High Speed Steels are perhaps the most important of alloy steels, and derive their name from the fact that they can be used as cutting tools when the cut on the machined member is being made at a high speed. This, of course, subjects the tool to severe operating conditions, which simple carbon steels could not stand. These steels have other notable characteristics, among which is that of "self-hardening" or "air-hardening", as it is sometimes called. This means, when the steel cools naturally in the air, from a red heat or above, it is not soft like ordinary steel, but is hard and capable of cutting other metals.
Another striking characteristic of high speed steels is their ability to maintain a sharp cutting edge while heated to a temperature far above that which would at once destroy the cutting ability of a simple tool steel. Because of this property, a tool made of high speed steel can be made to cut continuously at speeds three to five times as great as that practicable with other tools. The result of the friction of the chip on the tool may cause the tool to become red hot at the point on top where the chip rubs hardest, and the chip may, itself, by its friction on the tool, and the internal work done on it, by upsetting it, be heated to a blue heat, or even hotter.
ELEMENTS WHICH OCCUR IN ALL STEELS.
There are certain elements which are practically always found in any kind of steel. These elements are capable of producing many varied effects on the finished product. They are Iron, Carbon, Manganese, Silicon, Phosphorous and Sulphur.
IRON.
The base of all steels is Iron. It goes without saying that this element should be obtained in the best and purest state possible. Probably the best "base" iron comes largely from Sweden, which country seems to have produced the highest quality of iron on the market today.
CARBON.
Carbon has already been discussed under Carbon Steels, although, of course, its importance in Alloy Steels must not be under-estimated. The proportion of carbon aimed at in high speed tool steels is about 0.65%, which in simple steel would not be enough to give the maximum hardness, even if the steel were heated above the critical point and quenched in water, and still less so when the steel is cooled as slowly as these steels are in their treatment. This shows that the carbon element acts in a different way from what it does in simple carbon steels as previously discussed.
MANGANESE.
Manganese Steel is a typical self-hardening steel and so, obviously, is any steel which is in the austenitic condition at atmospheric temperatures, that is to say, whose critical temperature is below atmospheric temperature. Thus, self-hardening steels are non-magnetic. Because of its low-yield point, manganese steel does not give satisfaction in many lines, for which otherwise it might be eminently fitted.