We buy 15-5 VAR reforging stock (billet) to AMS 5659 from a reputable American mill for a particular aerospace product we make. The mill performs H900 capability testing on the MTR, which specifies a min YS/UTS of 170/190 ksi, respectively. Historically, they have averaged in the mid 170's for YS and upper 190's for UTS. The YS/UTS ratio is consistently in the 88-90% range.
Recently, however, we received two heats which reported YS/UTS of 196/199 ksi (ratio = 98.5%) and 200/201 ksi (ratio = 99.5%), respectively. Although these UTS values are certainly on the high side, they are not unprecedented - the YS values, however, are significantly higher than I have ever seen, either from this mill or elsewhere. The elongation and RoA were 14/43%, respectively, which is a little lower than we usually see but still comfortably passing for H900 (the specimen was transverse so the requirement is 6/20%, respectively).
I contacted the mill about this and we went through the standard due diligence of overchecking the stress strain curves, confirming that they extracted the specimens from the standard locations in the bar, checking the tensile specimen fracture surfaces, etc. and everything was done correctly; the material truly was that strong (it was tested at a 3rd party lab to boot). The mill commented that they do occasionally see YS/UTS ratios >98% and, while we could retest, there'd be no real point since the test didn't fail and also since another ingot from the same parent heat recorded average values (a third ingot reported 191/202 ksi, a 94.5% ratio).
I am reaching out to this forum hoping to gain insight into what might be happening mechanistically here. I agree with the mill that there is limited value in retesting and there is nothing wrong with the material, but when you see statistical outliers like this it makes you wonder what's going on. My assumption is that it has to do with lot-to-lot variation in response to the H900 aging cycle, which is only 1 - 1.25 hours instead of 4 - 4.5 hours for other aging conditions (including H925, which is only slightly hotter) so it's a relatively quick precipitation reaction where the difference between truly peak aging and slightly under/overaging is probably minutes at temp, if that. I am assuming that the stars just aligned with these particular heats/tests and they were pulled at exactly the right time to achieve a truly peak age, but that's mainly because it's the only explanation I can come up with.
Very interested to hear what you have to say!
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Sean Piper
Product / Process Metallurgist
Ellwood Texas Forge Houston
Houston TX
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