kilns and soak time

samuraistuart

samuraistuart

Joined
Dec 21, 2006
Messages
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A gentleman on another forum posed a question that hinted at something I have been contemplating. I've always heard it said "Start the soak time once the thermocouple has rebounded to your target temp." Should we actually wait a bit longer? My theory is the thermocouple will register 1475F much faster than the blade reaches 1475F. How much faster? Probably depends on the kiln, the amount of atmosphere, the thermocouple, etc. But shouldn't we give it a bit more time to come up to temp? If we need a 10 minute soak on a bowie knife, and the readout reaches back to 1475 within 30 seconds, the blade itself isn't 1475F in 30 seconds. It might take a couple of minutes MORE to reach an even 1475F, so then our soak time is actually reduced by a few minutes. And that may be the answer....two or three minutes isn't a big deal in the scheme of it all.
 
Well it kind of depends. IF your thermocouple is close enough or touching the blade, it should be reading very close. The thermal mass from the blade will be absorbing energy from the surroundings, aka the blade and the kiln will be rushing to meet each other at an equilibrium temperature. Once that happens then they will rise to the set temperature together. Kind of like boiling water in a paper bag. For them to be wildly different you would have to be putting in an incredible amount of energy somewhere.
 
After the kiln has equalized and I've put the blade in, I watch the read-outs. They will overshoot the mark as it ramps back up to temp... then cool back down and do a couple back-and-forths a few degrees above and below my target. When the temperature holds for more than just a few seconds, I start my timer. I always read the temp on the "down"(when the elements are off). I calibrate my kiln the same way. As the temperature falls, you are getting the true reading of the forge cavity, without interference from the radiating elements/coils.

In my kiln, there is a temperature difference of about 35F between my two TCs. One is more sensitive than the other. But, on the cool down, they are pretty much in sync with each other.
 
Good info Rick.

For practical purposes, a piece of steel the thinness of a knife blade will heat up and cool down pretty close to in-sync with the TC and kiln body. Is it exactly the same in any 1 second period ? ...no. If you start the 10 minute soak when it says 1475F will it be perfectly OK? ...YES.

Just let the temp restore to the target and start the timer. If you have a programmable kiln, that is how the programming work. If you are crazy obsessive, program the kiln for a first step of 1400F and hold. Place the blade in and hit "skip" or "next". The kiln will rise to the set temp for the next step ( say, 1475F) and start the timer ( set for Hold-10). The blade will be pretty much at the same exact temp as the TC when the time count starts.
 
For info here, when placing a blade into my pre-heated oven - I always wait 1/2 hour for temp equalizing - I use a high-temp laser thermometer. What I have discovered is that a medium sized blade takes about four minutes to reach to reach the temp eqaul to the oven when austenizing.
Of course, the cutting edge comes up quicker, but the main body of the blade takes that long.

Oddly, when tempering, and the oven is only at around 400F, it takes as long as 20 minutes for the blade to reach that same temp. So I always add 1/2 hour to my tempering times.
 
Good info Rick.

For practical purposes, a piece of steel the thinness of a knife blade will heat up and cool down pretty close to in-sync with the TC and kiln body. Is it exactly the same in any 1 second period ? ...no. If you start the 10 minute soak when it says 1475F will it be perfectly OK? ...YES.

Just let the temp restore to the target and start the timer. If you have a programmable kiln, that is how the programming work. If you are crazy obsessive, program the kiln for a first step of 1400F and hold. Place the blade in and hit "skip" or "next". The kiln will rise to the set temp for the next step ( say, 1475F) and start the timer ( set for Hold-10). The blade will be pretty much at the same exact temp as the TC when the time count starts.
 
Thanks so much for all of that input! I get a little too nit picky sometimes and start to over-analyze stuff. KISS....I need to remember to do that!
 
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