金属的热处理外文翻译

1Heat treatment of metalIn industry today there are more than a thousand different metals being used to manufacture products. The modern 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 alloy The 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 are often called by the principal alloying element present. Examples are 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 totemperature. 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 element that remains in solid solutions has a tendency to vary with the temperature and grain structure of the alloy 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 are heavier than the gangue. 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 are 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 ofmetalworking 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 are 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 canusually be greatly improved by annealing, as they are often too hard and strong to be easily cut at any but their softest condition.The procedurefor annealing hypoeutectoid steel is to heat slowly to approximately 60℃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 80℃above the AC3 or Accm3 temperature for hypoeutectoid or hypereutectoid.Steels, respectively; providing soaking time for the formation of austenite; and cooling slowly in still air. Note that the steels with more carbon than the eutectoid composition are heated above the Accm instead of the Ac13 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.SpheroidizingMinimum 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 nodulesin 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 A1 temperature (usually slightly below)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, spheroidizing 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 60℃above the Ac3 temperature and allowed to soak to obtain temperature uniformity and austenite homogeneity. Hypereutectoid steels are soaked at about 60℃above the Ac1 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 eliminatedA 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 are 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 A1(lower transformation )temperature. 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, thereverse. 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℃has little noticeable effect on hardened plain carbon steel. Between 100℃ and 200℃, there is evidence of some structural changes. Above 200℃marked changes in structure and properties appear. Prolonged heating at just under the A1 temperature will result in a spheroidized structure to that produced by the spheroidizing process.In commercial tempering the temperature range of 250℃-425℃is usually avoided because of an unexplained embrittlement, or loss of ductility, that often occurs with steels temp ered in this range. Certain alloy steels also develop a “temper brittleness” in the temperature range of 425℃-600℃, 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℃and quenched for rapid cooling. Quenches from this temperature, of course, do not cause hardening because austenitization has not been accomplished.As we 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 a desired 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 are 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.Almostall steel is hot-working from the ingot into someform 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 in ter med iate s ize b y ro llin g,mak in g it mo re read ily ha nd led.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 workingLower 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, are 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.金属热处理在现代工业中，有近千种金属应用于生产。

