Issue tempering a Nicholson file knife

[Red's Edge]

Hi there,

I forged a blade out of a file and hardened it. I planned to temper it in my toaster oven at 425 degrees for 1 hour, let it air cool and put it back in for another hour at 425.

I let the oven get up to temp via an oven thermometer, let it sit for a while to make sure it was staying at 425 because in the past it has gone up and down, so I check it constantly to make sure its about 425 and adjust the knob accordingly.

I placed the blade on the rack and let it sit for 15 minutes and when I checked it, the thermometer was reading around 550, so I immediately took out the blade and ran it under the faucet to cool it off. The color is a dark straw all around.

I asked a fellow bladesmith I know and he suggest I pop it back in for another hour at 400. I don't think that's a bad idea, but I'm really not sure what to do in this circumstance. I read briefly another thread saying that it is very important that the heat radiate through the steel, so perhaps only the outside of the blade has been "cooked" like a seared steak?

Thanks for your help!

Red's Edge

 

[Drew Riley]

At the very least, the edge may have reached 500 to 550 before you were able to pull it out. If you're worried at all, why not just re-heat treat the blade? At worst, you're out a couple hours of time and a little bit of gas; at best, you've restored a proper treat and temper to a blade that you'll likely invest several more hours into finishing.

All said, it might be ok as is, or it might not. We really have no way of knowing without a hardness tester. Better safe than sorry I say, and if anything, you get a little more practice heat treating.

 

[Drew Riley]

Out of curiousity, what kind of toaster oven do you have, and where are you placing the thermometer?

 

[Karl B. Andersen]

Test the edge.
A good temper on a blade should exceed well over an hour - 2 times.
If it was only at 550 for even a brief time, little took place to hurt the blade, if, in fact, you did your job well when hardening. That is very key here.
Keep in mind that the tempering temps we so often refer to are relative to the blades and steels we're using.
For those temps to have any meaning, they must be used on known steel with accurate forging, post forging, thermal control, proper austenizing temps and quench media.
If you don't have all those elements in order, don't sweat the small stuff.
If your file knife is still a bit hard - it's a good knife. :thumbup:

 

[Red's Edge]

@knife to a gunfight I think you are right. That's probably the best idea. I'm using a second hand Hamilton Beach toaster with the thermometer a little further back from the middle on the grill.

@Karl B. Andersen The only thing for sure I can say is that I quenched it in Parks 50 oil with M.S. J.D. Smith's assistance. It is still rather hard, but because of all the unknown variables, I think it'd be safe to say that it's worth re-heat treating.

Thank you both for your time and wisdom.