Heat Treating Old Chisels

[Timberwolf]

Well thanks for the info and now I am confused. I have 2 witherbys and one noname which afile wont even scrape the surface-not just the cutting edge the whole chisel. What do you suggest I do tomake thes chisels usable. Unfortunately I already have put handles on all 3. What can a do to anneal these chisels ?

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Just anneal the area one inch or so above the edge..Play the flame a couple inches above the edge and let the color the heat produces, flow towards the edge...as if you were using the flame as a brush..When the color turns straw yellow, it wont hurt to quench it, but it is not essential to do so. However, IF you set the handles into the tang using epoxy, I would definitely quench in water to cool it before the heat melts the epoxy.

EDIT...This process can be repeated if the hardness is not to your satisfaction.If you let the color go past straw yellow all the way to a purple/blue, the hardness should be gone and the chisel easy to file. The procedure is so much easier to do than it is to describe.

[adamcherubini]

Anneal is the wrong word. Annealing removes all hardness and changes the structure of the steel molecules. Annealing involves heating steel above its critical temp, which differs for different alloys, then cooling it very very slowly (sometimes as long as 24 hours). The critical temp of all steels is very high and cannot be achieved by waving a propane torch over it. You need a way to trap the heat. A clay pot, as Timberwolf suggested, could be one solution. Making a mini forge out of refractory materials is another typical solution. Just a word of caution: you can easily melt the brass tip of the propane torch attempting to generate forging temperatures using refractory materials. There is nothing easy or safe about generating steel melting temperatures in your garage with a propane torch.

To remove excess hardness post heat treating, the steel is heated to a much lower temperature and soaked for a period of time at that temperature then air cooled. This process is called tempering. To temper a chisel, remove your handles, degrease, and place your chisels in a pre-heated oven at 400F for 2 hours. Remove from oven and allow to air cool. They should be light straw in color as Timberwolf says.

Waving a propane torch over your chisels and watching the formation of oxide colors to temper them will work, but it will result in local softening which is the opposite of what you want.

[Timberwolf]

I have been collecting and refurbishing old chisels to resell mostly and keep the good ones. I have noticed that there seems to be a wide variation of hardness in the various brands. I bought a diamond plate with 400/1000 grit to ease the sharpening pain. I have DMT plates coarse,fine,very fine and they do seem to work well. Luckily I bought these 20 years ago and now they would be out of reach. Anyway I have a few Witherby chisels 1/2 and 3/4 that resist sharpening even on the new diamond plate ! I wish I had a hardness tester as these 2 are harder than I have seen before.

How did the old tool makers heat treat and keep the temperature consistent. I assume the quench had a lot to do with the steel hardness and how it was done-water,oil ? I'm no blacksmith or knife maker just curious about how these toolmakers regulated temperature and quenching. For the most part the steel seems to be uniform for maker to maker so I wonder if the tool makers bought billets from steel foundrys as opposed to making steel in house.

Cheers
Mike

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Mike, here's Ron Hock's take on what we have been talking about...Notice how similar his procedure is compared to what I have been trying to convey...albeit rather poorly. He has the advantage of knowing the type of steel he is treating...you don't have that luxury, but you can assume it is some type of high carbon steel. The procedure is "low-tech" but with modifications, it has worked for hundreds of years...way before much science was known about it. The video is only a few minutes long..

https://www.youtube.com/watch?v=JbsTpabc85k