The best grade of High Speed Steel ought to have a tungsten content of about 18.00% and a carbon content of about 0.65%. Thus whenever a steel is needed which must operate under especially severe conditions, this would be the steel to use. Such conditions are usually met in the case of rapid turning, boring, planing, slotting and shaping tools, also with twist drills and all forms of milling cutters, gear cutters, taps, reamers, special dies, etc. MOLYBDENUM. Molybdenum was once thought of as being somewhat in a class with tungsten, but its use in high speed tool steels is being generally discontinued. The reason for this is that it was found that in rapid steels this element caused irregular performance, such as large variations in the cutting speeds which they would stand. This element is also likely to make the steels seamy and contain physical imperfections. Molybdenum steels were also found to crack on quenching, and possess decided variations in internal structure. VANADIUM. Vanadium steels are still in their infancy. Therefore, the true value of this element in rapid steels must probably be held as not yet fully determined. With the single exception of carbon, no element has such a powerful effect upon steel as vanadium, for it is only necessary to use from 0.10 to 0.15% in order to obtain very noticeable results. In addition to acting as a very great strengthener of steel, especially against dynamic strains, vanadium also serves as a scavenger in getting rid of oxygen and possibly nitrogen. It is also said to decrease segregation, which we may readily believe, as most of the elements which quiet the steel have this effect. “Vanadium Steels” demand a somewhat higher price than do those steels which do not contain this element in appreciable amounts. It is, of course, especially useful for all purposes where strength and lightness are desired, such as springs, axles, frames and other parts of railroad rolling stock, and automobiles. COBALT. The valuable effect of cobalt is claimed to be that it increases the red hardness of high speed tool steel, enabling the steel to cut at a higher speed. However, this element much resembles nickel, which has been largely condemned as not being a desirable ingredient for high speed tool steels, because it has the effect of making the edge of the finished tool soft or “leady”. URANIUM, TITANIUM AND ALUMINUM. These elements are generally classed as scavengers, although recently important claims have been advanced for their effect upon the physical properties of steel. This is especially true for the first two. In present practice, however, they are used almost entirely as deoxidizers or cleansers, and are added to the metal for this purpose only. IMPURITIES.