Harold Hall

Workshop Processes


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Hardening and Tempering Silver Steel,    Harold Hall

For one who does not frequently carry out the task of hardening and tempering, and I suspect that this is the case for most home workshop owners, completing the task, resulting in a workable tool, is very satisfying. This process alone can make the project, typically the T Slot cutters seen in the Photograph 1, worthwhile, even discounting any financial savings.


As I do not often have need of the process, or that of brazing, I find a small brazing torch with its own gas canister adequate for the size of parts I harden. This, together with a small brazing hearth and an old pair of large pliers, completed the necessary kit for the job, Photograph 2. One other item, a large container filled with water for cooling the cutter.


For the cutters shown, I placed one into the corner of the hearth and applied the flame to the cutter head and waited for the cutter to turn bright red, A good description for this is the colour of boiled carrots. I then held the cutter at this colour for a few minutes and then plunged it into the water, cutter head first.


The shank and cutter head were then cleaned and polished with a fine emery paper to enable the tempering colours to be seen and re-heated the cutter, but this time with the flame at the threaded end. When the end took on a deep blue colour I observed the colours running up toward the head and when this became a straw colour the part was then quickly placed head first in the water and agitated rapidly.


More detailed explanation

For those for whom hardening and tempering is a new process the following expands on my brief account above.


It is preferable to carry out the activity in the open unless you have a workshop large enough for an area specifically set up for the task, I suspect, like me, most will not. If though it is windy, you must find a sheltered spot, also, if sunny, find a shaded area as this makes for consistent judging of the hardening and tempering colours. The task should not be done in bright sunlight if it can be avoided.


When hardening, the colour required should, when viewed in subdued lighting, approximate to that of boiled carrots and when the item arrives at this colour keep it at this for a minimum of five minutes, more if the part is large. To avoid the part becoming hotter during this period move the flame slightly further away, if it appears to be cooling the flame should be brought closer for a while.

Hardening and Tempering Cutting Tools, T Slot Cutters,
Brazing Hearth