Classification and Uses of Plain Carbon Steels

CLASSIFICATION AND USES OF PLAIN CARBON STEELS Production and Consumption.-Any classification which accounts for the destination of steel made by any combination of the above described processes must evidently be impossible on account of its complexity. As to process, it may be said that any of them can be made to produce steel which easily passes present commercial specifications, even for so severe a duty as railroad rails. The following figures show the distribution of American production of steel, 1938, according to The Iron Age: In England the quantity of plates and track material is rela tively larger, because of a more active ship-building industry and large exports respectively. The feature of the American industry is the large increase in •sheet production, and the rapid decline of track material.

A similar classification of the chief lanes of consumption in the United States is given in the table which follows. It should be noted in this connection that in England and the Continent the proportion of exports is very much larger, and the relative posi tion of automobiles and other machinery would probably be reversed. Figures are average for 5933-38 inclusive.

Grades of Carbon Steel.

Carbon is the principal alloying element used for hardening and strengthening iron. Consequently the utility of various iron-carbon alloys—the plain carbon steels which comprise such a large majority of the entire production— depends upon the amount of carbon present, and has led to the separation of the entire possible range into approximate grades: Ingot iron, made in a basic open-hearth furnace, has the lowest possible content of carbon and other alloying elements. It is used largely for sheets on account of its corrosion resistant properties. Extra soft or dead soft steel contains from 0.08 to 0.18% carbon, used where ductility, toughness, weldability or ease in cold work ing is desirable, and strength or stiffness a minor consideration, i.e., for pipe, rivets, sheet, wire, and case hardened parts. Struc tural grade or mild steel contains from 0.15 to 0.25% carbon, used where a combination of strength and easy machineability is de sired, for forging or flanging operations, i.e., for buildings, bridges and railroad rolling stock, for boilers, drop forgings, and bolts. Me dium grade contains 0.25 to 0.35% carbon and is even harder and stronger than mild steel, yet it may be hot-forged without diffi culty, used for ship building, shafting, and forged machine parts. Medium hard steel contains from 0.35 to 0.65% carbon, and enters into the range where the final condition and reliability is much dependent upon the past mechanical and heat treatment ; it is used for large forgings for locomotives and other prime movers, car axles, and in the higher carbons for railroad rails. Hard steel ranges from 0.65 to 0.85% carbon, and is used only after it is heat treated to a correct combination of strength and ductility, used for wrought steel wheels, locomotive tires, wood cutting tools, and sledges. Spring grades range from 0.85 to 1.05%, used in a heat treated condition, wherein a great range of hardness, resiliency and brittleness is possible, i.e., automobile springs, cold chisels, rock drills and shear blades are made of these steels. High carbon tool steels range from 1.05 to 1.20%, used only after heat treatment for railroad springs, metal cutting and forming tools. A characteristic of these carbon steels is that each o•o1% of car bon increases the ultimate strength in the annealed condition by approximately ',coo lbs. per sq.in., but at the expense of the ductility or toughness. Hardness also is increased.

Combinations of even higher strengths with considerable duc tility and impact resistance may be had only in alloy steels, of which a number have been developed with special properties.

Use of alloy steels, which prior to 1910 was confined mostly to big guns, armour and projectiles, has spread rapidly into all metal working industries, among which the automotive leads. Produc tion statistics are either lacking or uncertain because there is no agreement as to what constitutes an alloy steel. The trend (1939) is toward lower and lower contents of alloy made by basic open-hearth furnaces, operating under very special control for deoxidation and grain size. As pointed out in the article on STEELS, ALLOY, they are special steels, not merely a carbon steel to which some alloys have been added. Production of such material is at least 3% of the total steel made (1939).

Nickel and Chromium.—Although there are infinite possibilities in alloying, by far the most important steels are those containing nickel and chromium, used for crank shafts, axles, gears and other automotive and machine parts. Relatively few analyses are in use, the present tendency being to minimize the number of alloy steels in process so as to avoid mixtures of the stock. The popular nickel steels contain up to 3.5% nickel and o.io to 0.50 carbon (Plate III., figs. 15 and 16). Chromium steels range from 0.60 to 1.50 chromium with the carbon in direct ratio from 0.15 to 1.10.

