proper kiln use with knives

samuraistuart

samuraistuart

Joined
Dec 21, 2006
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I recently acquired a glass kiln, top loading, elements on the lid. Not ideal, for sure, but somewhat better than a paint can forge. The temp is accurate, within 15 degrees anyway, and I have a pyro/thermocouple to check it often. I've searched but can't find what I'm after. It does take a while to come up to 1500, maybe 30-45 minutes. But the temp is regulated really well. Is it OK to place a blade into the kiln cold, and then turn it on and let it come up to 1500F (would take about 45 minutes)? Besides decarb and scale to worry about, is the steel affected in any bad way by slowly coming up to austenitizing temps? I'm so used to placing a cold blade into a hot forge and watching it come up much quicker than this kiln. It would be nice to be able to place it in there cold, instead of bringing the kiln up to temp, opening door to place blade in, and have to wait to come back up. Not a big deal, but it's nice to avoid opening that lid. Only carbon steel like 1080 1084 52100 15n20 W2 CFV are being used. Thanks in advance for your suggestions!!!!

I know this has got to one of those questions that everyone else is saying, "This is too easy to even bother answering". But I seriously don't know if bringing a knife up slowly is bad or not.
 
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Some people allow their blades to come up to temperature with the kiln, some prefer to put the blade in once the kiln is already hot.

I prefer to allow my blades to heat with the kiln personally. I actually restrict the rate at which the kiln comes up to temp to around 900ºF per hour, as I've found that spending more time warming up before the soak starts yields higher rockwell hardness in the finished blades, without sacrificing any toughness as far as I've seen. (This is for A2, there likely won't be the same effect with eutectoid plain carbon steels like you're using)

I'd recommend trying both ways and seeing what you like, though I'm guessing in your kiln it will be safer to start from cold.

EDIT: I just realized you said you were using plain carbon steels. If you're going to let the blade heat up with the kiln you'll have to make sure it has adequate protection from oxygen to prevent decarb. I've used an anti-scale compound (ATP-641) on O1 steel in the past for this purpose and it worked well. If you're not going to use any anti-scale compound on your blade, then you want to minimize the time that the blade is exposed to heat and air, and so it might make sense to put it in once the kiln is pre-heated...
 
Aaron, thanks so much. I believe you answered my question. My main concern was heating too slowly will screw up the heat treat. I assume that's not the case. Decarb was my next concern, and if I am going to put it in cold, I will need to cover it with some anti-scale. I've heard that a thiiiiiiiiiiin wash of satanite does the same. I have some of that.
 
Let your kiln come to temperature and equalize before putting your blades in. No matter how slow you ramp up, unless your elements are shielded, you are overheating the steel if you let them heat with the kiln. Everybody(myself included) who has tested this comes to the same conclusion. I recently calibrated mine again with a two channel digital pyrometer and saw spikes of almost 200F where the kiln's readouts never went past a 25F variance as it reached temperature.
 
Rick Marchand said:
Let your kiln come to temperature and equalize before putting your blades in. No matter how slow you ramp up, unless your elements are shielded, you are overheating the steel if you let them heat with the kiln. Everybody(myself included) who has tested this comes to the same conclusion. I recently calibrated mine again with a two channel digital pyrometer and saw spikes of almost 200F where the kiln's readouts never went past a 25F variance as it reached temperature.
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Rick, out of curiosity, was the pyrometer inside a bar of steel, or attached to it, or in open air? I ask because I assume that the steel itself will have thermal mass, and won't cycle up to the same temperatures that an un-shielded thermocouple would.

I'd be curious to read through the thread on the subject if there is one! I've had good results from what I'm doing so far, but that's not to say that there isn't room for improvement!
 
The radiant energy from the coils can raise the steel temperature much faster and higher than the chamber air temp. The refractory body will slowly catch up, and the blade will slowly equalize to that temperature... but by then the blade may have gone several hundred degrees past the target and stayed there for a good length of time.
 
bladsmth said:
The radiant energy from the coils can raise the steel temperature much faster and higher than the chamber air temp. The refractory body will slowly catch up, and the blade will slowly equalize to that temperature... but by then the blade may have gone several hundred degrees past the target and stayed there for a good length of time.
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So on my evenheat kiln there is a ceramic jacket around the thermocouple, I thought that part of the job of that jacket was to help account for the directly radiated energy from the coils, is that not so?

I'm very curious about this topic as obviously any uncontrolled factors in my heat-treat will be doing things that I'm not accounting for.
 
The ceramic jacket shields the TC from the direct radiation and thus provides a more reliable reading. The ceramic does not absorb heat rapidly, and heats up as the chamber does. This transfers to the TC tip and the reading is closer to the actual chamber temp than it would be if the TC was openly exposed.

All this is why it is very important to fully soak the oven/forge refractory and chamber to the desired temperature for a while BEFORE putting in the blade. I get a chuckle when folks say, "My new forge gets to 2000°F in less than one minute. What they are reading is the flame. The chamber will take a good while to heat up to 2000°. To check the chamber temp, I always shut off the gas and read only the radiant temperature of the refractory. That is one of the advantages of PID control of the burner or oven coils....it reads the off temp to turn the gas/power on...and the running temp to turn the gas/power off. This prevents any great swing in temperature ONCE THE REFRACTORY IS SOAKED. The program of the PID should account for most of the swing problem once at temperature, but at start-up, the radiant energy absorbed by a blade can be much higher than the kiln shell and TC are absorbing.

