Tempering question

Daniel W

Daniel W

Gold Member
Joined
May 1, 2019
Messages
672
Good morning everyone, I was tempering a 1084 blade last night and when I pulled it out after the second cycle it has a hint of blue at the cutting edge. Should I re do the heat treat or will this be ok.

Danny
 
You probably should. You also should not rely on colour. How did you temper the blade? Did you put it in a tray with sand or on a firebrick? Did you wait for the oven to be at temp before you put the blade in?
 
When i used to temper at 400 in a toaster oven it always came out blue. Once i built my fancy temper oven (can austenize in it if i want to, it serves as a backup HT oven and i can easily do spring tempers without worry) i got consistent amber straw color at 400. Take that to mean whatever you want, just my experience.
 
Thanks Randy I have not seen this before I posted a picture like I said it’s just this hue at the edge
 
Do you sand the scale off or anything before tempering? Your pic looks like you don't, but if you did then that is pretty dark temper color!

What temp do you temper at? And have you verified this with a thermometer?
 
No Randy I did not clean it completely as I wanted to get it into tempering while it was still warm.
 
Randy I had it set at 395° F as that has been where most of my other knives were done at.

I’m a geographical bachelor due to the .mil and in a small apartment so I have to use my regular oven. I can’t seem to find a used toaster oven on the cheap.
 
I've heard of folks that use home/toaster ovens putting their blades in sand to shield the blade from overshooting the temp on the thinner parts of the blade during the heat cycles. I've never done that but it sounds like a cheap fix to temp swings.
 
If the colors are not on a bright clean blade but rather observed over forge scale it might just be a surface contaminant like quench oil residue. Do you put a couple of rack type thermometers in your oven with the blade to verify temps? Not that they are particularly accurate but it's another point of reference to see if it agrees with your oven. I would be more worried by the sharp angles of your jimping and the stress risers they may have created.
 
i4Marc said:
If the colors are not on a bright clean blade but rather observed over forge scale it might just be a surface contaminant like quench oil residue. Do you put a couple of rack type thermometers in your oven with the blade to verify temps? Not that they are particularly accurate but it's another point of reference to see if it agrees with your oven. I would be more worried by the sharp angles of your jimping and the stress risers they may have created.
Click to expand...
Marc, thanks for the help the jimping was rounded with the file as I was cutting them I normally use a round file but wanted to try something a little different. I do use a digital temp prob in the oven never saw it go above 400. Guess I never thought of the scale changing colors.
 
Ignore the color of a slow tempered blade. They can be any sort of peacock color … and mean little to nothing in regards to hardness.

Use time and temperature to determine the temper.
 
I recently did a temper test to verify what I thought I had been seeing. Quenched blade was ground thru decarb and acetone then alcohol to make sure it was clean. 400f for 2 hours gave a light straw color. The next cycle was a deeper straw color. The third cycle showed a little blue on certain areas. Temp inside oven was verified accurate with 2 thermometers.

Temper colors are not to be used to verify temps.
 
The colour is due to thin film interference in a thin layer of iron oxide, with different thickness giving different colours. The thickness is determined by both temperature and time. So with the sort of tempering we are doing you can get different colours with longer tempering cycles.
Using it as an indicator is really only useful for torch tempering

Tldr: what Stacy said
 
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