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A material is generally used because it offers the required properties at reasonable cost.Appearance is also an important factor.Perhaps the most common classification that is encountered in materials selection is whether the material is metallic or nonmetallic.The common metallic materials are such metals as iron,copper,aluminum,magnesium,nickel,titanium,lead,tin,and zinc and the alloys of these metals,such as steel,brass,and bronze.The metallic material is further classified as ferrous (iron and its alloys)and non-ferrous (all other metallic materials)metal.They possess the metallic properties of luster,thermal conductivity,and electrical conductivity;and they are relatively ductile;and some have good magnetic properties.The common nonmetals are wood,brick,concrete,glass,rubber,and plastics.Their properties vary widely,but they generally tend to be less ductile,weaker,and less dense than the metals,and they have no electrical conductivity and poor thermal conductivity.
Although it is likely that metals always will be the more important of the two groups,the relative importance of the nonmetallic group is increasing rapidly,and since new nonmetals are being created almost continuously,this trend is certain to continue.In many cases the selection between a metal and nonmetal is determined by a consideration of required properties.Where the required properties are available in both,total cost becomes the determined factor [1] .
One material can often be distinguished from another by means of physical properties,such as color,density,specific heat,coefficient of thermal expansion,thermal and electrical conductivity,magnetic properties,and melting point.Some of these,for example,thermal conductivity,electrical conductivity,and density,may be of prime importance in selecting material for certain specific uses.However,those properties that describe how a material reacts to mechanical usages are often more important to the engineers in selecting materials in connection with design.These mechanical properties relate to how the material will react to the various loading service.
Mechanical properties are the characteristic response of materials to applied forces.These properties fall into five broad categories:strength,hardness,elasticity,ductility,and toughness.
(1)Strength is the ability of a material resist applied forces.Elevator cables and buildings beams all must have this property.
(2)Hardness is the ability of a material resist penetration and abrasion.Cutting tools must resist abrasion,or wear.Metal rolls for steel mills must resist penetration.
(3)Elasticity is the ability to spring back to original shape.All springs should have this quality.
(4)Ductility is the ability to undergo permanent changes of shape without rupturing.Stamped and formed products must have this property.
(5)Toughness is the ability to absorb mechanically applied energy.Strength and ductility determine a material’s toughness.Toughness is needed in railroad cars,automobile axles,hammers,and similar products.
The main advantage of metals is their strength and toughness.Concrete may be cheaper and is often used in building,but even concrete depends on its core of steel for strength.Not all metals are strong,however.Copper and aluminum,for example,are both fairy weak,but if they are mixed together,results in an alloy called as aluminum-bronze alloy,which is much stronger than either pure copper or pure aluminum.Alloying is an important method of obtaining whatever special properties required:strength,toughness,resistance to wear,magnetic properties,high electrical resistance or corrosion resistance[ 2 ].
Plastics have specific properties,which may make them preferable to traditional materials for certain uses.In comparison with metals,for example,plastics have both advantages and disadvantages.Metals tend to be corroded by inorganic acids,such as sulphuric acid and hydrochloric acid.Plastics tend to be resistant to these acids,but can be dissolved or deformed by solvents,such as carbon tetrachloride,which have the same carbon bases as the plastics[ 3 ].Metals are more rigid than most plastics,while plastics are very light,with a specific gravity normally between 0.9 and 1.8.More plastics do not readily conduct heat or electricity.Plastics soften slowly and can easily be shaped while they are soft.
It is their plasticity at certain temperatures,which give plastics their main advantages over many other materials.It permits the large-scale production of molded articles,such as containers,at an economic unit cost,while other materials require laborious and often costly processes involving casting,shaping,machining,assembly and decoration.Plastics are lighter and more corrosion-resistant,but they are not usually as strong.Another problem with plastics is what to do with them after use.Metal objects can often be broken down and the metals are recycled;plastics can only be dumped or burned.
[1]Where the required properties are available in both,total cost becomes the determined factor.
在两种材料都能提供所需性能时，总成本就成了决定性因素。（句中where引导状语从句，这一段讨论的是金属和非金属两组材料，所以句中的both是指这两组中的双方）
[2]Alloying is an important method of obtaining whatever special properties...corrosion resistance.
合金是获得各种所需特殊性能（强度、韧性、耐磨性、磁性、高电阻或耐腐蚀性）的重要方法。（句中whatever是“不管什么”“诸如此类”的意思。冒号引起罗列各种相关的性能，相当于such as）
[3]Plastics tend to be...,which have the same carbon bases as the plastics.
