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Unexplained Failure of 17-7PH Part (Cont)
2

Unexplained Failure of 17-7PH Part (Cont)

RE: Unexplained Failure of 17-7PH Part (Cont)

To my opinion it is fatigue. You can see the "beach lines" at the upper side of the broken surface.

RE: Unexplained Failure of 17-7PH Part (Cont)

(OP)
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.

RE: Unexplained Failure of 17-7PH Part (Cont)

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.

ncik

RE: Unexplained Failure of 17-7PH Part (Cont)

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
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RE: Unexplained Failure of 17-7PH Part (Cont)

(OP)
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.

RE: Unexplained Failure of 17-7PH Part (Cont)

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
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RE: Unexplained Failure of 17-7PH Part (Cont)

(OP)
Edstainless,

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?

Thanks

mighoser

RE: Unexplained Failure of 17-7PH Part (Cont)

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
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RE: Unexplained Failure of 17-7PH Part (Cont)

(OP)
Edstainless,

Is there another alloy which would be better suited for this purpose which is similar in cost.  If there is what would be the trade off?

RE: Unexplained Failure of 17-7PH Part (Cont)

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
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RE: Unexplained Failure of 17-7PH Part (Cont)

(OP)
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.

RE: Unexplained Failure of 17-7PH Part (Cont)

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.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Unexplained Failure of 17-7PH Part (Cont)

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.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Unexplained Failure of 17-7PH Part (Cont)

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.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Unexplained Failure of 17-7PH Part (Cont)

(OP)
mcquire,

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???

Thanks again,

Mark

RE: Unexplained Failure of 17-7PH Part (Cont)

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.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Unexplained Failure of 17-7PH Part (Cont)

(OP)
Mcquire,

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?

Thanks,

Mark

RE: Unexplained Failure of 17-7PH Part (Cont)

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.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Unexplained Failure of 17-7PH Part (Cont)

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.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Unexplained Failure of 17-7PH Part (Cont)

(OP)
Mcquire,

We pulled about 100 of these undamaged links from the system which failed.  Every link which tested 45 and higher failed prematurely in terms of elongation. The heat treat certs show they were using the correct process. Those links which were below 45 failed in a nice grouping at almost twice the displacement on average (.003 vs .0055).   Failure loads were still well over factor of safty. One of our engineers thinks our failed link could have been scratched on the surface (see photo) and because of notch sensitivity this initiated the failure which then propagated to the d-ferrite layer.  Do you think this is a sound conclusion?  This part should see very little bending in normal operation.

RE: Unexplained Failure of 17-7PH Part (Cont)

Maybe you should re-write the spec to lower the hardness slightly.  I hope that you are actually testing tensile properties and not hardness.  I would go with a minimum yield strength, min and max UTS, and minimum elongation.
This may require test heat treatment of a sample from each master coil.  Coil number are important, heat numbers are secondary.

If you need help making the correct contacts let me know.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Unexplained Failure of 17-7PH Part (Cont)

This just gets more interesting. What percentage failed? How was the centerline banding distributed between the failed and the good?
I don't think a part with no stress across the centerline can fail there, unless the crack was there prior to the normal loading, so, no, I don't see the need to hypothesize a surface scratch.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Unexplained Failure of 17-7PH Part (Cont)

(OP)
Mcquire,

They all failed since we were pulling them on our instron machine.  I imagine that the sub 45 HRC links were made from one lot of material and the others were made from another.  We have comingled stock (no first in first). I haven't looked at those specimens.  Great idea!  Can you clarifiy this is statement: "I don't think a part with no stress across the centerline can fail there, unless the crack was there prior to the normal loading, so, no, I don't see the need to hypothesize a surface scratch".  The engineer isn't saying that the centerline cracked from the scratch but that the scratch was where the failure initiated (normal to the centerline crack)and then propagated along the d-ferrite layer. I haven't found any data which correlates hardness and notch sensitivity for 17-7PH TH1050.  Have you?

