304Cu steel microstructure 2

[Martinos]

Hi dudes.

I have here drawn bar made of 304Cu (1.4567) steel in solution annealed condition. Chemical composition C 0.011 Si 0.40 Mn 0.75 S 0.02 Cr 17.1 Mo 0.32 Ni 8.6 Cu 3.1%.
Please take a look at a photomicrographs linked below (1x transverse cut, 2x longitudinal cut). I have a dispute regarding prolonged stringers. May it be deformation induced martensite? My first conclusion was delta ferrite but now I am not sure about it.

Thank you!

Link
Link
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[EdStainless]

What etch?
What mag?
Check to see how magnetic it is.
What is the S and P content?
Was this strand (continuous) annealed?
How much of a temper reduction was made after annealing?

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P.E. Metallurgy, Plymouth Tube

 

[Martinos]

1. Kalling's 1 (Kalling's with water)
2. Original magnification 500, but camera magnified itself and make it almost 2x original magnification
3. I will check the magnetism - on tuesday. But I assume delta ferrite is magnetic as well as martensite (?)
4. sulphur 0.02%, definitely the stringers are not MnS (maybe a few, but not all). phosphorus 0,03 %
5. regarding annealig I have no informations about. bars were solution annealed and cold worked (see link below). That is all I know about...
Link

Is there any etching technique to prove if is it ferrite/martensite? (please, no electrolytic) Thanks.

 

[EdStainless]

Give me some photos re-polished with not etch, and then etch with oxalic acid (10%, electrolytic).
And lower the mag a bit.

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P.E. Metallurgy, Plymouth Tube

 

[MagBen]

Most likely ferrite stringers.
Make a calculation of percentage ferrite, it seems too much! The solution anneal did not work (too slow cool, too low temp etc.), or there existed severe segregation during solidification. Cu actually helps to stabilize austenite.
Cu also helps to suppress martensite formation at cold work.

 

[EdStainless]

Ben, I thought ferrite also, but by my calculation I am getting 3-4% (depending on the method).
That is fairly low for austenitic stainless.
So unless this product had very little cold work put into it I find it hard to believe that there is that much.
I really wonder how it is distributed though, if this is just along the centerline then maybe.

Martinos, have you tried annealing a piece in a lab furnace. A few minutes at 1100C and water quench should give you a clean structure.

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P.E. Metallurgy, Plymouth Tube

 

[Martinos]

Ben, Ed,
I have no possibility to electrolytic etch, but please take a look at the pictures linked below. After re-polishing only a few (Cr)MnS inclusions were found. But these photomicrographs linked below are after another etching this time with waterless Kalling's2 up-to 20 seconds. In my opinion the first etching with Kalling's1 was not appropriate and influenced the results because to etch primary austenitic grains was necessary to overetch the stringers in the direction of drawning. This time the microstructure looks more common with delta ferrite stringers, but... Please focus on fig 4 and 5... there are some needles through austenite (not in the direction of drawning). Could be those martensite?

fig. 2 Link
fig. 4 Link
fig. 5 Link

 

[EdStainless]

The ones in #2 could be delta ferrite, but I still suspect that some of it is other stuff.
Is it possible that the Cu rich phase formed during the final work? (I know that it will form during in-service creep)
I would suspect that the needles in #4 and 5 are transformation martensite. It looks to formed along twins.

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P.E. Metallurgy, Plymouth Tube

 

[Martinos]

Yes. I think you could have a truth. I think to myself what can 3% of copper do. Some can be dissolved in austenite, but some not - where is it?. Unfortunately, I do not have a better laboratory necessaries to have more precise analysis.

Thank you for great discussion guys.

 

[Dhurjati Sen]

Dear Martinos,

I was silently following the discussion, it is very interesting. I feel you have no other way but to send the sample to a metallurgical laboratory and ask them to carry out a SEM (Scanning Electron Microscopy) or a TEM (Transmission Electron Microscopy) to conclusively diagnose the particular phase.

Regards.

DHURJATI SEN

 

and ask them to carry out a SEM (Scanning Electron Microscopy) or a TEM (Transmission Electron Microscopy) to conclusively diagnose the particular phase.

Why? Optical techniques with appropriate etchants will tell you more. View it first without etching, the best way to confirm inclusions vs. anything else. MnS inclusions are always light gray in colour. Examination of the 'stringer' alignment will also help to confirm them. Sulphur content on the CMTR will tell you if there is potential for large stringers.
My first shot at etching would be 20~40% aqueous NaOH or KOH, electrolytic.
.
A quick and easy way to identify ferrite is to put a ferrite scope on it. Fischer is my weapon of choice.
Copper is added to 304SS to accommodate the extreme deformation inherent in cold heading.

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

 

[Martinos]

Yes. SEM/TEM may be helpful. This strain induced needles were not found in the past, til now and along with this we have some problems with deformations - residual strains after machining of bars. I think there are some changes in drawning process from side of our supplier (of course they say that nothing changed). Surface of bars is affected too. Below attached two micro-images of surface for comparison.

 

Microindentation hardness testing near the surface cracks would be informative.
I would also ask questions about lubrication during forming.
.
TEM for practical failure analysis? Never in my experience. (Is there an assumption here that more advanced technology is somehow automatically more informative or better suited?)
When I find a testing lab that applies SEM to anything and everything, I move on to another lab; they are just doing it to pump the invoice, and probably are lacking in metallurgy fundamentals.

p.s., every failure analyst should carry a magnet with him/her at all times.


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

 

[MagBen]

Martinos, how much cold work was material experienced? if it was heavily deformed, the needles could be mechanical twins. whatever, it looked to me typical shear bands and martensitic features. Note 304 is easier to form strain-induced martensite than 316, especially the composite is low C, N.
Another thing not clear is that, were these structures from solution annealed, or from as-drawn materails? The elongated (banded) structures seemed as-drawn condition.

 

[Martinos]

Ben, I understand that material was cold worked after solution annealing (see attachment). How much cold work? I have no idea.

Irony, SEM more likely than TEM may be helpful in a special cases, e. g. where the chemical composition of complex carbides is important (gasoline turbocharger VTG casting materials). Now it is not all about failure analysis. It is a deeper knowledge as well - in this case, copper distribution. But yes. Some laboratories are making useless SEM/TEM analyses everytime is possible.
And yes. Magnet is useful unless it is too strong... owing to Lenz's effect.

 

[EdStainless]

Martinos (OP) said that it annealed, but if so then it sure looks under-annealed to me.

Judicious polish and etching will go a long way.
You can electrolytic etch with a 9V battery.
First always look at unetched samples. Use polarized light and DIC.
Oxalic acid is my first echant of choice for austenitic SS, everytime.
After that I go to color tint etch, such as 10-20% HCl in water with 2-5% potassium metabisulfite.
The biggest issue with annealed stainless samples is smearing the surface during polishing.

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P.E. Metallurgy, Plymouth Tube

 

[Martinos]

Ed, At this time I have no possibility to electrolytic etching, but regarding oxalic acid I share your opinion. Coincidentally I am going to try color tint etch with Beraha. A few hours ago I have ordered potassium metabisulfite to our lab. I will post some photos after.

 

[Martinos]

Ed, Ben, please, take a look at some photomicrographs after Beraha tint etching.