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? 通過畢業(yè)設(shè)計(jì)基本掌握機(jī)械加工工藝設(shè)計(jì)及夾具設(shè)計(jì)方法，同時(shí)把所學(xué)知識(shí)運(yùn)用到設(shè)計(jì)中去。
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1. 根據(jù)給定的“閥體”零件圖，進(jìn)行工藝分析，完成毛坯圖；
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5. 編寫設(shè)計(jì)說明書。
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一、給定零件的機(jī)械加工工藝過程設(shè)計(jì)及工序設(shè)計(jì)
二、夾具設(shè)計(jì)圖紙
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四、英文資料翻譯
4．主要參考文獻(xiàn)：
[1] 主編 楊黎明 機(jī)床夾具設(shè)計(jì)手冊(cè) 國防大學(xué)出版社 1996年第一版
[2] 主編 陳旭東 機(jī)床夾具設(shè)計(jì) 清華大學(xué)出版社 2012年12月第四次印刷
[3] 主編 馬敏麗 機(jī)械制造工藝編制及實(shí)施 清華大學(xué)出版社 2013年1月第2次印刷
[4] 主編 陳宏鈞 金屬切削工藝技術(shù)手冊(cè) 機(jī)械工業(yè)出版社 2013年5月第1次印刷
[5] 主編 東北工學(xué)院機(jī)械設(shè)計(jì)機(jī)械制圖教研室 冶金工業(yè)出版社 1976年第二次印刷
[6] 主編 游文明，李業(yè)農(nóng) 機(jī)械設(shè)計(jì)基礎(chǔ)課程設(shè)計(jì) 高等教育出版社 201年2月第1次 ?印刷
[7] 主編 陳廣建 金屬切削方法與設(shè)備選用 南通職業(yè)大學(xué)出版
[8] 主編 李益民 機(jī)械制造工藝設(shè)計(jì)簡明手冊(cè) 機(jī)械工業(yè)出版社 2012年7月第1版
[9] 主編 張龍勛 機(jī)械制造工藝學(xué)課程設(shè)計(jì)指導(dǎo)書及習(xí)題 機(jī)械工業(yè)出版社 2012年7月?第1版
[10] 艾興 肖詩綱 切削用量簡明手冊(cè) 機(jī)械工業(yè)出版社 2012年7月 第1版
[11] 主編 李業(yè)農(nóng) 機(jī)械制圖 上海交通大學(xué)出版社 2010年8月第5版
[12] 王智平,王延露,徐建林,朱小武,肖榮振.? 鑄造工藝的計(jì)算機(jī)輔助設(shè)計(jì)[J]. 蘭州理工大學(xué)學(xué)報(bào). 2006(02)
[13] 周小平,胡紅軍.? 鑄造工藝CAD軟件的研究及開發(fā)[J]. 鑄造技術(shù). 2003(02)
[14] 宋永恒,荊濤.? 三維鑄造工藝工裝CAD/CAE系統(tǒng)研究[J]. 現(xiàn)代制造工程. 2002(06)
[15] 王政編著.焊接工裝夾具及變位機(jī)械[M]. 機(jī)械工業(yè)出版社, 2001
[16] 王娟等編著.表面堆焊與熱噴涂技術(shù)[M]. 化學(xué)工業(yè)出版社, 2004
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譯文題目：Principle of Heat Treatment of Steal
鋼的熱處理原則
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20xx年 2月 27日
Principle of Heat Treatment of Steal
The role of heat treatment in modern mechanical engineering cannot be overestimated. The changes in the properties of metals due to heat treatment are of extremely great significance.
1、 Temperature and Time
The purpose of any heat treating process is to produce the desired changes in the structure of metal by heating to a specified temperature and by subsequent cooling.
Therefore, the main factors acting in heat treatment are temperature and time, so that any process of heat treatment can be represented in temperature—time () coordinates.
Heat treatment conditions are characterized by the following parameters: heating temperature, i.e. the maximum temperature to which an alloy metal is heated；time of holding at the heating temperature; heating rate and cooling rate.
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.
With a varying heating (or cooling) rate, the actual rate should be attributed to the given temperature, more strictly, to an infinite change of temperature and time: that is the first derivative of temperature in time:.
