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hardening,in metallurgy, treatment of metals to increase their resistance to penetration. A metal is harder when it has small grains, which result when the metal is cooled rapidly. Sometimes small areas on the surface of a casting are given a fine-grain structure by chill hardening; metal pieces (chills) are inserted in the wall of a sand mold. The area next to the chill cools faster and becomes harder than the surface next to the sand. Metals worked cold, as by being rolled into thinner pieces, become hardened, partly by reducing grain size and partly by distorting the shape of the grains so that they increasingly resist further distortion. Alloying may harden a metal by changing its chemical composition. In hardening by precipitation, one constituent of a supersaturated solid solution separates from the solution. Usually the process is carried out at above room temperature. At room temperature the process takes longer; it is then known as age-hardening. Aluminum-copper alloys are hardened by precipitation. Iron-carbon alloys, steel and cast iron, for example, respond well to heat treatments. By varying the percentage of carbon and the rate of cooling from a high temperature, many gradations of hardness, softness, toughness, and other properties are achieved. To impart hardness the metal is rapidly cooled from a high temperature by quenching in water, oil, or molten salt. Later heat treatment by tempering or annealing modifies the metal slightly to give other desirable qualities. Steels with a low percentage of carbon can be given a hard surface by increasing the amount of carbon at the surface so that they will respond to heat treatment, a process known as carburizing, or casehardening. One way to do this is to pack steel in charcoal and then heat it. Another way is to heat the metal in a furnace with a hydrocarbon gas atmosphere; still another is to heat the metal in a molten-salt bath containing potassium and sodium cyanides. If the salt bath cited is of a lower temperature, the steel surface will also pick up nitrogen, which helps harden it; the process is then called cyaniding. At even lower temperatures the steel picks up only nitrogen, and is nitrided.
a type of heat treatment of materials that consists of heating and subsequent quick cooling and is intended to fix the high-temperature state of the material or to avoid or suppress any undesirable processes that occur during slow cooling. Hardening can be achieved only in materials whose high-temperature and low-temperature states of equilibrium differ (for example, in crystalline structure). Hardening is effective only if the cooling rate actually attained is sufficiently high to prevent the occurrence of the processes it is intended to suppress. The structures formed as a result of hardening are only relatively stable; upon heating they undergo a transition to a more stable state. Many substances (metals and their alloys; glass) may undergo hardening, either under natural conditions or in a particular technological process.
Hardening of steel. Steels are the largest group of materials that undergo hardening. According to the phase diagram for iron-carbon alloys (Figure 1), austenite, a solution of carbon
in gamma iron, is a thermodynamically stable state of steel at temperatures above the lineGS£ of the phase diagram; below the line PSK, the thermodynamically stable state is a mixture of ferrite (a solution of carbon in alpha iron) and cementite (an iron carbide, Fe3C). In cases of slow cooling, starting from temperatures above the line PSK, austenite must decompose into ferrite and cementite, according to the phase diagram. The rate of transformation depends on the temperature, and at sufficiently low temperatures it becomes so slow that the decomposition of austenite virtually ceases. Upon a further decrease in temperature, austenite becomes marten-site, the appearance of which in steel structure leads to a drastic increase in hardness, strength, and magnetic saturation and to a decrease in ductility. The goal of steel hardening is the production of a completely martensitic structure (without products of the decomposition of austenite)—that is, the suppression of the decomposition of austenite by quick cooling and the preservation of the decomposition up to temperatures at which the martensite transformation takes place. The minimum cooling rate sufficient to prevent the decomposition of austenite is called the critical cooling rate for steel hardening.
In practical applications of heat treatment of metals, various kinds of hardening are used to produce particular properties of metals, particularly steels. Hardening may be full or localized, depending on the heating conditions. In full hardening the steel is cooled quickly after having been heated to temperatures above the line GSE. Thus, the steel is completely converted to an austenitic state. In localized hardening, which is used mainly for tool steels, the metal is heated to temperatures above the line PSK’, after cooling, the structure may still contain so-called excess phases (ferrite or cementite and some more complex carbides), which remained undissolved during heating. Depending on the cooling conditions, hardening may be classified as austempering or martempering. In austempering, the steel is heated to temperatures above the line GSE (full hardening) or above the line PSK (localized hardening), then quickly cooled to temperatures below the line PSK and subjected to so-called isothermal holding, during which austenite is converted to other structures (pearlite or bainite). In this case the properties of the final products are determined by the temperature of isothermal holding; the hardness and strength of the material increase as the temperature is lowered. In martempering, the cooling is conducted very rapidly down to a temperature slightly above the temperature of the martensite transformation. This is followed by holding, which is necessary for the equalization of temperature throughout the entire thickness of the piece (a step). Subsequent cooling takes place slowly to the point of formation of martensite in the structure. External factors, mainly the hardening medium (water, oil, or molten salt), also influence the results of hardening.
Hardened steel is very brittle. Therefore, after hardening it usually undergoes tempering. Given equal hardness, a steel that was hardened with subsequent tempering will be more ductile (and hence more workable) than a steel that was subjected to slow cooling, which leads to the conversion of austenite into ferrite and cementite. This is the reason for the extremely wide use of hardened steel in technology: hardened steels may be used not only to produce steels with great hardness but also, after the proper tempering, steels of medium or low hardness, which have good structural properties.
