Random Temper Colouring

Rick Marchand

Rick Marchand

Donkey on the Edge
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Jan 6, 2005
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Every once in a while, I find my knives turning brown/blue during my temper cycles. It is blotchy and I suspect its from quench oil residue. Sometimes after the quench, I will just run the blade across a scothbrite wheel to expose the metal and throw them into the oven. I'm sure they are just oily. Also, if I don't clean the blade after the first temper, it will turn brown/blue.

I use 5160, 1070 and 1080.... I keep my temper oven set at 400F and let it stabilize before I put anything in it.

Should I be concerned and spend more time cleaning the blade prior to tempering?

Rick
 
Rick, I understand that are a number of factors which will influence the "heat coloring" of a blade. Beyond that, I can't say.

I do know my knives come out of the oven a nice uniform straw color (shade varies depending on the temp I use), and I don't clean them beyond just wiping them off (or occasinoally rinsing in water). I use my kitchen oven, soaked at heat for an hour, with a baffle to prevent radiant overheating, and with two seperate oven thermometers placed at the level of the knife. My kitchen oven runs about 20-30 degrees hotter than it reads.

What oven are you using? You may want to try some sort of baffle if the blades are in proximity with the elements.

--nathan
 
Rick, I also have had almost identical results as Nathan. Nice even straw color. I think ideally that's about 400 degrees if all is well and steel is uniform and clean. Blue might be something too hot but as Nathan said, a number of things could influence that.

I've noticed that even oil from your hands can make brown, purple and blue spots during heating. You're probably right about just being oils of one kind or another.
 
You think your getting color try some D2 or W2, the temper color threw me the first time I did either of those steels.
 
+1 on the kitchen oven temps.

Mine will run a steady 400 degrees when the ovens temp is set on 355 degrees.

400 setting was giving me 475 degrees actual

-Josh
 
If I don't clean blades in between tempers they get darker and darker each time. Also length of time in a single temper will cause a darker color.

Colors on the blade are simply oxides (rust) and I believe are quite an unreliable way to judge temper of a steel, really meaningless unless you are using a torch to temper. Use a good baffle/muffle system in your oven and a thermocouple to read temps.
 
Don't worry about it.
Judging steel temps by color only works in a perfect world/environment.
Those colors can be effected by humidity in the kitchen!
Just trust your thermometer.
 
Well I'm covered on temperature control.... although the baffel idea is good... here is my set up. I have alarms for temperature spikes. There is a 15deg difference between the frontand the back and it fluctuates +/-5degs.


080819112252.jpg


Rick
 
You could use something like a tray of sand or a brick that you place your knife on after it has come to temp. That will give you more thermal mass and maybe help reduce temperature swings, though 5 degrees isn't bad.

--nathan
 
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