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(shí)，轉(zhuǎn)變是不可能進(jìn)行的，因?yàn)樽畛醯膴W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(shí)，轉(zhuǎn)變才能在一個(gè)特定的過(guò)冷下開(kāi)始。</p><p>　　較低的轉(zhuǎn)變溫

36、度，較高程度的過(guò)冷度，并且較大的在較高的速度時(shí)的自由能和轉(zhuǎn)變效益中的差異。</p><p>　　在珠光體轉(zhuǎn)變過(guò)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(méi)有的鐵素體和含碳量為6.67的滲碳體。因此對(duì)珠光體轉(zhuǎn)變的奧氏體是伴隨著碳原子的擴(kuò)散和再分布進(jìn)行的。隨著溫度的增加，擴(kuò)散的速度明顯地降低，因此，這個(gè)轉(zhuǎn)變應(yīng)該在較大的過(guò)冷下緩慢進(jìn)行。</p><p>　　因此，我們可以得到一個(gè)重要結(jié)

37、論即過(guò)冷（降低轉(zhuǎn)變溫度）也許對(duì)轉(zhuǎn)變速度上有兩個(gè)迥然不同的效果。</p><p>　　一方面，較低的溫度（較大的過(guò)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(kuò)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(guò)冷增加到某一個(gè)最大值然后減少與進(jìn)一步過(guò)冷。</p><p>　　溫度在727℃（A1）和200℃以下時(shí)，轉(zhuǎn)變的速度為零，因?yàn)?/p>

38、哉727℃時(shí)自由能的差異為零和在200℃以下時(shí)碳原子的擴(kuò)散速度為零（更嚴(yán)格地說(shuō)，太低為了轉(zhuǎn)變能進(jìn)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(kāi)發(fā)，珠光體的形成是珠光體晶核形成和珠光體晶粒長(zhǎng)大的過(guò)程。</p><p>　　因此，珠光體轉(zhuǎn)變?cè)诟鞣N各樣的過(guò)冷度時(shí)，它的轉(zhuǎn)變速度不同，這歸結(jié)于事實(shí)不同的過(guò)冷影響晶核N的形成率和晶粒G的長(zhǎng)

39、大率。溫度在A1線和200℃以下時(shí)，結(jié)晶N和G的參數(shù)是相等的到零并且在150~200℃的過(guò)冷時(shí)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(guò)冷，碳原子擴(kuò)散不為零，結(jié)晶的中心出現(xiàn)，以轉(zhuǎn)變的形式，這種過(guò)程隨時(shí)間發(fā)生和以一種叫做轉(zhuǎn)變的運(yùn)動(dòng)曲線的形式可以代表，這顯示珠光體的數(shù)量形成當(dāng)時(shí)流逝從轉(zhuǎn)變開(kāi)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(shí)期。在轉(zhuǎn)變過(guò)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(shí)，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?yàn)镹或G很低，轉(zhuǎn)變的速度取決于過(guò)冷度，在低和高的過(guò)冷時(shí)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(lái)，由于原子的低擴(kuò)散流動(dòng)性。在最大轉(zhuǎn)變速度是運(yùn)動(dòng)曲線有尖的頂點(diǎn)，并且轉(zhuǎn)變是在短的時(shí)間內(nèi)完成的間隔時(shí)間。</p><p>　　在高溫（

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

42、溫度在500~600℃時(shí)這個(gè)階段是最短的，即轉(zhuǎn)變?cè)诙虝r(shí)間開(kāi)始是在那個(gè)溫度。</p><p>　　右手曲線在特定的過(guò)冷下轉(zhuǎn)變完成需要的時(shí)間，在同一溫度（500~600℃）這個(gè)時(shí)間是最短的。注意圖的橫坐標(biāo)是對(duì)數(shù)的。這種做法更方便，因?yàn)橹楣怏w的形成速度看得出不同（在A1線的臨界點(diǎn)的數(shù)以萬(wàn)計(jì)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(wú)擴(kuò)散馬氏體的轉(zhuǎn)變溫

43、度。馬氏體轉(zhuǎn)變是靠不同的機(jī)理發(fā)生，并且以后將會(huì)被討論。</p><p>　　我門討論的圖表類型通常被叫做TTT圖表（時(shí)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(shí)間。</p><p>　　在高溫，即在低的過(guò)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粒混合物被形成，這個(gè)結(jié)構(gòu)稱珠光體。</p><p>　　在較

44、低的溫度，并且，較大的過(guò)冷度時(shí)，更多的分散劑和堅(jiān)硬的產(chǎn)物被形成。這個(gè)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(duì)高溫設(shè)定一個(gè)更加均勻的結(jié)構(gòu)的形成的有利的條件和因而促進(jìn)片狀結(jié)構(gòu)的出現(xiàn)。非均勻的奧氏體在所有的過(guò)冷度下產(chǎn)生顆粒狀珠光體，因此，加熱到低溫（過(guò)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