Alessandro Anzalone Ph DAlessandro Anzalone, Ph.D.Hillsborough Community CollegeBrandon Campus1.Heating and Cooling of Metals2.The Iron Carbon Phase DiagramThe Iron-Carbon Phase Diagram3.Nonferrous Phase Diagrams4.Principles of Heat Treating5.Heat Treating Ferrous Metals6.Solution Heat Treating and Precipitation Hardening(Hardening Nonferrous Metals)7.Strengthening by Plastic Deformation and Alloying8.Annealing9.Heating Equipment10.ReferencesTemperature versus Time Curves Temperature versus Time CurvesThe states (liquid and solid) and the different atom lattices are referred to as phases (a phase being something separate, distinct, and homogeneous), so when these changes occur they aref d t h h d di f h threferred to as phase changes, and a diagram of where thesechanges take place is called a phase diagram. On the iron—carbon phase diagram lines marked liquidus and solidus areshown. Liquidus indicates the temperatures at which the various compositions of the alloy begin to become solid as thetemperature is reduced. Solidus indicates the temperatures atwhich the various compositions of the alloy are completely solid, and thus all liquid is absent. The ferrite and austenite phases areq p very important in heat treating and manufacturing.Fe-Fe3C Phase Diagram, Materials Science and Metallurgy, 4th ed., Pollack, Prentice-Hall, 1988/work/pAkmxBcSVBfns037Q0LN_files/image003.gifEutectic The root of this word means the lowest melting point. In Figure 3.4 the lowest melting point can be seen at 4.3 percentcarbon, and this point is identified as the eutectic in Figure 3.3. A t ti b th t t l i l d i th lleutectic can occur because the two metals involved in the alloysystem lack complete solubility; that is, they have partialsolubility, which also means they are partially insoluble. Eutectoid This word has the same root as eutectic but now refers to metals that are solids, so instead of being the lowest meltingpoint it is the lowest temperature at which one solid phasetransforms into other solid phases. The eutectoid also occurspbecause of insolubility, and in Figure 3.4 it can be seen that the single-phase austenite transforms into two phases, ferrite andcementite.Heat treating is generally identified with processes in which a metal is heated to an elevated temperature from which it is cooled very rapidly,and in the process the metal somehow gets harder and stronger. Butwhat is really going on?The phase diagrams of Figures 3.3 and 3.9 tell us what happens when the various compositions of alloys are heated or cooled under “equilibrium”conditions; for our purposes we can interpret equilibrium to mean thatthe alloys are heated or cooled very slowly. If, instead of cooling slowly,say we cool very rapidly by quenching in water, what can the phasediagram tell us? If our attempts at cooling are completely successful, wewill retain at room temperature whatever phase existed at the highertemperature. That is, we will have made the metal do something that byits original nature it was not supposed to do. Thus, for Figure 3.3 if weheat a steel (iron with 2 percent or less of carbon) into the austeniterange and then quench it we will have austenite at room temperaturecontaining much more carbon than iron should at that temperature.Note that the ferrite phase that normally exists at room temperaturecontains almost zero carbon./Quality_clip_image007.jpg /classes/MSE2094_NoteBook/96ClassProj/examples/icnew2.gif/work/pAkmxBcSVBfns037Q0LN_files/image004.gifHardening SteelsPlain carbon steel contains no alloying elements other than carbon and small percentages of elements such as manganese that are necessary in steel manufacture. It is used for knives, files, and fine cutting tools such aswood chisels because it will hold a keen edge. The hardness andstrength of alloy steels is determined by the carbon they contain, andother elements contribute properties to the steel such as corrosionresistance, and high-or-low-temperature strength. One of the majorreasons for using alloying elements is to gain hardenability, or as theword suggests, the ability to become hard. When alloying elements are added they usually slow down the rate at which austenite can changeinto the softer products that result from cooling, for example, pearlite.The effect of the alloy-ing elements is to move the TTT diagram to the right, giving the steel the time needed to cool to the Ms temperature and transform to martensite.The process of hardening steel is carried out in two operations. The first step is to heat the steel to a temperature that is slightly above the A3 andA3,1 lines on the iron—carbon phase diagram. This operation is called austenitizing by metallurgists. The austenitized steel (FCC crystalstructure) contains all the carbon in the interstices (spaces or voids in the lattice structure). The second step is to cool the red-hot metal soquickly that it has no opportunity to transform into softermicrostructures but still holds the carbon in solution in the austenite.This operation is called quenching. Quenching media, such as brine, tap water, fused salts, oil, and air, all have different cooling rates.Slower cooling is necessary for tool steels, and rapid rates are neededfor plain carbon steel. Rapid quenching can produce cracking in thicker sections and therefore is normally used on small or thin sections with low mass and for plain carbon steels.Hardening Cast IronsIn Figure 3.4 it can be seen that in the cast iron region, above the A31 transformation line, is an area containing austenite. This austenite can be cooled slowly to form pearlite, or rapidly to form martensite. Thus, all the forms of cast iron (white, gray, ductile, malleable) can beproduced with the same options as steels. Austenite can also be cooled on a path that will hold it above the Ms temperature and allow it totransform to bainite. This process is known as austempering and acurrently popular form of cast iron is ADI, or austempered ductile iron.TemperingMartensite, whether in a carbon or alloy steel, is very brittle until it is tempered. A tool that is hardened and not tempered will break in pieces when first used. Tempering involves reheating the hardened steel to a much lower temperature than that used for hardening, but it is morethan just a low-temperature anneal. During tempering some of thecarbon leaves the BCT martensite lattice and forms a complex carbide.In this process the steel loses some of its hardness, depending on thetemperature, and gains toughness, reducing brittleness. The higher the tempering temperature used, the softer the metal becomes. Oxide colors that form on the clean surfaces of steel in a given temperature rangeshow heat treaters the approximate temperature of the metal. This color method of tempering was used by black-smiths to determine thetemperature before they plunged the part into a water tank to stop the heating action. It is still used to some extent in small shops, but moreexact methods are used in the manufacturing of heat-treated steel parts.Surface HardeningOften, it is desirable to harden only the outer surface of a steel part, or to surface harden it to create a hard case around a softer core. A gear, for example, needs to be hard on its surface to resist wear but tough andimpact-resistant in its interior so it can resist sudden and repetitiveloads. There are two basic approaches to meeting this requirement: (1) adding carbon at the surface of an otherwise low carbon steel to change the chemistry of the surface and then heat treating the whole gear or (2) starting with sufficient carbon in the steel to achieve the hardnessrequired and heat treating only the outer surface. In the first methodthe surface chemistry of the steel is changed, and in the second it isselectively heat treated.Changing the Surface Chemistry. Carburization has been used for many years as a means of raising the car-bon content of the surface.This is done by diffusing carbonaceous or nitrogenous substances into the surface followed by heating and quenching in most cases. Some of these processes are carburizing, nitriding, carbonitriding, andcyaniding. Low-carbon steel can be carburized and surface hardened toa depth of about 0.003 in. by heating it with a torch to about 17000 F(927°C) and rolling it in a carbon compound such as Kasenit® followed by reheating and water quenching. In order to harden to 1/16 