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Coil Spring Knife Temper Gone Wrong?


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Hi,

I was tempering my coil spring knife in the oven, and looked at some heat treat charts. I saw that a straw to dark staw was between 400-450F. So I put my knife in the oven for two hours, from 375, to 450 for a bit, then finished off at 415. The tang was perfect golden, but how come the edge was purple? The blade should have gotten no hotter than 450, and that temp is below dark straw. What could be the issue? Is it maybe because I left scale on the knife, so it is playing tricks with me? Will my knife be too soft now since it got purple by the edge? In theory, just because the edge is thinner does not mean that it should get any hotter than the rest of the blade correct? And my oven was on convection, to get a nice even heat. Thanks for the help

Edited by Matt Behnke
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To be honest, it is hard to say. It could be a few things. There could have been residual oil on the blade or some other chemical that would have cause discoloration like that.

But, if there was none of that, then it is possible that your oven got to hot. The oven works to stay hot, and a convection oven just uses a fan to blow that hot air around to try and eliminate hot spots. But it isn't perfect. I turn on my oven and leave it on for at least 30 minutes to try to have the oven at as even a temp as possible.

Also, are you sure that your oven is accurate? Do you use an oven thermometer? Get two. Use them both at the same time. Check them after the oven is at the temp and if they both match the oven, then you are good.

Edited by Wes Detrick
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Thinner steel will of course gain heat faster; less mass, and so if the oven is too hot then that could explain it. However if I'm correct, the coloring is also built up over time as well as temperature. The heat color is due to an oxide layer after all, which grows thicker by temperature over time. Hypothetically if you start heating at 1:00 in an oven at 400 F, if at 4:15 the edge reaches 400 and gets straw yellow, but the spine is 250 and has not colored, then some time later the spine will have gained straw yellow, while the edge having a head start develops a deeper layer, showing blue.

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The blade edge could have also been hit by direct IR and gone a bit past the target temp as oven thermometers don't measure that, that is why I wrap mine in aluminum foil shiny side out (it also reduces the chance of letting off fumes into my oven). But more likely it is what Caleb said, but the aluminum will add more mass and cut it off from atmosphere

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Some thoughts for the record. Your oven is going to have an internal temperature swing, on the order of +/- 30 deg F or more. This is due to how they work. It's gonna heat up, sense that it is above the set temperature (by some amount set forth by the oven manufacturer) it is the gonna turn off until it falls below the set temperature by the same amount, turn on and then rinse and repeat.

 

Secondarily, you have the issues with how accurately the oven senses the set temperature. 450 deg F may really be 420 or it could be 480. Add these all together and your oven may be up to 60 degrees off or so.

 

Your spine, being thicker is gonna better deal with this temperature swing than the edge will. It has more thermal mass. I have a big cast iron skillet that I use as thermal mass. Others use pans filled with sand to help even out the thermal swings and to not overheat the blade.

 

I can't stress enough though, get an oven thermometer.

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Another thing people do is use some thermal mass to even out the temperature swings that the blade sees. Putting a dish with sand in it in the oven, let it get up to temp, then put the blade into the sand. While the air in the oven will swing as noted above, the greater mass will be more constant.

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