Solution heat treat of aluminum.

[TomNativeNewYorker]

First off, some background info about me. I am a sheet metal mechanic(airframes) at an overseas location, semi-isolated location. Very little experience with heat treatment other than basic heat treatment of solid rivets in the past. Jumping into the lions den for this project.

Tasked to fabricate small brackets with standard shop practices and box and pan brake for multiple bends.

First item is made from .063" 7075-0 clad requiring heat treat to T6. Second Item is made from .063" 6061-0 clad requiring heat treat to T6.

Heat treatment apparently isnt done very often around here, last time was about 1 1/2-2 years prior to my arrival.

The equipment is a dual chamber oven. The top chamber is a still air furnace to 2500*F; and the lower is a circulating air oven to 1600*F. Was told the still air chamber is for steels, and the circulating air is what aluminum has to be processed in. The lower oven has 2 honeywell udc-2300 controllers, and an ATC-365 countdown timer. the best I can get out of the installation manuals is that the top controller is to set the high and low limits set points, and the lower controller is to set the target temp for the process.

The pub we have to go by for the treatment of 7075-0 to T-6 calls for a 40 minute soak cycle at 860-930*f followed by a 20%/80% ethlyne glycol/water quench and then a 23-25 hour ageing at 250*F plus/minus 10*F.

After a lot of trial and error while using some of the information from the installation manuals. I came up with the following: a desired temp set point 870* soak requires the low set point of 890* and the high s/p of 970*.

OK someone correct me if I am right or wrong here, the high s/p is in case you get a runaway oven to kill it, and the low s/p is to kick the heat on when temp drops below this point? shouldnt the low s/p be the same as the desired soak temp?

This is how the run went on a piece of scrap 7075-0 went. The 870* heat treat worked like a champ and quenched as required. had to allow the oven door to remain open for awhile to get the temp to cool down and stabilized for the age cycle. it took over an hour to get the oven stabilized and kept the test plate in the upper chamber with ambient air temp of about 100*F.

The age soak cycle appeared to work fine for the several hours remaining before end of day. The next day, a visual showed the internal temp was about 130*F, and the chart recorder displayed a slow and steady spiral decline since quitting time the day before. it was determined we needed to run the timer(we didnt) assuming that we could just let it run on its own until we cam in to kill the process at the end of the age cycle.

according to our pubs, we were looking for a rockwell b target between 77 and 83 and we hit it right in the middle at 80.

Can anyone see any discrepancies that can be corrected or any other insight?

Is it normal to require the low set point to be 20*F higher than the desired soak set temp? Bad thermocouples? Controllers are due for calibration December of this year.

Thanks for any advice.

 

[CastMetal]

Depending on the size and configuration of the chamber you may have the thermocouples placed in a hotter zone than the parts or it could just be that your oven ran at 890*F. Depending upon the meaning of "several hours" I would expect you could see higher hardness values with fully artificially aged parts. A slightly raised solution heat treat will give higher hardness values.

Our ovens when solution heat treating 6061 actually are set 20*F bellow to account for poor temperature regulation of the oven.

Really at the end of the day its what works for you +/- 20 deg will vary your results a couple points of hardness but its more about being precise than accurate.

 

[EdStainless]

We used to take them home and age them in the oven there. At low temps most furnaces don't loose enough heat to force them to cycle enough to hold good regulation.

You should have a sample tested for strength. All that you need to to cut a tensile test coupon, put reference marks on it, and pull it until it breaks. All that you really need to know is load at failure (calculate ultimate tensile strength) and how much it stretched (elongation at failure). These will tell you if you are in the right range.

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Plymouth Tube

 

[Compositepro]

I think your description of the controllers is somewhat confusing but what you are doing ma be okay. They are not for "upper and lower set point". As you say later one is for controlling process temperature. That is the set point temperature for your process. You may need to set this higher than your desired part temperature if the thermocouple is located near the heating elements (not a good design). The second controller is an over-temperature safety limit that shutdown the oven in case the first controller malfunctions.