塑料能抵抗这些酸，但会被诸如同样有酸基的四氯化碳这样的溶剂溶解或致变形。（注意到句中采用的是非限定性定语从句，其中的动词是复数形式。这个定语修饰的是solvents而不是carbon tetrachloride）
材料通常因为它能以合理的成本提供需要的性能而被选用。外观也是一个重要的因素。材料选择中碰到的最普通的分类或许在于材料是金属还是非金属。常用的金属材料有铁、铜、铝、锰、钛、铅、锡、锌和它们的合金，如钢、黄铜、青铜。金属材料被进一步分为黑色金属（铁及合金）和有色金属（所有其他金属材料）。黑色金属具有金属的导热性、导电性等特性，外表光泽相对而言延展性好，有些还具有良好的磁性。常用的非金属有木材、砖、混凝土、玻璃、橡胶和塑料。它们的性能变化很大，但通常趋于延展性差、强度低、比金属密度小、无导电性且导热性差。
尽管金属可能总是在这两类材料中更为重要，但非金属的相对重要性正在迅速攀升，并且由于不断有新的非金属产生，这种趋势必定还将继续。在许多情况下，在金属和非金属之间的选择根据对所需性能的考虑来决定。在两种材料都能提供所需性能时，总成本就成了决定性因素。
一种金属常常通过物理性能来区别于另一种金属，例如颜色、密度、比热、热膨胀系数、导热性和导电性、磁性和熔点。在为某一特定用途选用材料时，某些性能，如导热性、导电性和密度，可能是首要的。材料对各种机械用途所表现的性能的反应，对机械工程师在设计选材时往往更为重要。这些性能关系到材料对各种负载工况的反应。
力学性能是材料对所施外力的响应特征。这些性能分为五大类：强度、硬度、弹性、延展性和韧性。
1.强度是材料抵抗外力的能力。电梯吊缆和建筑梁都有这样的性能。
2.硬度是材料抵抗渗透和耐磨损的能力。刀具必须要耐磨损，钢铁碾磨机的金属滚轮必须能抵抗渗透。
3.弹性是弹回到原始形状的能力。所有的弹簧必须有这种性能。
4.延展性是承受永久变形而不破裂的能力。冲压成型产品必须有这种性能。
5.韧性是吸收机械能量的能力。强度和延展性决定材料的韧性。铁路车辆、汽车车桥、锻锤及类似产品需要这种性能。
金属的主要优点在于它们的强度和韧性。混凝土也许因价廉而经常用于建筑，但需依赖钢筋提高强度。并非所有的金属都有高强度，例如，紫铜和铝的强度都低，但如果它们混合在一起，生成一种称为铝青铜的合金，就比纯铜或纯铝的强度高得多。合金是获得各种所需特殊性能（强度、韧性、耐磨性、磁性、高电阻或耐腐蚀性）的重要方法。
塑料有特殊的性能，这使它们对某些特定用途比传统材料更可取。例如，与金属相比，塑料有优点也有缺点。金属易于被诸如硫酸、盐酸等无机酸锈蚀。塑料能抵抗这些酸但会被诸如同样有酸基的四氯化碳这样的溶剂溶解或致变形。金属比大多数塑料刚性更好，而塑料非常轻，相对密度在0.9~1.8之间。大多数塑料不能导热或导电，软化缓慢，但极易在软化状态成型。
正是在特定温度下的塑性，赋予了塑料超越其他材料的主要优点，如容器之类的模压产品在大规模生产时使用塑料可降低单价，使用其他材料往往耗时费力，其昂贵的加工过程包括铸造、成型、机加工、装配和装饰。塑料更轻、更耐腐蚀，但强度低。另一个问题是塑料用过之后怎么处理。金属通常可以解体回收；塑料只能丢弃或焚烧。
Heat treatment is thermal cycling involving one or more reheating and cooling operations after forging for the purpose of obtaining desired microstructures and mechanical properties in a forging.
Few forgings of the types are produced without some form of heat treatment.Untreated forgings are usually relatively low-carbon steel parts for noncritical applications or are parts intended for further hot mechanical work and subsequent heat treatment.The chemical composition of the steel,the size and shape of the product,and the properties desired are important factors in determining which of the following production cycles to use.
