Spiral crack on Alloy 800 tube under tensile test

[isoca]

Dear colleagues,

I am performing some analysis on seamless tube of alloy 800. During tensile test at 350°C I have found that the tube, in all the cases, break forming a spiral crack.
At room temperature, the material break in a standard way (forming a neck). In both cases the fracture surface shows ductility.
I tried on different testing machines and different rates and the behavior is the same.
The material tested was previoully annealed at 980°C.

Could someone explain this phenomenon?

I have attached two pictures to better understanding.
Thank you

 

[metengr]

I would think it has to do with residual cold forming strain in the tube material after the solution anneal to straighten the tubes, and response of the material as the temperature increases. This reminds me of carbon steel boiler tubes that had graphitized along 45 degree planes in the tube material (Barber pole effect) at elevated temperature because of residual forming strains when the tube was manufactured. I would bet if you anneal the tubes to remove residual cold forming strain, the failure would appear as at room temperature.

 

By any chance is this a nuclear steam generator tube?

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

 

[isoca]

Thanks for your answers.
I have performed several trials increasing the annealing temperature up to 1000°C approx and this phenomenon still continues.
I have tested another tubes made of the same alloy but with lower OD and the same wall and this case the spiral crack is not observed. Could this situation be related with some geometrical issue. For example OD/WT ratio.
This tube is used on a heat exchanger.
Any suggestion will be welcome.
Best regards

 

[EdStainless]

We see this crack pattern in high strength thin walled samples for many alloys, but usually when there is limited ductility.
We have seen it in samples that have not been straightened.


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P.E. Metallurgy, Plymouth Tube

 

[isoca]

Many thanks for the reply.
In this case, elongation at room temperature exceeds 40% but I cannot measure the elongation at high temperature due to the crack morphology and becasue of there is no complete tube breaking.
In those case, how have you measured the elongation after break?
Best regards

 

[metengr]

isoca;
You might want to review the report below regarding thin walled tube testing;

Test methodologies for determining
high temperature material
properties of thin walled tubes

http://publications.jrc.ec.europa.eu/repository/bitstream/JRC105586/kjna28642enn.pdf

 

[EdStainless]

You do get a good Yield strength, correct?
After that we take the total extension (measured at the crosshead) at the initiation of fracture to be total sample elongation. Being in the US we make sure that there is 10" of sample exposed between the grips when we start just to make the math easy.
You might try cutting (milling) windows in the sides of a sample so that there is a reduced cross section portion. This effectively gives you two flat arched samples joined at the ends. Perhaps you can get good yielding that way.
Just remember that if you are pulling a flat sample (or a pair of them) you will see about 15% less elongation (say 30% vs 35%) than with a full body tube sample.


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P.E. Metallurgy, Plymouth Tube

 

[EdStainless]

I should also ask what strain rates are you using?
In some alloys when you test hot you can get very interesting strain induced recrystallization or grain growth issues.
Have you look at micros at the areas of these fractures and compared them to the rest of the sample?

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P.E. Metallurgy, Plymouth Tube

 

[dik]

Have you look at micros at the areas of these fractures and compared them to the rest of the sample?

sort of a before and after... you should be able to determine if there has been any change to the structure of the material.

Is there a possibility that there is some impurity in the base material to cause a hot short condition?

Dik

 

[Metalero]

During the test bands of Lüders are formed, favored by the temperature, this process is sensitive to the magnitude of the generated stress, which is why the behavior changes when modifying the size.

The inclined fracture follows the orientation of the bands.

Maybe if you do a test with the polished tube you can observe these bands and confirm the phenomenon.

Another probability is formation of slip bands and/or twins, at the end same behavior.

 

[WKTaylor]

isoca, all.. I'm a dumb aero material guy scratching his balding head...

So I can follow this discussion... please advise... what is 'alloy 800'... IE UNSxxxxx and/or typical spec number.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[metengr]

http://www.specialmetals.com/assets/smc/documents/alloys/incoloy/incoloy-alloys-800h-800ht.pdf

 

[isoca]

Thanks everyone for your usefull comments
Alloy 800 is an fully austenitic alloy stabilized with Ti. Similar to an asutenitic stainless steel but with higher content of nickel and chromium. (UNS N08800).

I agree with MarriolaV that this phenomenon could be related with Luder bands formation. Sometimes, lines with an angle of approx 60° can be see it a both sides of the fracture.

What is not clear for me yet is the effect of D/t ratio on formation of that slip bands.

So far I have not found any standard (ASTM , ASME, ISO, etc.) that considers this effect and the problem our client wants the tube to be tested in full size.

 

[WKTaylor]

Metengr... Ahhh.... UNS N088xx... I see cried the blind man. Sound like an alloy used in gas-turbines.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Metalero]

Isoca
ASTM E8 and E21 could be useful. I’m general ASTM methods are intended to use test pieces whit geometries where the stress distribution be good enough in the calibrated section..

Are you using correct length of test piece?
And remember avoid over press de tube whit the grip and internal support.

 

[isoca]

Hi,
I have tried with different probe configuration, always inside of ASTM E8 and E21 and the results remain the same. i have found some articles that relate this crack morphology with the OD / WT ratio. For example in aluminum tubes some authors indicate that if the ratio is grater than 20:1 the fracture is of spiral type. In this tube the ratio is 19:1.
In order to verify this situation I have tried with stainless steel tubes with ratios greater than 20 and in all cases the fracture has an anlgle of approx 60°. No spiral was produced but..
These SS tubes broke in a standard way (forming a necking in a middle) when tested at room temperature.

 

[EdStainless]

This is also a function of ductility, so testing hot will give different results.
When you test the 800 at RT you get regular fracture don't you?

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P.E. Metallurgy, Plymouth Tube

 

[Metalero]

isoca

Have you tried different speed of testing?

Slower speed (as permitted by test method) could minimize such behavior.

 

[isoca]

Yes, at room temperature alloy 800 shows a regular fracture with an elongation of approx 40%.
Regarding speed of test, I have tried in all range (even at lower speeds) specified by the standards.