Revealing Kiln Test.

Rick Marchand

Rick Marchand

Donkey on the Edge
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Hey Folks!

I have been talking about the quirks of electric kilns and conventional ovens for a while now. I thought it was time I learned just how serious it was. Two days ago, I placed one 1/8" x 1" x 4" pc of 1095(ground to bare metal) in my digitally controlled Sugar Creek kiln. I place four tabs from different Tempilstiks(1500F, 1600F, 1700F and 1800F) directly on the steel. Next to it, I placed another four tabs on a piece of broken firebrick. The controller was set to full ramp to 1500F and started cold. I let it equalize to where the temperature was fluctuating by only +/-2or3 degrees.(the maximum thermocouple reading was 1558F) When I opened the door I was shocked to see all four tabs melted on the steel and all but the 1800 melted on the firebrick.

Yesterday morning, I ran the same experiment but approached the ramping program differently. I set the controller for full ramp to 1400F, hold for 10mins, ramp of 300F/hr(or about 75F per 15mins) to 1475F. The thermocouple never went past 1484F. This time the 1500F tabs and 1600F(on the steel only) was melted, which told me one or more of the following...

1.The whole cavity was overshooting the mark.
2.The steel was overheating
3. My Tempilstiks were inaccurate

Lastnight, I gave it another go. This time, I added 1450F Tempilstik tabs and ran a slightly different program. Full ramp to 1250F, hold 10mins, ramp 200F/hr (50deg per 15mins) to 1475F. Everything went well this time. The kiln didn't go past 1479F. Both the 1450F tabs were melted(steel and brick) and all the rest were intact. The slower ramp did the trick.

Conclusions:

1. Your oven will overshoot by HUNDREDS of degrees at full ramping speed!:confused:
2. The steel gets hotter than the cavity during ramp cycles. I can only attribute this to radiant/infrared heat from the coils.
3. Having recessed/hidden coils doesn't make much of a difference, apparently.:rolleyes:
4. A slower ramping program will eliminate the excessive overheating, but it is still apparent that the coils have an effect on the bare steel.
5. Allowing the temperature to equalize a few hundred degrees below the target then ramping up at 50F every 15mins(200/hr) eliminated any chance of overheating.:thumbup:
6. Having a digitally controlled electric kiln doesn't mean you can nail heats relying on the technology, alone. Infact it can give a false sense of security with potentially disasterous results. KNOW YOUR KILN.

All this testing was done in my 24" Sugar Creek Big Knife Kiln. I'm interested to see how the more popular Evenheat and Paragon kilns perform in this kind of testing.

**ETA: Important afterthoughts not in original post**
Normally, I DO allow my kiln to equalize BEFORE I put in my blades. This test was set up to reflect the temperature spikes during ramp up proceedures and to validate the infrared/radiant heat scare that a few of us have been trying to get across to folks.

It is also important to note that the Tempilstiks can melt BEFORE the piece they sit on gets to temperature. I found this out while using the T-stiks to "calibrate my eye" when using an open forge. I was originally pre-marking the steel and gaging the temperature by when it melted.... that was the wrong way. I needed to apply the marker to the steel OUT OF THE HEAT SOURCE to get a true reading of the steel itself, rather than reading the forge.

The experiment in this thread was more about getting to know my own kiln than nailing the temperature for a good heat treat. If you have a kiln, I strongly urge you to do your own testing. Tempilstiks are easy to find online.

Rick
 
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Hello Rick,

I have seen the same results over the years. All was solved for me by perfoming a full ramp to 1200F, let it settle in and have the firebricks get nice and even. Then ramp 100F at a time to desired temp. Usually wait about 5 minute between each 100F jump. Have never overshot my temps once adopting this method.

This is a great topic as my HT was very inconsistent until figuring this out...

Eric
 
Rick,
Thank you for doing these tests. I have been trying to tell people exactly what you just proved to no avail. Maybe now, with your data, they will realize the folly of placing a blade in a cold oven and allowing it to heat up along with the chamber. I believe that during heat-up to 1450°F, the blade may easily reach 1800°F on a high wattage kiln.
 
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bladsmth said:
Rick,
I believe that during heat-up to 1450°F, the chamber may easily reach 1800°F on a high wattage kiln.
Click to expand...

I can confirm that this is the case in my HT oven. Slow ramp up is key once blade is placed in stabilized chamber.
 
Rick, I have the same model kiln. When heat treating simple carbon steel I usually let it equalize for a few minutes at the desired temp before putting the steel in the oven, now I am wondering if I could be over shooting temps when the oven is trying to recover from having the door open. I just ordered some 154CM, I will have to do some testing before I do any heat treating with the new steel.
 
bladsmth said:
Rick,
Thank you for doing these tests. I have been trying to tell people exactly what you just proved to no avail. Maybe now, with your data, they will realize the folly of placing a blade in a cold oven and allowing it to heat up along with the chamber. I believe that during heat-up to 1450°F, the chamber may easily reach 1800°F on a high wattage kiln.
Click to expand...