Hardening of aging alloys. If the equilibrium concentration of a solid solution undergoes a substantial change upon a change in temperature, the separation of the excess of one of the components will occur upon cooling. Such a process is a diffusion process and may be suppressed by hardening (Figure 2). In this case, the purpose of hardening is to fix a super-saturated
solid solution at a low temperature (for example, at room temperature). Aging of the alloy can then occur at room temperature or at a higher temperature. An alloy with a structure formed during hardening and aging has high strength properties and a high coercive force (magnetic alloys). The so-called age-hardenable alloys, which undergo hardening and subsequent aging, are widely used. For example, Duralumin is used as a structural material, nimonik is used as a refractory material, and alnico is used for manufacturing permanent magnets.
Hardening of ordered alloys. Ordering of an alloy leads to a change in its physical and mechanical properties, such as a decrease in ductility. If ordering is undesirable, alloys undergo hardening. Hardening causes the fixation of a disordered state at low temperatures. Such fixation is possible if the processes leading to ordering do not occur at an excessively high rate.
Hardening of pure metals and single-phase alloys. Hardening of pure metals and single-phase alloys is also used in the study of vacancies and their influence on the mechanical and physical properties of materials. In this case the purpose of hardening is the preservation at low temperatures of the same concentration of vacancies that was a state of equilibrium at high temperatures. The subsequent heating of materials to temperatures at which vacancies become mobile results in increased resiistance to plastic flow (“quench hardening”) and decreased internal friction. A study of the relationship between the equilibrium concentration of vacancies and temperature and of the rate of removal of the excess vacancies fixed during hardening makes feasible a determination of the energy of formation and activation of vacancy migration. The sum of these energies determines the activation energy for self-diffusion.
Hardening of liquids. Hardening can retard the crystallization of liquids. In this case hardening results in the transition of the liquid to a vitreous state. The crystallization of metals proceeds too rapidly, so that a successful conversion to a vitreous, amorphous state is usually not achieved.
Hardening from the liquid state. Hardening from the liquid state is possible for some systems, which have a certain type of phase diagram. Such hardening makes it possible to avoid liquation, which arises during crystallization with conventional cooling rates, to produce a supersaturated solid solution containing a significantly larger amount of the second component than the amount allowed by the phase diagram, and to produce metastable phases, which do not form during slow crystallization and do not appear on a phase diagram.
REFERENCESHardy, O.K., and T. J. Hill. “Protsess vydeleniia.” In the collection Uspekhi fiziki metallov, vol. 2. Moscow, 1958. (Translated from English.)
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Fizicheskoe metallovedenie, fasc. 1–3. Edited by R. Cahn. Moscow, 1967. (Translated from English.)
in metals technology, increasing the breaking strength or the resistance to permanent set of the material constituting a semifinished or finished article.
Hardening is characterized by the degree of hardening, which is an indication of the relative increase in the breaking strength or resistance to permanent set of a material, as compared with the original value, that results from a hardening treatment. In many cases, it may also be characterized by the depth of hardening, that is, the thickness of the hardened layer. Hardening is usually accompanied by a reduction in ductility. Therefore, in practice, the choice of the method and of the optimum conditions for a hardening treatment depends on the maximum increase in the strength of the material that can be obtained without reducing the ductility of the material to an unacceptable level. Such a combination of strength and ductility provides the greatest structural strength.
While a material is being produced, it can be hardened by using the effects of heat and radiation, by alloying, or by adding hardeners—such as fibers or dispersed particles—to a metallic or nonmetallic matrix (seeCOMPOSITE MATERIALS).
The material constituting semifinished or finished articles may be hardened by mechanical, thermal, chemical, and other methods, as well as by combined techniques, such as case-hardening or thermomechanical techniques. The most widely used type of hardening treatment is surface plastic deformation, which is a simple and efficient means of increasing the load-bearing capacity and durability of machine parts and structural members. Surface plastic deformation is particularly useful for parts and members subjected to alternating loads, for example, axles, shafts, gears, bearings, pistons, cylinders, welded structures, and tools. Depending on the design and material properties of the parts or members and on the size and nature of the service loads, various types of surface plastic deformation are used, including knurling and rolling with rolls and balls, rolling with serrated rollers, diamond burnishing, mandreling, hydroabrasion, vibration, and shot blasting. Surface plastic deformation often reduces surface roughness to a considerable extent, increases the wear resistance of parts, and—in the case of combined hardening and finishing—improves the appearance of the surface of parts.
In the heat treatment of metals, hardening is achieved, in particular, by quenching and subsequent tempering. Specific kinds of thermomechanical treatment, including hot and cold working, substantially promote the improvement of strength properties. Case hardening is achieved by nitriding, cyaniding, carburizing, or diffusion coating. Diffusion coating consists in the saturation of the surface of a part with aluminum, chromium, or other metals.
Hardening is also achieved by using electrical and electrochemical methods. Such methods include ultrasonic, electrical, electron-beam, photon-beam, electrochemical, electrical-discharge, laser-beam, and shock-wave machining, as well as magnetic-pulse and electrohydraulic forming.
The hardening treatment may be a surface, bulk, or combined treatment. For example, cold plastic deformation of the surface of a material is a surface treatment. Bulk treatments include isothermal quenching, and combined treatments include heat treatment followed by surface plastic deformation. The bulk and surface treatments may be carried out in succession by several methods.
REFERENCESGuliaev, A. P. Metallovedenie, 4th ed. Moscow, 1966.
Guliaev, A. P. Prochnost’ metallov pri tsiklicheskikh nagruzkakh. Moscow, 1967.
Papshev, D. D. Uprochnenie detalei obkatkoi sharikami. Moscow, 1968.
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Danilevskii, V. V. Teknologiia mashinostroeniia, 3rd ed. Moscow, 1972.
Kartavov, S. A. Tekhnologiia mashinostroeniia. Kiev, 1974.
D. L. IUDIN