Heat Treatment of Metals

a process involving the heating and cooling of metals and alloys in order to bring about desired changes in structure and properties. The process can also include deformation and chemical and magnetic effects.

History. Man has heat-treated metals since antiquity. In the Aeneolithic period, primitive man, in forging native gold and copper without heat, discovered the change in structure known as cold working. This effect hampered the production of articles with thin edges and sharp tips, and to restore ductility the smith would have to heat the forged copper in a hearth. The earliest evidence of the use of annealing to soften cold-worked metal dates from the end of the fifth millennium B.C. This annealing was the first instance of the heat treatment of metals. In making weapons and implements from iron obtained by blooming, the smith would heat the semifinished shape packed in charcoal as preparation for hot working (forging). The heating would carbur-ize the iron; that is, cementation—a form of chemical treatment—would occur. Upon cooling the forged article of car-burized iron in water, the smith would see an increase in hardness and an improvement in other properties. The process of hardening carburized iron by quenching in water was used from the end of the second to the beginning of the first millennium B.C. In Homer’s Odyssey (eighth-seventh centuries B.C.), we see the lines “As the smith dips the glowing ax or poleax into the cold water, the iron hisses and with gurgling becomes stronger, tempered in fire and water.” In the fifth century B.C., the Etruscans quenched mirrors made of bronze having a high content of tin in water, mainly to improve the shine after polishing.

Cementation of iron in charcoal or other organic matter, followed by hardening and tempering of the steel obtained, came to be widely used in the Middle Ages to produce knives, swords, files, and other implements. Not understanding the changes occurring within the metal, the medieval artisan often ascribed the improved properties obtained by heat treatment to supernatural causes. Until the mid-19th century, knowledge of heat treatment was limited to a number of set procedures accumulated from many centuries of experience.

Technological needs, especially the need to produce steel cannons, transformed heat treatment from an art into a science. In the mid-19th century, when the military was seeking to replace bronze and cast-iron cannons with stronger, steel counterparts, the problem of producing high-strength barrels took on greater urgency. Even though metallurgists knew procedures for melting and casting steel, gun barrels often exploded for no apparent reason. At the Obukhov Plant in St. Petersburg, D. K. Chernov, in examining microscopically pickled sections from a gun muzzle and observing under a magnifying glass the metal structure at the place of fracture, concluded that steel becomes stronger as its structure becomes finer. In 1868, Chernov discovered the internal structural transformations that occur in cooled steel at certain temperatures that he called critical points a and b. If the steel is heated to a temperature below point a, it cannot be hardened, and the steel must be heated to a temperature above point b to produce a fine-grained structure. Chernov’s discovery of critical points for structural transformations in steel provided the scientific basis for the choice of a heat-treatment cycle for obtaining the required properties in steel articles.

In 1906, A. Wilm of Germany discovered Duralumin and, in the process, age hardening—a highly important method for hardening alloys based on such metals as aluminum, copper, nickel, and iron. Thermomechanical treatment of copper alloys undergoing aging was developed in the 1930’s, and the 1950’s saw thermochemical treatment of steel, making possible a significant increase in the strength of steel articles. Among the combined methods of heat treatment is thermomagnetic treatment, which can bring about an improvement in magnetic properties by cooling an article in a magnetic field.

An elegant theory of heat treatment has been developed through numerous studies on changes in the structure and properties of metals and alloys upon heating.

The classification of the types of heat treatment is based on the nature of the changes occurring in the metal’s structure. The classification comprises heat treatment per se, chemical heat treatment, which combines thermal and chemical effects, and thermomechanical treatment, which includes both thermal effects and plastic deformation. Heat treatment per se encompasses annealing, stress relieving, and heating for homogenization, annealing with phase transformations, hardening with and without polymorphic transformations, age hardening, and tempering.

Annealing, stress relieving, and heating for homogenization. Distortions in structure arising from casting, welding, forming, or other technological processes can be wholly or partly corrected by annealing, stress relieving, and heating for homogenization. The processes that remove the distortions proceed on their own, and heating merely effects an acceleration. Key factors in annealing, stress relieving, and heating for homogenization are the temperature and the length of time that the metal is held at the temperature.

Heating for homogenization is an operation for removing the effects of dendritic segregation, as a result of which the chemical composition within the crystals of the solid solution after crystallization is nonuniform. Segregation can also be accompanied by the appearance of a nonequilibrium phase, for example, a chemical compound that embrittles the alloy. The diffusion that occurs with the heating applied to effect homogenization leads to the dissolution of the nonequilibrium excess phases, resulting in a more homogeneous alloy. Heating for homogenization also raises ductility and corrosion resistance.

