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1、CHAPTER 2 HEAT TREATMENT OF STEELCHAPTER 2 HEAT TREATMENT OF STEEL热处理在现代机械工程中的作用不可能评价的过高。由热处理而产生的性能改变是特别重要的。-1 1-The role of heat treatment in modern mechanical engineering can not be overestimated.The changes in the properties of metals due to heat treatment are of extremely great significance.第1页/
2、共53页CHAPTER 2 HEAT TREATMENT OF STEELCHAPTER 2 HEAT TREATMENT OF STEEL2.1.1 temperature and time 2.1.1 temperature and time 温度和时间温度和时间-2 2-任何热处理的目的都是（通过）将金属加热到一定的温度并(随后)冷却,以使金属组织产生所需变化。The purpose of any heat treating process The purpose of any heat treating process is to is to produce the desired p
3、roduce the desired changes in the structure of metal changes in the structure of metal byby heating to a specified heating to a specified temperature and by temperature and by subsequentsubsequent cooling.cooling.第2页/共53页CHAPTER 2 HEAT TREATMENT OF STEELCHAPTER 2 HEAT TREATMENT OF STEEL2.1.12.1.1温度和
4、时间温度和时间-3 3-因此，热处理的主要因素是温度和时间，所以任何热处理工艺都以用温度-时间为坐标轴进行表示。Therefore,the main factors acting in heat treatment are Therefore,the main factors acting in heat treatment are temperature and time,temperature and time,so that so that any process of heat treatment can any process of heat treatment can be be
5、representedrepresented in temperature-time(t-r)in temperature-time(t-r)coordinates.coordinates.第3页/共53页2.1.1 temperature and time 2.1.1 temperature and time 2.1.1 temperature and time 2.1.1 temperature and time 温度和时间温度和时间温度和时间温度和时间热处理工艺主要有以下几个参数：加热温度tmax，既合金加热的最高温度；在加热温度下的保温时间；加热速率和冷却速率。-4 4-Heat tr
6、eatment conditions are characterized by the following parameters:heating temperature tmax,i.e.the maximum temperature to which an alloy metal is heated;time of holding at the heating temperature ;heating rate and cooling rate .第4页/共53页2.1.1 temperature and time 2.1.1 temperature and time 2.1.1 tempe
7、rature and time 2.1.1 temperature and time 温度和时间温度和时间温度和时间温度和时间如果以不变速率加热或冷却，则温度和时间的关系可以具有不同倾斜角的直线。-5 5-If heating(or cooling)is made at a constant rate,the temperature-time relationship will be described by a straight line with a respective angle of incline.Respective分别的,各自的第5页/共53页2.1.1 temperature
8、 and time 2.1.1 temperature and time 2.1.1 temperature and time 2.1.1 temperature and time 温度和时间温度和时间温度和时间温度和时间如果加热速率（或冷却速率）变化，则实际速率由给定温度确定，更严格的说，是温度对时间的无穷变化：即温度对时间的一阶导数。-6 6-With a varying heating(or cooling)rate.the actual rate should be attributed to the given temperature,more strictly,to an infi
9、nite change of temperature and time:that is the first derivative of temperature in time .Infinite无限的东西(如空间、时间),数无穷大第6页/共53页2.1.1 temperature and time 2.1.1 temperature and time 2.1.1 temperature and time 2.1.1 temperature and time 温度和时间温度和时间温度和时间温度和时间加热可以是复杂的过程，可以包括几个加热阶段、间断式或阶梯式的加热/冷却，冷却到0以下等。任何热处理
10、过程都可以描述成为以温度和时间为坐标轴的曲线。-7 7-Heat treatment may be a complex process,including multiple heating stages.interrupted or stepwise heating(cooling),cooling to subzero temperatures,etc.Any process of heat treatment can he described by a diagram in temperature-time coordinates.interrupt中断stepwise楼梯式的,逐步的su
11、bzero-零度以下的（温度）第7页/共53页2.1.2 formation of austenite2.1.2 formation of austenite-8 8-根据Fe-C相图，在平衡临界点缓慢加热，珠光体转变为奥氏体。一般条件下，转变会延迟导致guore，即转变温度略高于Fe-C相图中的指定温度。The transformation of pearlite into austenite can only take place at the equilibrium critical point on a very slow heating as follows from the Fe-
12、C constitutional diagram.Under common conditions,the transformation is retarded and results in overheating,i.e.occurs at temperatures slightly higher than those indicated in the Fe-C diagram.Constitutional diagram相图(构成的,增强体质的,宪法的）pearlite珠光体 retard延迟,使减速,阻止,妨碍,阻碍第8页/共53页2.1.2 formation of austenite2
