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1、7.1 NATURE OF PLASTIC DEFORMATIONPlastic deformation is the deformation which is permanent and beyond the elastic rang of the material of ten , metals are worked by pfastic deformation because of the beneficial effect that is imparted to the mechanical properties by it. The necessary deformation in
2、a metal can be achieved by application of large amount of mechanical force only or by heating the metal and then applying a small force.7.1 塑性变形本质塑性变形是超过弹性变形范围之后的一种永久变形。通常，金属采用塑性变形加工是因为可以通过其获得良好的机械性能。金属所需要的变形可以只通过施加大量的机械力或者通过加热金属并且施加少量的应力来获得。The deformation of metals, which is caused by the displace
3、ment of the atoms is achieved by one or both of the processes called slip and twinning. The details of the microscopic deformation methods can be found in the textbooks of metallurgy. On the macroscopic scale, when plastic , deformation occurs the metal appears to flow in the solid state along speci
4、fic directions, which are depedent on the type of processing and the direction of applied force. The crystals or grains of the metal are elongated in the direction of metal flow. This flow of metal can be seen under microscope after polishing and suitable etching of the mental surface . These visibl
5、e lines are called fibre flow lines, some representative specimens of which are presented in fig 金属的变形是由原子排列引起的，这种排列是由被称作滑移和孪生过程中的一种或者两者共同作用造成的。微观的变形方式可以从冶金学课本中找到。在微观范围内，当塑性变形产生的时候，在固体内部金属的流动倾向于特定的方向，这个方向取决于加工类型和应力方向。在金属流动方向上金属晶体或者晶粒将会延长。通过显微镜观察抛光和蚀刻后的表面，可以看到这种金属的流动方式。这些可见的线条叫做纤维流线。Since the grains
6、are elongated in the direction of flow, whey would be able to offer more resistance to stresses acting across them . As a result, the mechanically worked metals called wrought products would be able to achieve better mechanical strength in specific orientation ,that of the flow direction .since it i
7、s possible sible to control these flow lines in any specific direction by careful manipulation of the applied forces as shown in Fig. 7.1, it is possible to achieve optimum mechanical properties. The metal, of course, would be weak along the flow lines.因为晶粒在流动方向上被拉长了，这样他们就更能够抵抗作用于其上的应力。因此，在被称为锻造产品的机
8、械加工零件的金属流动方向上会获得优良的强度。这是因为，只要小心谨慎地控制应力，就能让纤维流线朝着任何一个方向流动并且获得最佳的机械性能。当然，金属在沿着纤维线的方向也会变得疲软。The wastage of material in metal-working processes is either negligible or very small, and the production rate is in general very high .these two factors give rise to the economy in production材料的浪费在金属加工过程中是很少的，甚
9、至可以忽略不计，并且其生产率一般来说是非常高的。这两个因素使其在生产上更为经济。7.1.1 Hot Working and Cold WorkingThe metal-working processes are traditionally divided into hot -working and cold-working processes. The division is on the basis of the amount of heating applied to the metal before applying the mechanical force.7.1.1 冷加工和热加工传
10、统的金属加工工艺分为热加工和冷加工。这种区分是基于机械应力施加前金属的受热程度。Those processes , working above the recrystallization temperature, are termed hot-working processes whereas those below are termed cold-working processes在这些加工工艺中，加热温度在再结晶温度以上的叫做热加工，在再结晶温度以下的叫做冷加工。Under the action of heat and force when the atoms reach a certai
11、n higher energy level, the new crystals start forming which is termed as recrystallization . Recrystallization destroys the old grain structure deformed by mechanical working and entirely new crystals, which are strain-free are formed, the grains, in fact ,start nucleating at the points of severest
12、deformation. Recrystallzation temperature as defined by the American Society of Metals is “ the approximate minimum temperature at which complete recrystallization of a coldworked metal occurs within a specified time ”.在加热和应力的作用下，当原子达到某一更高能量的等级的时候，新的晶粒就开始形成的过程叫做再结晶。再结晶破坏了由机械作用构成的原有晶体结构，并且产生了无应变的新的晶体
