possible? I have attached one more photo at 200 magnification.

 

Silicon rubber is easier to apply but I am not a fan. Some skill is required for acetate; work on your technique, including wasting a few strips on the periphery of the polish location to make sure you do it right for the environmental conditions. Alcohol leaves stains while acetone does not. Something went wrong with this job.

That said, overlooking the artefacts you have a good etch, good image and the metal looks in OK condition.

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

 

[IRstuff]

I don't get it; have you examined the actual metal surface using dark-field?

It seems to me that if you've already polished and etched the metal, why not examine the metal directly?

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[Liuk]

No it is replica surface, but I used DIC (Nomarski prizm) for 500x magnification photo.

Too difficult condition on site to examine surface directly

 

[MagBen]

: TP 347, do you mean 347 stainless? long thermal exposure, do you mean material was exposure long time at 500-800C? This alloy is Nb/Ta stabilized, to sensitize grain boundary, hundreds, even thousands of hours are needed to form carbides at grain boundaries. The precipitates (if this is the real fixture) are randomly distributed, the amount is so high, I do not think this is NbC, but rather possibly Laves phase.

 

1) Laves phase? does not have the appearance of large rounded blobs.
2) Viewing the etched surface in situ is always useful as a check that helps confirm replica artefacts, and it confirms the quality of your handiwork.

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

 

[EdStainless]

I am leaning toward either sample prep artifacts or replication artifacts.
I am presuming that the smaller solid colored ones are NbCN, but the large ones don't make any sense to me.
Nice polish and photo work (I love DIC).

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P.E. Metallurgy

 

[Liuk]

Dear all,

Thank you for your reply!

Yes I mean 347 steel, service time is tens of thousands hours.

Of course it is very important to do examined area checking during work on site as well as select polishing/etching mode more carefully.

I would like to clarify: it isn't carbides (these voids could be "fallen" carbides only) acc. to focus changing.

As far as I can see it likes strong etched intermetallic phases (possible Laves phases). I have performed some "investigation" in lab: sample with sigma phase etching by Kallins's. Increasing time of etching gives some voids like I can see on replica (it isn't clear experiment I used duplex steel).

 

[Dhurjati Sen]

Dear Liuk,

A chemical analysis could help you. Also, look for similar photomicrographs in Material Handbook Vol 13 Corrosion.

Regards.

DHURJATI SEN
Kolkata, India

 

[MagBen]

What is the service temperature that fundamentally determines the precipitates (refer to TTT curve).

@ironic metallurgist: Laves phase precipitates intragranularly in the form of equiaxed particles, occasionally on grain boundaries. However, large size up to 10 microns does seem questionable

 

[Liuk]

Service temperature is 650C (as I was informed). Of course size isn't typical for Laves phase and shape of voids is doubtful...