in. deep, the part must be packed in the carburizing compound and held at that temperature for about 8 hours. Nitriding produces a harder case with a lower temperature and less distortion. Other methods produce a more uniform, harder case than carburizing in a shorter time. Some of thedisadvantages to these methods of surface hardening are that the entire part must often be heated and quenched, altering its entire chemicalstructure. Rising energy costs and the need for increased productionefficiency have brought about the development of better surfacehardening methods./fp/0/251/374.jpgSelective Heat Treating of the Surface. Although induction hardening has been used for many years to harden small parts or the ways onmachine tools newer processes make use of this hardening process in a selective manner so that wear surfaces are hardened only in stripesmoving progressively along the surface. This is done on flat surfaces,inside cylinders, and for hearing races on shafts. In these quick-heating processes the heated area is self-quenched by the adjacent cold metal, resulting in a shallow hardened area in the form of a line (stripe) orspiral. Electron beam equipment is also capable of producing selectively hardened areas, but it usually is done in a vacuum. Laser systems can operate in ambient conditions for selective heat treating./img2/laserhaerten03.jpg /lsm3.jpgExample of 2014 Aluminum DataPhysical Data :Density (lb / cu. in.) 0.101Specific Gravity 2.8Melting Point (Deg F) 950Modulus of Elasticity Tension 10.6Modulus of Elasticity Torsion 4Chemistry Data :Aluminum Balance Chromium 0.1 maxCopper 3.9 -5Iron 0.7 maxMagnesium 0.2 -0.8Manganese 0.4 -1.2Remainder Each 0.05 maxRemainder Total 0.15 maxSilicon 0.5-1.2Specifications The following specifications cover Aluminum 2014* ASTM B209* ASTM B210* ASTM B211* ASTM B221S co 05Titanium 0.15 maxTitanium + Zinc 0.2 maxZinc 0.25 max * ASTM B241 (Pipe-Seamless)* ASTM B247 (Forging -Open Die)* ASTM BB241* DIN 3.1255* MIL T-15089* QQ A-200/2* QQ A-225/4* QQ A-250/4* QQ A-367 (Forging -Open Die)* SAE J454* UNS A92014The behavior of metal crystals under load depends on a number of factors:✓the interatomic bonding strength:✓irregularities in the lattice—vacancies and discontinuities: and✓the lattice type.The third factor, lattice type, determines two other very important factors:✓the density of the atoms in the atom planes of the lattice and✓the space or distance between the planes of atoms in the lattice.AlloyingMetals may also he hardened by blocking the slip planes with atoms of other elements or compounds, by alloying. The diameters of the atoms of two metals can vary by as much as ±14 percent and the two metals will still have some solubility. Although these differences may seem small, thecombining of two such atoms in the same lattice can double the strength of the alloy. This phenomenon is referred to as solid solutionstrengthening. Such an increase is not as dramatic as what can heaccomplished with heat treating, but alloying can still improveproperties enough to make some alloys useful as engineering materials. The term annealing refers to any one of several thermal processes: stress relieving, process annealing, normalizing, full annealing, orspheroidizing. In general, the purpose of these processes is to return a metal to a softer, more workable condition than before the treatment.Compared with heat treating annealing involves much slower coolingrates; in effect it is the opposite of heat treating. It should beappreciated that metals do not have to undergo one of these controlled thermal treatments for the effects to occur. That is, if a part is heated to cure an epoxy adhesive, and the temperature and time are sufficient,then the part can experience stress relief. Also, a heat-treated part may be fully annealed if it is welded.Metals go through three stages in turn as they are heated at increasing temperatures: stress relief, recrystallization, and grain growth. These stages occur in all metals, ferrous and nonferrous. We shall thereforeconsider these stages first and then apply that knowledge to specialapplications with ferrous metals including normalizing, full annealing, and spheroidizing.Stress ReliefAs its name suggests the stress relief process requires that the metal has experienced a forming or heating process (for example, welding) thathas left behind stresses that are called residual stresses. Such stresses are not the stresses that produced the plastic deformation of a rolling or forging process, for example, but rather these are elastic stresses leftresiding in the metal by these operations.Stress relief is often needed for castings and weldments. Large welded structures such as tanks are sometimes stress relieved by covering them on the outside with thermal insulation blankets and heating them onthe inside with propane burners.Thermal stress relief is preferred for most manufacturing processes;however, vibratory stress relief (VSR) is often used for cast or weldedstructures that are too large to fit into a heat-treat furnace. To use VSR effectively, there must be1.loading of a structure by means of resonance by close control of avibrator’s frequency,2.proper instrumentation to display the pertinent VSR data. RecrystallizationRecrystallization takes place when cold-worked metals are heated to their specific recrystallization temperatures. The stored energy from coldworking combines with the heat energy of the annealing furnace,enabling small nucleating sites to form that contain unstrained atomlattices. With time additional atoms form up on these lattices, andgradually the whole cold-worked structure is replaced with a new“recrystallized” structure. Because the number of nucleating sites isdetermined by the amount of cold work, highly cold worked metals will have the smaller grains after annealing.The following factors are important in recrystallization:1. A minimum amount of deformation is necessary forrecrystallization to occur, regardless of the temperature.2.Similarly, a minimum temperature is required for recrystallizationto occur regardless of the amount of cold work present.3.The larger the grain size before cold working the greater theamount of cold work, or temperature, is required to cause a givenamount of recrystallization.4.Increasing the time of anneal decreases the tempera-ture necessaryfor recrystallization.5.The recrystallized grain size depends mostly on the degree ofdeformation and, to some extent, on the annealing temperature.6.Continued heating after recrystallization (re-forming of grains) iscomplete increases the grain size.7.The higher the temperature at which the cold work is done, thelarger the amount of deformation required to cause an equivalentpercentage of cold work.Grain GrowthIn performing such annealing treatments it is important to avoid heating fortoo long a time and/or at too high a temperature that causes the metalto go into the third stage of heating grain growth. The large grains that to go into the third stage of heating—grain growth. The large grains thatresult improve a metal’s ductility but may cause the surface to beroughened in a condition called orange peel. Full annealing, with someamount of grain growth, is some-times done to facilitate a difficultforming operation, in which case the orange peel will probably beremoved. Full annealing is usually accomplished by cooling the metalfrom the annealing temperature in the furnace with the doors closed toachieve a very slow cooling rate. The resulting metallurgical structure isvery similar to what is predicted by equilibrium cooling on the phasediagram.Full annealingSpheroidizing/capabilities.html/images/SSL10542.JPG /yahoo_site_admin/assets/images/HEAT_TREAT_OVEN.166101828.jpg/00074169/b/0/Electrode-Salt-Bath-Furnace.jpg /images_di/photo-g/salt-bath-furnace-36594.jpg/unique/forcedconvection.jpg/images_di/photo-g/paternoster-furnace-352728.jpg/images_di/photo-g/aluminum-heat-treatment-bell-type-furnace-396269.jpg/upload_file/prod/emp/2008/oimg_GC00030132_CA00030133.jpg /Images/manufacturing/DSCN5810.JPG1.R Gregg Bruce, William K. Dalton, John E Neely, and Richard R Kibbe, , ModernMaterials and Manufacturing Processes, Prentice Hall, 3rd edition, 2003, ISBN:97801309469802./default.asp3./propertypages/2014.aspAlessandro Anzalone Ph DAlessandro Anzalone, Ph.D. Hillsborough Community College Brandon Campus。