I would have totally agreed in the past Stacy but every one of my tests was done from cold and the second test showed that the internal temperature of the kiln stayed below 1600F. The steel however, exceeded 1600F but stayed below 1700F. This is why I think it has more to do with the infrared radiation, than the air in the chamber.

ETA: I just reread your post and totally missed the "high wattage" reference.... yes, I believe you could easily hit those temps with a more powerful kiln.

BTW... I had to edit a couple temps in my original post after reading through my notes. Nothing major... but a little different.
 
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rocketmann said:
Rick, I have the same model kiln. When heat treating simple carbon steel I usually let it equalize for a few minutes at the desired temp before putting the steel in the oven, now I am wondering if I could be over shooting temps when the oven is trying to recover from having the door open. I just ordered some 154CM, I will have to do some testing before I do any heat treating with the new steel.
Click to expand...

I think you'll be fine. The steel takes several minutes to reach ambient... your kiln should be equalized and firing at less of a duration by then.
 
I have a 1/4" thick steel plate that wraps around the inside back of my kiln to help keep from overheating the tips of my longer blades (my oven is a 22.5 Evenheat).
The added thermal mass of the plate helps keep a more even and constant temperature in the kiln. I ramp at full speed to my desired temp and let it sit for about 10 min. before even putting my blades in, I am fairly new to the "kiln world" but this seems to work well for me.
Granted I am using more forgiving steels such as 5160 and L6.
 
Rick, thanks for starting a great topic, and performing an even better experiment. As a process and equipment engineer I've come across very similar situations, where established protocols wouldn't work with new ovens or hot die bonders. I applaud your intuition, approach, conclusion and sharing of all.
Best,
Steve
 
After reading some of Stacy's posts in the past and reasoning it out in my head, I usually set my oven to either a pre-soak temp or full critical temperature and let it sit for at least 30 minutes after it hits the target temp. This is probably overkill, but I'm usually working on something else at the same time, so it hasn't really slowed me down too much in the past.

Letting the oven have a pre-soak allows the entire oven's thermal mass come up to temperature and stabilize. We have to consider that heating elements are an on/off proposition, and when they are on in most ovens, they put out well in excess of 2100F. When the oven is cold, the coils are on for a longer period of time as they attempt to heat the oven's interior and mass. After the oven is up to temp, the coils cycle on briefly and with longer "off" times, thus limiting the radiant over-heating.

--nathan
 
Another great post Rick! Thanks for documenting the experiment. my question is not so much where the thermocouple sits, but is it in a ceramic sheath? If I recall Nathan had a slower response on his PID controlled forge until he accidentally broke the ceramic cover off. Maybe the same effect?
Any way you see it, this appears to be a good protocol for ramp times to start my own experiments when I get my kiln!
Thanks!
 
i use a fire brick baffle to protect the blades from direct radiation from the coils sept for the back of the kiln (i put tangs back there as i know they can easy be over heated )

if i had a 24 inch kiln i would have a baffle all the way around
 
butcher_block said:
i use a fire brick baffle to protect the blades from direct radiation from the coils sept for the back of the kiln (i put tangs back there as i know they can easy be over heated ) if i had a 24 inch kiln i would have a baffle all the way around
Click to expand...

How do you arrange the bricks for that Butch?
 
Interesting info. Rick. I have the same kiln. I always bring my oven from cold up to the temp. I plan to use, then put the blade in, let it come back to the proper temp., & then let it soak if needed. The reason I've always done it this way is to reduce the time the blade is in the oven therefore theoretically reducing scale. I guess I've unknowingly been avoiding overheating by doing it this way. I've been thinking about buying either tempilstiks or tempilaq. Does anyone know if one is better than the other for knifemakers and why? Great thread, thanks for posting.
 
Why doesn't the thermocouple (of the unit) pick up and display the overshoot???
- thanks!
 
Yes, Rick, I should have said "Blade can reach 1800". I have edited it.

As to why the temperature display doesn't show this over heating, there are two reasons:

1) The TC is often encased in a white ceramic sheath, which reflects infrared radiation, and reads the ambient chamber air. Air is not as easily heated by infrared radiation as a metal blade is. The TC is also placed in a position quite a bit away from the blade.

2) The PID which controls the program "learns" as it cycles. The on/off control gets closer and closer as it figures out exactly how long to fire the coils each cycle. Once it gets it figured out, the oven will maintain a very even heat. In the early parts of the learning curve, the PID is like most new drivers. It only knows two positions - "Accelerator Flat on the Floor" and "Hard Brake". During these learning steps, any small mass metallic object that can absorb the heat energy ( like your blade) may overshoot the chamber temp. The readout only shows the air temp. not the blade temp. Page Steinhardt has had to do studies in his metallurgical occupation where they drilled holes in a metal object and placed a dozen thermocouples directly in the object to see how much variation there actually was. It made the company change its HT regimen to allow for fully soaking the oven chamber BEFORE placing the metal object in for HT.
 
Rick, it would be very interesting if in your testing procedures had you tested stainless temps. Would you be willing to perform the same tests aimed at 1950° instead of 1450°? How many watts is your kiln?
 
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