Heat treatment may be a complex process, including multiple heating stages, interrupted or stepwise heating (cooling), cooling to subzero temperature, etc. Any process of heat treatment can be described by a diagram in temperature-time coordinates.
2、Formation of Austenite
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-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.
When overheated above the critical point, pearlite transforms into austenite, the rate of transformation being dependent on the degree of overheating.
The time of transformation at various temperatures (depending on the degree of overheating) 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.
For instance, on quick heating and holding at 780℃, the pearlite to austenite transformation is completed in 2 minutes and on holding at 740℃, in 8 minutes.
The end of the transformation is characterized by the formation of austenite and the disappearance of pearlite (ferrite + cementite). This austenite is however inhomogeneous even in the volume of a single grain. In places earlier occupied by lamellae (or grains) of a pearlite cementite, the content of carbon is greater than in places of ferritic lamellae. This is why the austenite just formed is inhomogeneous.
In order to obtain homogeneous austenite, it is essential on heating not only to pass through the point 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.
The rate of homogenization of austenite 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.
3、Coarsening of Austenite Grains
At the beginning of pearlite to austenite transformation, the 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 formation of a multitude of fine grains. Therefore at the end of the transformation the austenite will be composed of a great multitude of fine grains whose size characterize what is called the original austenitic grains size.
Further heating (or holding) upon the transformation will cause coarsening of austenitic grains. The process of grain coarsening is spontaneous, since the total surface area of grains diminishes (the surface energy decreases) and a high temperature can only accelerate the rate of this process.
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.
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 coarsening; and (3) actual grain, i.e. the size of austenitic grains under given particular conditions.
The size of pearlitic grains at the same temperature 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 the inherent grain size of that steel determine the final grain size.
The properties of steel are affected only by the actual grain size and not by the inherent grain size. If two steels of the same grade (one inherent 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, and many properties of the two steels will also be different.
4、Decomposition of Austenite
The austenite to pearlite transformation is essentially the decomposition of austenite into almost pure ferrite and cementite.
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. The transformation can only start at a certain undercooling when the free energy of the ferrite carbide mixture higher the degree of undercooling and the greater the difference in free energies and the transformation proceeds at a higher rate.
In the pearlite transformation, the new phases 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. For this reason the austenite to pearlite transformation is accompanied with the temperature, redistribution of carbon. The rate of diffusion sharply diminishes with decreasing temperature; therefore, the transformation should be retarded at a greater undercooling.
Thus, we have come to an important conclusion that undercooling (lowering the transformation temperature) may have two opposite effects on the rate of transformation. On one austenite and pearlite, thus accelerating the transformation; on the other hand, diminished the rate of carbon diffusion, and thus slows down the transformation. The combined effect is that the rate of transformation first increases as undercooling is increased to a certain maximum and then decreases with further undercooling.
At 270℃（A1）and below 200℃, the rate of transformation 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).
As has been first indicated by I.L.Mirkin in 1939 and developed by R.F. Mehl in 1941, the formation of pearlite is the process of nucleation of pearlite and growth of pearlitic crystals. Therefore, the different rate of the pearlite transformation at various degrees of undercooling is due to the fact that undercooling differently affects the rate nucleation N and the rate of crystals growth G. at temperature A1 and below 200℃, both parameters of crystallization N and G are equal to zero and have a maximum at an undercooling of 150~200℃.
It follows from the foregoing that as soon as the conditions are favorable, i.e. austenite is undercooling below A1, the diffusion of carbon is not zero, and centers of crystallization appear which give rise to crystal. 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 has formed during the time elapsed from the beginning of the transformation.
The initial stage is characterized by a very low 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 roughly 50 percent of austenite has transformed into pearlite. The rate of transformation then diminishes and finally stops.
The rate of transformation depends on undercooling. At low and high degrees of undercooling the transformation proceeds slowly, since 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.
At a high temperature (slightly undercooling), the transformation proceeds slowly and the incubation period and the time of the transformation proper are long. At a lower temperature of the transformation, i.e. a deeper undercooling, the rate of transformation is greater, and the time of the incubation period and of the transformation is shorter.
