Tempering (metallurgy)

“ Thus, if a spring is to be made, the temper is drawn until a blue color shows, when the further drawing of the temper is stopped. The above is an outline of the ordinary tempering process as I have known it for the last twenty years.' The metal is partially restored to its original condition by heating it to a blue heat if the condition of spring temper is desired, or to a red heat if it is wished to have a perfect restoration of the metal. In making wire the metal is rolled into the form of a rod, which is drawn, when cold, through successively smaller holes in a drawplate. ”

“ When therefore a tool requires more strength than it possesses when hardened, it is strengthened by tempering it. Tempering proceeds in precise proportion as the temperature of the hardened steel is raised. When the steel is heated to redness the effects of the hardening are entirely removed, and the steel, if allowed to cool slowly, is softened or annealed. ”

“ Ordinary steel contains certain other chemicals, such as silicon, manganese, sulphur, and phosphorus, but these are mere natural impurities existing in the metal. The essential ingredients are iron and carbon. Steel is hardened by being heated to a high temperature and then suddenly cooled by contact with cold water, or in other like ways. Fixing the degree of hardness in a piece of steel is called tempering. The degree of hardness is dependent upon the suddenness of cooling. ”

“ If steel is heated to redness, and suddenly plunged into cold water, it is rendered extremely hard, but, at the same time, too brittle for use. On the other hand, if it is suffered to cool gradually, it becomes too soft and ductile. The great object of the operator is to give to the steel a quality equally distant from brittleness and ductility. The treatment by which this is effected is called tempering, which will be more particularly treated in the article on the cutler, whose employment is a refined branch of this business. ”

“ The Theory of Tempering.—Steel that has been hardened is generally harder and more brittle than is necessary, and in order to bring it to the condition that meets our requirements a treatment called tempering is used. This increases the toughness of the steel, i.e., decrease the brittleness at the expense of a slight decrease in hardness. ”

“ It is the comparative slowness of cooling in oil, the greater slowness of cooling in air, that make these by far the best tempering processes, because the molecular re-arrangement, in which tempering consists, requires time. Often the critical temperature, at which a desired re-arrangement takes place, is declared by the metal losing all power of response to a magnet: this fact affords the steel-maker welcome aid; he has only to shut off heat as soon as his steel ceases to attract a magnet and plunge the steel into water in order to obtain the hardness he wishes. ”

“ It happens that between the degree of hardness of hardened steel and the temper due to reheating it up to about 600° Fahr. lie all the degrees of hardness which experience has taught us are necessary for all steel-cutting tools. Hence we may use the appearance of colors as equivalent to a thermometer, and this is called color-tempering. The presence of these colors or of any one of the tints of color, however, is no guarantee that the steel has been tempered or possesses any degree of hardness above the normal condition, because they appear upon steel that is soft or has not been hardened. ”

“ Rough-grind while still hot and preheat to about carbon-steel hardening heat, then heat quickly in high-speed furnace to white heat, and quench in oil. If a very hard substance is to be cut, the point of tool may be quenched in kerosene or water and when nearly black, finish cooling in oil. Tempering must be done to suit the material to be cut. For cutting cast iron, brass castings, or hard steel, tempering should be done merely to take strains out of steel. ”

“ Steel containing from 0.5 to 1.5% carbon is characterized by the property of "taking temper." When the hot steel is suddenly cooled by plunging it into water or oil it becomes very hard and brittle. On carefully reheating this hard form it gradually becomes less brittle and softer, so that by regulating the temperature to which steel is reheated in tempering almost any condition of temper demanded for a given purpose, such as for making springs or cutting tools, can be obtained. ”

“ For all uses for tools steel must be hardened, or hardened and tempered. The operations of hardening and tempering, including the necessary heating, are the most important, the most delicate, and the most difficult of all of the manipulations to which steel is subjected; these operations form an art in themselves where skill, care, good judgment, and experience are required to produce reliable and satisfactory results. It is a common idea that all that is necessary is to heat a piece of steel, quench it in water, brine, or some pet nostrum, and then warm it to a certain color ”

“ As was stated before, martensite is the useful and proper structure for carbon steel tools.
No. 73. Annealed Steel has Fine Grain
(Magnification 70 Diameters)
Tempering or Drawing “Tempering” is done to relieve the intense brittleness of steel after quenching to martensite. While we dislike to sacrifice any of the hardness, it pays to temper or “toughen” the steel, as the toolmaker calls it, by reheating it to somewhere between 400° and 570° F.
The higher the temperature, the freer and quicker is the change from one structure to another, as, for instance, the austenite to martensite. ”

“ Machined or forged steel parts are packed with case-hardening material in metal boxes and subjected to a red heat. Under such conditions, carbon is absorbed by the steel surfaces, and a carburized case is produced capable of responding to ordinary hardening and tempering operations, the core meanwhile retaining its original softness and toughness.
Such case-hardened parts are stronger, cheaper, and more serviceable than similar parts made of tool steel. The tough core resists breakage by shock. The hardened case resists wear from friction. ”

“ If any parts are required to be tempered or hardened it must be remembered that they become brittle and can not afterward be bent without annealing or softening. Tool or drill steel is a name given to uniform or rather reliable grades of steel adapted to heat treatment as tempering or annealing. ”

“ The second is more difficult, because the quality of the steel employed for such purposes is variable, and hence the tempering process must be varied to suit the steel. The third is of the greatest importance, because the articles to be tempered are the most expensive to make, the duty obtained is of the greatest consequence to manufacturing pursuits, and the fine grade of steel employed renders it more liable to crack in the hardening process. ”

