Question about tempering

izafireman

izafireman

Well-Known Member
I do a bit of wood stabilising and at the moment I use a small toaster oven. The oven is totally unreliable and so as I am also looking into heat treating my own steel and building a kiln I thought I would convert my toaster oven too a PID controller/thermocouple/body/wiring upgrade where necessary so that it is accurate to within a degree or so. As I am doing this it will obviously be great for tempering when I need to in the future.

Now my question is, what would be the max I would require in degrees C for tempering. I was thinking 500c as this is what I had my Elmax tempered too (industrial bulk tempering) and to be honest I would probably go with a lower temperature next time having spoken to the manufacure.

The unit PID I am buying comes with a K type thermocouple rated to 400 c , would this be adequate for the normal ranges of steels used for knives in particular AEB-L which I want to try?

Thanks in advance.
 
Toaster ovens are never a good choice to be used for tempering. The problem is the "swing" (the amount of temp variance between when the oven kicks on to reach the set temp, and when it shuts off once it reaches that temp.... very often the "swing" will be as much as 100F (or more). When it comes to tempering blade steels, every 25F is a step in the tempering process, so if your tempering oven "swings" more then 25F, then it's unacceptable for tempering. Toaster ovens simply are not designed/insulated to a level that allows for anything close to "tight tolerances" when it come to holding a steady temp. The other issue is that I can't imagine you'd ever find a toaster oven that would go to 500c (that's over 900F). Most commercially available toaster ovens will go to 500F (260c) on the dial, but in reality they will rarely exceed 450F (232c). Unless you tore everything out of a toaster oven, and started from scratch, with only the shell, it just wouldn't work well....and doing so would likely cost more than a new heat treat oven. I've worked with/use PIDs, and can tell you that simply installing a PID won't aid in an ovens accuracy.....and in some cases a PID can drive you nuts trying to get all the internal parameters set (and with some PIDs it's a fight just to find those settings :) ) I think you'd be time and money ahead with just purchasing a heat treat oven.
 
I'm with Ed just buy a electric Kiln. I was going to buy 1. But then after researching I new it would take way to much time and I would rather make Knives then build an oven. I found my oven used. An evenheat 28" for $500 and it had new coils in it.
 
As the OP said, his toaster oven is totally unreliable, hence the idea of using a PID to control. GREAT idea. My toaster oven with PID and a fire brick inside to add some mass holds within one degree either way of setpoint. About 425°F is as high as it will go. "IF" you want hotter you'll have to tear the toaster oven apart and install some good insulation. I don't remember who did that to a toaster oven but now can temper up to 500°F I think I remember him saying.

While it would be nice to have another electric kiln for tempering, a toaster oven with PID control and a tad of extra mass are just as good, and actually hold a closer temperature than does my EvenHeat oven - as long as I'm not asking for more than 425°F.
 
The bigger questions in my opinion is what steel do you want to use, what heat source do you intend to use for hardening? There is wide variance in temp, soak and quench rates for different steels. Answering these questions first is likely to answer any question about what source to use for tempering.
 
Like yourself I used a toaster oven. It was a real pile of junk. I even put a PID on it. Like Ed said. The heat swing was way to much. That went to the junk yard. If I was going to buy another it would be a "convection" toaster oven w/PID. Ken has a good idea with the fire brick. I went back to my kitchen oven. I would love to find a real heat treat oven for knives used. Good luck!
 
I use a conventional electric oven. It works well...cost $50 used. Yes the space issue might be a deal killer for some. I believe that the huge capacity is what keeps it even on heat. Toaster ovens are just too small. The cubic difference between the two is huge. You don't want your steel that close to the heat source....toaster fails at this...standard kitchen oven excels. I initially moved my thermometer all over to check for cool/hot spots...none. I can hit my RC numbers dead on with it. Bakers claim electric is better for baking...I don't know if this has any bearing on tempering.

Also you can heat soup on the range for lunch...lol
 
Thanks for the replies and I take on board work your saying epically where guys have converted them themselves. Even if the oven failed as a tempering oven then it would surely be a huge step up for my stabilising. If I did this the oven would have a complete strip down by my father who is a sparky so would be safe.

I know when setting them up some folk buy the incorrect thermocouples and so the readings are crazy after they have been set up,

Im also looking for the largest oven I can.

I also tried my household oven and the temperature differentials were crazy.

Additionally i have never seen a tempering oven as such

Thanks
 
Were you using electric or gas?

Also...I wonder how an extra deep roaster would do...don't know if they would get high enough temp though.
 
It was an electric oven. I tried it again last night but it still lies as to the actual temperature but that's no issue as I can just set that via an temperature probe which I can have via a thermocouple in the oven / thermometer outside. I reckon a few fire bricks inside will stop any fluctuations.
 
I reckon a few fire bricks inside will stop any fluctuations.
Click to expand...

That's likely a good practice for any oven used for tempering. Since I forge almost all my blades, I use steels that all have tempering temps below 500F.... and like Ted, I use a typical kitchen type oven.......and actually prefer it to an actual heat treat oven for tempering. Anyway.... I have three full sized fire bricks sitting in the bottom of my tempering oven. With all that thermal mass, it takes it about an hour to heat up/level out, but once it does, it rarely swings more then a few degrees either way of the setpoint.
 
EdCaffreyMS said:
That's likely a good practice for any oven used for tempering. Since I forge almost all my blades, I use steels that all have tempering temps below 500F.... and like Ted, I use a typical kitchen type oven.......and actually prefer it to an actual heat treat oven for tempering. Anyway.... I have three full sized fire bricks sitting in the bottom of my tempering oven. With all that thermal mass, it takes it about an hour to heat up/level out, but once it does, it rarely swings more then a few degrees either way of the setpoint.
Click to expand...