13、。事实上，新的晶粒是在严重变形的时候开始形核的。再结晶温度是由美国金属学会这样定义的“在一定的时间段内，冷加工金属完成再结晶时的最小温度”。The recrystallization temperature generally varies between one-third to half the melting point of most of the metals. Typical values of recrystallisation temperatures are given in table 7.1,the recrystallization temperature also d
14、epends on the amount of cold work a material has already received. the higher the cold work the lower would the recrystallization temperature as shown in Fig 7.2大多数的金属再结晶温度一般在金属熔点的三分之一到二分之一内。表7.1给出了具有代表性的再结晶温度值。再结晶温度取决于材料已经改变的冷加工量。冷加工量越高，再结晶温度越低，如图7.2所示。In hot working, the process may be carried abo
15、ve the recrystallization temperature with or without actual heating.For example,for lead and tin,the recrystallization temperature is below the room temperature and hence working of these metals at room temperature is always hot working.Similarly for steels,the recrystallization temperature is of th
16、e order of 1000,and therefore working below that temperature is still cold working only.在热加工中，在加热或者不加热的实际条件下再结晶可能在再结晶温度以上发生。例如，铅和锡，它们的再结晶温度都在室温以下，所以室温下的加工都属于热加工。同样，钢也是一样的，钢的再结晶温度达到1000摄氏度，因此，在这温度以下的加工都是冷加工。In hot working.,the temperature at which the working is completed is important since any extra
17、 heat left after working will aid in the grain growth,thus giving poor mechanical properties.The effect of temperature of completion of hot working is shown schematically in Fig.7.3.A simple heating is shown in A,where the grains start growing after the metal size would be larger than when started i
18、n A.After heating,when the metal is worked,because of recrystallization,the grain size is reduced.This is made of possible because the working of metal gives rise to a large number of nucleation sites for the new crystals to form.But if the hot working is completed much above the recrystallization t
19、emperature as in.C,the grain size starts increasing and finally may end up as a coarse grain size.This increase in size of the grains occurs by a process of coalescence of adjoining grains and is a function of time and temperature.This is not generally desirable.If the hot working is completed just
20、above the recrystallization temperature as in D then the resultant grain size would be fine.The same is schematically shown for hot rolling operation in Fig.7.4.在热加工中，加工完成后的温度是很重要的，因为加工完成后任何余热都能让晶粒长大，这样会得到很差的机械性能。热加工最后温度的影响如图表7.3。如A中的一个简单加热过程，当温度超过再结晶温度后，晶粒就开始长大。当在没有热加工的条件下冷却时，如B，晶粒最后的尺寸将会比A开始的时候大。加
21、热之后，由于再结晶作用，当金属加工的时，晶粒尺寸就会减小。金属的加工将会增加在大量的形核点形成新核的可能性。但是，热加工的最后温度比再结晶温度高太多的话，如C,晶粒的尺寸就开始增大并且最后会得到粗大的晶粒尺寸。晶粒的尺寸的增大是通过合并相邻晶粒的过程发生的，这是时间和温度相互作用的结果。一般情况下，这都不是我们想要的。如果热加工的最终温度在再结晶温度以上，如D，这样得到的细小的晶粒尺寸。HOT Working Advantages1. As the material is above the recrystallization temperature,any amount of working
22、can be imparted since there is no strain-hardening taking place.2. At a high temperature,the material would have higher amount of ductility andtherefore there is no limit on the amount of hot working that can be done on a material.Even brittle material can be hot worked.3. Since the shear stress get
23、s reduced at higher temperatures,hot working requiresmuch less force to achieve the necessary deformation.热加工益处1. 当材料温度在再结晶温度以上时，加工量的多少都是可以的，因为这时候没有加工硬化的产生。2. 在较高温度下，材料将会有较高的韧性，因此材料的热变形量没有限制。即使脆性材料也可以进行热加工。3. 因为在较高温度下剪切应力将会减小，热加工中少量的应力就能获得所需的变形。4. 热加工中，晶粒的继续改良是有可能的，如果温度和加工率得到适当的控制，将会获得优良的晶粒尺寸和良好的机械性
24、能。Disadvantages1.Some metals cannot be hot worked because of their brittleness at high temperatures.2.Higher temperatures of metal give rise to scaling of the surface and as a result,the surface finish obtained is poor.Also,there is a possibility of the decarburization of skin in steels due to the h
25、igh temperature.3.Because of the thermal expansion of metals,the dimensional accuracy in hot working is hart to achieve since it is difficult to control the temperature of workpieces.4.Handling and maintaining of hot metal is difficult and troublesome.弊处1. 由于在较高温度下材料脆性的原因，一些金属不能进行热加工。2. 高温下，金属表面比例将会
