Vendor provided over-strength material, is our old testing valid?

[bc1080]

We purchased a run of specialized stainless forgings and the product the vendor delivered is testing about 30% higher than the yield and ultimate properties we requested in the spec. While the additional strength itself is not problematic, it is causing concern among our team because it's above the strength level we have done material testing on previously and are confident in. We are not sure if the different properties might cause a change in the fatigue, corrosion (particularly SCC), or other reliability characteristics. The vendor does not share our concerns and claim they provided superior material at no extra cost.

The material certs indicate they provided the condition we asked for (and tested in the past), but both the mechanical properties and the hardness are well over the "min" values provided in the ASTM spec. This has caused disagreement over what the validity of our past testing should be, whether it is the material heat treat condition (regardless of properties) or the as-delivered strength/hardness.

 

[metengr]

Are these forgings martensitic or PH stainless steel? Typically, the issue for acceptability is meeting minimum mechanical strength properties, which would include fatigue strength (which correlates with tensile properties). Your real concern at elevated tensile strength is stress corrosion cracking resistance and notch toughness behavior. However, acceptability is dependent on the final strength level, type of stainless steel and environment related to design. You are going to have to re-evaluate adequacy of design using the high strength material.

 

[tbuelna]

Depending on the specific alloy, HT condition and application, stress corrosion might be a concern. You'll have to check with whoever did the analysis work to see if the condition of this batch of forgings will be acceptable in terms of stress/fatigue/fracture. If you received certs with the forgings there should be some documentation of the HT processing, so make sure you check them against the HT spec.

You did not mention the specs for the material and HT process. But for example, say the material and HT specified is 17-4PH condition H1025 (Ftu 155ksi min) and a batch of forgings tested 30% above the min UTS (~202ksi). That would mean the forgings were probably not heat treated properly since 202ksi UTS is around the upper limit for an H900 condition. Most HT specs (MIL-H-6875, AMS 2759/3) usually control age hardening temperature for 17-4PH to something like +/-10degF. So if the HT vendor is working to a proper HT spec they would not get H900 properties using an H1025 process. This would also pose an issue with stress corrosion since 17-4PH H1025 is usually the upper strength limit allowed for applications sensitive to stress corrosion, and H900 condition would not be acceptable.

Regardless, it is good that you are looking into this issue. Initiate some MRB activity so that others can provide input on how to disposition (accept, reject, rework, etc) this batch of forgings. The corrective action might also include changes in the engineering documentation for this part no. so that this problem does not re-occur.

Lastly, I got a chuckle at the comment from your vendor, "The vendor does not share our concerns and claim they provided superior material at no extra cost.". You've got to admire their salesmanship efforts.

 

[bc1080]

Thanks for the responses! It’s a martensitic stainless. Their certs seem to indicate they followed the correct conditioning procedure, but I don’t see data on the times/temps they used. Heat treat was per ASTM 564/564M, and that spec doesn’t look as well defined as the ones mentioned. It says you can vary the time and temperature to achieve the ductility and tensile properties needed. However, like pointed out, my concern is that if the properties are so high above the mins, it seems likely they achieved a stronger condition by accident or oversight. Analysis is fine with it on a stress/fatigue level, it’s more of the fact it crosses an SCC threshold we normally avoid and have no data for. I think we are going to be doing some additional testing on this batch and will likely have to figure out a way to specify an upper bound on strength and/or hardness in the future. We have previously done this with other steels like 4340 to avoid certain strength ranges for corrosion reasons.

Thanks again!

 

[EdStainless]

Martencitic, I would look into the temper temperature. It may be that you want to look into re-tempering these parts.
In these alloys the toughness (notch or low temp) may be hurt by higher strength.
BUT, if the old material was just above the mins then 30% higher is very reasonable.

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