“ If the heat is run up until the oil flashes, the tempering is pretty thorough and is about right for good springs. If the oil be all burned off, there will be little temper left except in very high steel. High steel becomes much harder when quenched than low steel; consequently very high hardened steel may be heated until it begins to show color and still retain considerable hardness or temper, whereas a milder steel, under 90 or 100 carbon, when heated to such a degree will retain no temper, it will be soft. ”

“ Introduced at the right time into the Bessemer converter or the Martin furnace, a small proportion of the alloy of iron and aluminum communicates to the melted metal a fluidity which facilitates the disengagement of the gases that would otherwise remain imprisoned in the metallic bath, producing blow-holes, and destroying homogeneity and resistance in large pieces.
New uses are constantly found for the pure metal; less employed in jewelry, it is more used in the modest ranks of plated ware and kitchen vessels. ”

“ Published by Albert Sauveur, Cambridge, Mass., 1906.
Tempering and Annealing.
For rock drills, cold chisels, milling and other tools it is necessary to use steel carefully tempered, so that brittleness is greatly reduced while considerable hardness and cutting power remain. Other changes of properties, as remarkable, follow upon subjecting steel to greater heat than that used for tempering. Says Professor Roberts-Austen:—“Three strips of steel identical in quality are taken. By bending one it is shown to be soft ”

“ The use of uniform metal from the “mixer,” which was described in the article on the Bessemer process, has aided the open-hearth process also. Of course, when molten metal is added none of its silicon and manganese is reduced by the flame as occurred with the cold metals during the melting down, so the molten metal charged is usually low in these elements to compensate. By use of “hot” (molten) metal the time necessary to produce a “heat” of steel is considerably shortened. ”

“ It is possible to harden the surface of tool steel extremely hard and yet leave its inner core soft and tough for strength, by a process similar to case-hardening and known as "pack-hardening." It consists in using tool steel of carbon contents ranging from 60 to 80 points, packing this in a box with charred leather mixed with wood charcoal and heating at a low-red heat for 2 or 3 hr., thus raising the carbon content of the exterior of the piece. The article when quenched in an oil bath will have an extremely hard exterior and tough core. ”

“ Steel with 0.2 per cent carbon and 15 per cent nickel is entirely martensite. Higher percentages of nickel change the martensitic structure to austenite, the steel then being non-magnetic. The higher percentages, that is 30 to 35 per cent nickel, are used for valve seats, valve heads, and valve stems, as the alloy is a poor conductor of heat and is particularly free from any tendency towards corrosion or pitting from the action of waste gases of the internal-combustion engine. ”

“ The texture of carbon steel will be weakened by the prolonged high heat of carburizing, so that if we need a tough core, we must reheat it above its critical range, which is about 1,600°F. for soft steel, but lower for manganese and nickel steels. Quenching is done in either water, oil, or air, depending upon the results desired. The steel is then very carefully reheated to refine the case, the temperature varying from 1,350 to 1,450°F., depending on whether the material is an alloy or a simple steel, and quenched in either water or oil. ”

“ The malleability and ductility of metals lie at the basis of the work of the gold- and silver-smiths at one extreme, and of the boiler-maker at the other. Sheet metals can be made to assume almost any shape under the hammer, or by pressure, provided they are subjected to annealing to restore the property of malleability. The most awkward shapes, involving excessive extensions of metal, are produced by drawing processes between dies of iron and steel in power presses. ”

“ Steel for guns and armor is made in the open-hearth furnace where the quality of the metal may be regulated to a nicety. Gun tubes are cast in vertical molds and during the cooling of the ingot it is subjected to pressure so as to prevent segregation and the forming of “pipes.” Pipes are cavities that are liable to form in the center of the ingot due to contraction during cooling. Steel, as we have learned, is not pure iron, but an alloy, and the various constituents have different temperatures of solidifying, consequently they exhibit a tendency to [324] segregate. ”

“ The cast steel which is in demand for this purpose has a slightly lower permeability than the ingot-iron for low flux densities, but for flux densities above 16,000 the required result may be more cheaply obtained with a steel casting than with a forging. When high tensile strength is required in addition to considerable magnetic permeability, it has been found advantageous to employ a steel containing 5% of nickel. ”

“ Swift cooling, even such as results from the dumping of castings from the molds while at nearly white or high red heat results in insufficient graphite and otherwise harder metal. Cooling of very hot castings on a cold floor or in a current of cool air has considerable hardening effect even when the composition of the alloy would otherwise give very soft and machinable metal. In an extreme and very interesting case a few very hard cast iron flanges were each day found among the many thousands of habitually soft castings regularly produced. ”

“ In the open-hearth method the metal from the blast-furnaces is not sent to the converter, but is poured into oven-like structures built of fire brick, and in these heated to a terrific temperature. This heat has the same effect upon the metal as the blast of air in the Bessemer converter, and this open-hearth process has become very popular for manufacturing certain kinds of steel. ”

“ Power Presses in Metal Working.
From glass let us now turn to metals. It is their tenacity that chiefly gives them value; this tenacity is usually accompanied by a hardness which disposes us to regard nickel, for example, as of a solidity quite unyielding. But the coins in our pockets prove that under the pressure of minting machinery they are as impressible as wax. In molds and dies, each the counterpart of the other, brass, bronze, iron, steel, and tin-plate take desired forms as readily as if paste. Solid though these metals appear they yield under severe stress with a semi-fluid quality. ”

“ The silver-gold alloy left from the cupellation is soft and bends like lead; but after hammering or rolling it becomes harder, gets a spring in it like a piece of mainspring and cracks or splits somewhat easily. There should be no cracks or stripping or even roughness on the flattened metal, since such defects may cause the loss of small particles either during the flattening or in the subsequent treatment with acid. The softness of the metal is restored by heating. In bullion assays the flatting of the buttons requires care and practice for its skilful working. ”