Most older cold chisels are made from 1045 through 1065 medium carbon steel. Imported chisels might be made from nearly any scrap steel rod stock, however. Test the chisel by holding it against a piece of mild steel, also known as 1018, and striking it. If it does not bite into the steel, it should probably be hardened before further use. Chisels that have been sharpened may lose temper during grinding, and need to be hardened before use as well.
Don wrap-around eye protection, heavy leather work gloves and full leathers.
Normalize the chisel by heating the entire tool to black in a forge or using a torch. According to the Tempil Color Guide for heat-treating steel, black begins at 100 and ends around 725 degrees F. This prevents crack formation in the steel during machining.
Heat the first 2 inches of the chisel to dull red and allow to slow cool to room temperature.
Heat the first inch of the chisel to bright orange. Quench only that first inch in vegetable oil until no red color remains.
While still black-hot, scuff the tip of the chisel with an 80-grit abrasive wheel on a bench, belt or right-angle grinder so you can see the shine of the steel.
Watch the tip change color until the tip is light blue. Cool it in vegetable oil.
Test chisel hardness by cutting into a piece of 1018 mild steel by striking the chisel with a hammer. It should cut into the steel without dulling if you have followed the previous steps correctly.
According to Roy Beardmore, on heat treating processes, "Steel often requires heat treatment to obtain improved properties e.g increase hardness or strength, or to neutralize negative effects resulting from previous manufacturing processes e.g.remove internal stresses generated by fabrication processes."
Always wear wrap-around eye protection, heavy leather work gloves and full leathers when heat-treating steel. Keep a large, fully-charged ABC fire extinguisher within fingertip reach at all times when working or quenching hot steel. Keep a 5-gallon bucket of sand in reach to smother fires, and another 5-gallon bucket of water to immerse hands, arms or legs if splashed during quenching.