机械专业英语金属热处理

Heat treatment of metalIn industry today there are more than a thousand different metals being used to manufacture products. The modem automobile has more than one hundred different metals used in its construction. An attempt will be made in this passage to give an understanding of the basic classification of metals.Metals were formerly thought to be those elements that had a metallic luster and were good conductor of heat and electricity. Actually, metals are generally defined as those elements whose hydroxides from bases (such as sodium or potassium).the nonmetals* hydroxides from acids (such as sulphur). Metals may exist as pure elements. When two or more metallic elements are combined,they form a mixture called an alloyThe term alloy is used to identify any metallic system. In metallurgy it is a substance, with metallic properties, that is composed of two or more elements, in timately mixed. Of these elements one must be a metal. Plain carbon steel, in the sense, is basically an alloy of iron and carbon. Other elements are present in the form of impurities. However, for commercial purposes, plain carbon steel is not classified as an alloy steel.Alloy maybe further classified as ferrous and nonferrous. Ferrous alloys contain iron. Nonferrous alloys do not contain iron.All commercial varieties of iron and steel are alloys. The ordinary steels are thought of as iron-carbon alloys. However, practically all contain silicon and manganese as well. In addition, there are thousands of recognized alloy steels. Examples are special tool steels, steels for castings, forgings, and rolled shapes. The base metal for all these is iron.Steels arc often called by the principal alloying clement present. Examples arc silicon steel, manganese steel, nickel steel, and tungsten steel. Even nonferrous alloys may contain iron in a small amount, as impurities. Some of the nonferrous alloys are bronze, brass, and monel.Although pure metals solidify at a constant temperature, alloys do not. The first nuclei have a tendency to form at a higher temperature than that at which complete solidification occurs. Each element in an alloy has its own peculiarities relative to temperature. Thus, the change in temperature as solidification progresses causes the solid being formed to change in chemical composition.Many alloying elements dissolve in the base metal in different proportions in liquefied and solidified steels. The proportion of the alloying clement that remainsin solid solutions has a tendency to vary with the temperature and grain structure of the allov that is formed.Nonferrous metals are seldom formed in the pure state. They must be separated from the gangue before the ore can be reduced. Thus, a process known as ore-dressing is performed. Metals and metal compounds arc heavier than the ganguc. They settle to the bottom if such a mixture has been agitated in water. This process is similar to the method used by the early miners who panned for gold. However, refinements have been developed to speed up the accumulation of metal compound of metal compounds by using this principal.The reverberatory furnace is the type most often used in the smelting of nonferrous metals. This furnace is constructed of refractory brick with a steel structure on the outside. The charge is placed in the furnace and heated indirectly by the flame. Slag inducers or fluxes are added to the charge to reduce oxidation.Properties of metalsMetals have properties that distinguish them from other materials. The most important of these properties is strength, or the ability to support weight without bending or breaking. This property combined with toughness, or ability to bend without breaking, is important. Metals also have advantages regarding resistance to corrosion. They arc responsive to heat treatment.Metals can be cast into many shapes and sizes. They can be welded, hardened,and softened. Metals also possess another important property-recycling and reuse. When a particular product is discarded, it can be cut into convenient sections. These sections can be put into a furnace, remelted, and used in another product.The properties of metals may be classified in three categories: chemical properties, mechanical properties, and physical properties. Here we will emphasize the primary mechanical properties of metals. In understanding the related areas of metal working and methods used today, the mechanical properties of metals are of the utmost importance.The hardness of metals varies greatly. Some, like lead, can be indented easily. Others like tungsten carbide, approach diamond hardness. They arc of great value as dies for cutting tools of various types. Heat treatment causes changes in the hardness. Annealed tool steel can readily be machined. Often, this is difficult after it has been hardened and tempered. Annealed brass is comparatively soft. When cold-worked the hardness is greatly increased.A tough metal possesses very high strength. It also has the capability to deform permanently and resist rupture. Toughness enables the metal to survive shock or impact without fracture.The strength of a metal is its ability to resist deformation or rupture. In certain items, a combination of strength and plasticity is desirable. Machine tools are an example.AnnealingThe word anneal has been used before to describe heat-treating processes for softening and regaining ductility in connection with cold working of material. It has a similar meaning when used in connection with the heat treating of allotropic materials. The purpose of full annealing is to decrease hardness, increase ductility, and sometimes improve machinability of highcarbon steels that might otherwise be difficult to cut. The treatment is also used to relieve stresses, refine grain size, and promote uniformity of structure throughout the material.Machinability is not always improved by annealing. The word machinability is used to describe several interrelated factors, including the ability of a material to be cut with a good surface finish. Plain low carbon steel, when fully annealed, are soft and relatively weak, offering litter resistance to cutting, but usually having sufficient ductility and toughness that a cut chip tends to pull and tear the surface from which it is removed, leaving a comparatively poor quality surface, which results in a poor machinablity of many of the higher plain carbon and most of the alloy steels can usually be greatly improved by annealing, as they are often too hard and strong to be easily cut at any but their softest