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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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Old 03-26-2014, 09:58 PM
LABlade LABlade is offline
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Furnace Temperature Variances

I've got an old (1992) Vcella heat treating oven that the analog controller went out on. I always wondered what the true variance in high and low temperatures was because you can't easily tell with an old analog scale with 20 degree graduations. I replaced it with a digital controller that's reportedly accurate to within 1/4 of 1%. The module simply controls a 2 pole switch that turns the elements on when it calls for heat and turns elements off when the set point is reached, and it just cycles in and out this way. Now with the digital controller and readout I can accurately see what's happening. As a test, I set it at 1920 deg. F, let it reach temperature and soak for an hour, then recorded 50 cycles of the elements turning off and on and recorded the high and low temps. When the elements turn off at 1920 deg. the temperature still climbs to an average of 1931 deg. before starting to come down. When the elements activate again at 1920 deg. the temperature will still drop to an average of 1906 deg. before starting the climb back up. The total lag time between the high and low temperatures averaged 41 seconds. That's a 25 deg. variance every 41 seconds. My question is, is that too far out of range for reliable heat treating? I've been using mostly CPM 154, 154 CM and ATS 34 and they have tested 60 to 61 Rockwell after tempering BUT is this enough of a variance to cause unseen problems WITHIN the metal, i.e., regarding toughness, edge holding, etc.? With the old analog controller I now see that I've been heat treating about 30 degrees HOTTER than I thought, but have routinely tested blades using the brass rod test, cutting into heavy cardboard, sisal rope, wood, and leather, and the blades have performed well.


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Old 03-28-2014, 10:53 AM
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Gary Mulkey Gary Mulkey is offline
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Les,

25 degrees won't make a huge difference. I wouldn't be overly concerned about it.

Some of this variance may be the thermocouple or thermocouple placement in the oven. They aren't overly expensive. You might try replacing the thermocouple to see if that makes a difference.

Make sure that you place the blade(s) in the center of the oven and away from the elements as much as possible. This will help with the temperature variance. Also, use a 20 minute soak time and make sure that the oven is completely up to temperature before placing the blades inside (let the empty oven stay at temp for a while before placing the blades inside so that the entire oven is up to temp).

Gary


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Last edited by Gary Mulkey; 03-28-2014 at 10:57 AM.
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back, bee, blade, blades, brass, digital, edge, heat, hot, knife, leather, make, metal, pole, rod, scale, toughness, wood


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