Embrittled 17-7
Embrittled 17-7
(OP)
One of our plants supplies a Belleville made from 17-7 bar, 3" in diameter, with thickness of .25" thick. It is heat treated to the TH1050 condition. So far, so good. But, for some yet to be determined reason, the plant gives these a supplemental age of 1060F for 3 hours. A high percentage of the parts crack as they are loaded up in the assembly. My question: is this overage the plant is performing embrittling the 17-7?





RE: Embrittled 17-7
I will ask the obvious question, have you or someone performed a proper metallurgical analyis of the failed parts?
RE: Embrittled 17-7
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RE: Embrittled 17-7
RE: Embrittled 17-7
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RE: Embrittled 17-7
RE: Embrittled 17-7
If they are grinded the grinding process may create micro cracks. See MIL-STD-866 GRINDING of Chrome Plated Steel and Steel Parts Heat Treated to 180,000 PSI or Over at http://
The way to convince the customer to drop the hardness requirement is to show him that standard specs for the bars require only tensile properties. To my best knowledge hardness can vary up to ~10RC for same H1050 treatment with no change in tensile properties.
RE: Embrittled 17-7
I would look very carefully at any cleaning operations.
The parts need to be very clean before aging.
Also look to make sure that the ID bore and edges are smooth.
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RE: Embrittled 17-7
maybe the post heat treat was a stress relieve?
it may be as Israelkk states there may be cracks in the parts. to remove any posibilities recomend to NDT flo pen.
if any parts are cracked before assembled then do a section & micrograph.
is it a material issue or a workmanship problem.
if these parts are process like double disk grind
or just turned with high feed rate inducing work hardening or stress. this can cause the above.
where parts ever pickled?
RE: Embrittled 17-7
RE: Embrittled 17-7
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: Embrittled 17-7
If you baked your parts at 500F for a few minutes in air to remove any hydrogen and could then assemble them without failure, you would know if hydrogen was the culprit.
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: Embrittled 17-7
RE: Embrittled 17-7
RE: Embrittled 17-7
Generally, belleville spring are made sheet or strip of 1t-7 treated the CH900 condition. Therefore, the TH1050 treatment seems to be too low in strength.
According to NASA specifications only the CH900 (and in your case I assume HT900) condition has a high resistance to stress corrosion. See MSFC-STD-3029 GUIDELINES FOR THE SELECTION OF METALLIC MATERIALS FOR STRESS CORROSION CRACKING RESISTANCE IN SODIUM CHLORIDE ENVIRONMENTS st htt
If the parts are not grinded I will suspect the base material may be bad and may include inclusions and internal defects. Did you checked the bars? Are these a premium quality bars?
What are the calculated tensile stresses in the disks? Usually belleville spring are governed by compressive stresses but in this case it seems the tensile stresses are the problem. Can you supply the full dimensional details of the disks and applied deflections?
RE: Embrittled 17-7
RE: Embrittled 17-7
can you pull parts from stock & do a flo pen?
turning & heat treat should have not caused this problem.
it is possible it was a bad batch of material.
this material is susceptible to IGA & corrosion.
general at this heat treat TH1050 the material is easily machined. however it is easier to machine from full bar stock for this type of part.
RE: Embrittled 17-7
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: Embrittled 17-7
A.R. "Andy" Nelson
Engineering Consultant
anelson@arnengineering.com
RE: Embrittled 17-7
1. Work hardening in machining giving you CH conditions and low ductility at the edges.
2. and because you are machining soft material you are getting small tears smears along the highest stressed edges.
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