17-7 PH heat treat and project manufaturing process

[nickjk]

I am currently working on a prototype project making tubular blanks that will be machined then hydro-formed. The tubular blanks after hydro-form are designed to work like a radial spring. An FEA study showed maximum bending stresses in tension to be around 90,000 psi. The fist prototypes were made from 304 SS. Because of the high bending stress I am looking to change material to 17-7PH. I have been doing research here about the heat treating process of 17-7PH and found a great deal of information but still have some questions. Because the blanks are to be made from tubing the material will be purchased in the annealed condition. I currently have quotes out to several companies for 1 ¼” O.D. 7/8” I.D. 17-7PH Condition A tubing. Several post indicate it is better to machine the material in the Precipitation Hardened state (TH 1050). This should prevent the surface from smearing or tearing during turning. Because of fatigue loading I need to minimize surface tearing during machining and hydro-forming. If I understand correctly my process might look like this.
a) Purchase 17-7 PH Tubing Annealed
b) Heat treat material to condition TH1050
c) Machine (Turn) blanks as required
d) Solution anneal blanks
e) Hydro-form blanks
f) Degrease blanks
g) Heat treat blanks to condition RH950
h) Passivate
i) Hydrogen embrittlement relief treatment
Do any of the 17-7 tube manufactures sell the tubing in a hardened state for ease of machining?
Is Final stress relieve not required because parts will be stress relived during heat treat to RH950?
Also because blanks will contract during heat treat, should turning and forming them .0007” oversize help compensate?
Will the Approximate mechanical properties for the above described process (17-7PH RH 950) be?
• UTS 210,000 psi minimum
• Yield 190,000 psi minimum
• Hardness Rock “C” 44 minimum
Notes:
Maximum material elongation during hydro-form is 11%
Tube blank wall thickness .020”
Minimum hydro-form inner bend radius .062”
Lubricant used during hydro-form ProDraw722
I look forward to all responses.
Thank you in advance for comments, suggestions and idea’s.
Nickjk

 

[EdStainless]

Why harden and then machine?
Why not machine in the A condition and then form.
I would be more concerned about the need to anneal after forming. Some areas may end up much harder, almost in the CH condition.

I am even more curious about the part configuration. If some sections need to be 0.188" wall and others only 0.020" wall then maybe you should have two tubes and weld them together.
Though I have to admit that I am not excited about making either of those sizes.

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Plymouth Tube

 

[nickjk]

EdStainless
My reason for hardening prior to machining was based on information from related 17-7 threads (thread723-179830). In that thread you stated “The A condition is rather variable and smearing can be a real issue. The higher hardness aged improves cutting and surface condition.” I would rather machine from the annealed state to save cost and time but I am concerned with final surface finish quality.

I made a mistake in the process description. From my understanding it would be best to re-anneal before aging to prevent the problem you are talking about.

The center of the tube blank has the thin .020 wall. It is in that center where the tube is hydro-formed and subjected to high bending stresses. The top and bottom have thicker walls that are used for piloting the blank.

Do you feel I could remove the heat treat process before machining which may allow for the removal of the anneal process before hydro-form. Would the turning during machining work harden the material to the point where hydro-forming a .062 radius in .020 wall may be difficult to do.
Thank you for your time and help.
Nickjk

 

[EdStainless]

Are the thicker ends just used in forming or are they part of the finished part?
The machining must cost more than the tube.
I don't think that the machining will harm formability.
You will need an anneal after forming though.

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Plymouth Tube

 

[nickjk]

EdStainless
The thicker ends are part of the finished part and are used to provide pilot diameters for the formed blank into the housing.
The ends are also used to provide a sealed surface and shoulder.

The revised process may look like:

a) Purchase 17-7 PH Tubing Annealed
b) Machine Blanks as Required
c) Hydro-Form Blanks
d) Degrease Formed Blanks
e) Solution Anneal Blanks
f) Heat Treat Blanks to Condition RH950
g) Passivate
h) Hydrogen Embrittlement Treatment

Also, I am trying to get a quote from Plymouth Tube for 17-7 PH Tubing Annealed.
1 ¼” O.D. x 7/8 I.D. x minimum length sold. Do you know if this material is readily available from Plymouth Tube?



Thank you for your expert advise

Nickjk

 

[nickjk]

I cannot find 17-7 PH available in seamless tubing.
17-7 PH ASM 5568 is for welded tubing.
How will the tubing being welded instead of seamless affect ductility for hydro-forming, fatigue strength or ability to be to hold high pressure (5000 psi)?
Although I am trying to avoid it, would it be best to machine complete from round stock then hydro-form?

Thank you,
Nickjk

 

[nickjk]

The following link helped answer some of the above questions.

Does "Drawn-Over-Mandrel Tube" mean SEAMLESS TUBE?
thread367-92368

Thanks

Nickjk

 

[EdStainless]

17-7 is made as welded tubing. It is welded, annealed, cold drawn, re-annealed.
This tubing have very uniform properties and wall thickness. Fro fatigue applications it is favored over seamless.
We make a lot of it for aerospace applications.

First of all this is a very heavy wall that you are asking for. Are you sure that you can't go lighter?
The minimum purchase will probably be a few thousand pounds.

We haven't seen the inquiry here. Drop me a note edstainless at earthlink.net

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Plymouth Tube