If you wanted to test it, tightly wrap a TC to a blade with binding wire and set the blade on a blade rack in the cold oven. Connect the blade TC to a reading device ( PID or meter). Set your heating rate to 9999 ( full heat) and watch the oven temp readout compared to the blade temp readout and see how much they vary as the oven heats up. It may surprise you how much hotter the blade gets in the beginning. Depending on the oven's program and the oven's refractory type, the swing can be a lot. Once the oven is fully soaked, the blade and oven readings should be about the same all the time.
 
bladsmth said:
The ceramic jacket shields the TC from the direct radiation and thus provides a more reliable reading. The ceramic does not absorb heat rapidly, and heats up as the chamber does. This transfers to the TC tip and the reading is closer to the actual chamber temp than it would be if the TC was openly exposed.

All this is why it is very important to fully soak the oven/forge refractory and chamber to the desired temperature for a while BEFORE putting in the blade. I get a chuckle when folks say, "My new forge gets to 2000°F in less than one minute. What they are reading is the flame. The chamber will take a good while to heat up to 2000°. To check the chamber temp, I always shut off the gas and read only the radiant temperature of the refractory. That is one of the advantages of PID control of the burner or oven coils....it reads the off temp to turn the gas/power on...and the running temp to turn the gas/power off. This prevents any great swing in temperature ONCE THE REFRACTORY IS SOAKED. The program of the PID should account for most of the swing problem once at temperature, but at start-up, the radiant energy absorbed by a blade can be much higher than the kiln shell and TC are absorbing.

If you wanted to test it, tightly wrap a TC to a blade with binding wire and set the blade on a blade rack in the cold oven. Connect the blade TC to a reading device ( PID or meter). Set your heating rate to 9999 ( full heat) and watch the oven temp readout compared to the blade temp readout and see how much they vary as the oven heats up. It may surprise you how much hotter the blade gets in the beginning. Depending on the oven's program and the oven's refractory type, the swing can be a lot. Once the oven is fully soaked, the blade and oven readings should be about the same all the time.
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Thanks for the thorough response Stacy!

I'm curious: if you replaced the ceramic shroud around the thermocouple with one made from a high-temp stainless (say 310) would that make the kiln track the temperature of the blades better?
 
It is not the thermocouple that is the problem ... it's the radiant heat from the coils heating the thermal conductive metal(your blades and the TC). Think of a wrench sitting on a picnic table in the hot Sun. The air might be 90F, the table 95-100F maybe ... but the wrench could get to 120-140F easily.

If you could shield your blades from the coils(or vice-versa) it would be much better. When I temper in my kiln, I use an overturned long tin bread pan to shield the blade from the radiation.

When I did my tests, I had my kiln's TC with digital readout, an analog pyrometer/TC, another 2-channel digital pyrometer with one TC reading the ambient air and a second shrouded wire TC touching a 1/16" thick piece of 1" x 8" 15N20. That is four independent readings. I ran the experiment seven times and averaged the results. Then I was able to set my programs accordingly.
 
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Rick Marchand said:
It is not the thermocouple that is the problem ... it's the radiant heat from the coils heating the thermal conductive metal(your blades and the TC). Think of a wrench sitting on a picnic table in the hot Sun. The air might be 90F, the table 95-100F maybe ... but the wrench could get to 120-140F easily.

If you could shield your blades from the coils(or vice-versa) it would be much better. When I temper in my kiln, I use an overturned long tin bread pan to shield the blade from the radiation.

When I did my tests, I had my kiln's TC with digital readout, an analog pyrometer/TC, another 2-channel digital pyrometer with one TC reading the ambient air and a second shrouded wire TC touching a 1/16" thick piece of 1" x 8" 15N20. That is four independent readings. I ran the experiment seven times and averaged the results. Then I was able to set my programs accordingly.
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Yes I understand. What I was trying to get at was: if you were to couple the thermocouple of the kiln controller to a blade like you did with your pyrometer in the experiments, then theoretically the controller should work to control the temp of the steel, instead of the temp of the refractory.

I understand that this may not play well with the PID parameters for the kiln controller. When I suggested changing the thermocouple shroud to match the steel I was thinking that might be a way to get the kiln controller to 'see' the temp of the steel instead of the temp of the refractory. Hope I'm explaining that in an ok manner.

How do they shroud the thermocouples in commercial vacuum furnaces? They rely on radiative heating for most of the cycle, so I assume they must have some mechanism for dealing with this...

Also: I assume that heat-treat foil must offer some kind of shielding as it has to intercept and then re-radiate the heat from the coils. I'm curious as to whether that has a noticeable effect or not, do you guys have any info on that?
 
I thought this was more complicated than just "put it in there and turn it on". I'm thinking now to let the kiln come up to temp, and then place blade in, and just be careful. I do NOT like the top opening lid with the elements in the lid. What I have been doing since I got it (two heat treats), is unplug it immediately before opeing the lid. No risk of shock that way. Like Stacy mentioned, it might be a good idea to minimize the amount of time the blade could be affected by temperature swings because of the radiation. I thought I had asked something really stupid here. Seriously.
 
I have a rule that I'm learning to accept and live by:

"Everything is more complicated than it seems."
 
aarongough said:
... then theoretically the controller should work to control the temp of the steel, instead of the temp of the refractory.
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The goal is to have the steel you are heating and internal temperature of the kiln in sync. Everything struggles to be in equilibrium and there is a teeter-totter effect until that happens. Even after your kiln has equalized, once you open the door, the struggle begins again ... though, to a smaller degree. It is also important to have thermal mass. The more heat you can retain, the better. That's why you don't see many kaowool kilns ... castable refractory holds heat better, kaowool insulates from it.

I'm sure the stainless foil shields the blade to a certain extent. It would be better to have some pace between the two but that isn't the purpose of the foil to begin with.
 
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