Post machine heat treat of Invar 36 2

[CRT1]

I designed two optical mounts out of Invar 36 and had them machined with no heat treating. They are basically 1/2" thick plates 4" x 6" with a bunch of pockets that each hold a 3" diam mirror.

After aligning at room temp, we lose alignment when we run the assembly down to operating temp of -70º C. This can not be attributed to thermal expansion (contraction) of the material alone.

I was thinking about a Stress Relieve Anneal (600º F) to remove the machining stresses. Is this advisable or will my machining tolerances go to pot? What special considerations do I have to make (heat up, cool down times, etc)?

Thanks!

 

[arunmrao]

Stress relieving is essential for Invar parts during/after machining.. Perhaps atleast before finish machining,stress relieving would have been beneficial.

I am not sure if the dimension would go to pots now. For confirmation,you might try out on a test piece,before checking on work piece. Hope it helps.

 

[EdStainless]

You need to give a long soak at 600F and then a number of cycles. If you are going to -70C I would cycle between 400F and -100C.
You may need to re-machine to bring back tolerances, but residual stresses and uneven expansion will kill you otherwise.
You should have rough machined and then done soaking and cycling before the final machine steps.

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Plymouth Tube

 

[CRT1]

Ed, Thanks for the info. What do you mean by "long soak" at 600F? Alloy digest says 1hr, should I go longer?

On the 400F to -100C cycles, should I be concerned about ramp rates and soak time at high/low set points?

Thanks,
Chris