Stress Relieving INVAR 36 - what is the correct cycle?
Stress Relieving INVAR 36 - what is the correct cycle?
(OP)
Hello -
I have an Invar 36 cast facesheet (a contoured sheet, almost like a horse-saddle) that is approx 2" thick, by 20" x 48". A support structure of 1/2" thick invar36 ribs will be stitch welded perpendicular to the facesheet, to create a common plane for the weldment to rest on.
After welding the support structure and rough-machining the facesheet, we need to do a stress-relief cycle before finish machining. What spec or stress relief cycle should I use?
The goal is to relieve all stresses, but not fully anneal. I'd like to keep the hardness as high as possible. The device needs to be stable for final machining, and the final usage of the device needs to withstand repeated cycles to 400F and be dimensionally stable while doing so.
Any insight or leads would be appreciated.
I have an Invar 36 cast facesheet (a contoured sheet, almost like a horse-saddle) that is approx 2" thick, by 20" x 48". A support structure of 1/2" thick invar36 ribs will be stitch welded perpendicular to the facesheet, to create a common plane for the weldment to rest on.
After welding the support structure and rough-machining the facesheet, we need to do a stress-relief cycle before finish machining. What spec or stress relief cycle should I use?
The goal is to relieve all stresses, but not fully anneal. I'd like to keep the hardness as high as possible. The device needs to be stable for final machining, and the final usage of the device needs to withstand repeated cycles to 400F and be dimensionally stable while doing so.
Any insight or leads would be appreciated.





RE: Stress Relieving INVAR 36 - what is the correct cycle?
RE: Stress Relieving INVAR 36 - what is the correct cycle?
Do you know the difference or have you heard of the higher spec? (I'd rather just use the lower spec but I'm looking for a spec or something to confirm this).
I think the 1400 is for full anneal, which I wouldn't want.
RE: Stress Relieving INVAR 36 - what is the correct cycle?
RE: Stress Relieving INVAR 36 - what is the correct cycle?
Also, I assume you're speaking from direct experience?
Thanks again for helping.
RE: Stress Relieving INVAR 36 - what is the correct cycle?
RE: Stress Relieving INVAR 36 - what is the correct cycle?
With a lot of welding involved I would suggest that you go back to anneal. Say 2 hours at 1400F and slow cooled to room temp, followed by 1 hour at 600F. but this would leave it soft and difficult to machine cleanly.
The other option would to weld an then give it 600F for 2 hours, rough machine and then 1hour at 600F, and after final use the 400F treatment.
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Plymouth Tube
RE: Stress Relieving INVAR 36 - what is the correct cycle?
Ed - in your 2nd option you are basically saying to stress relieve it twice, once after weld but before rough machine, and once between rough and final machine, and both cycles can be to 600F? I'm not sure what you mean by "the 400F treatment" unless you just mean proceed to use it at 400F in regular service.
RE: Stress Relieving INVAR 36 - what is the correct cycle?
Thanks to everyone so far for your input.
RE: Stress Relieving INVAR 36 - what is the correct cycle?
You may need to expend the time at 600F after the welding. Overnight wouldn't hurt anything and might give you a better chance at minimizing distortion.
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Plymouth Tube
RE: Stress Relieving INVAR 36 - what is the correct cycle?
Weld and rough machine
Stress at 600F minimum 2 hours, longer better
Finish machine
400F (for a few hours minimum)
RE: Stress Relieving INVAR 36 - what is the correct cycle?
RE: Stress Relieving INVAR 36 - what is the correct cycle?
And then another after the rough.
Finishing with a 400F cycle will assure you that you will hold dimensions in service. Not a bad practice.
It will also reduce the thermal expansion prior to usage.
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Plymouth Tube
RE: Stress Relieving INVAR 36 - what is the correct cycle?
RE: Stress Relieving INVAR 36 - what is the correct cycle?
AFTER MAJOR MACHINING IS COMPLETE, STRESS RELIEVE AS FOLLOWS: HEAT
TO 1500F (815C), HOLD FOR 30 MINUTES, WATER QUENCH. REHEAT TO 600F
(315C), HOLD FOR ONE HOUR, AIR COOL. FINISH MACHINING IS PERMISSIBLE
AFTER HEAT TREAT, .030 MAXIMUM MATERIAL REMOVAL FROM ANY SURFACE.
As far as I know it's working.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Stress Relieving INVAR 36 - what is the correct cycle?
RE: Stress Relieving INVAR 36 - what is the correct cycle?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Stress Relieving INVAR 36 - what is the correct cycle?
.030" sounds like very very minimal stock removal after stress relieve. Can you share the configuration of the parts you are using this for? (approx size, thin or thick, casting/wrought stock, etc...)?
I'm just wondering as we would typically leave more than that for a part the size we are working with. Thanks.
RE: Stress Relieving INVAR 36 - what is the correct cycle?
I think the part is now machined from billet though originally it was from a casting.
Also, originally the stress relieve was the done after all machining but they couldn't meet the tolerances, which is why it was changed to allow minimal machining after heat treat.
This is from memory, I might be off a little as it's 18 months or so since I was tangentially involved in this one.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?