Hardened Steel Core To see if the knife has been fully hardened I test it with a file. When the Knife is fully hardened a file should not bit into the steel because the steel core is harder than the file itself. Now that is Hard!!! Here is a video of me trying to drill through a hardened blade. I had got in a hurry and forgot to drill the knife before I heat treated it so I ran into problems trying to drill it. I can take the temper out of the handle by putting the blade in water and heating the handle red hot while the blade is in the water. If you try to do this without the blade in water you will remove the temper out of the blade itself.