Heat treat temperature calibration

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BadDog
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Heat treat temperature calibration

Post by BadDog »

I picked up what appears to be a decent little heat treat oven for very reasonable money. It's based on a pottery kiln with a nice little digital thermal control. No timer, no steps, just turn it on, reach temp, hold temp until done. It was used in a small job shop for just that, heat treating things as required. They eventually upgraded to a larger commercial heat treat oven with more features, and no longer needed this one. Seems fairly nicely done, typical DIY of internet plans for such things.

The problem is that I have nothing but their description of it's temp accuracy. I have no reason to doubt, but would rather verify that it holds (reasonably) accurate temps as reported on it's digital display.

Can anyone suggest a method to calibrate this thing? Something reasonable for a home shop?

I suppose I could just heat something to 900 and use an IR thermometer like I've used for checking exhaust ports, but no idea how actually accurate it is. I've never cared about the actual numbers, it's mostly a ballpark value and relative difference, much like a DTI in that's it's really not about accurate quantification

I hope to be heat treating some 17-4 to H900 in the near future, and it needs to hold pretty close to 900* for 1 hour, and wanted to make sure that's what I was going to get.
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GlennW
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Re: Heat treat temperature calibration

Post by GlennW »

As a start...Heat treat a test piece and then find a friend with a hardness tester.
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Russ Hanscom
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Re: Heat treat temperature calibration

Post by Russ Hanscom »

Look into thermocouples. They are very accurate and relatively inexpensive. You can probably find a unit ready to go on ebay.
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Bill Shields
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Re: Heat treat temperature calibration

Post by Bill Shields »

Thermocouple type J or K will fill your need.

In addition to the actual thermocouple you will need a readout to which it is connected.
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BadDog
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Re: Heat treat temperature calibration

Post by BadDog »

Thanks

It would be cool if I had ready access to a hardness tester. Sadly, I don't know of any within an hour drive.

The control unit uses a thermocouple for it's reference. No idea of it's quality or accuracy. But you reminded me that my Fluke MM has provision for a thermocouple. Maybe I should look into that.
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Bob D.
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Re: Heat treat temperature calibration

Post by Bob D. »

Tempil temperature indicating crayons. Anywhere from 100f to 2500f. Probably the cheapest known accuracy method.
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John Evans
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Re: Heat treat temperature calibration

Post by John Evans »

Russ : Pick up a set of "hardness files" , not extremely accurate but will give you a idea in a 5 point window. Use mine often.
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BadDog
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Re: Heat treat temperature calibration

Post by BadDog »

Thanks.

I have used and considered tempil (sp?) sticks, but you have to watch those for the transition. I'm more concerned with whether the digital controller takes it to temp (900*f in this case) and holds that temp within a few degrees as required for my pending 17-4 bearing race project. And I need to do that with the oven closed.

I've also got a set of hardness files. But the problem is that if you miss the HT on 17-4 to H900, you've got a problem in that the way forward is unacceptable. Annealing goes to ~2k*F for some time, then slow cool to room temp, then repeat. But that leads to scale and distortion, basically scrapping the fairly high precision part. The whole point of the exercise is that turning will (can) produce very fine finish and HT to acceptable hardness (H-900) without need for post grinding. If I miss it the first time, I have to remake them.
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tornitore45
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Re: Heat treat temperature calibration

Post by tornitore45 »

Mauro Gaetano
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BadDog
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Re: Heat treat temperature calibration

Post by BadDog »

Thanks. Interesting. I ran across them some time back, but had forgotten about them.

Looks like they start a bit above 1000*F, so not able to calibrate at my target temp of 900*F. Also seems that (quick glance) are a minimum of ~60*F between each number (for lowest temp range 19, 18, 17), which is less precise than I would like verify accurate temp reading/maintenance.

Maybe I'm just overthinking it as I often do. He claimed it was accurate to within a few degrees, and to have used it to good effect. It's based on a thermocouple for reference, so assuming it's good as is the processor, maybe I should just move on and worry about it if it proves problematic. Maybe just a sanity check with the IR thermo gun.
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liveaboard
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Re: Heat treat temperature calibration

Post by liveaboard »

cones are not suitable for this, as their melt point is based on temperature as well as time.

The digital thermocouple thermometer that you got with the oven is probably quite accurate, the bimetal thermocouple produces a known voltage according to temperature. The readout is just a digital voltmeter.

If you need to check it, the best you can do is get another one.
Alternately, find a hobby ceramic supply shop nearby and ask them to check it against theirs next time they fire.
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John Hasler
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Re: Heat treat temperature calibration

Post by John Hasler »

>Maybe just a sanity check with the IR thermo gun.

More of a sanity check for the gun. Those things are notoriously inaccurate. Thermocouples are almost always either quite accurate or don't work at all.
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