Stainless Steel 316 annealing temperature 1050C (1922 F) vs 1950 F....is there a big difference? 2

[AnsAvi]

Hi all,

Over the last few years, I have come across some cases where some of the 316 material we use has failed ASTM A262 Practice A (Test for sensitization) but passed ASTM A262 Practice E testing. This material seems to also show a higher susceptibility to SCC. A common thread in all these cases has been that the mill heat was solution annealed at 1922 F as opposed to the "good" heats that were solution annealed at 1950-1955 F. Just FYI, The final product is made out of material that is heavily cold worked to different sizes after the solution anneal treatment.

So I would like to ask this community if anyone has observed that a 1950 F on 316 treatment is "significantly" better than 1922 F treatment, especially in terms of IGC and/or SCC.

Thanks

 

[racookpe1978]

Do you know the time-at-temperature (soak time) for the failing items?
Any differences in thickness or cross-section (thermal mass effects) between the failing items and the good items?

Could be temperature - don't know but crystals do change at specific temperatures. It's just possible that there are other differences that may make the slightly lower heat treat temperature more important. Theoretically, an open pot set on a stove top at 101 degree C will boil the water. Practically speaking, the water will never really get to 100 C, but instead will evaporate out at 98-99 C.

 

[EdStainless]

No I haven't, but then we use 1975-2000F.
I have seen issues with material annealed in nitrogen rich atmospheres.
It will fail A because of nitrides, but pass E.
In service the performance is somewhere in between.
I have never seen a scientific article that showed a link between IGC and SCC, except in a few very specific environments.
Remember that SCC requires an active corrosion site, so anything that lowers the corrosion resistance of the alloy can contribute to SCC.
But if all that stands between success and failure due to SCC is the anneal temp then you have selected the wrong alloy.

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P.E. Metallurgy, Plymouth Tube

 

[MagBen]

Not supposed to observe "significant' difference between 1922 and typical 1950F, but a little lower temperature could cause subtle difference that might trigger a big one.

One thing i am thinking of is the Sigma phase, if existed after hot work, lower solution temperature is harder to remove it. The hard sigma phase could cause failing practice A.

By the way, practice A is a screen test, any time passing A will pass E. Vice versa is not true. I never experienced that 316 failed E.

 

[EdStainless]

I'll second Ben's comment, I have done a few thousand Pract E tests, and never failed one.

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P.E. Metallurgy, Plymouth Tube

 

[AnsAvi]

Thanks Racookpe1978, EdStainless and MagBen!

 

[AnsAvi]

Wow, I asked the question just 2.5 months ago on this thread and now I have a real twister. We just received a test report saying that the material exhibited Step Structure in Practice A (ASTM A262) but when the lab conducted Practice E test, the material failed. This is a heavily cold drawn 316 material. Any explanation on how that could be possible?

 

[EdStainless]

So you tested in the the heavily cold worked condition?
That is why.
This is intended for annealed material.
The stress can have an impact on response.
If you are buying cold worked material the A262 should be done before the final cold working, after the last anneal.

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P.E. Metallurgy, Plymouth Tube

 

[AnsAvi]

Thanks Ed for the response. So just to clarify, you mean that if I have to check a cold worked 316 for intergranular corrosion susceptibility, I should stick to only Practice A and not necessarily Practice E? In other words, as long as the cold worked material passes Practice A and its prior solution annealed bar meets Practices A and E, the material can be considered to be free from sensitization?

Or do you mean that the entire A262 (including Practice A) should only be done in the "as solution annealed" state? In that case, what way do we have to check for sensitization in any cold rolled 316 bar?

 

[EdStainless]

The results of the Pract E test will be distorted by the cold work (the same is true of pitting tests also).
I would require Pract E with no screening allowed prior to the final cold work.

Are you sensitizing these? Have you looked at different times and temps?
I recall that for cold finished wire we had to modify the procedure, but I don't remember the details.

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P.E. Metallurgy, Plymouth Tube

 

[AnsAvi]

Ed, Thank You. That could explain the phenomenon. The bars are cold drawn to the desired dimensions and mechanicals by our vendor at a draw shop. They claim the entire process is low T and thus should not cause sensitization. SO I guess in the cold drawn state, we should only look for acceptable structures as per Practice A in order to qualify the product.

 

[WKTaylor]

RE the 316 CRES material You are processing...

What is the raw material procurement spec and 'condition' on the procurement documents.

Does each shipment include accurate [trustworthy] procurement certifications? IF SO, do the CERTS consistently include the basics of chemistry, grain analysis, mechanicals, etc?

Curious: WHY do You have to anneal the raw stock... why-not get it certified annealed?

Does the material always meet the procurement requirements... or is there theme/variations??

Is your raw stock consistently from the same heat/lot... or is it mixed?

What heat treat spec are you performing to? Are You sure Your A-HT process is consistent? NOTES.

AMS-H-6088 Table 1C allows annealing temperature range of 1900-to-2050F, then water quench after required soak.
AMS2759/4 Table 1 requires 'set temperature' at 1950F, then water quench after required soak.

Is Your HT equipment routinely maintained/calibrated... EX atmospheres, furnaces, thermocouples... to ensure consistent/accurate distribution throughout the cavity?

Are You aware of [or is your shop aware of] the warning in AMS2759/4 regarding contaminates from residual furnace atmospheres and surfaces, thus...
3.2.1 Atmospheres
Shall be controlled so as not to contaminate the parts being heat treated. Furnaces used to heat treat other classes of steel
using atmospheres that could contaminate austenitic corrosion-resistant steel parts, such as endothermic, exothermic,
carbon-containing nitrogen-base, etc., shall have purge cycles (see 8.2) run and then shall be tested as required in 3.4.1 to
ensure that the surfaces of parts are free from nitriding and intergranular oxidation. Parts being heat treated shall be suitably
isolated from products of combustion. Materials that could attack or contaminate metal shall not contact parts during heat
treatment. Composition and maintenance of salt baths shall be such as to prevent contamination of the parts. Salt baths
shall be tested in accordance with AMS2759.

Etc...

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]
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[MagBen]

Practice E is kinda bending test after etching. For heavily cold worked material, it is very easy to fail E. so when you do practical E, test it right after solution anneal prior to cold work!
You got a "step" structure in practice A, this is the best result, meaning no intergranular corrosion at all! you have nothing to worry about.
Normally when you get step or dual structure in A, you are having a pass, no even need to do E.

 

[AnsAvi]

Thanks MagBen, WKTaylor and EdStainless for your responses and help in this matter! Perhaps then it does seem it would be good if the ASTM A262 Practice E test should have a footnote or something that says that this test might be too severe for heavily cold worked material and therefore, in such cases, one must use Table V (classifying the etch structures from Practice A as acceptable and not-acceptable) as an acceptance criterion. At least that's what my test data seems to suggest.

 

[MagBen]

ASTM is pretty good at the details. Clause 43.3 in ASTM A262 indicates Practice E applies only to annealed austenitic material.
Intergranular cracking or crazing is evidence of susceptibility. The condition, including surface preparation, specimen size, is critical to this test.