To keep the productivity at a maximum it’s important to sharpen the chisels when the chisel tip eventually wears out. It’s not always necessary to harden the chisel every time you sharpen it.
Check the status of your chisel:
If the chisel tip is slightly worn out you only need to sharpen it. In this case it is not necessary to have a heat treatment. If the chisel is very worn it may be necessary to reshape it before sharpening.
If the chisel is through hardened and the reshaping is made carefully without inducing too much heat in the chisel, hardening afterwards is not necessary.
If the chisel is case hardened it’s likely that reshaping will expose the unhardened core of the chisel. It’s therefore important to harden the chisel after the reshaping.
If the chisel is severely damaged it can be better to reforge it instead of reshaping it. Hardening is always needed after reforging as the heat from the process will cause the hardening to disappear. It is recommended to sharpen the chisel after hardening: a sharp chisel will increase productivity!
Heat treatment for chisels
Forging and other processes should always be carried out in accordance with the following instructions. When reforging worn chisels and when forging a chisel to the shape wanted, be cautious to restrict the heat treatment to as small a portion of the chisel as possible, to avoid unnecessary loss of strength.
Heat the chisel slowly to the temperature specified.
The final forging should be carried out at the lower temperature limit.
Let the chisel cool in dry charcoal (breeze).
Forging a chisel at 850-1050°C
Don't harden the chisel directly from the forging heat.
Heat slowly from a low temperature to 800°C. The slower the heating process is the better the hardening of the chisel.
Cool the chisel in oil.
Cool down hardened chisel in a bucket with oil
Temper the chisel for one hour at the temperature specified.
After hardening and tempering, the chisel will have the hardness of 55 to 58 HRC.
Tempering a chisel at 220°C
Heat treatment not carried out in the prescribed manner may result in an excessively brittle chisel tip that can chip or crack. An incorrect heat treatment can also lower chisel hardness, leading to rapid wear or deformation of the chisel tip.