17-7 CH900

[newmacnow]

We are designing a spiral torsion spring using 17-7 Condition C. Our question is after forming the spring do we have to harden it to CH 900? Or can it be used reliably in Condition C?

We were told that we can only use half the maximum yield stress for our factor of safety calculations, if we stress relieve it after forming.
CH900 is 260 ksi, so we could only use 130 ksi. The stress at the at use position is 92ksi and we are required to have a factor of safety of 1.5. So we only have a 1.4 and that will be considered unacceptable. If we use it in Condition C we were told we could use the full 190 ksi limit, yielding 2.1 factor of safety.

Thanks.

 

[TVP]

Can you please clarify the following:

1. Is the material in the form of wire or strip?
2. What is the diameter (or thickness) of the wire (strip)?
3. Is this a static, or dynamic application?
4. What is the end-use environment (temperature, chemicals, etc.)?

 

[newmacnow]

1. Is the material in the form of wire or strip?
Strip
2. What is the diameter (or thickness) of the wire (strip)?
0.049x0.500"
3. Is this a static, or dynamic application?
Static.
4. What is the end-use environment (temperature, chemicals, etc.)?
It's going to be a one use application, outer space, deployment.

 

[israelkk]

newmacnow

"We were told that we can only use half the maximum yield stress for our factor of safety calculations..." is it related to stress corrosion cracking? Can you explain why? Do you have a spec and requirements for the spring?

How do you account for relaxtion with time?

Why use 17-7PH condtion C at all if you do not heat treat it to H900. Instead you can use AISI 301/302 hard or extra hard?

 

[TVP]

The reason the allowable stress is lower for the aged/stress-relieved condition vs. the as-rolled condition is due to residual stress effects from the spring coiling operation. This type of spring, similar to helical torsion springs, have a favorable residual stress after coiling when the spring loading will cause a reduction in the spring's radius of curvature. The Associated Spring Design Handbook has a small section on these springs, and the allowable stresses are similar to helical torsion springs.

israelkk is correct that you may as well use Type 201/301/302 in the Spring temper or Type 301 in the SPL Spring temper instead of paying for Type 631 (17-7) and not using it in the CH900 condition. The following brochure from Somers Thin Strip (part of Olin Brass) shows the properties for Spring and SPL Spring temper stainless steel spring materials:

http://www.olinbrass.com/OlinSteelBrochure.pdf

 

[israelkk]

TVP

Why stress relieving is needed at all? isn't the H900 aging do the relieving and strengthening? As I understand the spring is rolled in the C condition and then aged at 900F. This will relieve the coiling stresses but will raise the tensile strength. Therefore, this spring will not have the favorable stresses at all.

To regain or intentionally induce the favorable stresses it will need to be "prestted" in torsion. On the other hand if you use a cold worked alloys that doesn't need a subsequent heat treatment such as AISI 201/301/302 you have the favorable stresses as mentioned in Associated Spring Design Handbook.

 

[TVP]

Israelkk,

You and I have the same understanding. You are quite correct, no separate "stress relief" operation is required-- it just occurs if the normal aging treatment is performed, and therefore the allowable stress is reduced, unless it can be regained by a torsional preset. Apparently the spring producer for newmacnow is not proposing to preset the spring, and would prefer to just use the 17-7 spring material in the C condition-- as coiled, no stress relief, no preset. I agree with your proposal to use a strain-hardened, high spring temper austenitic stainless steel instead of their current plan.

 

[israelkk]

TVP and newmacnow

NASA approves 17-7PH only in the CH900 condition for HIGH RESISTANCE TO STRESS CORROSION CRACKING see NASA MSFC-SPEC-522B and MSFC-STD-3029 which superseded it. All other conditions are considered low or moderate resistance to stress corrosion cracking. Newmacnow said it is used for outer space use therefore, I would carefully check if it is approved at all to be used in the C condition. Not to mention that the spring will be under constant load until deployment and that can be a long time. Therefore, high resistance to stress corrosion cracking risk should have a major consideration.