外文原文Metal heat treatmentMetal heat treatment is a kind of craft to heat pieces of metals at the suitable temperature in some medium and to cool them at different speed after some time.The metal heat treatment is one of the important crafts in the machine-building, comparing with other technologies, the heat treatment seldom changes the form of the work pieces and chemical composition of the whole .it improve the serviceability of the work piece through changing their micro- work pieces, chemical composition, or surface. Its characteristic is improving inherent quality of work pieces which can not be watched by our eyes.In order to make the metal work piece have mechanics , physics and chemical property which are needed, besides the use of many materials and various kinds of crafts which are shaped , the heat treatment craft is essential. Steel is a wide-used material in the mechanical industry, its complicated micro-composition can be controlled through the heat treatment , so the heat treatment of the steel is a main content of the metal heat treatment . In addition aluminium, copper, magnesium, titanium and their alloys also can change their mechanics , physics and chemical property through the heat treatment to make different serviceability.During the process of development from the Stone Age to the Bronze Age and to the Iron Age, the function of the heat treatment is gradually known by people. As early as 770 B.C.~222 B.C., the Chinese in production practices had already found the performance of the copper and iron changed by press and temperature . White mouthfuls of casting iron’sgentle-treatment is a important craft to make farm implements.In the sixth century B.C., the steel weapon was gradually adopted. In order to improve the hardness of the steel, quench craft was then developed rapidly. Two sword and one halberd found in YANXIA, Hebei of China , had “MA structure” in its micro-composition which was quenched.With the development of quenching technology, people gradually found the influence of cold pharmaceutical on quality of quenching. Pu yuan a people of the Three Kingdoms(now, Shanxi province Xiegu town)made3000 knives for Zhu Ge-liang.the knives were quenched in Chengdu according to legend. This proved that the chinese had noticed the cooling ability of waters with different quality in ancient times, and the cooling ability of the oil and urine at the same time were found. People found a sword in Zhongshan tomb which were up to the Western Han Dynasty (B.C. 206 -A.D. 24 ),in whose heart department carbon was about 0.15-0.4%, but on whose surface carbon was about more than 0.6%.this has shown the use of the carburization craft. But as the secret of individual's " craftsmanship " at that time, the development was very slow.In 1863, Britain metallo graphy expert and geologist's discoverity that six kinds of different metallography organizations existed in the steel under the microscope, proved that the inside of steel would change while heating and cooling. the looks of steel at the high temperature would change into a harder looks when urgently colded. Frenchmen Osmon established Allotropic theory , and Englishmen Austin first made the iron- carbon looks picture .these tow theories set the theoretical foundation for the modern heat treatment craft . Meanwhile, people also studied the metal protection in the heating to avoid the metal's oxidizing and out of carbon inthe course.1850~1880s, there were a series of patent to use kinds of gases to heat (such as hydrogen , coal gas , carbon monoxide etc. ). Englishman's Rec obtained the patent of bright heat treatment of many kinds of metal in 1889-1890.Since the 20th century, the development of metal physics and transplantation application of other new technologies,make the metal heat treatment craft develop on a large scale even more. A remarkable progress was carburizition of gas in a tube of stoves in industrial production during 1901~1925; 1930s the appeariance of the electric potential different count and then the use of carbon dioxide and oxygen made stove carbon of atmosphere under control . In 1960s, hot treatment technology used the function of the plasma field, developed the nitrogen, carburization craft.The application of laser , electron beam technology, made the metal obtain new method about surface heat treatment and chemical heat treatment.The metal heat treatment craftThe heat treatment craft generally includes heating, keeping and cooling and sometimes only heating and cooling two progresses . The course links up each other.Heating is one of the important processes of the heat treatment . There are a lot of heating methods of the metal heat treatment . the first heat source were the charcoal and coal , then liquid and gaseous fuel. The application of the electricity is easy to control the heating, and no environmental pollution. the heat source could be heated directly or indirectly by the use of salt or metal of melting or the floating particle.While metal heated, the work piece in air , is often oxidized or take off carbon ( steel's surface carbon contentreduces).this does harm to the metal's surface performanc which is heated. Therefore metal should heat in the the vacuum or the melted salt, in controlled atmosphere or protected atmosphere . Sometimes it is heated in the protect means of coating or pack .Heating temperature is one of the important craft parameters of the heat treatment craft , choosing and controling heating temperature is a main matter of guaranting heat treatment quality. Heating temperature may change according to the different purposes of the heat treatment and different metal materials , but usually it is up to the temperature at which high temperature frame could be abtained.it must keep some time at the high temperature to make the inside and outside of the metal reach the some heating level,so that its micro-frame would turn out wholely.we call this period of time "keep-heat"time. There is no "keep-heat"time when adopting density heating and surface heat treatment of high energy because of the rapidity. But the chemical heat treatment often need much more time to sustain the heat .Cooling is an indispensable step in the craft course of heat treatment too . cooling methods are different because of crafts , mainly at controling the speed of cooling. generally anneals is slowest in speed, the cooling normalizing is a little fast in speed, the quenched cooling is much faster in speed. But there are different demands according to the kindof steel, for example empty hard steel can be cooled with normalize as quick as the speed by hard quench .The metal heat treatment craft can be divided into whole heat treatment , surface heat treatment and chemical heat treatment.Every kind could be divided into different crafts according to heating medium , heating temperature and coolingmethod. The same kind of metal adopting different heat treatment crafts can get different organizations which have different performance . The steel is the widest-used metal on the industry, and its micro- organization is the most complicated, so the steel heat treatment craft is various in style.The whole heat treatment is to change the whole mechanics performance of work piece through heating the work piece wholely and then cooling at the proper speed. The whole heat treatment of steel roughly has four basic crafts of annealing , normalizing , quenching and flashing back .Annealing means heating the work piece to the proper temperature ,then adopting different temperature retention time according to the material and size of work piece and then cooling slowly, whose purpose is to make the metal organization to achieve or close to the balance state, obtain good craft performance and serviceability, or prepare for quench further. normalizing is to cool in the air after heating the work piece at suitable temperature , its result is similar to annealing except that the organization out of normalizing are more refined which is often used to inhance the cutting performance of the material and is occationally used for the final heat treatment of material which are not high-requested. .Quenching is to cool work piece which has been heated and kept in warm fast in the cold medium as water , oil , other inorganic salts ,or organic aqueous solution and so on . The steel quenched becomes hard and fragile too. To reduce its fragility , we must first keep the quenched piece of steel in a certain temperature which is higher than room temperature but lower than 650℃for a long time,and then cool it again. this progress is called the flashing back . Annealing , normalizing,quenching , flashing back is " four fires " in the whole heat treatment . the quenching contact close to flashing back ,and they are often used together." Four fire "is divided into kinds of heat treatment crafts by different heating temperatures and diferent ways of cooling. What is " quality adjust " is a kind of craft combining "quench" with "high-temper a ture flash back" to make the work piece obtain certain intensity and toughness. Some alloy saturation out of quench can improve its hardness, intensity, electricity and magnetism after it is kept in the high proper temperature for a little long time . Such heat treatment craft is called “effective dealing”.Deformation-heat-treatment is the combination of pressure-deformation and heat treatment on work piece ,this mothod could enhance its intensity; and vacuum-heat-treatment is that work piece is heated in atmosphere or vacuum.It can make the work piece not oxidize or take off carbons , keep its surface bright and neat and improve its performance. At the same time ,it can carry on the chemical heat treatment by the pharmaceutics.Surface heat treatment on work piece is only to heat its cover to change the metal-layer's mechanics performance. In order to only heat the layer of work piece without making too much heat spreading into the inside, the heat source used must be of high density of energy , namely it can offer greater heat energy on the unit's area of the work piece and make its layer or parts reach high temperature in short-term or instantaneously. The main method of the surface heat treatment is "flame quenching" and "reaction heat" treatment and the heat source used commonly are flame as oxygen acetylene or propane, reaction electric current, laser and electron beam,ect.The chemical heat treatment is to alter the chemical composition, organization and performance of the top layer of work piece.The difference between Chemical and surface heat treatment is that the latter just change the chemical composition of the top layer of work piece . The former is to set the work piece heating in the medium (the gas , liquid , solid ) including carbon , nitrogen or other alloying elements,and then to keep it warm for longer time, thus to make elements as the carbon,nitrogen,boron and chromium,etc permeate through the top layer of work piece.Sometimes after permeation, there is other heat treatment craft to carry on such as quenching and flashing back . The main method of the chemical heat treatment include carbon,nitrogen, and metal permeation.The heat treatment is one of the important processes in machine components and tool and mould manufacture. Generally speaking, it guarantees and improves various kinds of performance of the work piece , for instance wear proof and anti-corrosion. It also improve the organization and state of the tough work piece to ensure various kinds of cooling and heating work.For example tin are annealed for a long time to turn into malleable cast iron which is of plasticity. proper heat treatment craft can prolong the gear wheel's service life at double or dozens of times than these without heat treatment ; In addition, the cheap carbon steel with some alloying elements permeated will own the alloy steel performance whose prices hold high so that it can replace some heat-resisting steel , stainless steel ; all tool and mould need to be through the heat treatment before in use..中文译文金属热处理金属热处理是将金属工件放在一定的介质中加热到适宜的温度，并在此温度中保持一定时间后，又以不同速度冷却的一种工艺。