Thanks for the reply Ed, the tempering oven idea is in the bin :) and I will follow your advice and that of the others who replied as I you guys are the ones with the expertise and wise heads where this is concerned so thanks for saving me the wasted cash and hassle that I would have had.
 
Just an FYI but I temper in my kitchen oven as well currently and it has huge (50 F) temperature swings. I use a deep rectangular pan filled with sand and burying the blade deep in the middle. Using a digital temperature probe in the sand I can usually keep the temp within 10-15 degrees F of my target temp.

YMMV
 
jaxxas said:
Just an FYI but I temper in my kitchen oven as well currently and it has huge (50 F) temperature swings. I use a deep rectangular pan filled with sand and burying the blade deep in the middle. Using a digital temperature probe in the sand I can usually keep the temp within 10-15 degrees F of my target temp.

YMMV
Click to expand...

Thanks for the tip
 
EdCaffreyMS said:
That's likely a good practice for any oven used for tempering. Since I forge almost all my blades, I use steels that all have tempering temps below 500F.... and like Ted, I use a typical kitchen type oven.......and actually prefer it to an actual heat treat oven for tempering. Anyway.... I have three full sized fire bricks sitting in the bottom of my tempering oven. With all that thermal mass, it takes it about an hour to heat up/level out, but once it does, it rarely swings more then a few degrees either way of the setpoint.
Click to expand...
Hadn't thought about the bricks! Great idea. I do leave my oven on for an hour before I put steel in it. That seems about when it "settles". Gonna add some brick next time.
 
springer82 said:
Like yourself I used a toaster oven. It was a real pile of junk. I even put a PID on it. Like Ed said. The heat swing was way to much. That went to the junk yard. If I was going to buy another it would be a "convection" toaster oven w/PID. Ken has a good idea with the fire brick. I went back to my kitchen oven. I would love to find a real heat treat oven for knives used. Good luck!
Click to expand...
I should have replied to this post before. You mention your toaster oven with PID didn't hold an even heat? with a PID it "should" have held well <5±F, and closer to ±1°F. Most likely the problem was PID programming - right out of box they typically need to be placed in the "learn" mode so PID can automatically determine correct correct parameters for the settings. I've done 3 toaster ovens with PID control, and a couple other controllers (lead pot, etc) and have never had a problem getting temperature control using a PID.

The fire brick in oven to add mass does help stabilize temp a good bit.

Ken H>
 
would a pid controller be of benefit on a conventional oven? How difficult to install? Does the PID override the manual control on the oven?

asking cause I heard "closer to +/- 1 degree F..." !!! Lol.
 
I used to believe that a PID and a quality thermocouple would increase accuracy, but I've found that to not be true...at least in the majority of applications.
A PID does very little to improve the actual accuracy of an oven. It's just another method of control. The true accuracy of an oven has more to do with the build quality (IE: type and quality of insulation) then the method of control. While it's true that a PID, in certain applications, MAY increase the accuracy of an oven (like a toaster oven which has an El-cheapo control system, but unless you add insulation to a toaster oven, a PID WILL NOT give you anything close to the accuracy needed for tempering blades)..... I've tried PIDs on a number of different types of ovens..... and frankly, the lower the quality of the oven, the less impact a PID and thermocouple control has.

A kitchen oven is a bear to wire with a PID.....first you gotta find a 220V PID that with handle the amps....... and the control circuits on kitchen oven tend to be over complex.

When I tried wiring a kitchen oven with a PID, I gave up after 3 days.....put the wiring back the way it was...... and just used a couple of oven thermometers and experimented over a couple of days to determine what the actual temps were versus the dial settings.

A while later I ran across a "digital" controlled oven, still in the box at a yard sale...... I managed to snag it for $150...... brought it home, wired it into the shop, and Lo and behold!!!! Once I put three firebricks in the bottom, it actually provides the temp that the digital readout says! WOOP! The caveat is that with the added mass of the bricks, I have to set the pre-heat, and let it cycle for about an hour to completely heat the bricks...... then it holds with about 5-10F degrees.....provide someone doesn't keep opening the door to look at thing! :)
 
While I do admit it takes a bit of understanding how process temperature control works, it can be done fairly easy with PID. The PID controller does NOT care what voltage or amps the oven requires. The only the the PID controller controls is the relay setup that does the controlling. About the easiest way to control a toaster oven (or kitchen oven) is to mount the PID in a box with relays and recept. I use 20 amp 120 volt recept for toaster oven. Plug the toaster oven into recept, set toaster oven to max temperature on T'stat. This puts the toaster oven's T'stat out of circuit because it'll be calling for heat all the time.

You can tear the toaster oven apart and build PID inside and make a really neat installation, but that doesn't make it work any better.

Mount the TC so the tip (tip is the only part that senses temperature) about an inch above middle of blade location. Here's a pic of my setup. The PID and recept is shown to the right side with brick inside. The 400°F setpoint on bottom is for Powder Coating cast bullets.
IMG-3196.jpg
 
That's VERY interesting Ken. I tried that very PID in you pic on a kitchen oven.....wired it with relays, and the moment the power of switched on. the PID went up in a cloud of smoke. I may have done something dumb in the wiring, and caused it, but after contacting the guy I buy PIDs from, he laughed at me and told me that even though the PID I'd tried to use was labeled for 220V, the amps fried it. Honestly, I never revisited trying to wire another kitchen oven with a PID...... didn't really have any need to once I got my hands on the digital kitchen oven.

I guess the take away is.....if it works for ya....by golly use it!! :)
 
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