The object of heat treating metals is to impart certain desired physical properties to the metal or to eliminate undesirable structural conditions which may occur in the processing or fabrication of the material.In the application of any heat treatment it is desirable that the “previous history”or structural condition of the material be known so that a method of treatment can be prescribed to produce the desired result.In the absence of information as to the previous processing,a microscopic study of the structure is desirable to determine the correct procedure to be followed.
The commercial heat treatments in common use involve the heating of the material to certain predetermined temperatures,“soaking”or holding at the temperature,and cooling at a prescribed rate in air,liquids,or retarding media [1] .
Spheroidizing is heating of iron-based alloys at a temperature slightly below the critical temperature range followed by relatively slow cooling,usually in air.Small objects of high carbon steel are more rapidly spheroidized by prolonged heating to temperatures alternately within and slightly below the critical temperature range[ 2 ].The purpose of this heat treatment is to produce a globular condition of the carbide.
Normalizing is heating iron-base alloys to temperatures approximately 50℃above the critical temperature range followed by cooling in air to below the range.The purpose is to put the metal structure in a normal condition by removing all internal strains and stresses given to the metal during some processing operation.
Hardening is a process to increase its hardness and tensile strength,to reduce its ductility,and to obtain a fine grain structure.The procedure includes heating the metal above its critical point of temperature,followed by rapid cooling.As steel is heated,a physical and chemical change takes place between the iron and carbon.The critical point,or critical temperature,is the point at which the steel has the most desirable characteristics.When steel reaches this temperature,somewhere between 1 400 and 1 600°F,the change is ideal to make for a hard,strong material if it is cooled quickly.If the metal cools slowly,it changes back to its original state.By plunging the hot metal into water,oil,or brine (quenching),the desirable characteristics are retained.The metal is very hard and strong and less ductile than before.
Steel that has been hardened by rapid quenching is brittle and not suitable for most uses.By tempering or“drawing”,the hardness and brittleness may be reduced to the desired point for service conditions.As these properties are reduced,there is also a decrease in tensile strength and an increase in the ductility and toughness of the steel.The operation consists of the reheating of quench-hardened steel to some temperature below the critical range,followed by any rate of cooling.Although this process softens steel,it differs considerably from annealing in that the process lends itself to close control of the physical properties and in most cases does not soften the steel to the extent that annealing would.The final structure obtained from tempering fully hardened steel is called tempered martensite.
Tempering is possible because of the instability of the martensite,the principal constituent of hardened steel.Low temperature draws,from 300 to 400°F(150 to 205℃),do not cause much decrease in hardness and are used principally to relieve internal strains.As the tempering temperatures are increased,the breakdown of the martensite takes place at a faster rate,and at about 600°F(315℃)the change to a structure called tempered martensite is very rapid.The tempering operation may be described as one of precipitation and agglomeration,or coalescence of cementite.A substantial precipitation of cementite is at 600°F(315℃),which produces a decrease in hardness.Increasing the temperature causes coalescence of the carbides,with continued decrease in hardness.
The primary purpose of annealing is to soften hard steel so that it may be machined or cold-worked.This is usually accomplished by heating the steel to slightly above the critical temperature to form austenite,holding it there until the temperature of the piece is uniform throughout,and then cooling at a slowly controlled rate so that temperature of the surface and that of the center of the piece are approximately the same.This process is known as full annealing because it wipes out all trace of previous structure,refines the crystalline structure,and softens the metal.Annealing also relieves internal stresses previously set up in the metal.
When hardened steel is reheated to above the critical range,the constituents are changed back into austenite,and slow cooling then provides ample time for complete transformation of the austenite into the softer constituents.For the hypoeutectoid steels,these constituents are pearlite and ferrite.
The temperature to which given steel should be heated in annealing depends on its composition,and for carbon steels it can be obtained readily from the iron carbide equilibrium diagram.The heating rate should be consistent with the size of sections so that the center part is brought up to temperature as uniform as possible.When the annealing temperature has been reached,the steel should be held there until it is uniform throughout.For maximum softness and ductility,the cooling rate should be very slow,such as allowing the parts to cool down with the furnace.The higher the carbon content,the slower this rate must be.
[1]The commercial heat treatments in common use involve the heating of the material to certain predetermined temperatures,“soaking”or holding at the temperature,and cooling at a prescribed rate in air,liquids,or retarding media.
经常使用的商业热处理是指把材料加热到某一预定的温度，在此温度下进行均热，即进行保温，并按规定速度在空气、液体或在保温介质中冷却。（句中的the heating...“soaking”...and cooling是involve的宾语;or holding是soaking的同位语,or译为“即”）
[2]Small objects of high carbon steel...the critical temperature range.
在临界温度和略低于临界温度的范围内长时间重复加热，小型高碳钢零件可以更迅速地球化。
热处理是锻后一次或多次重新加热和冷却操作的热循环过程，使锻件获得所需的显微组织和力学性能。
几乎所有锻件都需要进行某种形式的热处理。没有经过热处理的锻件，要么是应用场合不太重要的碳含量相对较低的零件，要么是那些需要进一步热加工后再热处理的锻件。钢的化学成分、产品的规格和形状以及所希望的性能是选择接下来所使用的生产工艺的重要因素。
金属热处理的目的是使金属获得所需的物理性能，或者是消除那些在材料生产和加工中可能出现的组织结构缺陷。应用任何热处理方法都应该知道“以前的历史”或材料的组织状态，这样才能够确定一种具体的处理方法并达到预期的结果。如果缺乏前面加工的有关信息，必须对组织结构进行显微研究，以便确定要采用的正确的步骤。
经常使用的商业热处理是指把材料加热到某一预定的温度，在此温度下进行均热，即进行保温，并按规定速度在空气、液体或保温介质中冷却。
球化处理是指在略低于临界温度的条件下，对铁基合金持续长时间加热，紧接着以较慢的速度在空气中冷却。在临界温度和略低于临界温度的范围内长时间重复加热，小型高碳钢零件可以更迅速地球化。这种热处理的目的是生成球状碳化物组织。
正火是指把铁基合金加热到临界温度50℃以上，紧接着在低于临界温度下自然冷却。其目的是消除金属的组织结构中因某种加工操作产生的全部内应力，以便使金属恢复到正常状态。
淬火工艺可以增加材料的硬度、抗拉强度，减少其延展性并获得细微的晶格，方法是把金属加热到材料的临界温度之上后迅速冷却。加热时，铁和碳之间发生物理化学反应。而在临界点或者临界温度，钢可以获得最理想的特性。当钢被加热到1400°F~1600°F（760℃—871℃）后，如果迅速冷却，将有利于生成一种坚硬、强韧的材料。如果慢慢冷却，金属会回变成原来的状态。把热金属投入水、油或者盐水（淬火液）中，金属理想的特性被保持——非常坚硬、强韧，但延展性比原来有所降低。
迅速淬火的钢很脆，应用十分有限，回火可以降低硬度和脆性，使材料满足工作条件。随着这些特性的降低，钢的抗拉强度同时降低，但延展性和韧性有所提高。回火操作是指把淬过火的钢重新加热到临界温度以下，然后以任意的速度冷却。虽然这种工艺使钢变软了，但与退火不同的是，这种工艺本身可以精密控制材料的物理特性，使钢在大多数情况下要硬一些。充分淬火的钢经回火所得的最终结构称为回火马氏体。
淬过火的钢的主要成分是马氏体，由于不太稳定，因而可以进行回火处理。低温回火的温度为300°F~ 400°F（150℃~205℃），硬度降低不多，主要用于释放内应力。随着回火温度的增加，马氏体的分解速度越来越快，在大约600°F（315℃）时迅速变成回火马氏体。回火操作可以描述为渗碳体的沉淀、积聚或结合。渗碳体在600°F（315℃）时充分积聚，硬度降低。增加温度碳素体继续积聚，硬度继续降低。
退火的主要目的是使硬度较高的钢变软，以利于机加工或冷加工。通常的操作是把钢稍微加热到临界温度以上形成奥氏体并保温，当整个零件的温度完全均衡后再慢慢冷却，以保证零件表面与中心的温度大致相同。这种工艺称之为完全退火，因为它消除了所有前面工序遗留的结构缺陷，细化了晶格，使金属变软。退火还可以释放金属中残留的内应力。
当淬过火的钢被重新加热到临界温度之上时，其成分会变成奥氏体，此时缓慢冷却可以提供充分的时间把奥氏体完全转换成为较软的组织。对于低碳钢（亚共析钢），这些组织就是珠光体或铁素体。
退火时的加热温度依赖于钢的成分，碳素钢的加热温度从铁碳平衡图中很容易获得。加热的速度应该与零件截面的大小一致，以便使零件中心部分的温度尽可能均衡。当加热到退火温度时，应该保温，使零件彻底热透。为了尽可能使钢变软并获得最好的延展性，冷却速度应该十分缓慢，例如可以随加热炉一起冷却。碳的含量越高，冷却速度必须越慢。