英文原文HEAT TREATMENT OF METAL 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 o decrease hardness, increase ductility, and sometimes improve machinability of high carbon 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 steels, when fully annealed, are soft and relatively weak , offering little resistance to cutting, but udually having sufficient ductility and toughness that acut chip tends to pull and tear the surface from which it is removed, leaving a comparatively poor quality surface, which results in a poor machinability rating.1 For such steels annealing may not be the most suitable treatment. The machinability of many of the higher plain carbon and most of the alloy steels can usually be greatly improyed by annealing, as they are often too hard and strong to be easily cut at any but their softest condition.2 The procedure for annealing hypoeutectoid steel is to heat slowly to approximately 60 above the Ac3 line,3°°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 fumace or burying it in lime ot some other insulating material. The slow cooling is easential to the precipitation of 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 the exceptions that the steel is not reduced to its softest condition and the pearlite is left rather fine instead of coarse. Refinement of grain size, relief of internal stresses, and improvement ofstructural uniformity together with recovery of some ductility 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 approximately 80°above the Ao3 or Accm3 temperature for hypoeutectoid or hyereutectoid sreels, respectively.Providing soaking time for the formation of austenite; and cooling slowly in still air, Note that the steels with more carbon than the eutectoid composition are heated abou the Accm instead of the Ac13 used for annealing. The purpose of normalizing is to attempt to dissolve all the cementite during austenitization to eliminate, as far 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 cementite1 SpheroidizingMinimum 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 ofprocessing are used, but all require the holding of the steel near the A1 temperature {usually slightly below } 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, spheroidizing is not performed nearly as much as annealing or normalizing.Hardening of SteelMost of the heat treatment hardening processes for steel ate based on the production of high percebtages of martensite.The first step,therefore, is that used for most of the other heat-treating processes—treatmentto produce austenite. Hypoeutectoid steels ate heated to approximately 60°above the Ac3 temperature and allowed to soak to obtain temperature uniformity and austenite homogeneity. Hypereutectoid steels ate soaked at about 60°above the Ac1 temperature,which leaves some 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—Tcurve.The cooling rate is determined by the temperature and 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 stresser 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, a long slender bar should be end-quenched, that is, inserted into the qudenching medium vertically so that the entire section is subjected 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 tempeiature before the other, change of dimensions would likely cause high stresses producing plastic flow and permanent distortion.Seyeral special types of quench are conducted to minimize quenching stresses and decrease the tendency for distortion and cracking. One of these is called martemoering and consists of quenching an austenitized steel in a salt at a temperature above that needed for the start of martensite formation (Ms).The steel being quenched is held in this bath until it is of uniform temperature but is removed before there is time for formation of bainite topletion of the cooling in air then causes the same hard martensite that would have formed with quenching from the high temperature,but the high thermal or “quench” stresses that are the primary source of cracks and warping will have been eliminated.A similar process performed at a slightly higher temperature is called austempering.In this case the steel is held at the bath temperature for a longer period,and the result of the formation of bainite.The bainite structure is not as hard as the martensite that could be formed from the same composition,but in addition 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.4 TemperingA third step usually required to condition a hardened steel for swevice 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 are not serviceable “as quenched”.The drastic cooling to produce martensite causes the steel to be very hard and to contain both macroscopic internal stresses with the result that the material this little ductility and extreme brittleness. Reduction pg these faults is accomplished by reheating the steel to sometimes referred to, drawing. With the exception of austempered steel, which is frequently used in the as-hardened cognition, most steels are not serviceable “as quenched”, The drastic cooling to produce martensite causes the steel to be very hard and to contain both macroscopic and microscopic internal stresses with the result that the material has little ductility and extreme brittleness. Reduction of these faults is accomplished by reheating the steel to some point below the A1 (lower transformation) temperature.The 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 tempering or drawing procedure.The magnitude of the structural changes and the change of properties caused bytempering 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 hardenss and strength to gain ductility and toughness. Reheating to below 100°has little noticeable effect on hardened plain carbon steel. Between 100°and 200°,there is evidence of some structural changes. Above 200°marked changes in structure and properties appear . Prolonged heating at just under the A1 temperature will result in a spheroidized structure similar to that produced by the spheroidizing process.In commercial tempering the temperature range of 250—425°C is usually avoided because of an unexplained embrittlement,or loss of ductility, that often occurs with steels tempered in this range of 425—600°C,particularly when cooled slowly from or through this range of temperature.when high temperature remperature tempering is necessary for these steels,they are usually headed to above600 ºC and quenched for rapid cooling. Quenchesfrom this temperature, of course ,do not cause hardening because austenitization has not been accomplished.附录B汉语翻译金属热处理一退火在前面描述冷拔加工材料的软化并重新获得塑性的热处理方法时，就已使用退火这个词。

