17-7PH heat treat didn't work!

[SwampBronco]

I have inventory of 17-7PH rods, 6mm diameter, in Condition C. I need to perform a forming operation to the rod that is not possible in condition C. Due to supply issues, I was not able to purchase the material annealed, so we used the following recipe to attempt to anneal it, form it, then heat-treat to TH 1050 . . . except it didn't work. We were able to anneal the rods and perform the forming operation, but after the subsequent heat treat the rods are as soft as when they were annealed. Is it not possible to anneal condition C to reprogram it? Would starting with annealed have worked?
[ol 1]
[li]Rods received in Condition C[/li]
[li]Solution heat treat existing rods at 1950±25°F for 30 minutes[/li]
[li]Form rods[/li]
[li]Austenite conditioning: 1400±25°F for 90 minutes minimum[/li]
[li]Transformation: Cool to 60-70°F for 30 minutes minimum within 1 hour of austenite conditioning[/li]
[li]Precipitation Harden: 1050±10°F for 90 minutes minimum[/li]
[li]Air cool to room temperature[/li]
[/ol]

 

[EdStainless]

First, how much forming? If it is much then you should reanneal after forming.
The HT you list works perfectly every time.
If it didn't work then you didn't follow it correctly.
Cooling small diameter is easy, just wave it in the air a bit.
Did you check hardness after the 1400F?
For 6mm you sure don't need 90min at 1400.
Annealed you have HRB 92max (85-90 is typical)
After the 1400 I would expect something around HRC 30.
And then after the 1050 you will have HRC 38 min (40-45 typical).
What is your atmosphere?
Please don't say that it is nitrogen rich.

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P.E. Metallurgy, consulting work welcomed

 

[SwampBronco]

Thanks so much for your reply! The forming is not that aggressive and is in a localized area. Unfortunately we did not check hardness after the 1400F step. As far as the atmosphere . . . um . . . well . . . let's say it's rather nitrogen rich at around 78% . . .
Would that effect properties or just aesthetics?

 

[EdStainless]

You will have to acid pickle off the outer layer of oxide and damaged metal under it.
The surface will be low hardness.
You need to either grind into the base metal for a test or pull a full section tensile.
For a part this small wrap it in HT foil to minimize oxidation at the high temps (1950 and 1400).
It is easy enough to make a pouch that you can open quickly for cooling.

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P.E. Metallurgy, consulting work welcomed

 

[MagBen]

Soft almost indicated that austenite was not trnaformed to martensite. 1400+cool is more likely an issue than other processes.

N2 atmosphere could impact from two perspectives:
1. N stabilizes austenite to prevent martensite transfer.
2. N prevents NiAl precipitation, especailly on the surface of the material. Even if NiAl is precipitated, due to large lattice parameter difference between austenite and NiAl (need fcc ferrite or fct martensite to match NiAl), there is no coherent effect, so no hardening effect.

 

[EdStainless]

Air usually isn't that big of a deal.
The oxidation blocks most N uptake.
A nice clean N+H atmosphere is a disaster, because of what Ben said.


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P.E. Metallurgy, consulting work welcomed

 

[SwampBronco]

Thank you again for your replies. Once we remove the oxide layer, is the HT foil pouch enough to prevent issues with the N atmosphere or would you recommend we find a shop that can perform the work in a N-free or evacuated environment?

 

[EdStainless]

Are you in air? If so then it shouldn't be a big deal.
The foil will help a lot.
We purged out box furnaces with Ar.
Just enough to get the oxygen down to a few percent.
Samples still discolored but they didn't scale heavily.
And we didn't get N uptake issues. (there are other SS grades a lot worse than PH)

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P.E. Metallurgy, consulting work welcomed

 

[SwampBronco]

Yes, we are in air. We'll pickle the parts and try again with the foil pouches. Thanks for the tips!

 

[EdStainless]

What are you pickling with?
We usually used a weak Nitric/HF (maybe only 1-2%) and not hot, just above room temp.

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[WKTaylor]

Hmmmm... 17-17PH ROD is an unusual material aerospace use... typically I've seen sheet metal and plate. Further MMPDS has deleted Rod material Cond C... however I found MIL-HDBK-5 still have allowables.

ARE You absolutely certain You received 17-7PH Cond C rod? What specification [AMS, ASTM, etc?]. Is the vendor reliable and did You receive valid Material 'Certification' documents for Your exact rod materials? IF NOT, have You done any chem testing to validate the alloy?

SO... IF YOU HAVE 'good 17-7PH rod'... I said to myself... what does AMS-H-6875 [was MIL-H-6875] HT spec... now recently replaced by 'new' AMS2761... have to say about HT to TH1050? Unsure if AMS2759/3 is of any value for 'raw 17-7PH SStl materials'.

Both of these document follow Your heat schemes very closely... but QUENCHING schemes appear to be missing from Your brief process steps.

Suggest You look closely at 1-or-more of these specs [noted] for process comparisons.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[EdStainless]

WKT, Almost all PH rod ends up in springs and especially 17-7 in C.
When we asked in committee who was interested in seeing the condition C material retained there was a resounding 'np thanks'.
I still see this material commonly in springs in pressure relief valves and other similar devices.
I have no concern about quench rates in 6mm bar.
Forced air cooling (fan or waving around) should be fine.
Just keep it there long enough to finish the transformation.
You can mess up by going into aging too quickly.
There are some specs that say to cool to room temperature and some that below 70F.
I believe that the difference is if the person drafting them worked in an air-conditioned space or not.
For heats of 17-7 that I have worked with anything below 95F worked fine.


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P.E. Metallurgy, consulting work welcomed

 

[SwampBronco]

EdStainless, We'll use your recommendation above for pickling. I thought we had expertise on campus with this but looks like it will be another science experiment! I did pass a few under a TIG brush that we use to clean welds, and it certainly made the rods look marginally better.

WKTaylor, We purchase the material to ASTM A313, but I have not personally reviewed the material certs or done any chemical analysis to verify. It did respond nicely to the annealing recipe for 17-7!

Good to know that the ambient temperature isn't absolutely critical. We quenched in conditioned space, so pretty close to or even below the 70F "threshold".

We reviewed the feedback from above internally and it turns out that our HTer did enclose the rods in foil pouches during both the initial anneal at 1950F and the austenite conditioning at 1400F. Additionally, he also added a certain paper to remove the oxygen in the pouch. We're pretty sure the material properties were unchanged to the core as I was able to bend a "TH 1050" rod into a U shape with my hands. Certainly can't do that with Condition C and certainly wasn't expecting to be able to with TH 1050. Posting a picture below of how things look throughout the process.

At this point, I think our plan is to pickle the parts and give it another go. I'll certainly post the results.

Thanks so much!

 

[EdStainless]

We used to toss in pieces of Ti, Zr, or Ta foil to act as a getter to reduce oxidation.
After the 1400F treatment the material is stable.
So you can leave it like that for as long as you wish.
This is also where 90% of the size change takes place.
We actually used to ship tubing to people in that condition.
The they would machine and final age.

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P.E. Metallurgy, consulting work welcomed