Nickel-chromium steels are more numerous. For machine parts, one series has nickel about 1.25%, chromium o.6% and carbon 0.10 to o•5o; another has about 1.75% nickel, i.o% chromium; a third has 3.5% nickel, chromium 1.5%. Nickel and chromium when added to steel increase its strength and hardness without a corresponding increase in brittleness. Nickel steels—not heat treated—are used for structural steel in long span bridges, and high strength boiler-plates; heat treated for gun tubes, engine forgings and castings, and shafting. Chromium steels are used for projectiles, for grinding rolls and for roller bearings, and with nickel or vanadium for armour plate and all sorts of heat treated machine parts, axles, and gears. Rustless irons and steels (q.v.) contain 12 to 18% chromium and up to 8% nickel.

Manganese is present in all steel, but when it adds up to be tween 1.o and 2.0%, and carbon 0.30 to 0.50, the steel acquires special properties. These analyses are favoured in America for high strength castings, heat treated forgings, rifle barrels and for case hardening. So-called silicon structural steel used in long span bridges and for boiler plate, is really one of the lower manga nese steels of this classification. The tough Hadfield manganese steel, used in castings for shock and wear resistance, contains or 12% manganese and 1.o to 1.2% carbon.

Vanadium is an element which has a powerful deoxidizing and scavenging effect ; thus a vanadium-treatment on steel which shows very little of the element in the analysis makes for sound ness, fine crystalline grain and reliability in forgings. From 0.5 to 2.0% is added along with other alloys to make high speed steels and tools for hot work.

Molybdenum is added to chromium, nickel, and chromium nickel steels in from 0.2 to 0.3% to widen the range of heat treat ment for given values, and to improve the forging and machining properties, and their resistance to creep when working at high temperature. Carbon-molybdenum steels of controlled grain size and hardenability are favoured by some American metallurgists.

High-speed Steel.—Carbon tool steels for cutting metals are heat treated by quenching from a high temperature, which gives them hardness, and then by reheating or tempering to a moderate degree to restore toughness. In use they must be operated at such low speeds that the friction between chip and tool-nose will not reheat the cutting edge above the drawing temperature, else the hardness and usefulness of the tool is destroyed. Better speeds could be made by using self-hardening tool steels, those which are hard after a slow cooling from a high temperature, such as the patented tungsten-manganese alloy invented by Robert Mushet in England sometime between 1860 and 1870, or the chromium-tungsten substitutes adapted by some American steel makers. During a 26-year research into the art of cutting metals, Frederick W. Taylor and Maunsel White discovered, 1898 to 1900, that if these steels were quenched from a sweating tem perature, so high that it would utterly ruin a carbon steel, the hardness would be retained even when the cutting speed was so high that the nose of the tool and the chip leaving it were red hot. Modern high-speed steels possessing this property of red hardness fall into four general classes: These are crucible or electric steels carefully forged to break up the crystals of complex carbides forming during the solidification of the ingot and distribute them evenly throughout the bar. A well annealed bar is not very hard (Brinell 240) and can be ma chined into a complex tool like a milling cutter, and then heat treated; pre-heat to 85o° C, heat rapidly to 1,30o° C, cool in oil or an air blast, giving a Brinell hardness of at least 600. Re heating 3o min. at 600° C is then necessary to give the quality of red hardness; the Brinell hardness is even higher and the tough ness materially better. As an indication of the revolution which these steels have effected in machine shop practice, a plain carbon tool steel will cut 16 ft. per min. on a medium steel forging, an air hardening steel 26 ft. per min., the same after being given the high quench 6o ft. per min. and a modern high speed steel ioo ft. per min. or more, all cuts being of same depth and feed, and on the same material. Cobalt high speeds are for cutting extra hard or gritty substances.

Tungsten Steels.—Other alloys whose properties are due prin cipally to tungsten, in addition to the air hardening tool steels and the high speed steels, are tool steels to which from 1 to 5% tungsten is added to improve the quality of the wood working or other tools which would be made of the plain carbon steels. They are also good for making finishing cuts on metal after the most of it has been removed by deep cuts with high speed steel. Mag nets are also made of steels with 6% tungsten and 0.7% carbon, quenched but not tempered.