26、增大，这样将会导致最后得到的表面质量很差。而且，由于高温会使钢表面有脱碳的可能性。3. 由于金属内热扩散的原因，控制工件的温度是很困难的，所以在热加工中很难获得精确的尺寸。4. 操作和维持加热金属是很困难和麻烦的。Cold working advantages1. Cold working increases the strength and hardness of the material due to strain hardening which would be beneficial in some situation.Table 7.3 gives some comparative v
27、alues for t he properties obtained after hot working and cold working.Further,there is no possibility of decarburization of the surface.2. Since the working is due in the cold tate,no oxide would form on the surface and conseque ntly,good surface finish obtained.3. Better dimensional accuracy is ach
28、ieved.4 It is far easier to handle cold parts and also is economical for smaller size. 冷加工益处1. 由于加工硬化的原因，冷加工将会增加材料的强度和硬度，这在某些方面是有利的。图表7.3给出了热加工和冷加工后获得的性能的一些比较值。而且，冷加工不会有表面脱碳的可能性。2. 因为加工是在冷态下完成的，所以表面不会产生氧化物，最后会得到良好的表面质量。3. 能获得良好的尺寸精确性。较小尺寸的冷态零件更容易控制，而且也很经济Disadvantages1.Since the material has higher
29、yield strength at lower temperatures,the amount of deformation that can be given to is limited by the capability of the presses or hammers used.2.Since the material gets strain hardened,the maximum amount of deformation that can be given is limited.Any further deformation can be given after annealin
30、g.3.Some materials which are brittle cannot be cold worked.弊处1. 因为低温下材料拥有较高的屈服强度，所以变形量将会受到压力机和锤头能力的限制。2. 因为加工硬化的原因，最大变形量也会受到限制，任何后续的变形都需经过退火。 一些脆性材料不能进行冷加工。7.2.1 PrincipleRolling is a process where the metal is compressed between two rotating rolls for reducing its cross-sectional area.This is one o
31、f most widely used of all the metal working processes,because of its higher productivity and low cost.Rolling would be able to produce components having constant cross section throughout its length.many shapes such as I,T,L and channel sections are possible as shown in fig.7.6,but not very complex s
32、hapes.It is also possible to produce special sections such as railway wagon wheels by rolling individual pieces.7.2 辊轧辊轧是在两根旋转辊之间对金属施加压力使其横截面积减少的一种工艺。由于较高的生产率和较低的花费使其广泛应用于各种金属加工工艺。辊轧可以生产在沿长度方向上拥有恒定截零件。如图7.6是中的I、T、L和槽型等许多形状，但是不能生产复杂的形状。它也可以通过辊轧单独的零件来制造特定截面的零件，如铁路货车车轮。Rolling is normally a hot-working
33、 process unless specifically mentioned as cold rolling.The metal is taken into rolls by friction and subsequently compressed to obtain the finial shape.The thickness of the metal that can be drawn into rolls depend on the roughness of the roll surface.Rougher rolls would be able to achieve greeter r
34、eduction than smoother rolls.But the roll surface gets embedded into the rolled metal thus producing rough surface.除非特别提出将其作为冷加工，不然辊轧一般都属于热加工。辊轧通过摩擦力和之后的压力将金属放入旋转辊中来获得最终的形状。金属最终拉伸的厚度取决于辊的表面粗糙度。粗糙的旋转辊比光滑的辊更能得到较少的横截面，但是旋转辊表面会嵌The reduction that could be achieved with a given set of roll is designed as
35、 the “angle of bite”and is shown in Fig.7.5.This depends on the type of rolling and the conditions of the rolls an shown in Table 7.4.The volume of the metal that enters the rolling stand should be the same as that leaving it except in initial passes when there might be some loss due to filling of v
36、oids and cavities in the ingots. Since the area of the cross section gets decreased,the metal leaving the rolls would be at a higher velocity than when it entered.Initially,when the metal enters the rolls,the surface speed of rolls is higher than that of the incoming metal,whereas,the metal velocity
37、 at the exit is higher than of the surface speed of rolls.Between the entrance and exit,the velocity of the metal is continuously changing,whereas the roll velocity remains constant.Somewhere in the contact length,the velocities of the metal and rolls are same,which is designated as neutral plane in