Recrystallization annealing removes deviations in structure from the equilibrium state caused by plastic deformation. When the metal is hardened through cold (pressure) working, there is a rise in strength but a drop in ductility owing to an increase in the density of dislocations within the crystals. Upon heating the cold-worked metal above a certain temperature, primary recrystallization commences and then gives way to secondary recrystallization, whereupon the density of the dislocations falls sharply. As a result, the metal becomes more ductile at the expense of strength. Recrystallization annealing is used to improve formabil-ity and to impart to the metal the necessary combination of hardness, strength, and ductility. The goal in recrystallization, as a rule, is to produce a textureless material devoid of anistotropy. Recrystallization annealing is used in the production of plates from transformer steel to obtain a desired metal texture upon recrystallization.

Stress relieving is used for articles in which during forming, casting, welding, heating, and other technological processes excessively high residual stresses are introduced. Residual stresses can cause distortions in the shape and dimensions of articles during processing, use, or storage. The yield point decreases upon heating, and when it becomes less than the residual stresses, the stresses are quickly relieved by plastic flow in various layers of the metal.

Annealing with phase transformations. Annealing involving phase transformations is used only for metals and alloys that undergo phase transformations with changes in temperature. With this type of annealing, qualitative or purely quantitative changes in the phase composition (types and volumes of the phases) occur with heating, and reverse changes are seen upon cooling. The main parameters of this type of annealing are temperature, length of time that the temperature is held, and cooling rate. The annealing temperature and time are chosen so as to ensure the necessary phase changes, for example, polymorphic transformations or dissolution of an excess phase. The operation is usually conducted so as to prevent the growth of large grains in the phase that is stable at the annealing temperature. The cooling rate should be low enough so that reverse phase transformations, for which diffusion forms the basis, can occur upon lowering the temperature. Either furnace cooling or air cooling can be used, and in the latter case the process is called normalization. Annealing with phase transformations is used most frequently in steels to effect grain refinement, softening, and better machinability.

Hardening without polymorphic transformations. Hardening without polymorphic transformations is used for any alloy in which, upon heating, the excess phase either wholly or partly dissolves in the main phase. Again, the most important parameters are temperature, length of time that the temperature is held, and cooling rate. The cooling rate should be high enough to avoid separation of the excess phase. (The process of phase separation is brought about by the redistribution through diffusion of the components of a solid solution.) This rate will be high enough in the case of Duralumin and copper alloys if the alloys are quenched in water; magnesium alloys and certain austenitic steels can be hardened through air cooling. A supersaturated solid solution is formed as a result of hardening.

Hardening without polymorphic transformations can both strengthen and weaken an alloy, depending on the phase composition and structural features in the original and hardened states. Aluminum-magnesium alloys are hardened to raise the strength; in beryllium bronze, the strength after hardening is lower, and the ductility higher, than after annealing, and hardening can be used here to raise ductility prior to cold working. The main purpose of hardening without polymorphic transformations is to prepare an alloy for age hardening.

Hardening with polymorphic transformations. Hardening with polymorphic transformations is used for any metal or alloy in which the crystal lattice is rearranged upon cooling. The main parameters of the operation are temperature, length of time the temperature is held, and cooling rate. The temperature is raised above the critical point to form a high-temperature phase. Cooling should proceed at a rate that will prevent a “normal” diffusive transformation and foster a lattice rearrangement through the mechanism of a nondiffusive martensitic transformation. Since martensite is formed through hardening with polymorphic transformations, this treatment is called martempering. Carbon steels undergo martempering and are quenched in water, and many alloy steels, in which diffusive processes proceed slowly, can be subjected to martempering with cooling in oil and even in air.

The main purpose of martempering is to increase hardness and strength and to prepare the metal for tempering. The high strength of steels from martempering is explained by the formation of a supersaturated solution of carbon atoms occupying interstitial positions between atoms of α-iron, by the appearance of a large number of twin bands, and by the increase in the density of the dislocations upon martensitic transformation. The high strength also derives from the anchoring of the dislocations by carbon atoms and dispersed carbide particles, which can separate out at the dislocations at sites of carbon segregation. Carbon steel becomes extremely brittle with martempering, mainly because of the impeded motion of the dislocations in martensite. Iron alloys not containing carbon remain ductile after martempering.

Age hardening. Age hardening is applied to alloys that have been subjected to hardening without polymorphic transformations. The supersaturated solid solution in such alloys is thermodynamically unstable, tending to spontaneous decomposition. Age hardening consists in the formation, through diffusion within the grains of the solid solution, of zones enriched in a dissolved element (Guinier-Preston zones) and/or of dispersed particles of excess phases, most often chemical compounds. These zones and dispersed particles of precipitated phases prevent the dislocations from sliding and thus increase hardness. Age-hardened alloys are therefore called precipitation-hardened.

