High Speed A great variety of steels of the class known as self- or air-harden ing, have been put upon the market within the past few years. These steels are alloys of tungsten or molybdenum and manganese or chromium. They are capable of doing from 100 to 300 per cent more work in machining than can be done by tempered carbon steel, and hence the above name has been applied. Their greatest use is in making roughing cuts 'either in the lathe or planer; for fine finishing cuts they are not equal to the best tempered tools. Hardened high-carbon steel for metal working has its output limited by the fact that in re moving chips from the piece being machined most of the work is transformed into heat at the point of the tool, which consequently loses its temper at that point. Under normal ing conditions the heat generated at the tool point is conducted and radiated away to such an extent that the temperature is maintained uniform and proportional to the speed of the cutting. The output of a tool depends upon its strength and upon the heat that endure without losing temper. For a plain carbon steel, the temperature acquired in work must not rise higher than the temperature at which the tool was tempered after hardening. Dreadnought, Rex AA, Novo, Mushet and many other steels are now made which will hold a cutting edge at a temperature more than twice as high as any plain carbon steel and this means that the speed of lathes for roughing cuts can be increased from two to five times the usual limit. Thall ner recommends very high speed with limited depth of cut, rather than a very deep cut and low speed. The former condition is the more economical.
The adoption of high-speed steels in large shops has revolutionized this kind of work and has necessitated the installation of more power and heavier machinery. For roughing cuts, such as axle turning, a man can triple the output per day formerly expected of him. High-speed steels are used also in wood turn ing and with the same good results. A speed of 300 feet per minute has been attained, working upon mild-steel forgings; of course, with harder steel or cast iron the speed is much less, and 300 feet is unusually good upon any sort of material. The number of steels now on the market, all of which lay claim to the possession of properties suiting them to high-speed work, is very large. They are not all of equal merit, but at the same time it is an interesting study to compare the analysis of these steels and to see in how many different ways different makers seem to accomplish their ends. Their compositions are nearly as numerous and as different as those of bearing, or so-called anti-friction metals. Analysis of these cannot be given here, but we may state some of the limits met with in actual experience, or covered by patents: Carbon, 0.3 to 2 per cent; tungsten, 0 to 25 per cent ; silicon, 0.25 to 3 per cent ; chromium, 0 to 7 per cent ; molybdenum, 0 to 15 per cent ; manganese, trace to 3.9 per cent; vanadium, 0
to 2.50 per cent. Sulphur and phosphorus al ways low. Usually three of these elements are present in quantities which show them to have been intentionally added.
Edwards and Kikkawa have recently shown that the hardness of properly hardened high speed steel decreases slightly and successively on drawing the temper up to 700° or 800° F. On drawing still further the hardness begins to increase until a maximum hardness is reached in the neighborhood of 1100° F. and this hard ness is greater than the original quenched and undrawn steel. When drawn at temperatures above 1100° F., the hardness falls off rapidly. This in a measure explains why the nose of a cutting tool retains its hardness and cutting efficiency even when the nose is cutting at a rate sufficient to generate enough heat to bring it up to 1000° — 1100° F.
Special Properties of Some Steel Alloys.— The alloys, with some of the rarer metals, will now be mentioned so far as reliable information concerning them is to be had and recent scien tific work upon alloys, which has been pro ductive of practical results, will be briefly con sidered. The advent of the automobile and aeroplane has placed a demand on the modern steel manufacturer for special steels and alloys which shall combine the greatest strength con sistent with the maximum toughness. Ma terials of construction. have been developed dur ing the last decade which have virtually revolu tioned the high-grade steel industry. Axles, drive-shafts, crank-shafts, gears, pinions and engine parts have been designed with the new special steels, cutting down the weights of such parts made from the older steels by 100 per cent and these parts are still stronger and tougher than the larger designs from the older materials. The advantages accruing from the special steels are in most instances developed through the medium of heat treatment In the annealed condition they are in general but little stronger or tougher than the corresponding straight carbon steel. Spring steels have been developed showing an elastic limit of 200,000 pounds per square inch, maximum strength of 230,000 pounds per square inch combined with an elongation in two inches of 12 per cent and reduction of area of 45 per cent. Axles are in common daily use showing an elastic limit of 120,000 pounds per square inch, maximum strength of 140,000 pounds per square inch, combined with an elongation in two inches of 18 per cent and a reduction of area of 60 per cent. Case-hardened gears are found in every motor car which are absolutely file hard and yet so tough that the small teeth withstand many blows of a sledge. All of the alloy steels have a wider range of heat treatment than the corresponding carbon steels of same carbon con tent; that is, they can be heated hotter or cooler than the designed temperature for heat treatment without injury than can the straight carbon steel.