I need a metallurgist...What is this microstructure found in A240 Grade 347 1

[replica]

Dear all,

I am puzzled with the microstructure found in the vessel made of A240 Grade 347. We used replica technique to pick up the microstructure using 10% oxalic acid. I can see twins inside the prior austenite grains with black dots along the boundary. Are they carbides of artefact due to etchant? Why the microstructure is so banded ie. patches leaving some area unetched? Is it due to manufacturing of the plate? I never see this microstructure before. Is this microstructure detrimental to the material? ..Any comment will be highly appreciated...Please see the microstructure attached at 100X, 200X and 500X.

 

[EdStainless]

Was the material ordered stabilized?
I am judging that most of the grain boundary stuff is carbides.
I am a bit disappointed that I don't see more of them within the grains.
The variation in grain size is not uncommon, partial recrystallization.
It depends a lot on how it was rolled and heat treated.
Unless you need coarse grain material for creep resistance I don't see any issues with that.

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P.E. Metallurgy, consulting work welcomed

 

[replica]

EdStainless,
Thank you for the comments. It was ordered stabilized...but who knows....Can we really know if the material is stabilized or not?..Thank you very much the information regarding partial recrystallization. Based on ASTM A262, Detecting susceptibility to inter-granular attack in austenitic stainless steel, ..Look like the material is dual structure (step and incomplete ditch)
I am a bit disappointed that I don't see more of them within the grains.
Yes, no carbides at the twin boundaries.

 

[MJCronin]

Where is the country of origin of your 347 SS ?

MJCronin
Sr. Process Engineer

 

[replica]

MJCronin,

From China...does it matter...We found crack at the weld deposit and try to figure out whether this material is stabilized or not?

EdStainless ...If I cannot find carbides in the subgrains boundaries ..does it mean that the material was not sensitisized during operation? If we get carbides at the austenite grains ..Are they have to be in the twins boundaries as well? What type of carbides that we will see at the grain boundaries ..cr carbides or Nb carbides? The vessel operated between 520C and 600C

 

[mrfailure]

I wonder if you still see grain boundary effects if you pulled the replica in the as-polished condition.

 

[weldstan]

How long has it been in operation? How many cycles?

 

[EdStainless]

If it had been properly stabilized I would expect to see carbides along most of the grain boundaries as well as a dusting of them within each grain.
If you mees up in processing and actually fully solution anneal the material (dissolve nearly all NbC) then when you stabilize you will only get grain boundry carbides. This lowers toughness and if it was not done hot enough will result in sensitization.
The only way to detect is by extracting a sample. Is there Cr depletion along the GBs?
If it wasn't well stabilized your service almost certainly sensitized it.

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P.E. Metallurgy, consulting work welcomed

 

From China...does it matter.

It matters a lot, and these days it should be your first question.

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

 

[EdStainless]

My real question is what QA was done on the HT originally?
Somehow I doubt that you did much on site, but maybe.

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P.E. Metallurgy, consulting work welcomed

 

[Chumpes]

Hello
Just a quick remark :
SS 347 is always chemically stabilized by chemical addition of Nb.
In addition, SS 347 may be ordered with a stabilization heat treatment that forces the precipitation of Nb-carbides, which should promote the resistance of SS 347 to further sensitization
++

 

[replica]

mrfailure
I wonder if you still see grain boundary effects if you pulled the replica in the as-polished condition.
Great
I think we cannot see anything other than mirror image polishes surface as it has to be etched to reveal the grain and carbides.

weldstan
about 10 years.

Edstainless
If it had been properly stabilized I would expect to see carbides along most of the grain boundaries as well as a dusting of them within each grain.

What carbides ..NbC or CrC...how can we tell

If you mees up in processing and actually fully solution anneal the material (dissolve nearly all NbC) then when you stabilize you will only get grain boundry carbides. This lowers toughness and if it was not done hot enough will result in sensitization.


Thank you very much for the comments and info

The only way to detect is by extracting a sample. Is there Cr depletion along the GBs?

Very hard to find out whether stabilised carbides (NbC) or sensitization (CrC)...I think it is a very demanding task...May be analysed using TEM attached with EDS...

If it wasn't well stabilized your service almost certainly sensitized it.

Agree..but we do not know whether it was stabilised or not...how to distinguish wheather it was sensitised because of not being stabilized or being sentised during operation.

My real question is what QA was done on the HT originally?
Somehow I doubt that you did much on site, but maybe.

None...not much work on site...only did it when cracking at weld deposit.