13、.1.2 formation of austenite当加热温度高于临界点时，珠光体转变为奥氏体，转变速率依赖于过热的程度。(过热、过冷的概念)-9 9-When overheated above the critical point,pearlite transform into austenite，the rate of transformation being dependent on the degree of overheating.第9页/共53页2.1.2 formation of austenite2.1.2 formation of austenite各温度下的转变时间（依赖
14、于过热度）表明在较高的温度下相变产生较快（时间短）；加热速度快时，相变在较高温度产生。-1010-The time of transformation at various temperatures(depending on the degree of overeating)shows that the transformation takes place faster(in a shorter time)at a higher temperature and occurs at a higher temperature on a quicker heating.第10页/共53页2.1.2
15、formation of austenite2.1.2 formation of austenite例如例如，780780加热速度较快时，珠光体完全转变加热速度较快时，珠光体完全转变为奥氏体需要为奥氏体需要2 2分钟，分钟，740740时则需要时则需要8 8分钟。分钟。-1111-For instance,on quick heating and holding at 780，the pearlite to austenite transformation is completed in 2 minutes and on holding at 740C,in 8 minutes.centigr
16、ade 第11页/共53页2.1.2 formation of austenite2.1.2 formation of austenite-1212-转变结束是以奥氏体形成和珠光体(F+渗碳体)消失为标记的。然而，即使在单个晶粒中奥氏体的也不均匀。The end of the transformation is characterized by formation of austenite and the disappearance of pearlite(ferrite+cementite).This austenite is however inhomogeneous even in th
17、e volume of a single grain.第12页/共53页2.1.2 formation of austenite2.1.2 formation of austenite-1313-早期形成的片状(或li状)珠光体渗碳体，其含碳量高于片状铁素体，这就是奥氏体（成分）不均匀的原因。lamellae薄片状的,薄层状的In places earlier occupied by lamellae(or grains)of a pearlitic cementite,the content of carbon is greater than in places of ferritic la
18、mellae.This is why the austenite just formed is inhomogeneous.第13页/共53页2.1.2 formation of austenite2.1.2 formation of austenite为了获得均匀的奥氏体，加热温度不仅要超过珠光体转变为奥氏体的终点温度，而且要高于这一温度，并保温一段时间，使奥氏体晶粒内发生完全扩散。-1414-In order to obtain homogeneous austenite,it is essential on heating not only to pass through the poi
19、nt of the end of pearlite to austenite transformation,but also to overheat the steel above that point and to allow a holding time to complete the diffusion processes in austenitic grains 第14页/共53页-1515-奥氏体均匀化的速度主要依赖与钢的原始组织，特别是渗碳体的分布及晶粒形状。当渗碳体颗粒细小均匀，总表面积较大时，相转变更快。The rate of homogenization of austeni
20、te appreciably depends on the original structure of the steel,in particular on the dispersion and particle shape of cementite.The transformations described occur more quickly when cementite particles are fine and,therefore,have a large total surface area.Particle 粒子,点,极小量,微粒Appreciably 1.有一点儿的;可以感到的
21、;2.可观的,值得重视的2.1.2 formation of austenite2.1.2 formation of austenite第15页/共53页2.1.3Coarsening of Austenite Grains2.1.3Coarsening of Austenite Grains在珠光体向奥氏体转变开始时，奥氏体首先在铁素体和渗在珠光体向奥氏体转变开始时，奥氏体首先在铁素体和渗碳体（珠光体的组织组分）之间的晶界处形成。因为晶界碳体（珠光体的组织组分）之间的晶界处形成。因为晶界非常多因此转变首先形成于大量的细小晶粒。非常多因此转变首先形成于大量的细小晶粒。developeddeve
22、loped 发达的，引申为发达的，引申为“多多”。-1616-At the beginning of pearlite to austenite transformation.The first grains of austenite form at the boundaries between the ferrite and cementite the two structural constituents of pearlite.Since these boundaries are very developed,the transformation starts from formatio
23、n of a multitude of fine grains.第16页/共53页Coarsening of Austenite GrainsCoarsening of Austenite Grains因此，奥氏体转变结束时，会形成大量的细小晶粒，因此，奥氏体转变结束时，会形成大量的细小晶粒，它们的尺寸称之为它们的尺寸称之为奥氏体初始晶粒度奥氏体初始晶粒度。-1717-Therefore,at the end of the transformation the austenite will be composed of a great multitude of fine grains whos
24、e size characterizes what is called the original austenite grain size.第17页/共53页随着加热(保温)温度高于转变温度，将引起奥氏体晶粒的粗化。晶粒粗化的过程是自发的，因为晶粒总面积减少（表面能减少）和高温可以加速转变过程。-1818-Further heating(or holding)upon the transformation will cause coarsening of austenitic grains.The process of grain coarsening is spontaneous,since
25、 the total surface area of grains diminishes(the surface energy decreases)and a high temperature can only accelerate the rate of this process.第18页/共53页in that connection在那种情况下,如果情况是那样liability、liable有责任的,有义务的,易.的,有.倾向的,inherent固有的,内在的,与生俱来的在那种情况下，就可以形成两种钢：固有细晶粒(钢)和固有粗晶粒(钢)。前者晶粒粗化的倾向比后者小。钢中通过热处理形成的晶粒
