We have performed quite a few studies with cryogenic HT in AM17-4. The question that we have is, how appropriate is an LN
2 quench in an industrial setting? Right now, we believe the most effective approach is to pull the N
2 out of the matrix to get the Ms temperature as high as possible beforehand. That way a quench will be more effective, and we may not need to go to LN
2, which can introduce distortion. Wrought 17-4 transforms to approx 95 % martensite with an air cool, so unless we have a major breakthrough, it would appear that some form of quench may be something that we have to live with to get a comparable condition-A for N
2-atomized powder. That said, if the material isn't fully homogenized, an LN
2 quench won't get you any improvement in the martensite phase fraction.
Your other comment is on point, the lower the N
2 content the better. One of our models shows that the transition is around 0.08 % N
2, and if the N
2 content exceeds 0.15 %, the material won't respond much at all.
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Mark R. Stoudt, FASM
Staff Scientist
National Institute Standards & Technology
Germantown MD
(301) 975-6025
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Original Message:
Sent: 03-15-2022 11:00
From: Stephen Rooney
Subject: Impact of additive manufacturing
Have sub-zero heat treatments for 17-4 PBF components been explored for complete martensite transformation? It would be relatively simple to perform a cryogenic treatment on small AM parts than on larger forged/cast parts.
Getting low N powder would of course be the simplest solution for the part manufacturer.
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Stephen Rooney
R&D Metallurgist
Ellwood Materials Technologies
Original Message:
Sent: 03-15-2022 08:30
From: Mark Stoudt
Subject: Impact of additive manufacturing
We did several studies to determine whether a nitrogen cover gas would influence the total content in the solid and the results showed the effect was negligible (same with argon). The largest influence was the solubility of the nitrogen gas in the liquid phase prior to solidification. FWIW, chemical analysis showed the nitrogen content actually decreased by a small amount (≈ 0.5%) during the build.
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Mark R. Stoudt, FASM
Staff Scientist
National Institute Standards & Technology
Germantown MD
(301) 975-6025
Original Message:
Sent: 03-15-2022 08:16
From: John Grubb
Subject: Impact of additive manufacturing
Is it possible that there was some pickup of nitrogen during the powder bed fusion? That too would increase retained austenite.
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Original Message:
Sent: 3/14/2022 7:02:00 AM
From: Jaime Berez
Subject: RE: Impact of additive manufacturing
I'd like to just follow up on this with some supporting evidence. Again, specifically for 17-4, some literature [1-2] has speculated that the extremely fine grain sizes produced in an AM process like laser powder bed fusion alter the kinetics of austenite-to-martensite phase transition during the quenching stage of the solutionizing process for this alloy. I've seen Takaki et al. referenced to explain this [3]. It's certainly true that lots of authors (pick a paper w/ phase analysis of LPBF 17-4) have observed more retained austenite than expected in LPBF processed 17-4, even after a standard solutionizing and aging heat treatment.
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Jaime Berez
j.m.berez@gmail.com
Original Message:
Sent: 03-08-2022 09:09
From: Mark Stoudt
Subject: Impact of additive manufacturing
As with most new technologies, the materials research and documentary standards often lag the 'wave' by a considerable period, and this is the case with AM. The microstuctures and properties are substantially different from what we see in wrought/cast materials with the same composition. That means the heat treatments designed for wrought/cast do not apply to AM. One needs to consider AM as 'new alloys' that need heat treatment protocols designed specifically for AM processing. For example, when made via AM processing with nitrogen-atomized powder, SS17-4PH martensitic stainless steel components do not have the same yield stress or ductility as their wrought counterparts. After proper homogenization and austenitizing steps followed by an H900 age, the yield and ultimate stresses and ductility generally exceed the wrought values. Over the next few years, there will be new standards/specifications for composition that report the O2 and N2 levels in the powder as well as AM alloys with tweaked compositions to better accommodate AM processing.
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Mark Stoudt
Staff Scientist
National Institute Standards & Technology
Germantown MD
(301) 975-6025
Original Message:
Sent: 03-06-2022 09:36
From: Stephen Maus
Subject: Impact of additive manufacturing
What do you see happening in additive manufacturing of metallic components, and what impact do you think that will have on traditional heat treating in the next five years?
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Stephen Maus
Partner
MetalPro Resources LLC
Greenwood IN
3174057047
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