外文翻譯--奧氏體的分解

1、<p>　　Decomposition of Austenite</p><p>　　The austenite to pearlite transformation is essentially the decomposition of austenite into almost pure ferrite and cementite .</p><p>　　At the e

2、quilibrium temperature,the transformation is impossible, since the free energy of the original austenite is equal to that of the final product , pearlite .</p><p>　　The transformation can only start at a cer

3、tain undercooling when the free energy of the ferrite carbide mixture (pearlite) is lower than that of austenite .</p><p>　　The lower the transformation temperature ,the higher the degree of undercooling an

4、d the greater the difference in free energies and the transformation proceeds at a higher rat .</p><p>　　In the pearlite transformation , the new phases sharply differ in their composition from the initial

5、phase ; they are ferrite which is almost free of carbon , and cementite which contains6.67percent carbon. For that reason the austenite to pearlite transformation is accompanied with the diffusion , redistribution of ca

6、rbon . The rate of diffusion sharply diminishes with decreasing temperature , therefore , the transformation should be retarded at a greater undercooling .</p><p>　　Thus ,we have come to an important conclu

7、sion that undercooling (lowering the transformation temperature ) may have two opposite effects on the rate of transformation .</p><p>　　On one hand , a lower temperature (greater undercooling ) gives a grea

8、ter difference in free energies of austenite and pearlite , thus accelerating the transformation ; on the other hand , it diminishes the rate of carbon diffusion ,and thus slows down the transformation . The combined eff

9、ect is that the rate of transformation first increases as undercooling is increased to a certain maximum and then decreases with further undercooling .</p><p>　　At 727℃(A1) and below 200℃ ,the rate of transf

10、ormation is zero , since at 727℃ the free energy difference is zero and below 200℃ the rate of carbon diffusion is zero (more strictly ,too low for the transformation to proceed ) .</p><p>　　As has been fir

11、st indicated by I. L. Mirkin in 1939 and then developed by R. F.Mehl in 1941 ,the formation of pearlite is the process of nucleation of pearlite and growth of pearlite crystals .</p><p>　　Therefore ,the diff

12、erent 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 .A temperature A1 and below

13、200℃ ,both parameters of crystallization N and G are equal to zero and have a maximum at an undercooling of 150～200℃ .</p><p>　　It follows from the foregoing that as soon as the conditions are favorable ,I .

14、e .austenite is undercooled 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 kin

15、etic curve of transformation ,which shows the quantity of pearlite that has formed during the time elapsed from the beginning of the transformation .</p><p>　　The initial stage is characterized by a very low

16、 rate of transformation ; this is what is called the incubation period . The rate of transformation increases with the progress in the transformation . Its maximum approximately corresponds to the moment when rougly 50 p

17、ercent of austenite has transformed into pearlite .The rate of transformation then diminishes and finally stops .</p><p>　　The rate of transformation depends on undercooling the transformation proceeds slowl

18、y , since N or G are low ;in the former case , owing to a low difference in free energies , and in the kinetic curves have sharp peaks , and the transformation is finished in a short time interval .</p><p>　

19、　At a high temperature (slight undercooling ) ,the transformation proceeds slowly and the time of the incubation period and the time of the transformation proper are long .At a lower temperature of the transformation , i

20、.e. a deeper undercooling , the rate of transformation is greater , and the time of the incubation period and of the transformation is shorter .</p><p>　　Having determined the time of the beginning of austen

21、ite to pearlite transformation (incubation period ) and the time of the end of transformation at various degrees of undercooling ,we can construct a diagram in which the left hand curve determines the time of the beginni

22、ng of the transformation , i.e. the time during which austenite still exists in the undercooled state ,and the section from the axis of ordinates to the curve is the measure of its stability . This section is shortest at

23、 a temp</p><p>　　The right hand curve shows the time needed to complete the transformation at a given degree of undercooling . This time is the shortest at the same temperature (500～600℃) .Note that the absc

24、issa of the diagram is logarithmic .This is done for more convenience ,since the rates of formation of pearlite appreciably differ (thousands of seconds near the critical point A1 and only one or two seconds at the bend

25、of the curve ) .</p><p>　　The horizontal line below the curves in the diagram determines the temperature of the diffusionless martensite transformation. The martensite transformation occurs by a different me

26、chanism and will be discussed later.</p><p>　　Diagrams of the type we discussed are usually called TTT diagrams (time temperature transformation), or C curves, owing to the specific shape of the curves. The

