The tool steel that is generally used for cutting tools is made by the crucible process. If the steel depends on the carbon in it for its hardening qualities, it is called carbon tool steel. High-carbon steels harden better, stand higher speeds, and allow heavier cuts than the same quality of steel with lower carbon percentages.
In order to produce a steel that will stand higher speeds and heavier cuts than carbon steel, various elements have been added. Each of these steels are generally given a distinguishing name, usually that of the added element, such as vanadium steel, manganese steel, silicon steel, tungsten steel, etc.
Fig. 21. Water Spray for Cyanide Bath.
If tungsten is added in a small percentage, a steel is produced that allows higher speeds and will cut harder stock. Steel with a higher percentage of tungsten hardens if heated red hot and allowed to cool in the air, but better results follow if it is cooled in an air blast, or in oil. This is called air-hardening or self-hardening steel. Tools made from air-hardening steel allow speeds from 50 to 60 per cent higher than can be obtained from similar tools made from carbon steel. This steel proves particularly satisfactory for heavy roughing cuts, but not for finish cuts as it does not hold a fine cutting edge. It has given way to the modern high-speed steel in most shops, but for certain classes of work it is still used to some extent.
Carbon tool steels, when hardened by the ordinary fire-and-water method, show a tendency to get out of shape, or to change in length measurements. To do away with this difficulty, oil-hardening steels are extensively used in many shops for making taps, dies for screw cutting, blanking dies for punch-press work, etc. Under many conditions these steels work very satisfactorily, if a brand adapted to the particular work to be done is procured. The method of treatment for the steel of different makes varies so much it would not be wise to attempt to give any specific instructions without knowing the particular make of steel and the purpose for which it is to be used. The makers of these special steels always furnish instructions for working their particular brands, so there need be no difficulty encountered in their use. Directions should be carefully followed, except in cases where experience has shown the advisability of a different method. To secure the best results, a furnace equipped with a good pyrometer should be used, as this enables the operator to adjust the temperature to the proper point.
To show the variation in treatment for the different makes of alloy steels, we shall cite two cases, both well-known brands. One make that is specially adapted for taps, dies, and similar tools should be hardened at a temperature of 1350° F., while another make, to be used for the same purpose, shows best results when hardened at 1500° F., a variation of 150 degrees. Yet both steels give excellent results when treated according to instructions.
If, besides tungsten, certain proportions of chromium are added, a steel is produced that has revolutionized machine-shop methods. It allows extremely high speeds, heavy cuts, and coarse feeds. It is possible with a good grade of high-speed steel to increase the cutting speed of tools from 50 to 200 per cent above that possible with ordinary carbon steel. Unlike carbon steels, the high-speed steels grow harder as they become heated, until they are red hot when they are soft enough to forge.
This steel should not be heated too rapidly; in fact, it requires comparatively slow, careful heating in a good, heavy fire of blacksmithing coke. It should be worked at a high heat, with rapid blows, which should cease as the temperature goes down. Never hammer when the steel is at a low red. Although the steel should be reheated as soon as it is below a forging heat, as much as possible should be done at each heat.
After forging, the tool should be reheated to a high red heat, and allowed to cool slowly in the air; this is done to remove forging strains which might cause the steel to crack when hardened. When the tool has cooled down below a red, place it in the fire, and reheat for hardening.