26、尺寸称之为实际晶粒度。-1919-In that connection,two types of steels are distinguished:inherent fine grained and inherent coarse grained,the former being less liable to grain coarsening than the latter.The size of grains formed in a steel by heat treatment is called the actual grain size.第19页/共53页因此，区分如下：(1)原始晶粒
27、度，即珠光体刚转变为奥氏体时奥氏体的晶粒尺寸；(2)固有晶粒度，即奥氏体粗化的倾向性；(3)实际晶粒度，也就是在给定条件下的奥氏体实际晶粒尺寸。-2020-Thus,a distinction should be made between:(1)original grain,i.e.the size of austenitic grains immediately after the pearlite to austenite transformation;(2)inherent(natural)grain,i.e.the liability of austenite to grain coa
28、rsening;and(3)actual grain,i.e.the size of austenitic grains under given particular conditions.第20页/共53页2.1.3 Coarsening of Austenite Grains2.1.3 Coarsening of Austenite Grains-2121-奥氏体转变为珠光体时珠光体晶粒的大小决定于奥氏体形成时的晶粒大小。奥氏体仅在加热时长大（冷却时不细化），因为奥氏体区的最高温度和固有晶粒度决定了最终晶粒尺寸。The size of pearlitic grains at same te
29、mperature of the austenite to pearlite transformation depends on that of the austenitic grains from which they have formed.Austenitic grains grow only during heating(but are not refined in subsequent cooling),because of which the highest temperature a steel is heated to in the austenitic state and t
30、he inherent grain size of that steel determine the final grain size.第21页/共53页2.1.3 Coarsening of Austenite Grains2.1.3 Coarsening of Austenite Grains-2222-钢的性能仅仅由最终晶粒大小而不是固有晶粒大小决定。具有相同等级的两种钢(一是固有粗晶粒，另一是细晶粒)根据不容温度的热处理，将有相同的实际晶粒尺寸，它们的性质也将相同。否则，两种钢的许多性能将不同。The properties of a steel are affected only by
31、 the actual grain size and not by the inherent grain size.If two steels of the same grade(one inherently coarse grained,the other fine grained)have the same actual grain size upon heat treatment at different temperatures,their properties will also be the same;if otherwise,many properties of the two
32、steels will also be different.第22页/共53页2.1.4 Decomposition of Austenite 2.1.4 Decomposition of Austenite -2323-奥氏体向珠光体转变本质是奥氏体分解为近乎纯的铁素体和渗碳体。The austenite to pearlite transformation is essentially the decomposition of austenite into almost pure ferrite and cementite.第23页/共53页2.1.4 decomposition of A
33、ustenite 2.1.4 decomposition of Austenite -2424-在平衡温度时，不发生转变，因为初始奥氏体的自由能等于最终产物珠光体的自由能。At the equilibrium temperature,the transformation is impossible,since the free energy of the original austenite is equal to that of the final product,pearlite.第24页/共53页2.1.4 decomposition of Austenite 2.1.4 decompo
34、sition of Austenite -2525-只有珠光体(铁素体和碳化物的混合物)的自由能低于奥氏体，在一定的过冷度下，转变才发生。The transformation can only start at a certain undercooling when the free energy of the ferrite carbide mixture(pearlite)is lower than that of austenite.第25页/共53页-2626-转变温度越低、过冷度越大，自由能差越大，转变速度越快。The lower the transformation tempera
35、ture,the higher the degree of undercooling and the greater the difference in free energies and the transformation proceeds at a higher rate.2.1.4 Decomposition of Austenite 2.1.4 Decomposition of Austenite 第26页/共53页-2727-在珠光体转变中，新相的成分与原始相完全不同，铁素体几乎不含碳，渗碳体含碳6.67。，in the pearlite transformation,the ne
36、w phase sharply differ in their composition from the initial phase;they are ferrite which is almost free of carbon,and cementite which contains 6.67 percent carbon.2.1.4 Decomposition of Austenite 2.1.4 Decomposition of Austenite 第27页/共53页-2828-因此奥氏体转变为珠光体伴随着碳的扩散和重新分布。随着温度的降低，扩散率显著降低，因此，转变需在较高的过冷下，转
37、变会被延迟。For this reason the austenite to pearlite transformation is accompanied with the diffusion,redistribution of carbon.The rate of diffusion sharply diminishes with decreasing temperature,therefore,the transformation should be retarded at a greater undercooling.2.1.4 Decomposition of Austenite 2.
