Heat treating in kilns - or how to know the temperature of something you cant touch

S

SimonB

Joined
Jul 5, 2018
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6
Hi guys!

i'm fairly new around these parts, so sorry in advance for any dumb questions or wrongdoings.

anyways, thing is. i usually heat treat my knives in a salt pot, because for some reason building one of those was cheaper and easier than building an electric kiln. so usually i have no trouble with knowing when the steel reaches austenitizing temperature. i am currently making a blade that because of size constraints, i cannot heat treat in the salts, so i have borrowed access to a kiln that will fit said knife.

my question is then, when austenitizing in a kiln, how do you know when to start the soak timer? in salts this is simple, because the temperature of the salts will quickly match that of the steel. but in a kiln, how do you know when the blade is up to the set temperature? is it just a timing issue, I.E. it takes X amount of minutes to heat up the steel to 1500F, of can you rely on the thermocouple to be somewhat accurate.

sorry in advance for the rant, but my hour or so of googling didn't reveal an answer, might be that i'm just bad at it.
 
First off what steel if this blade made out of? Second most of us set the oven to our desired temp and give us a few minutes. Give it a quick peak and if the the same color as the inside if the oven then start the timer.
 
125sc, a plain 1.25% carbon steel with 0.58% manganese. so i dont have too much leeway on soak time.
 
When the readout reaches the set temperature, start timing. Most ovens are automatic/programmable, and don't start the soak/hold step until at the correct temperature.

In an oven not run by a controller, if you open the door and put the blade in the hot oven, then watch for the temp to return to the set point. When it is back, start timing.
 
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Stacy E. Apelt - Bladesmith said:
When the readout reaches the set temperature, start timing. Most ovens are automatic/programmable, and don't start the soak/hold step until at the correct temperature.

In an oven not run by a controller, if you open the door and put the blade in the hot oven, then watch for the temp to return to the set point. When it is back, start timing.
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This is a related question. For carbon steels like 8760, 80CRV2, 15N20, 1084, do you wait for the oven to reach desired temp and then insert blade, or can you keep them in there as the oven warms up to desired temperature?
 
I let my oven come up to temp, and let it sit for a while, so that the heat really soaks in. Then I put my blades in, wait for temp to come back up, then start my timer. When I open the door to put my blades in, temps usually drop a little over 100 degrees, depends how fast you are! My timer automatically starts when it reaches desired temp!
 
ashwinearl said:
This is a related question. For carbon steels like 8760, 80CRV2, 15N20, 1084, do you wait for the oven to reach desired temp and then insert blade, or can you keep them in there as the oven warms up to desired temperature?
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Either way is potentially fine, but leaving the blade in while coming to temp will exacerbate decarburization to some degree (thouh decarb only starts at around 1300, so depending on the rate of heating, it may not make a huge diference). You could possibly also overheat the blade from the radiant energy coming off your coils, depending on the design of your oven.

I put the blade in at temp but if you want to put it in to start, just test it on a cupon and see if you have any issues. I suspect that you wont.
 
ashwinearl said:
This is a related question. For carbon steels like 8760, 80CRV2, 15N20, 1084, do you wait for the oven to reach desired temp and then insert blade, or can you keep them in there as the oven warms up to desired temperature?
Click to expand...

definitely only put the blade into the already hot kiln. the oven firing on full blast is quite a different radiant heat from the oven just trying to maintain equilibrium. also, as a general rule of thumb, you want the heating speed to be as quick as possible.
 
SimonB said:
definitely only put the blade into the already hot kiln. the oven firing on full blast is quite a different radiant heat from the oven just trying to maintain equilibrium. also, as a general rule of thumb, you want the heating speed to be as quick as possible.
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Not untrue, but some kilns have shielded elements and you could always add a muffle or similar. Plenty of steels soak followed by a full power ramp to austenitizing temp. Most of these call for a SS wrap, but there is definately not a hard and fast rule that says you cant have steel in the oven when it is heating up.
 
well, he did specify a bunch of low alloy steels, none of which need a pre-heat. as you said, that is usually reserved for the air hardening steels, with their higher austenitizing temperatures. but i can definitely see why my above statement could cause confusion (english is my fifth language, so i do make those kinds of mistakes from time to time)
 
I am with you, I am just saying that if you do have some compelling reason to put the blade in to the cold oven, you should be able to work around the stated limitations and still achieve a good heat treat.

5th language? Impressive.
 
joedhiggins said:
I am with you, I am just saying that if you do have some compelling reason to put the blade in to the cold oven, you should be able to work around the stated limitations and still achieve a good heat treat.

5th language? Impressive.
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its a bit cheating, because danish, swedish and norwegian are very closely related. german and english we learn in school.
 
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