How sure are you that they are carbides? There is a bunch of stuff that you can form in 17-7, carbides usually aren't on the list.
Is this an actual part? Do you have some micros?
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Corrosion never sleeps, but it can be managed.
There are a lot of PH experts who will be contributing to this, but I can say from production experience that it is not difficult to get CR23C6 precipitates. Since these alloys are a delicate balance, this is not a good thing since, besides causing the normal loss of corrosion resistance, it also screws up austenite stability, which in turn changes Ms temperatures and austenite/ferrite tendencies.
mcguire is right on. I had checked my "Handbook of Stainless Steels" (Peckner and Bernstein) and on pages 7-8 thru 7-10 there is reference to the austenite pre-conditioning heat treatment, after fabrication of solution treated (mill annealed at 1950 +/- 25 deg F) Condition A, for PH grade 17-7, and the effects of solution-treatment temperature on mechanical properties, as well.
I would expect based on the information in this Handbook that depending on the solution treatment temperature, the actual carbon content, and the balance of ferrite formers (Cr, Si, Mo, Al) and austenite formers (C, Mn, Ni and N) one could have chromium carbide precipitation at ferrite stringer/austenite interfaces and possibly along grain boundaries for Condition A.
Further information on Condition A material after fabrication;
Depending on the temperature used for the austenite pre-conditioning heat treatment, the PH 17-7 will develop further chromium carbide precipitation at ferrite/austenite stringer interfaces, and possibly an intergranular network of M23C6 carbides (if the carbon content of the material is at the high end of the carbon specification). The formation of the carbides that occurs during this pre-conditioning treatment removes chromium and carbon from solution resulting in transformation to martensite at higher temperatures. As the pre-conditioning heat treatment temperature is increased to 1500 deg F, this lowers the martensite start temperature. Pre-conditioning at 1750 deg F results in fewer carbides, thus sub-cooling is necessary to achieve complete transformation to martensite.
First, some disclaimers.
Most of the 17-7 that we run is light gage, rarely over 0.083". And we run toward the low C end of the range.
Some of the strip that we recive mill annealed does have singificant amounts of carbides and other cats and dogs in the structure. Since this material is rolled sheet the structure tends to have bands (like a sandwich) and not strictly following grain boundaries.
I have looked at micros of our mill annealed product (1900-1950F WQ) and there are small scattered secondary phases. No masive grain boundary pahses.
I have looked at samples that were both TH1050 and RH950. Yes, the structure of the TH material is a mess. Pimary and secondary carbides, retained austenite and phases that I don't have a clue about. Some in GB, but a lot along the prior ferrite/autenite interfaces.
The RH material is very clean by comparison. Well dispersed, uniform, fine pahses. And increadible mechanicals.
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Corrosion never sleeps, but it can be managed.
I have another question on this subject. Has any of you heard of a 2% max ferrite 17-7PH? I'm sure if it's possible, it would have to be controlled with the chemistry. But, what would this do to the mechanical properties after heat treat?
