Stress Relieving INVAR 36 - what is the correct cycle? 3

[Albigger]

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.

 

[arunmrao]

Stress Relieve: Heat to 600 - 700 oF for about two hours, air cool,or oven cool.

 

[Albigger]

arunmrao - Thank you. That's what I was thinking, but I've heard two different specs, the one you mentioned and another, at around 1400F, which a couple metallurgists told me was necessary to relieve stress.

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.

 

[arunmrao]

1400F may be the full anneal,which is not essential.

 

[Albigger]

Even though we will be doing quite a bit of welding to put the backup structure in place?

Also, I assume you're speaking from direct experience?

Thanks again for helping.

 

[arunmrao]

My interest in Invar is limited to the extent of manufacturing castings,their machining and assembly.

 

[EdStainless]

If you were only forming then the 600-650F would be fine.
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

 

[Albigger]

We were wondering if it would be appropriate to rough-machine, weld, stress relieve at 600F for 2-3 hours, and finish machine? Any thoughts on that?

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.

 

[Albigger]

Ed or anyone have any more options or opinions? We are leaning toward the lower stress relief cycle.

Thanks to everyone so far for your input.

 

[EdStainless]

You finish with the 400F for a couple of reasons. To make sure that it doesn't move when put into service and to minimize the expansion.
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

 

[Albigger]

So a process like this should be fine?

Weld and rough machine
Stress at 600F minimum 2 hours, longer better
Finish machine
400F (for a few hours minimum)

 

[metengr]

I really don't see the need for a 400 deg F post final machine thermal treatment. This is unnecessary.

 

[EdStainless]

I would suggest that the first 600F cycle be between the welding and rough machining.
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.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[Albigger]

OK, thanks. If time and planning allows, we will run 2 cycles, but it may not be possible. (i.e. we may end up roughing before welding)

 

[KENAT]

I just looked at an Invar 36 part we have that we had to fine tune the heat treatment on and we ended up with:

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.

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[metengr]

Good information.

 

[KENAT]

Yeah, well, don't ask me for the science behind it!

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[Albigger]

KENAT -

.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.

 

[KENAT]

It's a precision part, about 8" square maybe couple of inches tall. The .030 is on the small side but it's because of the nature of our situation - a chuck for a precision microscope.

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.

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