Re-heating of heat-treated materials? 1

[cbrf23]

Hi,
I'm looking for some general advice/guidance on heating a couple pre-heat-treated materials.

I have an assembly I'm building using 6061-T6 aluminum and Q&T 4340 (750°F temper) steel.
Both parts are machined with holes, and I need to heat both parts up so that I can drop in some carbide bushings and get a shrink fit.

I'd like to heat the aluminum to 350°F and the steel to 650°F.
I'm SWAGing a soak time of around 45-60 minutes for the AL and 60-90 for the steel.
Given the above materials and time/temperature soaks, will I be (appreciably) altering the mechanical properties?
Or put another way, is there a "safe" temperature I can raise the parts to without changing the mechanical properties?

The steel I'll be heating to about 100°F below the tempered temperature, but the aluminum I think is tempered right around those temperatures (google tells me 1hr@400F, 5hr@350, or 8hr@325 for T6).

I appreciate any advice.
Thanks!!

More details if interested:
The carbide bushings have an o.d. 2.5062 MAX while the machined parts have an i.d. of 2.500 MAX.
I've calculated needing a 191°F temperature raise for the aluminum, and 381°F for the steel to achieve 0 interference at max interference, so assuming 70°F room temperature I'd need to heat AL to ≈260°F and steel to ≈450°F to achieve this difference.

To allow for positioning in the jig, and to allow for temperature loss during handling, I wanted to raise the temperatures to what I listed above

Thanks!

 

[btrueblood]

You might over-age the aluminum somewhat, but the net result might be somewhat improved fatigue life, and a fairly minimal decrease in tensile yield strength. No real issue on the steel side that I can see, assuming your temperatures are well controlled.

 

[cbrf23]

@btrueblood - Thanks!
I've always run under the assumption that if I keep the second heating below the original tempering temperature I'll be safe with regards to mechanical changes. Is that a fair assumption?

 

[EdStainless]

Will thermal expansion give you 0.003" clearance? If not at least that you will never get them together.
We used to shrink fir ceramic sleeves, but just to contact. We then used a mechanical lock system to prevent motion.
We never could get enough compression on them to truly lock them. The ceramic is so hard and smooth that there is very little traction between it and the metal.

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

If you want the bushing to drop freely into the bore of the mating part, you'll want some clearance between them with your thermal fit temps. It should not be a problem heating the 6061-T6 to 260degF or the Q&T 4340 alloy steel to 450degF. You can also chill the bushings to get a bit extra clearance for the thermal fit.

Thermal fitting large diameter parts without much installation clearance can be tricky. As soon as they contact there is heat transfer and they seize up. So it would be a good idea to use some sort of installation fixture to make sure the bushing is aligned with the mating bore at assembly. The fixture could also include some feature (such as a depth stop or flange) to control the final axial location of the bushing.

 

[cbrf23]

@EdStainless - I should (based on my calculations) end up with about .003-.006 clearance (best/worst case stackup) just from heating the female parts. Theoretically I can get another .0013 if I chill the carbide with dry ice. WC doesn't respond much to temperature changes and actually I've heard of it being installed by setting the carbide piece in place, and heating both parts up, relying on the difference in thermal expansion create clearance! I actually don't need any rotational resistance, the carbide bushings soak up compressive forces in the final assembly, and serve as a guide for some shafts that pass through in the assembly as well.

@Tbuelna - Exactly! I've designed and am building a jig to help with alignment and axial positioning. I also have a 20 ton shop press available so if they shrink up and seize too quickly I should be able to press them on still.

 

[EdStainless]

Trust me, 20T won't budge them once they are tight, unless you tear the Al out.
If you could align them and then heat them both and let the SiC drop in that would give you the best chance for success.

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P.E. Metallurgy, Plymouth Tube

 

[metman]

cbrf23 (Mechanical) (OP)
If you do need to establish tonnage needed - assuming abrading of Aluminum is not an issue - Machinery Handbook has an excellent article on shrink and force fits with all the formulae needed to calculate.

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