The main parameters of age hardening are temperature and length of time the temperature is held. A rise in temperature accelerates the diffusive decomposition processes of the solid solution, and the alloy is quickly hardened. When the alloy is held at a sufficiently high temperature for a certain length of time, over-aging occurs; that is, the hardness of the alloy is reduced. The reason for overaging is the coagulation of the dispersed precipitate, that is, coagulation involving the dissolution of the smaller particles and growth of the larger particles of the precipitated phase. As a result of coagulation, the distance between these particles is increased and the motion of the dislocations within grains of the solid solution is less impeded. Some alloys, for example, the Duralumin alloys, become extremely hard when held at room temperature after hardening (natural aging). Most alloys are heated after hardening to accelerate the decomposition of the supersaturated solid solution (artificial aging). Age hardening is sometimes carried out in stepwise fashion, with the alloy held first at one temperature and then at another temperature. Age hardening is used mainly to raise the strength and hardness of structural materials (aluminum, magnesium, copper, and nickel alloys; certain alloy steels), as well as to increase the coercive force of magnetically hard materials. Aging time for obtaining desired properties varies from tens of minutes to several days, depending on the alloy composition and the aging temperature.

Tempering. Tempering is applied to alloys, mainly steels, that have undergone martempering. The main process parameters are temperature, length of time the temperature is held, and, in certain cases, cooling rate (to prevent temper brittleness). Marten-site occurs in steels as a supersaturated solution, and the basic structural changes upon tempering are similar to those upon age hardening, that is, decomposition of a thermodynamically unstable supersaturated solution. Tempering differs from age hardening mainly in the features of the martensite substructure but also in the behavior of carbon within the martensite of hardened steel. Martensite typically has a large number of crystal defects, such as dislocations. Carbon atoms quickly diffuse into the martensite lattice and concentrate at the dislocations. Dispersed carbide particles can also act this way immediately after hardening or even during quenching. As a result, hardened steel will be at or near its maximum precipitation hardness. Therefore, upon precipitation of dispersed carbide particles from martensite during tempering, the hardness of the steel is not increased at all and the strength is raised by only a small amount, if at all. The weakening of the martensite is explained by the reduction of the carbon concentration through the carbide precipitation. Thus, tempering usually leads to a reduction in hardness and strength and a simultaneous increase in ductility and resilience.

Tempering of iron alloys not containing carbon that have undergone martempering can result in high precipitation hardness, owing to the precipitation from the supersaturated solution of dispersed particles of intermetallic compounds. The reason for the strengthening in tempering is the same as in age hardening, and the terms “tempering” and “age hardening” are often used synonymously.

Heat treatment, in bringing about various types of structural changes, permits a control over the structure of metals and alloys and makes possible the production of articles with the required combination of mechanical, physical, and chemical properties. This ability, together with the simplicity and inexpensiveness of the equipment used, has made heat treatment the most common industrial method for modifying properties of metallic materials.

At metallurgical plants, heating for homogenization is carried out on ingots prior to forming operations in order to raise the ductility; annealing is used on plates, strips, pipes, and wires to eliminate the hardening effect of cold work; and hardening, tempering, age hardening, and thermomechanical treatment are used to strengthen rolled products and pressed articles. At machine-building plants, forgings and other semifinished shapes are annealed to decrease hardness and improve machinability. In addition, hardening, tempering, age hardening, and chemical heat treatment are applied to various machine parts and tools to raise the strength, hardness, resilience, and resistance to fatigue and wear; articles are also annealed to reduce residual stresses. In industries connected with instrument-making, electrical engineering, and radio engineering, such operations as annealing, hardening, tempering, and age hardening are used to modify the mechanical, electrical, magnetic, and other physical properties of metals and alloys.

The following examples show the degree to which heat treatment will change mechanical properties. Recrystallization annealing of cold-rolled copper lowers the ultimate strength from 400 to 220 meganewtons per sq m (MN/m²), or from 40 to 22 kilograms-force per sq mm (kgf/mm²), and at the same time raises the elongation from 3 to 50 percent. Annealed U8 steel has a hardness of 180 HB; hardening raises the figure to 650 HB. Steel 38 KhMIuA has a hardness of 470 HV after hardening, but after nitriding the hardness of the surface layer reaches 1,200 HV. The ultimate strengths of Duralumin D16 after annealing, hardening, and natural aging are, respectively, 200,300, and 450 MN/m² (20, 30, and 45 kgf/mm²). The elastic limit σ_0.002 of beryllium bronze Br. B2 after hardening is 120 MN/m² (12 kgf/mm²); after age hardening it is 680 MN/m² (68 kgf/mm²).