Home built Heat Treating Oven

[anthonyget]

Dan, thanks so much. appreciate everything you said. As it turns out I did do some of this, like stapling and double twisting the end pieces. I did not however install a door switch. When I replace the coil for a thicker one, I'll try and do that. Great ideal.

[anthonyget]

Thanks, Dan for pitching in: Nice blog btw.
Thanks for also pointing out the brittleness of wires, and that max temperature degrades with wire dia:
For instance: Kanthal A wire of 3 mm dia (not the average home shop kiln) could take 1350 deg,
but around 1 mm it'll take max 1250.


Sorry, Nick for perhaps being confusing in my previous post, I totally missed the obvious last night...
The discussion on IR thermometer and materials & temp is probably NOT relevant in your case.

I assume by "Wrong thermocouple" you meant you plugged a "K" type into a PID/ controller designed for a "J" type?

Then, IF you choose 600 C as the limit temp, the controller will open the throttle wide open,
until it gets 33 mV back from the thermocouple input.
Fine and dandy IF you'd hook a "J" type up, which will precisely return that voltage at 600 C,
- but a "K" type would give off 33 mV first when it's reached 830-ish degrees.
I guess this was exactly what happened
(but your IR thermometer could have had its emissivity factor off too, but NOT deviate by 200 degrees...)

Hope this helps!
Cheers
Johan

Johan, thanks so much for all your help. You did not misunderstand. I bought a K type that was only rated to 600. I have taken all your comments on board. Super useful, thank you so much.

[anthonyget]

I know it will hurt, but this is a rubbish design, and not because of the effort you went to in building it, but because the design demonstrates that you never understood how heat is flowing, through the walls and (closed) door, out of your furnace. You do need a more than nodding understanding of Fick's laws to understand the spread of heat inside furnace walls and thus the resultant temperature distribution inside the furnace. I am not here to belittle your effort, I am writing to provide an explanation for your furnace's behaviour. Once you understand why you get this behaviour, you should be able to think up possible means to improve on it.
First, temperature measurement. Forget about using a pyrometer (i.e. infrared radiation thermometer), because this will produce a meaningful reading only if you measure the radiation coming out of a black hole. With "black hole" I am not referring to the astronomical object, but to a box with a central, large cavity that is accessible only via a small hole. Only this small hole has an emissivity of 1, all other surfaces will have emissivities less then 1, emissivities that depend not only on the material, but also on surface roughness and time. Thus, forget about looking up any emissivity, it does not produce a trustworthy temperature reading without the black hole emitter.
You could also estimate your temperature using Seger cones.
Or you can attach some weight to the tip of a thermocouple and place that at different positions inside your furnace, to measure a temperature distribution chart. If you achieve a temparature variation of less than 50 ^oC, you have build well. From the pictures you provide on the placing of your heating element, I would expect a spread of 200 ^oC or more, just what you have observed.
Once you have measured that and understand this is not simple bluster, we can talk again, this time about where to place the control thermocouple, and where to place the heating wires.

Thanks for your feedback. I would be interested to know where I should have placed my element and thermocouple. Although I think Fink's law might be a tad over my head.

[anthonyget]

Here is part 2 for anyone who is interested. Thanks very much to those who commented already.

[DanCom]

One good thing is that my oven will never need to go as high as 1300C, Probably 900C max.

I would recommend setting your sights on about 1100°C for a top end temp. Many popular cutlery stainless steels require a soak at or around 1060°C. 13C26, Elmax, 154CM, CPMS35VN etc.

[anthonyget]

I would recommend setting your sights on about 1100°C for a top end temp. Many popular cutlery stainless steels require a soak at or around 1060°C. 13C26, Elmax, 154CM, CPMS35VN etc.

Good point. My new thermocouple is good to 1200C

[DIYSwede]

Thanks, Nick for vid #2 !
Now, I might seem a little OCD when it comes to your "K"-type element:

1) Realizing your ITC-100 has multi-choice possibility of element types:
"Support Multiple Thermocouples and Resistance Sensors (K, S, Wre, T, E, J, B, N, CU50, PT100)"
was the correct type chosen in its menu before startup?
And, if so - what's your take on the 800-ish deg top temp?
Did the element croak at 600 C, or was it still operative, pulsing the output to the SSR correctly?

2) Was the PID, per chance in a "J"-type factory setting - thus achieving the thermal runaway as I suggested previously?

Just curious after all these years - trying to learn, and avoid mistakes!

Small tip: Just adding an extra breaker (2-pole toggle, lo volt and amp) for the SSR pulse control,
to let the coils "cool off" a few secs before opening the door, might be a good idea for wire longevity.
Thermal shock isn't supposed to be something good for A-1, or any other wire type either.

Cheers
Johan

[DanCom]

All the low-cost temperature controllers I've had (Inkbird, MyPIN & CXTG) are defaulted to input Type K. The most common type K in this temperature range has a -100° to 1250°C. These are the eBay/Amazon $10 ones. I have learned to keep a spare handy. The high temperatures oxidize the probe's stainless shell over time. Or you can get a ceramic holder which prolongs the life of the probe.

[anthonyget]

Thanks, Nick for vid #2 !
Now, I might seem a little OCD when it comes to your "K"-type element:

1) Realizing your ITC-100 has multi-choice possibility of element types:
"Support Multiple Thermocouples and Resistance Sensors (K, S, Wre, T, E, J, B, N, CU50, PT100)"
was the correct type chosen in its menu before startup?
And, if so - what's your take on the 800-ish deg top temp?
Did the element croak at 600 C, or was it still operative, pulsing the output to the SSR correctly?

2) Was the PID, per chance in a "J"-type factory setting - thus achieving the thermal runaway as I suggested previously?

Just curious after all these years - trying to learn, and avoid mistakes!

Small tip: Just adding an extra breaker (2-pole toggle, lo volt and amp) for the SSR pulse control,
to let the coils "cool off" a few secs before opening the door, might be a good idea for wire longevity.
Thermal shock isn't supposed to be something good for A-1, or any other wire type either.

Cheers
Johan

Johan, please don't worry about seeming OCD. I am so grateful for the time you are taking to help me. Genuinely. I think Dan is right, they do default to type K. The correct one should be delivered tomorrow. rated to 1200C. I will start all by testing again but can't thank you all enough. I will keep this thread posted as to the outcome. You are all brilliant and generous. Thank you.

[anthonyget]

All the low-cost temperature controllers I've had (Inkbird, MyPIN & CXTG) are defaulted to input Type K. The most common type K in this temperature range has a -100° to 1250°C. These are the eBay/Amazon $10 ones. I have learned to keep a spare handy. The high temperatures oxidize the probe's stainless shell over time. Or you can get a ceramic holder which prolongs the life of the probe.

Dan, I feel this is a foolish question, but please bear with me. I am still worried about the PID showing accurate temp. If I place a blade cold in the oven at start up, and set the temp at 815C for 1070, should I expect the steel to have reached that temp at the same time as the oven, or does one have to wait for the steel to heat up? And if so and if the steel reaches the critical temp, how long does one leave it there? In my forge I wait 30 seconds after losing magnetism, but I can't keep opening the oven. Any advice would be greatly appreciated. Thanks in advance. Nick