金属热处理专业词汇一、退火（Annealing [əˈniːlɪŋ]，名词）1. 完全退火（Full Annealing）- 定义：将亚共析钢加热到Ac3以上30 - 50℃，保温足够时间，使组织完全奥氏体化后缓慢冷却，以获得接近平衡组织的热处理工艺。

2. 不完全退火（Incomplete Annealing）- 定义：将亚共析钢加热到Ac1 - Ac3之间，保温后缓慢冷却的热处理工艺。

二、正火（Normalizing [ˈnɔːməlaɪzɪŋ]，名词）- 定义：将钢件加热到Ac3（或Accm）以上30 - 50℃，保温适当时间后在空气中冷却的热处理工艺。

其目的是细化晶粒、调整硬度、消除网状渗碳体等。

三、淬火（Quenching [ˈkwentʃɪŋ]，名词）1. 单液淬火（Single - liquid Quenching）- 定义：将加热到淬火温度的工件迅速放入一种淬火介质（如水、油等）中冷却到室温的淬火方法。

2. 双液淬火（Double - liquid Quenching）- 定义：工件先在一种冷却能力强的介质（如水）中冷却到接近Ms点（马氏体转变开始点），然后立即转入另一种冷却能力较弱的介质（如油）中冷却，以减少淬火内应力和变形开裂倾向的淬火方法。