Chumpes
Just a quick remark :
SS 347 is always chemically stabilized by chemical addition of Nb.
In addition, SS 347 may be ordered with a stabilization heat treatment that forces the precipitation of Nb-carbides, which should promote the resistance of SS 347 to further sensitization
++

Yes it should be ...The issue is we do not know weather it was stabilized or not from the beginning....we have to trust the supplier. Is there any method to verify? any microstructural analysis?

Thank you everyone for very useful comments...

 

[weldstan]

Have you confirmed the material as 347 by OES or XRF methods? Mixed alloys were somewhat prevalent in SS from China a decade ago. Poor heat treatments were also common. But at this point there is really no way to confirm.

 

[EdStainless]

So the trick with 347 (and 321) is that Nb and Ti carbides are stable to higher temp than Cr carbides.
So in anneal you can dissolve the Cr carbides and leave the Nb and/of Ti ones.
In annealed material there should still be Nb carbides scattered within the grains.
Then when you stabilize these should grow.
There should not be heavy carbide growth at the boundaries, some but not heavy.
A micro before anneal, after anneal, and then after stabilization will serve to tell the story.
If you have inspected the inside thoroughly and you don't see any IGA, then don't worry about sensitization.
After 10 years all of the carbon is tied up in some form and it isn't going anywhere.
Grind your crack out and verify by NDT that you got all of it, and re-weld using your approved procedure.
Just remember that your material has lower ductility than it really should, but all should be ok other than that.
Checking the actual chemistry (not portable) is a good idea, just remember that each plate might be different (chem and micro).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[replica]

weldstan

Have you confirmed the material as 347 by OES or XRF methods? Mixed alloys were somewhat prevalent in SS from China a decade ago. Poor heat treatments were also common. But at this point there is really no way to confirm.

Yes ..we have confirmed that the material is 347..However we suspected that the plate was not stabilized properly/or not stabilized at all.

EdStainless
So the trick with 347 (and 321) is that Nb and Ti carbides are stable to higher temp than Cr carbides.

Yes..I think annealing between 800 and 1230C, dissolved the Cr carbides leaving Nb carbides. Above 1230C everything is in solution. Ideally, rapid quenching between this temperature range formed Nb carbides without Cr carbides.

So in anneal you can dissolve the Cr carbides and leave the Nb and/of Ti ones.

In annealed material there should still be Nb carbides scattered within the grains.
Then when you stabilize these should grow.

I suppose the stabilization temperature should be above 800C before it can be used between 600C and 800C

A micro before anneal, after anneal, and then after stabilization will serve to tell the story.

Thank you for the valuable suggestion.

Grind your crack out and verify by NDT that you got all of it, and re-weld using your approved procedure.
Just remember that your material has lower ductility than it really should, but all should be ok other than that.

This is exactly what we are doing now ..we reconfirmed that no crack left by NDE after grinding prior to re-welding..We actually removed the entire weld deposit including the root pass as we found the crack was deep.

Checking the actual chemistry (not portable) is a good idea, just remember that each plate might be different (chem and micro).
We have confirmed that the material met the specification for 347 by portable (not really portable lol) spark spectroscopy. We also found that the microstructure of the plate was not homogenous with some locations were fully recrystallised and some partially recrystallised. Does it meant that the annealing temperature or the stabilizing temperature were not homogeneous?

Thank you very much everyone for a very useful comments and suggestions. I really appreciated them.

 

Have you confirmed the material as 347 by OES or XRF methods? Mixed alloys were somewhat prevalent in SS from China a decade ago.

Here's my analogy for CRAs and other sophisticated alloys, and why you should think carefully before 'saving' a few dimes sourcing from China:

Making an alloy is like making soup: it is a lot more than just the list of ingredients (i.e., composition). It is how you cook them to make the soup that imparts the quality.

An alloy like 347 or duplex SS or (especially) advanced ferritic steels for steam plant I would never source from China.

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

 

[EdStainless]

I am with IM on this one.
Sub-optimal processing can destroy the best material.
Why people feel compelled to try and save $100k on a vessel that is part of $1B project is beyond me.
I don't care who makes the steel, fabs it, or tests it.
But in the end the level of QA and documentation needs to be the same.
And in China that often means sending a 2-5 person team for the full duration.
Don't allow 1" weld to laid without witness, don't allow as single NDT test without witness.
It isn't that they will try to cheat (usually) but often the operators don't understand why they are to do things a specific way.
And hence they don't understand the risks of deviations.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed