金属热处理-外文翻译

1
Heat treatment of metal
In 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 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 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 metals
Metals 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 of
metalworking 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.
Annealing
The 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 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.
Normalizing
The purpose of normalizing is somewhat similar to that of annealing with the
Exceptions 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 internal
stresses, and improvement of structural 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 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.
Spheroidizing
Minimum 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 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 steel
Most 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 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-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 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 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.
Tempering
A 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, 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℃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 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 life
There 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。

现代汽车制造就需要用一百多种金属。

下面，我们将试图通过这篇文章让大家对金属的基本分类有个了解金属过去被认为是具有金属光泽、良好的导电性和导热性的物质。

实际上，金属一般被定义为其氢氧化物为碱性的物质（如钠、钾），而非金属则定义为其氢氧化物为酸性的物质（如硫黄）。

金属可以是由单一元素构成的纯金属，也可以是由两种或两种以上金属元素组成的化合物，这种化合物就称为合金.
“合金”这个术语用于识别任何金属系统。

在冶金学中，合金是由两种或两种以上的元素均匀混合、具有金属性质的物质。

在所构成的元素中，必须是一种是金属。

例如，普通碳钢就是主要由碳和铁组成的合金，当然还包括其他的一些杂质元素。

然而，出于商业目的，普通碳钢没有被归类于合金。

合金又可细分为铁合金和非铁合金，其区别就在于是否包含铁元素。

所有的商业用钢、铁都是合金。

最普通的钢，就是铁合碳的合金，但实际上，它还包含微量的硅元素和锰元素。

另外，还有很多的合金钢，例如特殊工具钢（用于铸造、锻造和锟型）等，这些合金的基底元素都是铁。

通常可根据合金中的主加元素来区分钢的种类。

例如硅钢、锰钢、镍钢和钨钢等。

即使在非铁合金中铁也可以作为杂质少量的存在，如青铜、黄铜和锰乃尔铜。

纯金属具有固定的结晶温度，但合金没有。

最开始凝固的晶核要比后凝固的晶核温度高。

合金中的每一种的元素都有它自己的温度特性。

所以，在凝固过程中温度变化会促成固体的形成从而使合金的化学成分发生变化。

许多的合金元素在液态钢和固态钢的基底金属中的溶解率是不一样的。

固溶体里的合金元素的溶解率取决于温度及合金形成的晶粒结构。

非铁金属在自然界很少以纯金属的形态存在。

要想获得纯的金属，就要从矿石中将脉石分离出开。

在分离之前要选择合适的矿石，这个过程通常称为选矿。

纯金属或者金属化合物要比脉石重，当我们将矿石在热水中搅拌后，纯金属和金属化合物将会沉淀于底部被分离出来。

这种方法有点类似于早期矿工的淘金。

然而，利用这个原理发展起来的精炼技术提高了金属化合物的凝聚速度。

非铁金属的熔化最常用到反射炉。

此种炉的构造包含两层，里层由耐火砖组成，外层是钢。

原料放入其中，间接进行加热，在加热过程中，要在原料中加入一些诱导物或造渣物或熔剂，用于脱氧。

金属具有很多区别于其它材料的特性。

其中最重要的就是强度，它是一种承受载荷不发生弹性变形和折断的能力。

这种性质非常重要，它通常适合韧性-金属
发生变形而不断裂的能力联系在一起的。

除此之外金属还具有抗腐蚀的能力，这种能力的高低与热处理相关。

金属可以铸成许多不同的形状和尺寸。

它们既可以焊接处理，也可以进行淬火和退火处理。

回收利用是金属的另一个很重要的性质。

当某一产品不能再被使用时，我们就可以将它分割成便于移动的若干小块，然后将它们放进炉子内，重新熔化，铸成新的产品。

金属的特性可分为三类：即化学特性、机械特性和物理特性。

这里我们着重介绍金属主要的机械特性。

在了解金属加工的相关领域以及目前所采用加工方法时，金属机械特性就显得尤为重要。

金属硬度的变化范围很大。

有些金属，例如铅，很容易塑造成型。

还有些金属，例如碳化钨（硬质合金），它的硬度已经接近钻石的硬度。

它们在制造各种硬质合金刀具时具有很高的价值。

热处理可以导致金属硬度发生变化。

退火工具钢易于机加工，但经淬火和回火（调质处理）后，机加工就变得困难了。

退火以后的黄铜相对来说较软，当把它进行冷加工以后，它的硬度则大大提高。

韧性高的金属强度也很高，它承受载荷而不发生永久变形和断裂的能力也很强。

金属具备了韧性就能够承受撞击和冲击载荷而不发生断裂。

金属的强度是一种抵抗变形和断裂的能力。

在实际应用中，我们所希望是强度和塑性的完美结合，机床就是一个例证。

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。

当用于同素异晶材料的热处理时，该词具有相似的意义，完全退火的目的是降低硬度，增加塑性，有时也提高高碳钢的切削加工性，否则这种钢很难加工。

这种热处理方法也用来减少应力，细化晶粒，提高整个材料的结构均匀性。

退火不总是能提高切削加工性，切削加工性一词用来描述几个相关因素，包括材料加工时获得好的表面光洁度（即较小的表面粗糙度值）的能力。

当完全退火时普通低碳钢硬度较低，强度较小，对切削的的阻力较小，但通常由于塑性变形和韧性太大以致切削离开表面是会划伤表面，工件表面质量比较差，导致较差的表面切削加工性。

对这类钢，退火可能不是最好的处理方法。

许多高碳钢和大多数合金钢的切削加工性能通常可经过退火大大改善，因为除在最软条件下，它们的硬度和强度太高而不易加工。

共析钢的退火方法是将钢缓慢加热到Ac3线以上大约60摄氏度，保温一段时间，使整个材料温度相同，形成均匀奥氏体，然后随炉或埋在石灰或其它绝缘材料中缓慢冷却。

要吸出粗大铁素体和珠光体，使钢处于最软、最韧和应变最小
的状态，必须缓慢冷却。

正火
正火的目的多数类似于退火，但铁不是均匀状态且自古光体是细均而不粗大。

钢的正火能细化晶粒。

释放内应力，改善结构均匀性同时恢复一些塑性，得到高的韧性。

这种方法通常用来改进切削加工性能，减少内应力，减少因内部切削加工或时效产生的变形。

正火的方法是将亚共析钢或过共析钢缓慢加热到Ac3线或Accm线上约80摄氏度，保温一段时间以便形成奥氏体，并在静止空气中缓慢冷却。

要注意，含碳量超过共析钢成分的钢要加热到Accm线以上，而不是退火时的Ac1线以上。

正火的目的是在奥氏体化过程中试图溶解所有渗碳体。

从而尽可能多的减少晶界上的硬脆铁谈化合物，而得到小晶粒的细珠光体、最小自由铁素体和自由渗碳体。

球化退火
通过球化退火可以得到最小的硬度和最大的塑性，它可使铁碳化合物以小球状分布在铁素体基体上。

为了使小颗粒球化更容易，通常对正火钢进行球化退火。

球化退火可由几种不同的方法，但所有方法都需在A1线温度附近(通常略低）保温很长时间，使铁碳化合物形成更稳定，能及较低的小圆球。

球化退火的方法的主要目的是改进高碳钢的切削加工性，并对淬硬钢进行预处理，使其淬火后结构更均匀。

因为热处理时间长，因此成本高，球划退火不如退火或正火常用。

钢的硬化
钢的大多数热处理化方法是基于产生高比例的马氏体。

因此，第一步用的是大多数其它热处理用的方法-产生奥氏体。

亚共析钢加热到Ac3温度以上大约60摄氏，进行保温，使温度均布奥氏体均匀。

过共析钢在Ac1线温度以上大约60
摄氏度时保温，刚中仍残留部分铁碳化合物
第二步是快速冷却，力图比年在等温曲线鼻部产生珠光体转变。

冷却速度取决于温度和淬火时淬火介质从钢表面带走热量的能力以及钢本身传热的能力。

高的温度梯度产生高应力，会引起变形和开裂，所以淬火只有在非常需要产生特定结构时才使用。

淬火时必须小心，使热量均匀扩散以减小热应力。

比如，一根细长棒端部淬火，即将它垂直插进冷却介质中，这样整个截面，同时产生温度变化。

如果这种形状的工件的某一边比另一边早降温，尺寸变化很可能引起很高的应力，产生塑性流动和永久变形。

用几种特殊的淬火方法可减少淬火应力，减少变形开裂倾向。

一种称为分级淬火，其方法是：将奥氏体钢放入温度高于马氏体转变起始温度（MS）的盐浴中，
放置一定的时间直道温度均匀，在开始形成贝氏体之前取出，然后放在空气中冷却，产生与从高温开始淬火时同样硬的马氏体，而导致开裂和翘曲的高的热应力或淬火应力已经被解除。

在略高一点温度下的类似方法称为等温淬火，这时，将（奥氏体）钢放在盐浴中，保持很长时间，等温处理的结果是形成贝氏体。

贝氏体结构不如在同样成分时形成的马氏体硬，但除了减少钢在正常淬火时受到的热冲击外，不必要进一步处理，就可获得在高硬度时好的冲击韧性。

回火
调整淬硬钢以便使用的第三步通常是回火。

除了等温淬火钢通常在淬火状态下使用外，大多数钢都不能在淬火状态下使用。

为产生马氏体而采用的激冷使钢很硬，产生宏观内应力和微观内应力，使材料塑性很低，脆性极大。

为减少这种危害，可通过将钢再加热到A1线（低温转变）以下某一温度。

淬火回火时产生的结构变化是时间和温度的函数，其中温度是最重要的。

必须要强调，回火不是硬化方法，而是刚好相反。

回火钢是将经热处理硬化的钢，通过回火时的再加热，来释放应力、软化和提高塑性。

回火引起的结构变化和性能变化取决于钢重新加热的温度。

温度越高，效果越大，所以温度的选择通常取决于牺牲硬度和强度换取塑性和韧性的程度。

重新加热到100摄氏度以下，被淬火普碳钢影响不大，在100摄氏度到200摄氏度之间，结构会发生某些变化，在200摄氏度以上，结构和性能够显著变化。

在紧靠着A1温度以下的长时间加热会产生与球化退火过程类似的球化结构。

在工业上，通常要避免在250-425摄氏度范围内回火，因为这个范围围内回火的钢经常会产生无法解释的脆性或塑性丧失现象。

一些合金钢在425-600沙氏度范围内，也会产生“回火脆性”，特别是从（或通过）这个温度范围缓慢冷却时会出现。

当这些钢必须高温回火时，他们通常加热到600摄氏度以上并快速冷却。

当然，从这个温度快冷会产生硬化，因为没有进行奥氏体化。

我们都知道，铸造就是将溶化的金属浇注进预订铸膜中，并使其冷却成形的一个机械加工过程。

当金属没有按预定形状进行铸造的时候，就要通过机械加工方法使其成形。

当决定要采用哪种方法的时候，应该综合考虑多种因素。

如果零件的形状非常复杂，就必须采用铸造的方法，这样可以避免昂贵的机加工费用。

另一方面，在指定的零件中，如果材料的强度和质量是主要的因素，这时采用铸造就不合适。

正是由于这个原因，在飞机制造业上很少应用铸铁。

金属加工有以下三种基本的方法。

它们是热加工、冷加工和挤压。

虽然在加工某一个工件的时候，既可以用热加工也可以用冷加工，但还是应该根据所采用
的金属和对零件的要求来选择加工的方法。

几乎所有的钢都是由工业纯铁经过热加工得到的，把钢再进行热加工或冷加工就可以制成最终产品。

当把铁锭从模具中取出来时，它的表面是凝固的，但其内部却仍是液态。

然后再将铁锭放进均热炉内，减缓热量的损失，使内部熔化的金属渐渐的凝固下来。

此过程结束以后，纯铁各处的温度都相等，然后通过热轧使它的尺寸变得适中，这样就可以更加方便的处理。

热加工的主要原理是使材料发生塑性变形。

它所需要的力一般比冷加工小。

在热加工的过程中并没有改变金属的机械特性，原因就在于发生变形的温度高于金属的再结晶温度。

金属在高于再结晶温度下发生的塑性变形是没有任何应变强化的。

因此，对金属进行热加工而不使其断裂是有可能的。

热加工主要有以下优点：1、消除空隙；2、使杂质分布均匀；3、细化晶粒—获得较好的物理特性；4、降低金属成型需要的能量。

热加工也有以下缺点：1、尺寸精度较低；2、需要总能量较多（加热工件也需要热量）；3、工件表面氧化（剥落），表面光洁度差；4、缩短刀具寿命。

热加工有两种工作类型：热轧和锻造。

热轧即热的金属在轧制机上被挤压成需要的形状和厚度。

这样可以细化金属颗粒，是金属得到更好的物理特性。

锻造是另一种热加工的方法。

锻造时金属被锤连续撞击或者被一对成形模具挤压，使其成形。

模具就像锤子一样撞击金属使之成形。

锻造主要用于粗加工。

需要说明的是锻造通常不适于生产几何形状复杂的零件。

锻造经常用来处理那些热轧解决不了的复杂工件或者加工一些小批量的工件。

钢的锻造是一种作用于热金属表面通过挤压和锤击使其成为我们所需要的形状的一种机械加工操作。

当我们所需要处理零件的尺寸太大或太重时，这时我们就需要压制了。

它可以代替锤子来代替优质碳素钢。

由于加压的过程是缓慢的，它的力量可以缓慢的传到工件中心，这样就使整个工件内外获得了最好的组织结构。

用锤子击打只能用于小件产品中。

由于锤子传递的力是瞬时的，它的影响只能先订到很小的深度，因此，就有必要换个大锤或反复锤击工件，以确保工件的最终完成。

如果锤击的力量达不到中心，工件的表面就会凹下去，相反地，力量过大，表面就会凸起和膨胀。

锤击的好处就是操作者可以控制力量的大小以及结束温度，得到高质量懂得产品。

这种锻造方法通常称为自由锻，广泛的应用于小批量零件的加工。

当对工件要求不高时，通过此种方法加工工件可以节省很多的原料以及简化不少机加工工序。