Posted by HWooldridge on Friday, May 21, 2004 at 2:52PM :
In Reply to: OT: Re-tempering cold chisel? posted by David Sherman on Thursday, May 20, 2004 at 5:27PM :
Heat carbon steel until it reaches a temp where the steel loses its magnetic qualities. That's typically a dark blood red and is best done with a torch in a dark shop - don't try it in bright sunlight. Use a parts pickup magnet to test it and go slowly with the torch. Be careful not to overheat since that causes grain enlargement. It would be best to quench in oil - not water. Get a reasonably large volume of oil - like two quarts of motor oil, quench and agitate until it's not smoking then it can be cooled in water to allow handling. At this point, see if a file will cut it - it is too brittle to use but should resist a file. If it is not hard then you may have something that was surface hardened and can't be brought back using this method. If it is quite hard, put the chisel in the oven and bake it at 400 degrees for an hour or so. That should provide enough tempering so that it does not chip in use. I make a lot of tools from coil springs and use this method successfully. I have a cold chisel that is over 20 years old which cuts quite well.
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