四、回火（Tempering [ˈtempərɪŋ]，名词）1. 低温回火（Low - temperature Tempering）- 定义：回火温度在150 - 250℃之间，主要用于降低淬火应力、提高工件韧性，回火后得到回火马氏体组织，常用于高碳钢刀具、量具等的处理。

2. 中温回火（Medium - temperature Tempering）- 定义：回火温度在350 - 500℃之间，得到回火托氏体组织，可显著提高弹性极限和屈服强度，常用于各种弹簧的处理。

3. 高温回火（High - temperature Tempering）- 定义：回火温度在500 - 650℃之间，得到回火索氏体组织，可使工件具有良好的综合力学性能，生产中常把淬火加高温回火的复合热处理工艺称为调质处理。

中英文对照翻译Heat Treatment of SteelsHeat treating refers to the heating and cooling operations performed on a metal for the purpose of altering such characteristics as hardness, strength, or ductility. A tool steel in¬tended to be machined into a punch may first be softened so that it can be machined. After being machined to shape, it must be hardened so that it can sustain the punishment that pun¬ches receive. Most heating operations for hardening leave a scale on the surface, or contribute other surface defects. The final operation must, therefore, be grinding to remove surface de¬fects and provide a suitable surface finish.When a steel part is to be either hardened or softened, its temperature must be taken a¬bove the critical temperature line; that is, the steel must be austenitized. Usually a tempera¬ture of 50 to 100 degree above the critical temperature is selected, to ensure that the steel part reaches a high enough temperature to be completely austenitized, and also because fur¬nace temperature control is always a little uncertain.The steel must be held at furnace temperature for sufficient time to dissolve the carbides in the austenite, after which the steel can be cooled. How much residence time in the furnace is required is to some degree a matter of experience with any particular steel.Usually, for a 3/4 in. bar Clin=O. 0254m), 20 minutes or slightly more will do. Doub¬le the time for twice the diameter. Alloy steels may require a longer furnace time; many of these steels are best preheated in a lower-temperature furnace before being charged into the hardening furnace.When the heating time is completed, the steel must be cooled down to room tempera¬ture. The cooling method determines whether the steel will be hardened or softened. If the steel is quickly removed from the furnace and quenched into cold water, it will be hardened. If it is left in the furnace to cool slowly with the heat turned off, or cooled in air (small pieces of plain carbon steel cannot be air-softened,however), it will be softened. High-alloy steels may be hardened by air-cooling , but plain carbon steels must have a more severe quench, almost always water.There are several softening methods for steels , and the word softening therefore does not indicate what softening process or purpose was used. The method of softening by slow cooling from austenite is called annealing, not softening, Annealing leaves the steel in the softest possible condition (dead soft) .To conclude, the difference between hardening and annealing is not in the heating process but in the cooling process.Powder MetallurgyThe definition for the term powder metallurgy, as provided by the committee for Powder Metallurgy of the American Society for Metals, is＂The art of producing metal powders and objects shaped from individual, mixed, or alloyed metal powders, with or without the inclusion of nonmetallic constituents, by pressing or molding objects which may be simultaneously or subsequently heated to produce a coherent mass, either without fusion, or with the fusion of a low melting constituent only.＂Originally Developed as a Step in Refining. References to the granulation of gold and silver and subsequent shaping into solid shapes go back as far as 1574. It is also noteworthy that in the nineteenth century more metallic elements were produced in powder form than in any other form. For the most part, however, these were all precious or rare metals for which powder metallurgy was only practical method of manufacture, and it has only been within more recent year that this process has become competitive with more conventional processes in the manufacture of articles from iron, copper, aluminum, and the other more common metals.Two Unique Advantages. Early developments in powder metallurgy were based on two factors. During the production of platinum, tantalum, osmium, tungsten, and similar refractory metals, reduction was purely a chemical process from which the reduced metal was obtained as a precipitate in flake or powder form. Because furnaces and techniques were not available for complete melting of these materials, the only procedure for producing them in solid form was to press them into coherentmasses and sinter at temperatures below the melting point. This procedure still applies in the production of some metals, especially tungsten. A second major advantage of the process, which led to early use and is still applied today, is in the production of porous shapes obtained with lighter pressing pressures or lower sintering temperatures. Materials in this form are useful as chemical catalysts, filtering elements, and bearings.Process Involves a Series of Steps. Figure 12-13 shows the steps ordinarily required in the production of a part by the powder metallurgy process. Suitable powder must first be produced. While theoretically any crystalline material may be fabricated by powder metallurgy, the production of suitable has presented restrictions in many cases, either because of difficulty in obtaining adequate purity or because of economic reasons. After selection and blending of the powder and manufacture of a die for the shape to be produce, the powder is pressed to size and shape. The application of heat results in crystalline growth and the production of a homogeneous body.Properties Influcenced by Heat-Pressure Cycle. Various combinations of heat and pressure may be used. Some sintering takes place under high pressure at room temperature. However, cold pressing is usually followed by sintering at a temperature. Somewhat below the lowest melting point of any of the constituents. An intermediate elevated temperature may be used during pressing, then the shape removed from the press and subjected to higher temperature. In hot pressing, the final sintering temperature is applied simultaneously with the pressure.Application for Powdered Metal ProductsPowder metallurgy occupied tow rather distinct areas. It is a basic shape-producing method for practically all metals, in direct competition with other methods. In addition, for many refractory (high melting point) materials, both metals and nonmetals, powder metallurgy is the only practical means of shape production, tungsten is typical means of shape production. Tungsten is typical of the refractory metals; it has a melting point of 3400 ºC, and no satisfactory mold or crucible materials exist for using conventional casting techniques at this temperature. Tantalum and moly bdenum are similar. For some other metals, possible to melt, impuritiespicked up by the liquid from the containers would be undesirable, and powder metallurgy offers the most economical means of obtaining solid shapes.Cemented Carbides an Important Powder Product. Cemented carbides form one of the most important groups of materials that can be fabricated into solid shapes by powder metallurgy only. These materials will be discussed as cutting tools in later chapter, but their method of manufacture may be noted. The principal material used is tungsten carbide, although titanium carbide and tantalum carbide are also used. While it is possible to press and sinter these metal carbides in pure form, the resulting solid material is too brittle for most practical use. The addition of 3% to 20% cobalt or nickel powder yields a product with somewhat reduced hardness but with sufficient shock resistance to be useful for many applications in which high hardness and wear resistance at high temperatures are of importance.The final combination of hardness and ductility may be controlled by the percentage of nickel or cobalt added, with the smaller amounts yielding the hardest but most brittle products.Carbides have a high modulus of elasticity of 350 GP(50 million Psi) or more,acompared to 200 GP(30 million Psi) for steel. The quality of rigidity indicated byathis value is important in applications where minimum deflection under load is desired. Cemented carbides have high damping qualities, giving them an additional value m many machining operations where vibration might otherwise be a problem. Compressive strength varies from 3.5 to 6 GP(500,000-9000,000 Psi). Hardnessavalues of the different grades run from approximately Rockwell C 65 to 90. Cemented carbide was first used as a cutting-tool material prior to 1930. The earliest tools consisted only of tungsten carbide and cobalt, and their use was restricted to machining nonferrous materials and treatment has since developed a wide range of hardness and toughness properties that are applicable to most machining operations. Many of the variations have been classified into standard grades suitable for various cutting uses.At present there are four general classed of carides available for cutting tool usestraight tungsten carbide (WC); crater resistant steelcutting (We+Tic+Tac); straight titanium carbide (Tic); and coated carbides.As noted above, straight tungsten carbide is used principally for nonferrous metals and cast iron, both for roughing and finishing operations.The addition of titanium carbide and/or tantalum carbide to tungsten carbide increase the resistance to cratering and reduces the tendency for welding between the tool and work when machining steel.Straight titanium carbide with cobalt as a binder produces a very hard, wear resistant tool, suitable only for finishing operations on steel because of high brittleness and a tendency to chip under shock loading. Cemented carbide cutting tools are often made as composites by using two different grades of carbide or other materials. A tough, shock-resistant grade is used as the core of main body of the tool. This core is coated by vapor deposition with a very thin layer of titanium carbide, titanium nitride, hafnium nitride or aluminum oxide without cobalt or other binder. The core provides the toughness needed for shock loads and the coating provides a highly wear resistant surface, making the tools suitable for both roughing and finishing operations. Coated carbides are produced only as indexable inserts. Sintered Bearings. A further area in which powder metallurgy produces products not practical by other means is in the manufacture of materials with controlled low density. One of the first massproduced powder metallurgy products was sintered porous bronze bearings. After cold pressing, sintering, and sizing, the bearings are impregnated with oil, which in service is made available for lubrication. Although not true fluid film bearings, they provide long service with low maintenance. Porous materials are also useful as filters.Unusual Alloys Formed by Powder Metallurgy. Composite electrical materials form a group similar to the cemented carbides Tungsten and other refractory metals in combination with silver, nickel, graphite, or copper find wide application as electrical contacts and commutator brushes; powder metallurgy not only provides a means for producing the combination but also provides the finished shape for the parts. Many of the currently used permanent magnet materials are produced by powder metallutgy.钢的热处理热处理指对金属进行加热或冷却操作, 以改变其硬度、强度或延性等特性。