RE: Unexplained Failure of 17-7PH Part (Cont)

The stainless steel handbook shows that the notch tensile strength, a good measure of toughness, of 17-7 PH decreases fairly rapidly as tensile strength increases, so I would have to say that notch sensitivity unquestionably increases as hardness increases. This is irrespective of the ferrite layer, which, if your engineer is right, may be an innocent accomplice to the fracture initiated in the PH martensite.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Unexplained Failure of 17-7PH Part (Cont)

I had an issue with 17-7PH a few years ago, the dog bones were showing correct tensile and elongation, but were splitting perpendicular to the ductial failure.  I sent it in for failure analysis, and it was found the cause of the splitting was:
"EDX tests showed increased percentages of Al and Cr, and decreased Ni content."
The certs showed every thing was fine, but I guess mixture of the elements were not homogenious enough.

RE: Unexplained Failure of 17-7PH Part (Cont)

(OP)
Monkeydog,

What was the HT? TH1050?  Did the failure look like the photo I have posted?  See first post for link

Mark

RE: Unexplained Failure of 17-7PH Part (Cont)

mighoser,
The heat treat was close, TH1110 (we have to do things a little different).  I saw the photo, and it did not look like that, but after reading you had multiple problems, I thought I would throw my experience in for your thoughts.  I cannot post the photos of my problem (my company thinks it owns everything).  But, the dog bone had a normal ductal failure.

RE: Unexplained Failure of 17-7PH Part (Cont)

Step 1: require lot control and part marking (laser or acid etch) based on the master coil number for the material.  You can trace this back to heat number, but not every coil from the same heat will have the same thermomechanical history.

Is the sheet aged in the coil?  After it is slit?
What are the tensile properties?  Were any of hte tensile samples RC47?  If not someone is cheating.
I still suspect that there is non-uniform aginging and the 'extra hard' zone in some material is cracking during blanking and this results in the early failures.  There are a lot of segregation issues in these grades other than just delta ferrite.
If I had samples delaminate in tensile testing I would reject the coil as simply being unsound material.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Unexplained Failure of 17-7PH Part (Cont)

(OP)
Edstainless,

Thank you for the sugestions.  I'm sure we will implement some form of corrective action to prevent future occurances of this.  I don't have tensile prop data from the coil.  I still need to look at this.  Sheet is received in Contition A and then cut into squares.  It is then treated to TH1050 and then fine blanked.  During tension tests with actual parts (links) we were able to correlate performance reduction with hardness.  But none of the links delaminated the way the one in the posted photo did.  

RE: Unexplained Failure of 17-7PH Part (Cont)

I would be looking for some data on uniformity of properties after the heat treatment.  Are the sheets stacked when they are heat treated? Are they unstacked to cool?  Are there thermocouples inside the stack?
I can see all kinds of issues.  You need a bunch of tensile data on the sheet material after heat treatment.

I would play with bend testing.  Maybe even bending samples and then pulling them in tension.  You need a way to catch this condition early in the process.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: Unexplained Failure of 17-7PH Part (Cont)

As aging temperature increases, hardness drops and fracture toughness increases, so there is a strong inverse correlation. Toughness drops off by 50% for a 20% gain in strength, which equals hardness.

Segregation, as monkeydog cites, causes ferrite, but bear in mind that the non-centerline is depleted of Cr and Al, so it will have different heat treat response than a homogeneous alloy.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Unexplained Failure of 17-7PH Part (Cont)

That photo is incredible. If you ever needed any demonstration of the low toughness of delta ferrite, this is it.
But since it is on the neutral plane of mighoser's part, as with the tensile specimen, it shouldn't be the root cause of failure. I'm looking at the excessive delta ferrite as an indicator of inhomogeneous composition throughout,which could have caused the heat treat response to be abnormal.

To bad we don't have coil tracability. This could proably be solved by looking at coil history and comparing to statistics on other material of the same grade.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: Unexplained Failure of 17-7PH Part (Cont)

It is also possible to have other problems (than retained delta ferrite) in these alloys which result large variations in hardness.
My hunch is that there is a band (or two) of harder material in your strip.  When they were blanked you got some cracking in that band.
I have always hated the 'TH1050' type of nomenclature.  The inferance that 1050F will always give you specific properties just isn't true.  Due to variations in chemistry and thermomechanical history you have have to be prepared to use different temperatures.
We often lab heat treat samples, rather quick and dirty.  We compare the results with historical data and decide if that material need a 'little more' or a 'little less'.  A 25F change can work wonders some times to reach the required ductility

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

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