Tell Me Again - Why Do I Need An Electric HT Oven?

[MTBob]

It seems everything I read says that I need an electric oven to heat treat stainless steel. Apparently the electric oven provides better temperature control.
But, that's where the numbers don't add up for me. Let's say I'm shooting for a 1500F austenitizing temperature. Ovens like Paragon and Evenheat apparently have temperature tolerances that are around +/- 2-3%, and perhaps may go up to 5-7%. So, using a temperature variation of , say, +/-3% that means the electric ovens may vary as much as 45F, or 1455-1545F.
I built a 2 burner heat treating forge that gives me the ability to control temperatures in the +/- 25F range at austenitizing temperatures. I use a digital readout with a type K probe, with the probe mounted inside a tubular steel baffle next to the blade(s) being heated. The digital readout claims to have a tolerance of +/- .2-3%, a very small temperature tolerance.
So, my question is this - Is temperature control the only reason I need an electric oven to HT stainless? If that's the case, I think I can equal or beat the standard electric oven heat treating temperature tolerances. But, to be clear, controlling target HT temperatures on my forge does require some "nimble" maneuvering with the gas valves. Note the gas bypass valve (yellow handle) that lets me more accurately control the gas flow through a small needle valve.
Do Evenheat & Paragon ovens actually have notable temperature swings while trying to hold a target temperature? If so, what's your experience with your oven?

Here's the gas control panel & flame baffle.

 

[Andre Grobler]

I would say no… not if those are the spec.s all furnaces have variations in temp somewhere in them, the firther you go from the thermocouple, the further it goes from target temp… i use a variable output controller into a solid state relay and my oven overshoots mostly with a degree or two. Tested with silver melting point… so… i think those specs would have me in tears. But they can probably be worked around… keeping the temp there for 25-30min would be a bear with gas… but yours seem similar in concept to the don fogg style heat treatment forge… so i see no reason it wont work well

 

[CDHumiston]

I have a Paragon oven and it is very accurate. I have measured it with a standalone heat probe. As for holding temp, my oven never varies more than a degree or two when stabilized.

 

[timgunn]

For Austenitizing most of the stainless steels, the main advantage of an electric HT oven seems to be its (reasonably) tight temperature control and the way it can be set and left to run while the operator gets on with something else. If you can get similarly tight temperature control from a gas setup, and it makes sense for the work you do (this probably means your knifemaking is a hobby, rather than the means by which you put food on the table and pay the mortgage, so the time spent making manual adjustments doesn't really cost), by all means use the gas setup.

I have built gas-fired HT "forges" and electric HT ovens. I genuinely believe the gas-fired ones are better for Austenitizing Carbon steels because the reducing atmosphere significantly reduces scaling and decarb issues compared to the air atmosphere in an electric HT oven. To some extent the issue can be mitigated with anti-scale coatings, but even then, the reduced-Oxygen environment "seems" to give less decarb.

Stainless steels are going to need foil-wrapping in either an elecric HT oven or a gas-fired HT "forge", so there's no clear winner, assuming the temperature control is comparable.

Where the electric HT oven really wins is where ramping is required. Some of the fancier stainless steels may benefit from ramping up to temperature: I don't know for sure. I do know that if you need to anneal stainless steels, the ability to ramp down the temperature slowly is only really found in electric HT ovens.

 

[Gliden07]

This I agree with. If your gas forge is as accurate as you say it will be a lot quicker heat up and ramp up in the gas forge! Give it a shot on some sample coupons?

"Where the electric HT oven really wins is where ramping is required. Some of the fancier stainless steels may benefit from ramping up to temperature: I don't know for sure. I do know that if you need to anneal stainless steels, the ability to ramp down the temperature slowly is only really found in electric HT ovens"

 

[jmforge]

I have seen a small Fogg style drum forge maintain +- 5F at like 1435. With that said, I would not want to try that at 1935F. I have a Paragon. The newer ones can be had with zoned heating elements and a second thermocouple IIRC. So maybe even more accurate.

 

[Andre Grobler]

I have seen a small Fogg style drum forge maintain +- 5F at like 1435. With that said, I would not want to try that at 1935F. I have a Paragon. The newer ones can be had with zoned heating elements and a second thermocouple IIRC. So maybe even more accurate.

That would be progress, i have been playing with the idea of making a three controller / element / tc oven…

 

[MTBob]

Thanks guys, good comments. I should have mentioned this: yes, I'm a knife making hobbyist.

gas-fired ones are better for Austenitizing Carbon steels because the reducing atmosphere significantly reduces scaling and decarb issues compared to the air atmosphere in an electric HT oven

So, the reducing atmosphere of a gas fired forge can be an advantage; hadn't really given that much thought. I typically use ATP641 anyway and get very little scaling/decarb.

I have a Paragon oven and it is very accurate. I have measured it with a standalone heat probe. As for holding temp, my oven never varies more than a degree or two when stabilized.

Chris, when you open to oven door & insert your steel, does your oven rise up directly to your target temperature? Or, does the oven overshoot and undershoot while seeking to achieve the target temperature?

Where the electric HT oven really wins is where ramping is required. Some of the fancier stainless steels may benefit from ramping

Which stainless requires ramping?

Again, thanks for all the great comments

 

[Gliden07]

Thanks guys, good comments. I should have mentioned this: yes, I'm a knife making hobbyist.

So, the reducing atmosphere of a gas fired forge can be an advantage; hadn't really given that much thought. I typically use ATP641 anyway and get very little scaling/decarb.


Chris, when you open to oven door & insert your steel, does your oven rise up directly to your target temperature? Or, does the oven overshoot and undershoot while seeking to achieve the target temperature?


Which stainless requires ramping?

Again, thanks for all the great comments

The only stainless I use is AEB-L, if I remember the program (oven makes ya lazy too) 1650 then 1925. Plate quench till cool to touch, Cryo in RV Antifreeze and Dry Ice then temper.