condition.The procedure for annealing hypoeutectoid steel is to heat slowly to approximately 60 "C above the Ac3 line, to soak for a long enough period that the temperature equalizes throughout the material and homogeneous austenite is formed, and then to allow the steel to cool very slowly by cooling it in the furnace or burying it in the maximum ferrite and the coarsest pearlite to place the steel in its softest, most ductile, and least strained condition.NormalizingThe purpose of normalizing is somewhat similar to that of annealing with theExceptions that the steel is not to its softest condition and the pearlite is left rather fine instead of coarse. Refinement of grain size, relief of internalstresses, and improvement of structural uniformity together with recovery of someductility provide high toughness qualities in normalized steel. The process is frequently used for improvement of machinability and for stress relief to reduce distortion that might occur with partial machining or aging.The procedure for normalizing is to austenitize by slowly heating to approximate 80c above the AC3 or Accm3 temperature for hypoeutectoid or hypereutectoid.Steels, respectively; providing soaking time for the formation of austcnitc; and cooling slowly in still air. Note that the steels with more carbon than the eutectoid composition arc heated above the Accm instead of the Ac 13 used for annealing. The purpose of normalizing is to attempt to dissolve all the cementite during austenitization to eliminate, as for as possible, the settling of hard, brittle iron carbide in the grain boundaries. The desired decomposition products are smallgrained, fine pearlite with a minimum of free ferrite and free cementite.SphcroidizingMinimum hardness and maximum ductility of steel can be produced by a process called spheroidizing, which causes the iron carbide to form in small spheres or nodules in a ferrite matrix. In order to start with small grains that spheroidize more readily, the process is usually performed on normalized steel. Several variations of processing are used, but all require the holding of the steel near the Al temperature (usually slightly bclow)for a number of hours to allow the iron carbide to form on its more stable and lower energy state of small, rounded globules.The main need for the process is to improve the machinability quality of high carbon steel and to pretreat hardened steel to help produce greater structural uniformity after quenching because of the lengthy treatment time and therefore rather high cost, sphcroidizing is not performed nearly as annealing or normalizing. Hardening of steelMost of the heat treatment hardening processes for the steel is the based on the production of high percentages of martensite .The first step, therefore, is that Used for most of the other heat-treating processes-treatment to produce austenite.Hypoeutectoid steels are heated to approximately 60t: above the Ac3 temperature and allowed to soak to obtain temperature uniformity and austcnitchomogeneity. Hypereutectoid steels are soaked at about 60'c above the Acl temperature, which leavesSome iron carbide present in the material.The second step involves cooling rapidly in an attempt to avoid pearlite transformation by missing the nose of the I-T curve. The cooling rate is determined by the temperature and the ability of the quenching media to carry heat away from the surface of the material being quenched and by the conduction of heat through the material itself. Table 11 -1 shows some of the commonly used media and the method of application to remove heat, arranged in order of decreasing cooling ability.High temperature gradients contribute to high stresses that cause distortion and cracking, so the quench should only as extreme as is necessary to produce the desired structure. Care must be exercised in quenching that heat is removed uniformly to minimize thermal stresses. For example, along slender bar should be end-quenched, that is, inserted into the quenching medium vertically so that the entire section issubjected to temperature change at one time. If a shape of this kind were to be quenched in a way that caused one side to drop in temperature before the other, change of dimensions would likely cause high stresses producing plastic flow and permanent distortion.Several special types of quench are conducted to minimize quenching stresses and decrease the tendency for distortion and cracking. One of these is called martempering and consists of quenching and austenitized steel in a salt at a temperature above that needed for the start of martensite formation (Ms). The steel being quenched is in this bath until it is of uniform temperature but is removed before there is time for formation of bainite to start. Completion of the cooling in air then caused the same hard martenside that would have formed with quenching from the high temperature, but the high themal or "quench" stresses that are the primary source of cracks and warping will have been eliminated.A similar process performed at a slightly higher temperayure is called austempering. In this case the steel is the formation of bainite. The bainite structure is not as hard as the marten site that could be formed form the same form composition, but in addition n to reducing the thermal shock to which the steel would be subjected under normal hardening procedures, it is unnecessary to perform any further treatment to develop good impact resistance in the high hardness range.TemperingA third step usually required to condition hardened steel for service is tempering, or as it is sometimes referred to, drawing. With the exception of austempered steel, which is frequently used in the as-hardened condition, most steel arc not serviceable "as quenched", the drastic cooling to produce martensite causes the steel to be very hard and to contain both macroscopic and macroscopic internal stresses with the result that the material has little ductility and extreme brittle ness reduction of these faults is accomplished by reheating the steel to some point below the AI (lower transformation )tcmpcraturc. Structural changes caused by tempering of hardened steel are functions of both time and temperature, with temperature being the most important. It should be emphasized that tempering is not a hardening process, but is, instead, the reverse. A tempered steel is one that has been hardened by heat treatment and then stress relieved, softened, and provided with increased ductility by reheating in the tempered or drawing procedure.The magnitude of