Sunday 24 May 2020

Heat Treatment

In addition to having good machinability, the advantage of using O1 tool steel is that heat treatment – hardening and tempering - is straightforward.

Some years ago I purchased two small furnaces on ebay. They are quite old but were really cheap and have proved very reliable. The furnace that I use for hardening, shown below, has a small working section because it needs to be heated to around 800 0C but it is quite large enough for the gears, shafts, bearings etc that you find on motorbikes.

The other furnace has a much larger working section because it only needs to be heated to around 250 0C maximum. It is also used for heating castings for removing and fitting bearings and for preheating prior to welding. My missus would be really unhappy if I put large and, when heated, smelly chunks of metal into her kitchen oven!

The basic heat treatment process is:

1)    Heat gear to 820 0C in the small furnace and oil quench. Cheap vegetable oil from the local supermarket is used.

2)    Temper at 230 0C to give a HRc (Rockwell Hardness) of 60/61, which is a quite sufficient degree of hardness with good toughness for a gear like this.

A few practical points:

Before heating to high temperature, stainless steel lock wire is used to make a little “package” so that the gear can be easily taken out of the furnace when hot. An anti-scaling compound (see picture below) is also used to avoid a black oxide scale that would otherwise form after high temperature heating. Finally, the larger furnace is preheated to the correct temperature so that the gear can be put into an already warm environment for tempering immediately after quenching.

After heat treatment, the anti-scale compound can be easily removed because it forms a baked hard shell that peels off to leave a clean surface. The picture below shows the gear and the pinion after heat treatment but before cleaning up.

Keyways need to be cut in the gear (and also in the sprockets). I leave this to after heat treatment because we have a local engineer with 50+ years of experience in spark eroding and it makes no difference whether the material is in the annealed state or has been hardened. In the absence of a spark eroder, the keyway would need to be broached before heat treatment.

Finally, a tumbler using ceramic chips is useful both at the end of machining and as a final step to smooth any rough edges and to give the gears a uniform and clean appearance. 

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