“金属热处理”Unit 4 metallurgy 冶金(术) nickel 镍chromium 铬manganese 锰molybdenum 钼tungsten 钨vanadium 钒silicon 硅plain-carbon steel 普通碳素钢iron-carbon diagram 铁碳(合金相)图delta region 铁素体区eutectoid region 共熔体区，共析区austenite 奥氏体ferrite (plus carbide) 铁素体(加碳化物) fcc (face-centered cubic) 面心立方bcc (body-centered cubic) 体心立方in solid solution 固溶体状态cementite 渗碳体，碳化铁specimen样品，试样etching 侵蚀加工，洗nitric acid 硝酸lamellar 屈状的，多片的pearlite 珠光体mother-of-pearl 珠母层(珠光体层) hypoeutectoid 亚共析的hypereutectoid 过共析的tie-line 截线lever-law 杠杆定律leverage杠杆作用proeutectiod 先共析体equilibrium 平衡，均衡，安静hardening 淬火，硬化quench 急冷、淬火tempering 回火，调质annealing 退火drawing 回火martensite 马氏体normalizing 正火spheroidizing 球化处理，延期热处理carburizing 渗碳carbonitriding 碳氮共渗cyaniding 氰化处理nitriding 氮化处理0C(degree centigrade) 摄氏度0F(degree Fahrenheit) 华氏度Unit 4 HEAT TREATMENT OF METALS金属热处理(The understanding of heat treatment is embraced by the broader study of metallurgy.)深入研究冶金学可以掌握热处理的知识。