38、 Fig.7.5.横截面的减少可以通过一套设计有轧入角的旋转辊获得，如图7.5。这取决于辊轧类型和辊轧条件，如表7.4。金属进入辊轧机和离开辊轧机的体积应该保持不变，除了最初通过辊轧机填充铸锭中的空位和型腔时会使体积减少。由于横截面积的减少，金属离开辊轧机的速度会比进入时的高。金属开始进入辊轧机的时表面速度比将要进入的金属高，然而，在出口处的金属速度比旋转辊的速度高。在出口与入口之间，金属的速度持续变化的，但是旋转辊的速度却保持恒定。在接触长度上，金属和旋转辊的速度保持一致，这个地方被设计为中性平面，如图7.5。The pressure on rolls gradually builds up
39、 form the entry to the neutral point where it is highest and then decrease till it reaches the exit.The roll-separating force which separates the two rolls apart can be obtained by multiplying the average roll pressure with the total contact area.The average roll pressure can be decreased by reducin
40、g the maximum pressure,which is a function of the contact length,it is possible to decrease the roll-separating force.This in turn,can be achieved by reduction.The smaller rolls would not have enough regidity to support a large roll-separating force.Hence,backup rolls are attached to the small rolls
41、 to provide the necessary rigidity.Though higher friction between rolls and the metal is required for increasing the reducition achieved, it also increases the roll separating force.辊上的压力从进入部分到中点逐渐增大，其中中点的最大，然后到出口部分一直变小。分离两个辊的卷分离力可以通过辊的平均压力和总的接触面积的乘积得到。通过加少最大压力可以减小接触长度上的平均压力，这也有可能减小卷分离力。反过来说，也可以通过减小
42、辊的直径来获得，因为，减少相同的量，较小的辊比较大的辊拥有减小的接触长度。较小的辊用于减少量较大和卷分离力较大的冷轧的情况下。但是小辊没有足够的刚度来支撑较大的卷分离力，因此，支撑辊被附加在小辊上已提供所需的刚度。虽然辊和金属间较大摩擦力可以增大减少量，但是这也会增加卷分离力。7.2.2 Rolling Stand ArrangementThe arrangement of rolls in a rolling mill, also called rolling stand,varies depending on the application.The various possible con
43、figurations are presented in Fogs.7.7and 7.8.The names of the rolling stand arrangements are generally given by the number of rolls employed.The first one in Fig.7.7a,the 2-high non reversing rolling stand arrangement,is the most common on arrangement.In this ,the rolls always move in only one direc
44、tion .The arrangement shown in Fig.7.7b is a 2-high reversing rolling stand,where the direction of roll rotation can be reversed.This type of stand is particularly useful in reducing the handling of the hot metal in between the rolling passes.When all the metal has reached the right side of the Fig7
45、.7,the direction of the rolls is reversed and the metal is allowed to enter into the next pass.These stands are more expensive compared to the non reversible type because of the reversible drive needed.7.2.2 轧钢架布置辊轧机轧辊的布置也叫轧钢机架因应用的不同而不同。如图7.7和7.8是几种可能的布置。轧钢机架布局是通过所应用的轧辊的数量来命名的。图7.7（a）的第一个图是二辊不可逆轧钢机架
46、的布置，这是最常见的布置。在这种布置中，轧辊只会朝着一个方向运动。图7.7（b）是轧辊只能旋转可以倒转的二辊不可逆轧钢机布置。这种机架主要用于减少加热金属通过辊轧机时的操作。当所有金属到达图7.7的右边时，轧辊的旋转方向就会倒转过来，金属就有可以通过下一个通道。因为需要反转驱动，所以这种钢架比不倒转的钢架更贵。The 3-high rolling stand arrangement shown in Fig.7.7 is used for rolling of two continous passes in a rolling sequence without reversing the dr
47、ives.After all the metal has passed through the bottom roll set,the end of the metal is entered into the other set of the rolls for the next pass.For this purpose,a table-tilting arrangement is required to bring the metal to the level with the rolls.三辊轧机的布置如图7.7（c）。这种布置用于在轧制次序中不需要反转驱动的两个连续通道的轧制。当所有的
48、金属都通过轧制机的尾部的时候，金属的末端进入另一个轧制机的通道。为此，需要一个倾斜板布置将金属带到辊轧机的水平位置。A 4-high rolling stand shown in Fig.7.7 is essentially a 2-high rolling mill,but with small-sized rolls.The other tuo rolls are the backup rolls for providing the necessary rigidity to the small rolls.A better backup can be provided to the sm
49、all rolls with a cluster arrangement of rolls as shown in Fig.7.7.For the rolling arrangements requiring large reduction,a number of free rotating wheels instead of a single small roll,are fixed to a large backup roll in the planetary rolling mill arrangement shown in Fig.7.8.四辊轧机钢架布置如图7.7（d），它的本质其实是一个拥有小轧辊的二辊轧机。其他两个轧辊是为小轧辊提供所需刚度的支撑轧辊。如图7.7（e）是能够为带有一串轧辊的小轧辊机提供支撑的布置。对于需要大量缩减量的轧制机布置来说，大量的自由旋转轮代替了单独的小轧辊，这些旋转轮被安在行星轧制机中的大支
