Tue Feb 09 2021
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Tue Feb 09 2021

Troubleshooting heat treat oven

Not sure what happened with that last part (making this log update a couple weeks after the fact), but here's a scrap part ready to test the effectiveness of the heat shield while running the oven.

So, yeah...that didn't work at all.

Interesting...some of the blisters have water in them.

A couple more parts for testing in the oven. I did some shrinking on one of the parts to see if the blistering is related to the areas of the part which have expereinced work hardening.

I noticed something...the oven has a pretty significant overshoot of the temperature at the end of the heating ramp. Here, it peaked at 15 C over the target temperature. This is likely to be a significant problem, because some reading indicated that the solution heat treatment temperature is only a few degrees below the incipient melting temperature for 2024. I'm going to have to work on adjusting the PID gains in the controller to prevent this. Also worth noting, when attempting to maintain temp, it fluctuates as much as +/-7 C.

Ran another test after adjusting the gains. Although I had to manually prevent it from overshooting durning the ramp up, once it is at temperature, it holds that temp very tightly. Unfortunately, this part also blistered at the end which was further in the oven.

OK, so there are several things I'm going to do:

  • I'm going to add retainers at the back corners of the heat treat oven to retain the heating element. These will need to be able to handle high heat, and also be non-conductive since they may be in contact with the (electrified) heating elements. I plan to use ceramic insulators, but should the insulators break, I want to ensure that the retainers are not accessible from outside the oven
  • I'll be adding another thermocouple near the back so I can monitor the temperature both at the front and the back.
  • I'm going to add a fan to circulate air within the oven. This is going to be a challenge due to the high temperature. Obviously, the motor can't be inside the oven, so I'll need to have a shaft extending inside. However, when I checked the strength of stainless steel at, say, 2000 F it is quite close to zero. So, that probably means a ceramic shaft.
  • I may also add baffles in the oven to force air to circulate in a big loop rather than just local circulation near the fan. I'll see how it goes without baffles. I should be able to monitor how well it works using the two thermocouples.

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