Do you see the lighter gray layer which runs continous at the sheet centerplane (located on the ID of the failed ring)? Delta Ferrite? To me this failure look like what we would see with fiber filled PEEK or Lytex(minus the fibers). I think you are right about fatique at that location but I don't think it was the 1st failure mode.
Have you mounted and polished a section of the failed part? If this is a centerline problem then you should see it in the mount, and it wouldnt require too much material.
An SEM could tell you w/ mounting and polishing if the light grey area you describe has a differing chemical composition.
There look to be multiple initiation sites. This needs some serious SEM time.
Residual ferrite in the center of light gage sheet? That would be some seriously screwed up steel making.
We do see banding in 17-7, but it isn't residual ferrite. It may have been in the original slab, but all that is left at light gage is some ghosting.
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Rust never sleeps
Neither should your protection
We've hired a lab to do a full failure analysis. They will be doing some SEM. I'm the curious ME who loves materials (need to go back to school). My comments on delta ferrite were based on some input from mcguire. See thread1135-159883 He seemed to think that it was quite possible that this layer could have caused the longitudinal failure that is shown in the photo.
I recall those comments from McG.
I have been looking at more 17-7 micros recently. Depending on what you see I may try to get a bunch of strip samples to look at. We have strip from two different mills, in gages from 0.030" to 0.120".
I have seen 17-7 that when aged develops very uneven precipitation. Both the density and the size of hte precipitates changes through the cross section. This could be related to chemistry variations, and it could raise hell when you stamp the parts.
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Rust never sleeps
Neither should your protection
It seems like maybe we should be using 17-7PH RH950 which from what I have read has a more consistant microstructure and superiour engineering properties. But, I'm not sure if it would play nicely with our, post HT, fine blanking process??? Any thoughts?
Blanking RH material will be very difficult, unless you use a higher aging temp. The RH950 is really hard.
I'll look, I had some data on RH with different aging temps.
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Rust never sleeps
Neither should your protection
I would rather blank in the A condition and then age. You would need to look into the need to anneal after blanking, maybe yes maybe no.
Even if you had to use a sizing die to bring parts into tolerance this would not be a serious operation.
Of the PH grades 17-7 is one of the most forgiving.
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Rust never sleeps
Neither should your protection
Does the annealing process cause most of the shrinking/expanding? Would the sizing die be the last step in the process? We are holding +/- .0015 on the diameter. We also have a flatness requirement. For such small deflections (using the sizing die) wouldn't the link just spring back into its post aginging geometry? I'm not familiar with this process.
I don't think aging variables will mean too much because the centerline ferrite band is not affected. The crack will form during blanking when there is a stress acrossthe plane. Propagations will occur in service. Z dirction toughness is bad when ferrite is rolled flat.
This may all arise from transient conditions during continuous casting, such a first or last spot of the cast, speed change, etc. Once its at final guage and it's still there, it's going to stay there. Yoy would ahve to anneal for hours at 2000F+ to homogenize this material.
McG, I am speculating that these parts are cracking becuase they are blanking a high strength part with non-uniform microstructure. I suspect that if they blanked in a softer condition (with enough counter pressure) that they would not get cracks initiated at that stage of the process.
I may be wrong. If the cracks are not starting until the parts are in service then my theory is all wet.
Will pieces of this strip pass a bend test after ageing? I am trying to think of some way to evaluate material before you make parts out of it.
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Rust never sleeps
Neither should your protection
Ed, you're right. Softer condition would mean lower blanking forces. That may be enough to prevent centerline cracks. If that's true then adjusting the blanking clearances coud help also. Better yet, how about laser blanking.
I tend to think that this failure is related to a specific problem with the material. We did see high hardness: 47 rockwell C. I think that our post HT blanking process is not good but we've made millions of these parts this way which have undergone 2x lifetime testing, hot/cold testing, saltfog and saltwater submersion testing and centrifuge testing. Like I said before, this was a brand new system. I'm not the material specialitist but logic and law of probabilities is pointing me elsewhere. What was you impression of the photo I posted. Did you see the light gray layer?? Anything else I should have my lab looking at???
Don't get me wrong. I agree that the light gray layer is the culprit and that it should not be there. I do think that it is an expected abberation in 17-7 PH because the alloy has an unavoidable tendency toward delta ferrite. This is maybe a worst case situation, but one you will always have to worry about.
That having been said, I think the material is defective. The mill should have discovered this condition and rejected the material, melted it back down. Have you asked the producing mill to account for themselves? This material should be thoroughly traced back to the original melting/casting. I'll bet the cause will be evident. Give 'em hell. Make your purchasing guy earn his lunches.
On what basis would they (the mill) have rejected this material. In otherwords, is there a specific test which they would do which would detect this condition in process? Would the image I have posted justify rejection? Anything else I need to stuff up my sleeve before I give'em hell? We have all acquired all the certs. Material was from Allegheny Ludlum Corp. You were VP of technolgy for this company before they were acquired by them, right?
In order to force it down their throats you will need to know what it really is and how to test for it. I like the idea of aging a sample strip and then bending it, shaprly. I thought that there was a spec for this, but I can't find it right now. You could look at the spring wire specs for bend test guides.
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Rust never sleeps
Neither should your protection
I never worked for Allegheny, but they are a first rate operation who will, I'm sure, step up to the plate on this. I would simply tell them that you have what appears to be an unacceptable level of centerline segregation based on your examination. Ask them to confirm or deny. Mills rountinely do these examinations. I would always go to the mill very early for help in these things. They have extremely competent people if you can get a hold of the right one.
