The ASM Metals Handbook Eighth Edition Volume 10 - Failure Analysis and Prevention reports on page 221 in Example 6 on Stress-Corrosion-Cracking Failure of a sensitized 17-4 PH Stainless Steel Valve Stem in High-Purity Water.
Maybe there is some similarity.
According to NASA MSFC-STD-3029 titled "Guidelines for the selection of metallic materials for stress corrosion cracking resistance in sodium chloride environment", all conditions of 17-7 except CH900 has a low resistance to stress corrosion cracking in sodium chloride environment.
EdStainless--Part is a leaf spring formed from 17-7 strip, condition C, then aged to condition CH-900. Part reportedly was cracked when received by the customer. In manufacture, the parts are formed, then as WIP sit around in an uncontrolled environment until sent out for the CH900 age. The crack was transverse to the length of the leaf and was on the concave side, which is where the residual tensile stress from forming would be. There was also a nick at the initiation point, which would serve as a stress raiser. Some light rust was observed on a portion of the fracture at the initiation point.
I have never heard of this specific problem, but your description of the problem makes it seem quite plausible. Type 631 (17-7 PH) in the C condition will be similarly susceptible as a solution annealed austenitic grade like Type 302 or Type 304. If the spring was not passivated, which seems likely given that there was some rust observed at the fracture origin, and residual stress was likely, then SCC would be a possibility. Was the cracking through the entire cross section?
The discolored zone extends about 2/3 of the strip thickness, so the leaf held together through most or all of the processing. The spring was passivated, but not until after shot peening, so the peening could have closed up the crack and the passivation would not have affected the fracture.
We tested nearly all the PH alloys for stress cracking in low temperature salt baths after we lost many thousands of dollars worth of machined parts.
I recall that 17/7 was one of the alloys that failed very quickly. The problem is I can't recall the heat treated condition of the 17/7 part tested.
We found quite a few of the PH alloys susceptible to SCC in molten salt baths of the type we used for cleaning. We took this as warning to be very careful of the enviroment that any of the PH alloys was exposed to.
The slight rusting of your spring is unusual for a 17/7 part.
When do you peen the parts?
Is there a reason that you can't form and then age right away?
Even aged you are going to need to protect the parts somewhat. The CH900 condition isn't known for corrosion/cracking resistance.
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Rust never sleeps
Neither should your protection
