Heat Treating a Chisel

A few weeks ago, as I write this, I read the chapter in Mike Dunbar's book about heat treating chisels and other edged tools. I thought to myself, "This probably doesn't come up too often unless you make your own tools", but I read it since it was interesting.

What do you know, a few weeks later, I found myself needing to do this.

The same day that I'd gotten Mike's book, I'd picked up a slew of chisels at various flea markets. Included with some chisels I wanted was a poor orphaned 1/4 inch chisel that looked like it had been used to mix or spread adhesive of some sort.

It sort of sat around for a few weeks, but I got to feeling sorry for it, so I set out to scrape the goop off of it. No results to speak of, that gunk was tough. I took out a first cut file and started filing the stuff off. This worked, but I was surprised to discover that the chisel was much softer than the file.

After getting the gunk off, I noticed that the edge of the chisel was bent backwards, towards the flat. Not good, I felt that it should have broken before it bent that far...

Being an optimist (some would disagree :-) ), I sharpened the chisel. It felt weird, almost gummy on the stones, especially noticeable on the coarser grits.

I tried it. The very edge of the blade bent over backwards again, but not nearly so bad. I suspect that a previous owner had ruined the temper on an electric grinder. Poor chisel.

It was obvious that the chisel needed to be re-heat-treated, or discarded. Of course, I tried the former.

First, I ground the edge slightly blunt, figuring that it would make control of the temperature at the edge a bit easier.

Then, I got out a pan of water and a propane torch. Then I heated the chisel until the whole tip was glowing a medium red. Then I quenched it in the water.

Then I got out some 600 grit abrasive paper and shined up the metal.

Once again, I fired up the torch, but this time, just as I noticed the metal starting to turn yellow, I quenched it again.

Oops, the results didn't look so good. One side was straw colored, the other was more of a purple color. Looking this up later in a chart, that comes to about 70 degrees F. Not too good...

With visions of a blade that would wear unevenly, I decided to start again from the top (meaning re-hardening, then re-tempering). This time, I heated the blade further back from the tip, and made a point to get it into the water faster, as well as to swirl it in the water to prevent steam from insulating the blade.

It looked good! Sort of straw colored all the way across!

I sharpened the blade up. This time it felt normal on the stones. It also pared the endgrain of some oak with no trouble, and lived through pounding a shallow mortise in some chestnut scrap.

It was rehabilitated! A useful tool again!

Notes:

Some steels require oil quenching, they might crack in water. How to tell? I don't know, but it seems from what I've heard that older steels are more likely to prefer water.

If the steel doesn't harden in water, you can try a brine solution instead.