Inconel 718 S-R Failure (Notched/Smooth Combo) 2

[BMKR]

Good Morning All,

Kinda urgent issue. I had an N/S S-R test fail at 40+ hours in the notch for Inco 718 (AMS 5662/5663). Naturally we sent more material for the 3 redundant tests all of which broke in the notch in under 2 hours. I checked the solution H/T chart on our end and everything ran OK to the cycle and the lab was to perform the precipitation hardening then run the tests. On their end they say that the H/T ran smooth.
[ul]
[li]The specimens were taken from a thin walled forging in the longitudinal direction(0.5",~12.7mm)[/ul]
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[ul]
[li]I think that maybe the way specimen was machined in respect to the wall/surface defects may have been a factor.[/li]
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[metengr]

I would be more concerned with how the notch was machined. This tends to be a significant factor related to test performance.

Also, you don't mention test temperature, stress and expected rupture life so I have no idea how your data compares with expected results.

 

[BMKR]

I believe the S-R is done at 1200°F (649°C) according to the AMS. Required life is 23hrs failing on the smooth section. If a notch failure occurs the test is considered a failure regardless of time. In the notch section, we used to have a problem but as of recent (the last year or so), that has been panned out (or so I thought). I am beginning to suspect that they went out of practice to machine the specimens due to the small wall thickness.

 

[EdStainless]

Go back and look at the anneal and the annealed properties. What was the elong? If you get the anneal right it won't age correctly.

Did you run smooth S-R? What load were you using?

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Plymouth Tube

 

[BMKR]

It is done with a smooth and notched combo. loaded with 100ksi (689MPa) for 23 hours then Incrementally increased by 5ksi every 8 hours until failure. The solution anneal ran as planned within the 1725-1850°F for the appropriate wall thickness. The lab performed the Age 1325-1400°F @8Hrs reduce heat to 1150-1200°F (100°F per hour (2hrs)) held at that temperature for 8 hours. (18Hrs) then cooled at any rate.

 

[EdStainless]

Did you verify annealed tensile properties? Double check these.
What were the aged tensile properties?

I also suspect the notches based on personal pain.

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Plymouth Tube

 

[BMKR]

The AMS calls out for RT and Elev Tensile both passed with flying colors. Inco is usually the least of my worries as far as testing goes, but when it fails it sure gives me a headache. I am getting onto the notch train as well, seems a bit fishy that 3 samples failed within 2 hours of hanging on the retest. if it was the age cycle I would expect similar hanging times for the retests to fail (i.e. in excess of 23 hours, giving plenty of time for the dislocations to move through the notch, 2 hours seems a bit quick).

 

[EdStainless]

I have seen lots of material where annealed and aged elongs were well within spec, but toward the low side. This material would not pass S-R.

Yes, 2 hours really sounds fishy. Notch quality is one thing, but could they have gotten either the temp or stress wrong? 110ksi at 1300F sure gives a lot faster failures.

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Plymouth Tube

 

[BMKR]

We send this part out for testing probably 12 times a year or so (each time we make it) and this lab sees Inco 718 parts all the time(from us in addition to other companies). May just be a machining fluke.

 

[EdStainless]

I mentioned the 1300F because some of the specs use that and some use 1200F.

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Plymouth Tube

 

[BMKR]

UPDATE: got back yesterday with the lab, apparently the delta phase did adequately dissolve in the solutioning leg of the H/T cycle, there were semi-continuous delta networks in the grain boundaries instead of non-continuous precipitates. I guess the delta transus was higher on this heat. Now I just have to worry about my grain size getting too large(6.5 pre p/h)

 

[metengr]

Good follow-up. This is why met lab analysis is needed versus guessing.

 

[EdStainless]

In house we have very tight limits on yield and elong for the annealed material.
We have seen similar issues in the past.

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Plymouth Tube