Evenheat Oven First Firing

opaul

Well-Known Member
#1
So I wired up the 240v receptacle for the kiln so the Evenheat oven now resides in my shop rather than the shipping crate. I did a test to run it up to 400 degrees to see how it would do. It took just a littler over 4 minutes for it to reach 400. I think that's pretty good, but I really don't have anything to compare it to except the kitchen oven.
So for those that use an oven, do you set it on 400 and leave it or do you set it a little low? When I aborted the test fire, the kiln temperature read 424 even though it was set to hold on 400.
 

Self Made Knives

Well-Known Member
#2
Which Evenheat did you get? I don't think mine ever runs over the set temp much at all. Maybe a couple degrees tops. Then again, I hardly ever use it as a tempering oven, so my programmed setpoints are all 1475 or higher. I've also got one spot programmed for annealing.
 

Gliden07

Well-Known Member
#3
So I wired up the 240v receptacle for the kiln so the Evenheat oven now resides in my shop rather than the shipping crate. I did a test to run it up to 400 degrees to see how it would do. It took just a littler over 4 minutes for it to reach 400. I think that's pretty good, but I really don't have anything to compare it to except the kitchen oven.
So for those that use an oven, do you set it on 400 and leave it or do you set it a little low? When I aborted the test fire, the kiln temperature read 424 even though it was set to hold on 400.
I don't have one of these but I read that if you set it on the fastest setting to ramp up the temp it will coa
st over a bit. If I understood what I read correctly if you ramp the temp up slowly it won't be as apt to shoot over the set temp.
 
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timgunn

Well-Known Member
#4
This is something to be expected. Evenheat will almost certainly have found that their customers want fast heating to Austenitizing temperatures and will have tuned the controller accordingly. This will tend to result in substantial overshoot at the much lower tempering temperatures unless steps are taken to prevent it.

Assuming you have one of the Ramp/Soak-capable controllers, setting a slow ramp to temperature will prevent the overshoot.

Back when I built my first HT oven, I did some testing using a borrowed-from-work high-end industrial controller, along with a series of different-diameter thermocouples and a datalogger. The aim was to get some sort of idea of the effect of direct radiant heating on a blade profile and to see which of the higher-end features would actually be useful in this application.

Temperatures are in degC. The control thermocouple was a 6mm diameter Mineral Insulated type N. The others were 0.5mm MI, 1.5mm MI and an exposed-junction type K. I suspect all the MI thermocouples had Insulated junctions, but cannot be certain: the response of the 1.5mm suggests it may have had a Grounded junction.

The controller had been autotuned at 800 degC (1472 degF) and the setpoint was 250 degC (482 degF) in both cases.

Without a ramp, the peak overshoot was to 119 degC (214 degF) above setpoint. With a 30-minute ramp, peak overshoot was 19 degC (34 degF) above the setpoint.

No overshoot whatsoever was evident from the controller, which was only "seeing" the control thermocouple.

Lessons I learned from this included:

1/ Use the fastest-response control thermocouple possible.
2/ Use a Ramp/Soak controller.
3/ Always ramp to tempering temperature.


GraphPIDto250deg.GIF GraphRampto250deg (1).GIF
 

KenH

Well-Known Member
#5
Nice graphs Tim -

Opaul, how long did you leave the oven on? It's typical for the oven to overshoot when first heating up, even at 1900F setting. Give a while to settle down - might even take 15 to 30 minutes for a good solid temp. Once settled in my Evenheat oven will hold within a couple degrees of setpoint. BUT - it does require time to settle in. Also, ramping up to temp slowly will prevent some overshoot. I always set to max heat, then let it settle down.

I've never used my oven for tempering, except for backsprings where the tempering temp is around 1,000F to 1100F.
 

opaul

Well-Known Member
#6
Nice graphs Tim -

Opaul, how long did you leave the oven on? It's typical for the oven to overshoot when first heating up, even at 1900F setting. Give a while to settle down - might even take 15 to 30 minutes for a good solid temp. Once settled in my Evenheat oven will hold within a couple degrees of setpoint. BUT - it does require time to settle in. Also, ramping up to temp slowly will prevent some overshoot. I always set to max heat, then let it settle down.

I've never used my oven for tempering, except for backsprings where the tempering temp is around 1,000F to 1100F.
I wanted to see how quickly it would reach 400• so I used the Max revamp up setting.
Any reason you don’t use the oven for tempering? Just curious.
 

timgunn

Well-Known Member
#7
The main reason for not using the same oven for Austenitizing and tempering is the amount of time it takes to cool from Austenitizing temperature to tempering temperature. The bricks hold heat for a long time, so you can't just switch it off, open the door and wait until it shows tempering temperature. If you do, when you close the door, the temperature will gallop up as the heat soaks back out of the bricks. Cooling properly takes several hours and few of us want to have glass-hard blades hanging around that long.
 

KenH

Well-Known Member
#8
Tim's right on the slow cooling point. I've got a toaster oven with PID control I use for tempering blades, anything under 400F is just fine and holds ±1F just fine. As mentioned, I do use the EvenHeat oven for tempering backsprings because of the high heat requirement to drop Rc down to mid 40s.

Paul, now see how long it takes your oven to reach 1500⁰F from a cold start. Then watch it once it reaches 1500⁰F and see how it takes for it to stabilize at 1500 and how much swing you have after 10 to 15 minutes.

You gonna enjoy that oven!!

Ken H>
 

Self Made Knives

Well-Known Member
#9
I wanted to see how quickly it would reach 400• so I used the Max revamp up setting.
Any reason you don’t use the oven for tempering? Just curious.
Ditto the other guys. I've got the KO version which I think is insulated better, so it takes forever to cool down enough to temper. I've got an accurate thermometer and a big piece of slate in a toaster oven for tempering. The slate helps average out the on/off temps.
 

Ben Sellers

Well-Known Member
#10
Hey Opaul, you getting your oven is what made me "have" to have mine. I bought the KH18 with solid state electronics, Genesis controller, and BB mod. It doesn't seem to overshoot temps. I can see the power output on the screen and it reduces power as it approaches the target. To get to 400 degrees, it doesn't get above 50% power. It landed on 400 on the dot today without overshooting. Even though its a 110v, it gets to 2100 degrees fine assuming you have enough time. I only have 1 oven so I plan to cryo my blades between heat treat and temper, even the ones that may not benefit much. Its got to be somewhere and I have a dewar of ln2 just sitting there.
 

timgunn

Well-Known Member
#11
If you look at the graphs above, the "control" thermocouple which was the one also connected to the PID controller and therefore the temperature that would be displayed did not show overshoot at all. The other 3 thermocouples were .060", .020" and "very thin": my best guess would be around .010". They were intended to at least partially simulate the effect of radiative heating from the elements on the bevels of a blade, though I used the thermocouples I had to hand, rather than sourcing the best ones for the job. My expectation was that overshoot would become an increasing problem as the thickness reduces, making the overshoot/overtempering worst at the cutting edge. My reading of the data is that this seems likely to be the case. The 1.5mm/0.060" thermocouple reading shows the most overshoot and it "should" not, if my expectation is correct. I never got the opportunity to rerun the test with known good thermocouples.

The point I am trying to make is that the controller will tell you what is happening at the measuring junction of the control thermocouple. This is not necessarily the same as what is happening at the edge of the workpiece.
 

JJB11B

Well-Known Member
#12
Would you slow down and let me catch up please?! LOL I JUST GOT TO USE MY NEW FORGE and now you get an EVENHEAT?! just kidding.
Congratulations
 
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