38、1.4 Decomposition of Austenite 第28页/共53页-2929-因此，可得出一重要结论：过冷（较低的转变温度）对转变速率可以产生两种相反的影响。thus,we have come to an important conclusion that undercooling(lowering the transformation temperature)may have two opposite effects on the rate of transformation.2.1.4 Decomposition of Austenite 2.1.4 Decompositio
39、n of Austenite 第29页/共53页-3030-一方面，温度较低，过冷较大，使奥氏体和珠光体自由能差较大，因此加速相变，另一方面，它降低了碳扩散的速率，又减慢了相转变。On one hand,a lower temperature(greater undercooling)gives a greater difference in free energy of austenite and pearlite,thus accelerating the transformation;on the other hand,it diminishes the rate of carbon d
40、iffusion,and thus slows down the transformation.2.1.4 Decomposition of Austenite 2.1.4 Decomposition of Austenite 第30页/共53页-3131-整个影响就是：随着过冷度的增加，转变率增加到某一最大值后，随着进一步的过冷而降低.The combined effect is that the rate of transformation first increase as undercooling is increased to a certain maximum and then d
41、ecreases with further undercooling.2.1.4 Decomposition of Austenite 2.1.4 Decomposition of Austenite 第31页/共53页-3232-在727和低于200时，转变率为0，因为727时自由能差为0，低于200时，C的扩散率为0.（更严格的来讲，温度太低，转变难以进行。）at 727(A1)and below 200,the rate of transformations is zero,since at 727 the free energy difference is zero and below
42、 200 the rate of carbon diffusion is zero(more strictly,too low for the transformation to proceed).2.1.4 Decomposition of Austenite 2.1.4 Decomposition of Austenite 第32页/共53页-3333-珠光体的形成是珠光体形核和珠光体晶粒长大的过程，这一点首先由I.L.Mirkin 在1939年提出，并由R.F.Mehl 在1941年加以完善。As has been first indicated by I.L.Mirkin in 193
43、9 and then developed by R.F.Mehl in 1941,the formation of pearlite is the process of nucleation of pearlite and growth of pearlitic crystals.2.1.4 Decomposition of Austenite 2.1.4 Decomposition of Austenite 第33页/共53页-3434-因此，在不同过冷程度下珠光体的转变速率不同,(这)主要是因为过冷度影响形核率N和晶核长大速率G。在A1和200以下，N和G都等于0，并在在150200时过冷
44、度最大.Therefore,the different rate of the pearlite transformation at various degrees of undercooling is due to the fact that undercooling differently affects the rate of nucleation N and the rate of crystal growth G.At temperature A1 and below 200,both parameters of crystallization N and G are equal t
45、o zero and have a maximum at an undercooling of 150200.2.1.4 Decomposition of Austenite 2.1.4 Decomposition of Austenite 第34页/共53页-3535-如前所述，当条件允许，例如奥氏体过冷到A1点以下，C的扩散速率不为0，则产生形核中心，继而形成晶粒。随时间变化产生的这一过程可用动力学转变曲线表示，该曲线中会显示从转变开始后，随时间变化，珠光体的形成量。It follows from the forgoing that as soon as the conditions ar
46、e favorable,i.e.austenite is undercolled below A1,the diffusion of carbon is not zero,centers of crystallization appear,which give rise to crystals.This process occurs with time and can be represented in the form of so called kinetic curve of transformation,which shows the quantity of pearlite that
47、has formed during the time elapsed from the beginning of the transformation.2.1.4 Decomposition of Austenite 2.1.4 Decomposition of Austenite 第35页/共53页-3636-初始阶段，转变速率非常慢，称之为孕育期。转变进程中，速率加快，当大约有50%的奥氏体转变为珠光体时，转变速率达到最大。然后转变速率减小并停止。The initial stage is characterized by a very low rate of transformation;
48、this is what is called incubation period.The rate of transformation increases with progress in the transformation.Its maximum approximately corresponds to the moment when roughly 50 percent of austenite has transformed into pearlie.The rate of transformation then diminishes and finally stops.2.1.4 D
49、ecomposition of Austenite 2.1.4 Decomposition of Austenite 第36页/共53页-3737-转变速率依赖于过冷度，过冷度过低和过高都使转变进程变慢，这是由于N、G比较小，前者是由于自由能差过低，后者是因为原子扩散慢，在转变速率最大处，动力学曲线上有峰值，在较短的时间间隔里，转变结束。The rate of transformation depends on undercooling.At low and high degrees of undercooling the transformation proceeds slowly,sinc
50、e N and G are low:in the former case,owing to a low difference in free energy,and in the later,due to a low diffusion mobility of atoms.At the maximum rate of transformation the kinetic curves have sharp peaks,and the transformation is finished in a short time interval.2.1.4 Decomposition of Austeni