Hardening and Tempering Metalworking tools 02

Harold Hall

Workshop Processes


If the part being hardened is large a small brazing torch may not be powerful enough. In this case make a cavity out of a number of fire bricks to house the part and aiming the flame into the hole will help considerably. I have also used two torches together for extra heat.


Use a large pair of pliers to pick up the part and place in the water, cutting edges first. The steam produce around the part will reduce the cooling effect so it is essential to agitate it vigorously in the water. That completes the hardening process, but if left in that state the part would in most instances be too brittle for the task it has to perform, because of this it needs tempering.


The tempering will reduce the hardness the amount depending on how hot the part is made during the tempering process. Tempering is more critical than the hardening process as the task parts are called upon to withstand vary widely. A cutter for use on a metalworking machine will need to be harder and only slightly tempered whilst a screw driver needs to be less hard and also less brittle.


In theory, lathe tools which are normally well supported and the cut is continuous can be left harder than a cutter used in the milling machine where they are less well supported and the cut is intermittent. The colours are light straw for lathe tools and straw for milling cutters. However, when using the colour method to determine tempering temperature the colour difference will be too small to be totally certain. Such precise tempering is really only appropriate when items are heated in a controlled environment such as an oven.


After hardening the part it will have taken on a dirty black appearance, so that the tempering colours cannot easily be observed. This must be cleaned off and the surface brought to a polished finish. This will be easier if the part originally had a good surface finish.


Few parts need to be equally hard all over with many actually benefiting from a progressive tempering as is the case with the tee slot cutter.

In such cases apply the heat to the end needing to be less hard, the threaded end in this case. Wait till this becomes blue and watch carefully how the lighter colours commence to run towards the cutter head. Once the colours start to run, which will happen quite quickly, be ready with the pliers to once more place the part in the water, again agitating rapidly.


Be vary careful with the tempering process, the speed at which the colours travel makes it easy to over temper the cutter edge. If this is done it will be necessary to anneal the part and once more go through the hardening and tempering process. More reading will be necessary if you find yourself in this situation.


If a part needs to be equally tempered all over keep the torch further away and continually move the flame  until the colour required is even over the whole part.


I am sure that when you first make your hardened cutter, be it a tee slot cutter, or similar, you will gain much satisfaction when you see it cutting through the metal with ease.  However, your silver steel cutter is not the equal of a high speed steel tool so cutting speeds will need to be on the low side.


To expand on the subject of tempering colours the following detail may be of help.


Lathe tools, Light Straw

Milling cutters, Straw

Pin punches and similar, Dull orange/brown

Cold Chisels, Purple

Springs, Blue


Some publications include a colour chart as examples of the colours required.


The above should be adequate for the occasional lathe or milling cutter but if more detailed information regarding hardening and tempering is required, other reading on the subject should be studied.

"Hardening, Tempering and Heat Treatment" by Tubal Cain, Workshop Practice Series, No. 1 would be one possibility.