Features in drawn SS tube 1

[will123123]

My company is investigating a new source for stainless steel tubing and have obtained a sample for examination. Inspection of polished sections under an optical microscope source revealed particular features all around the internal surface (see attached image). From looking at the image can anyone suggest what they are and how they are caused?

The material is 316 and the cert states the material is to ASTM A269 and ASTM A213.

Thank you in advance.

 

[metengr]

will123123;
This appears to be corrosion attack of the ID tube surface. The surface is highly irregular and looks like there must have been a contaminant present if this is the only surface that was affected. I have seen both stress corrosion cracking and intergranular oxidation appear like this without further investigation.

What was the appearance of the OD surface?
Can you come back information as to how were the tubes manufactured?

 

[CoryPad]

I think this looks like InterGranular Oxidation (IGO).

 

[will123123]

The surface of the outer surface was good, finish was'smooth' and not sign of the second phase. Attached is a second image at lower magnification where the external surface can be seen.

Apologies I meant to have included that the tube was cold drawn and then bright annealed!

 

[unclesyd]

It's hard to tell but these appear to be stringers/inclusions apparently that were on or near the surface of material used to make the tube. There may be incipient cracking between the indications.
I've seen something like this to a far less degree, only 2 or 3 indications. This was attributed to problems in the mill that made strip for the tubes.

Can you comeback with the size of tube?
Seamless or Welded?
Was any testing like eddy current preformed?

 

[metengr]

If this tube material is dual certification as mentioned above it cannot be seam welded because A 213 material specification is for seamless heat exchanger and boiler tubing.

Per ASTM A269 specification was there a hydrotest performed or nondestructive electric test? This is important because if water quality was not controlled during hydrotest this could be all corrosion attack.

 

[will123123]

I have now attached an electronic copy of the cert. Which suggests some type of hydrostatic testing was carried out.

 

[metengr]

Yes, indeed a hydrotest was performed and I suspect this could possibly have been the source of your problem. Go back and find out the water quality for the hydrotest and how the tubes were processed in terms of any lubricants (if drawn over an ID mandrel).

 

[arunmrao]

If you have the pipe available with you,my advice will be to get the sample retested for chemistry. This will give you a certain lead,please do not rely on 3rd party reports ever.

Learn the rules,so you know how to break them properly.
Dalai Lama

_____________________________________

 

[moltenmetal]

I'm a rank amateur in this area, but we too have seen bad seamless tubing from China recently. The OD is fine and the ID looks good at the ends, but the IDs are filthy the further from either end you go. We did no analysis- we just sent it back to the supplier. We suspected that the tubing was improperly purged of air prior to the anneal. Could this be the problem here?

Out of curiosity, how precisely does one make 1/8" OD x 0.049" wall tubing anyway? OK, how do they make 1/16" x 0.004" ID capillary tubing? It amazes me that it's even possible.

 

[CoryPad]

This is IGO. The mechanism is oxidation of elements like Cr and Si during high temperature exposure, the cause is improper atmosphere control during either hot deformation or annealing.

Thin wall tubing is made by welding a sheet into a tube or piercing a rod. Then, the tube diameter and wall thickness are reduced by drawing them through a die with our without a plug or mandrel put inside the tube.

 

[arunmrao]

Cory,can you please explain as to why the IGO is observed on the ID and not OD?

Learn the rules,so you know how to break them properly.
Dalai Lama

_____________________________________

 

[moltenmetal]

Obviously no plug or mandrel at these sizes- and quite a bit of drawing to get them from a diameter at which this is feasible to the final dimensions. Thanks CoryPad!

 

[moltenmetal]

No problem: the OD is immersed in an inerted furnace- but a 20' long piece of tubing with a bore of only 0.025" cannot be passively purged. Once filled with air, it will remain full of air until such time as it is physically displaced.

 

[CoryPad]

The OD could have been treated by deformation, machining, wear or chemicals to remove the IGO. Or, the ID may not have been adequately purged during heat treatment.

 

[arunmrao]

Thanks!! Much appreciate .

Learn the rules,so you know how to break them properly.
Dalai Lama

_____________________________________

 

[will123123]

And I thank you as well.

I appreciate your help.

 

[unclesyd]

CoryPad
The tube itself should have been bright annealed after all mechanical forming operations are completed. If the post by metengr is correct as it pertains to this tube there should be not weld.
It is also hard to comprehend that the amount of O2 available in the tube of this size could caused this anomaly.

 

[CoryPad]

Unclesyd,

That is a good point. It is possible that the tube had intermediate anneal(s) to get it to final size, so there could have been exposure to oxygen at multiple times. I think you may be surprised at how little oxygen is required to produce this defect - it is < 100 [&micro;]m, so it isn't horrible. It isn't good, either.

 

[unclesyd]

I wish we could get a picture with a light Oxalic Etch (electrolytic) to bring out the grain and see if there are any cracks. This will be a little tricky at this size.
Another thing that would nice is look at the inside surface to see if the indications are truly just radial or say helical surface breaking form tearing while drawing.

I been trying to recall a visit I made many years ago to a manufacturer of small diameter tubes as to the exact process. I hope I have a trip report.