27、structure and properties of the products of austenite decomposition depend on the temperature at which the transformation has taken places.</p><p>　　At high temperature , i.e. low degrees of undercooling ,a

28、coarse grained mixture of ferrite and cementite is formed which is easily distinguished in the microscope .This structure is called pearlite .</p><p>　　At lower temperatures , and therefore ,greater degrees

29、of undercooling , more disperse and harder products are formed .The pearlite structure of this finer type is called sorbite.</p><p>　　At still lower temperature (near the end of the C curve ), the transforma

30、tion products are even more disperse ,so that the lamellar structure of the ferrite cementite mixture is only distinguishable in the electron microscope .This structure is called troostite .</p><p>　　Thus ,

31、pearlite , sorbite and troostite are the structures of the same nature (ferrite+cementite) but a different dispersity of ferrite and cementite .</p><p>　　Pearlite structure may be of two types: granular (in

32、which cementite is present in the form of grains ) or lamellar (with cementite platelets).</p><p>　　Homogeneous austenite always transforms into lamellar pearlite . Therefore , heating to high temperature se

33、ts up favorable conditions for the formation of a more homogeneous structure and thus promotes the appearance of lamellar structures . Inhomogeneous austenite produces granular pearlite at all degrees of undercooling, th

34、erefore, heating to a low temperature (below AC3 for hypereutectoid steels) results in the formation of granular pearlite on cooling .The formation of granular cementite is </p><p><b>　　奧氏體的分解</b>

35、;</p><p>　　奧氏體向珠光體的轉(zhuǎn)變本質(zhì)上是奧氏體分解成純凈的鐵素體和滲碳體。</p><p>　　在平衡溫度時，轉(zhuǎn)變是不可能進行的，因為最初的奧氏體的自由能和最后產(chǎn)品，珠光體的自由能是相等的。</p><p>　　當(dāng)鐵素體滲碳體混合物（珠光體）的自由能比奧氏體的低時，轉(zhuǎn)變才能在一個特定的過冷下開始。</p><p>　　較低的轉(zhuǎn)變溫

36、度，較高程度的過冷度，并且較大的在較高的速度時的自由能和轉(zhuǎn)變效益中的差異。</p><p>　　在珠光體轉(zhuǎn)變過程中，新形成相的成分明顯不同于初相；新相為含碳量幾乎沒有的鐵素體和含碳量為6.67的滲碳體。因此對珠光體轉(zhuǎn)變的奧氏體是伴隨著碳原子的擴散和再分布進行的。隨著溫度的增加，擴散的速度明顯地降低，因此，這個轉(zhuǎn)變應(yīng)該在較大的過冷下緩慢進行。</p><p>　　因此，我們可以得到一個重要結(jié)

37、論即過冷（降低轉(zhuǎn)變溫度）也許對轉(zhuǎn)變速度上有兩個迥然不同的效果。</p><p>　　一方面，較低的溫度（較大的過冷）給奧氏體和珠光體的自由能提供一種較大的差異，因而加速了轉(zhuǎn)變；另一方面，它減少了談原子擴散的速度，因而減慢了轉(zhuǎn)變。這種組合的效果是轉(zhuǎn)變的速度首先增加，當(dāng)過冷增加到某一個最大值然后減少與進一步過冷。</p><p>　　溫度在727℃（A1）和200℃以下時，轉(zhuǎn)變的速度為零，因為

38、哉727℃時自由能的差異為零和在200℃以下時碳原子的擴散速度為零（更嚴(yán)格地說，太低為了轉(zhuǎn)變能進行）。</p><p>　　在1939年首先由I.L.Mirkin提出的，然后在1941年由R.F.Mehl開發(fā)，珠光體的形成是珠光體晶核形成和珠光體晶粒長大的過程。</p><p>　　因此，珠光體轉(zhuǎn)變在各種各樣的過冷度時，它的轉(zhuǎn)變速度不同，這歸結(jié)于事實不同的過冷影響晶核N的形成率和晶粒G的長

39、大率。溫度在A1線和200℃以下時，結(jié)晶N和G的參數(shù)是相等的到零并且在150~200℃的過冷時有最大值。</p><p>　　像前面所述，當(dāng)條件是有利，即奧氏體在A1線以下過冷，碳原子擴散不為零，結(jié)晶的中心出現(xiàn)，以轉(zhuǎn)變的形式，這種過程隨時間發(fā)生和以一種叫做轉(zhuǎn)變的運動曲線的形式可以代表，這顯示珠光體的數(shù)量形成當(dāng)時流逝從轉(zhuǎn)變開始。</p><p>　　初始階段有轉(zhuǎn)變速度比較低的特征，這叫做孕育