the structural changes and the change of properties caused by tempering depend upon the temperature to which the steel is reheated. The higher the temperature, the greater the effect, so the choice of temperature will generally depend on willingness to sacrifice hardness and strength to gain ductility and toughness. Reheating to below 100 c has little noticeable effect on hardened plain carbon steel. Between 100'C and 200"C, there is evidence of some structural changes. Above 200"C marked changes in structure and properties appear. Prolonged heating at just under the Al temperature will result in a sphcroidized structure to that produced by the spheroidizing process.In commercial tempering the temperature range of 250'C-425'C is usually avoided because of an unexplained embrittlement, or loss of ductility, that often occurs with steels tempered in this range. Certain alloy steels also develop a "temper brittlcncss" in the temperature range of 425*C-600'C, particularly when cooled slowly from or through this range of temperature. When high temperature tempering is necessary for these steels, they are usually heated to above 600*c and quenched for rapid cooling. Quenches from this temperature, of course, do not cause hardening because austenitization has not been accomplished.As wc know, casting is a mechanical working process that forming a molten material into a desired shape by pouring it into a mold and letting it harden. When metal is not cast in a desired manner, it is formed into special shapes by mechanical working processes. Several factors must be considered when determining whether adesired shape is to be cast or formed by mechanical working. If the shape is very complicated, casting will be necessary to avoid expensive machining of mechanically formed parts. On the other hand, if strength and quality of material are the prime factors in a given part, a cast will be unsatisfactory. For this reason, steel castings arc seldom used in aircraft work.There are there basic methods of metal-working. They are hot working, cold working, and extruding. The process chosen for a particular application depends upon the metal involved and the part required, although in some distances you might employ both hot5-and cold-working methods in making a single part.Almost all steel is hot-working from the ingot into some form from which it is either hot-or cold-worked to the finished shape. When an ingot is stripped from its mold, its surface is solid, but the interior is still molten. The ingot is then placed in s soaking pit, which retards loss of heat, and the molten interior gradually solidifies. After soaking, the temperature is equalized throughout the ingot, which is then reduced to intermediate size by rolling, making it more readily handled.Hot working is the process in which the ingot is deformed mechanically into a desired shape. Hot working is usually performed at an elevated temperature. At high temperature, scaling and oxidation exist. Scaling and oxidation produce undesirable surface finish. Often times, most ferrous metals need to be cold-worked after hot working in order to improve the surface finish.The main principle behind hot working is to cause plastic deformation within the material. The amount of force needed to perform hot working is normally less than that for cold working. As such, the mechanical properties of the material remain unchanged during hot working. The reason that the properties of the materials are unaltered comes from the fact that the deformation is performed above the metal recrystallization temperature. Plastic deformation occurs with metals when deformed at above the recrystallization temperature. Plastic deformation occurs with metals when deformed at above the recrystallization temperature without any strain hardening. As a matter of fact, the metal usually experiences a decrease in yield strength when hot-working. Therefore, it is possible to hot-work the metal without causing any fracture.Hot working has the following advantage:Elimination of porosity.Uniform distribution of impurities.Refinement of coarse or columnar grain-better physical properties.Lesser energy requirement to deform the metal into shape.Disadvantages of hot working Lower dimensional accuracy.Higher total energy required (due to thermal energy to heat the work-piece). Work surface oxidation (scale), poorer surface finish, shooter tool lifeThere are generally two types of hot working process: rolling and forging. Rolling is a process whereby the shape of the hot metal is altered by the action of the rollers which acts to "squeeze" the hot metal into desired shape and thickness. One advantage effect of hot rolling is the fact that there is a grain refinement. Refined grain usually possesses better physical properties.Forging is another hot working method. In forging, the metal is pounded by hammer that or squeezed between a pair of shaped dies. The die acts as a hammer that can "pound" the hot metal into shape. The metal is desired. Forging is done either by pressing or hammering the heated steel until the desired shape is obtained.Complicated sections that cannot be rolled, or sections of which only a small quantity is required, arc usually forged. Forging of steel is a mechanical working of the metal above the critical range to shape the metal as desired. Forging is done either by pressing or hammering the heated steel until the desired shape is obtained.Pressing is used when the parts to be forged are large and heavy, and this process also replace hamming where high-grade steel is required. Since a press is show acting, its force is uniformly transmitted to the exterior to give the best possible structure as well as the exterior to give the best possible structure throughout.Hamming can be used only on relatively small piece. Since hamming transmits its force almost instantly, its effect is limited to a small depth. Thurs, it is necessary to use a very heavy hammer or to subject the part to repeated blows to ensure complete working of the section. If the force applied is too weak to reach the center, the finessed forging surface will be convex or bulged. The advantage of hammering is that the operator has control over the amount of pressure applied and the finishing temperature, and is able to produce parts of the highest grade.This type of forging is usually referred to as smith forging, and it is used extensively where only a small number of parts are needed. Considerable machining and saving when a part is smith forged to approximately the finished shape.。