40、時期。在轉(zhuǎn)變過程中，轉(zhuǎn)變速度不斷增加。當(dāng)大約奧氏體的50轉(zhuǎn)變成珠光體時，它的最大值大約相當(dāng)于片刻。轉(zhuǎn)變的速度然后減小和最后停止。</p><p>　　因為N或G很低，轉(zhuǎn)變的速度取決于過冷度，在低和高的過冷時轉(zhuǎn)變效益很慢；在前面的情況下，由于低的自由能差，和在后來，由于原子的低擴散流動性。在最大轉(zhuǎn)變速度是運動曲線有尖的頂點，并且轉(zhuǎn)變是在短的時間內(nèi)完成的間隔時間。</p><p>　　在高溫（

41、輕微的過冷）時，轉(zhuǎn)變慢慢地進行，并且孕育時期的時間和轉(zhuǎn)變的時間是長的。在轉(zhuǎn)變溫度較低時，即，更加較深的過冷，轉(zhuǎn)變的速度較快，并且孕育時期的時間和轉(zhuǎn)變的是更短的。</p><p>　　為了確定在各種各樣的過冷度時奧氏體轉(zhuǎn)變珠光體的開始時間（孕育時期）和轉(zhuǎn)變結(jié)束時間，我們可以構(gòu)造一個圖表，在圖表中的左手曲線確定轉(zhuǎn)變開始時間，即在這期間的時間里奧氏體仍然存在于過冷階段的圖，并且從縱坐標(biāo)的軸的部分對曲線的是它的穩(wěn)定措施。

42、溫度在500~600℃時這個階段是最短的，即轉(zhuǎn)變在短時間開始是在那個溫度。</p><p>　　右手曲線在特定的過冷下轉(zhuǎn)變完成需要的時間，在同一溫度（500~600℃）這個時間是最短的。注意圖的橫坐標(biāo)是對數(shù)的。這種做法更方便，因為珠光體的形成速度看得出不同（在A1線的臨界點的數(shù)以萬計的秒鐘和僅在曲線的彎曲處的一兩秒）。</p><p>　　圖表中在曲線以下的水平線確定了無擴散馬氏體的轉(zhuǎn)變溫

43、度。馬氏體轉(zhuǎn)變是靠不同的機理發(fā)生，并且以后將會被討論。</p><p>　　我門討論的圖表類型通常被叫做TTT圖表（時間溫度轉(zhuǎn)變），或者C曲線，這歸功于曲線的特殊形狀。奧氏體分解的產(chǎn)物的結(jié)構(gòu)和性質(zhì)取決于轉(zhuǎn)變發(fā)生的時間。</p><p>　　在高溫，即在低的過冷度下，在顯微鏡下容易被區(qū)分的鐵素體和滲碳體的粗晶?；旌衔锉恍纬桑@個結(jié)構(gòu)稱珠光體。</p><p>　　在較

44、低的溫度，并且，較大的過冷度時，更多的分散劑和堅硬的產(chǎn)物被形成。這個更細(xì)小的類型的珠光體結(jié)構(gòu)叫做索氏體。</p><p>　　仍然在較低溫度（在C曲線末端的附近），轉(zhuǎn)變的產(chǎn)物甚至更加分散，以至于鐵素體滲碳體的片狀結(jié)構(gòu)只有在電子顯微鏡下才能被區(qū)分。這種結(jié)構(gòu)叫做屈氏體。</p><p>　　因此，珠光體，索氏體和屈氏體是相同的自然結(jié)構(gòu)（鐵素體+滲碳體），但是鐵素體和滲碳體的分散度不同。<

45、/p><p>　　珠光體結(jié)構(gòu)也許是兩種類型：顆粒狀（在哪種滲碳體以谷粒的形式存在）或片狀（滲碳體片狀）。</p><p>　　均勻的奧氏體總是轉(zhuǎn)變成片狀的珠光體。所以，加熱對高溫設(shè)定一個更加均勻的結(jié)構(gòu)的形成的有利的條件和因而促進片狀結(jié)構(gòu)的出現(xiàn)。非均勻的奧氏體在所有的過冷度下產(chǎn)生顆粒狀珠光體，因此，加熱到低溫（過共析鋼AC3線以下）導(dǎo)致在冷卻中顆粒狀珠光體的形成。顆粒狀滲碳體的形成可能由奧氏體中