5、TTT Diagram or C-Curve
Having determined the time of the beginning of austenitic 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 beginning of transformation, i.e. the time during which austenite still exists in the undercooling state, and the section from the axis of ordinates to the curve is measure of its stability. This section is shortest at a temperature of 500~600℃, i.e. the transformation begins in a shortest time at that temperature.
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 abscissa of the diagram is logarithmic. This is done for convenience, since the rate of formation of pearlite appreciably differ (thousands of seconds near the critical point A1 and only one or two seconds at the end of the curve).
The horizontal line below the curves in the diagram determines the temperature of the diffusionless martensite transformation. The martensite transformation occurs by a different mechanism and will be discussed later.
Diagrams of the type we discussed are usually called TTT diagrams (time temperature transformation), or curve, owing to the specific shape of the curves. The structure and properties of the products of austenite decomposition depends on the temperature at which the transformation has taken place.
At high temperatures, i.e. low degrees of undercooling, a coarse grained mixture of ferrite and cementite is formed which is easily distinguished in the microscope. This structure is called pearlite.
At lower temperatures, and therefore, greater degrees of undercooling, more disperse and harder products are formed. The pearlitic structure of this finer type is called sorbite.
At still lower temperatures (near the end of the C curve), the transformation products are even more disperse, so that the lamellar structure of the ferrite and transformation products only distinguishable in electron microscope. This structure is called troostite.
Thus, pearlite, sorbite and troostite are the structures of the same nature (ferrite + cementite) but a different dispersity of ferrite and cementite.
Pearlitic structures may be of two types: granular (in which cementite is present in the form of grains) or lamellar (with cementite platelets).
Homogeneous austenite always transforms into lamellar pearlite. Therefore, heating to a high temperature sets 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, therefore, heating to a low temperature (below Accm for hypereutectoid steels) results in the formation of granular pearlite on cooling. The formation of granular cementite is probably promoted by the presence of undissolved particles in austenite, which serve as additional crystallization nuclei.
6、Quasi-eutectoid
We have discussed the austenite to pearlite transformation in steels whose composition is close to eutectoid. If the content of carbon in steel differs from the eutectoid value, the pearlite transformation will be preceded with the precipitation of ferrite or cementite (as follows from the iron carbon constitutional diagram).
In hypoeutectoid steels, the transformation of austenite begins with the formation of ferrite and the saturation of the remaining solution with carbon, and in hypereutectoid steels, with the precipitation of cementite and depletion of the austenite of carbon. Under equilibrium a condition, the decomposition of austenite into ferrite and cementite (pearlite transformation) begins when the content of carbon in austenite, remained upon precipitation of excess ferrite or cementite, corresponds to 0.8% carbon.
The eutectoid which forms from undercooled austenite and has a concentration differing from the eutectoid value is called quasi-eutectoid in hypereutectoid steels contains more 0.8 percent carbon and that in hypoeutectoid steels, less than 0.8 percent, the deviation from this value being greater at lower temperature of transformation. Therefore, the lower the temperature of transformation, the less the excess ferrite (or cementite) precipitates before the pearlite transformation begins. At temperature near the bend of C curve and at lower temperature, decomposition of austenite begins without precipitation of excess phases.
If we take a hypereutectoid steel instead of hypoeutectoid, the decomposition of austenite at small degrees of undercooling will be preceded with precipitation of cementite.
7、Martensite Transformation
If the cooling rate is higher, the transformation has no time to proceed in the upper temperature range. The austenite will be undercooled to a low temperature and will transformation martensite. Such a cooling will result in hardening. Therefore, to harden steel, it should be cooled at a high rate so that austenite has no time decompose in the upper temperature range.
The lowest cooling rate needed to undercool austenite up to martensite transformation is called the critical rate of hardening. If steel is to be hardened, it should be cooled at a rate not less than the critical rate. The critical rate is lower for steels whose curve of the beginning of transformation passes farther to the right. In other words, with a lower rate of austenite to pearlite transformation, it is easier to undercool the austenite to the temperature of martensite transformation and the critical rate of hardening will be lower.
If cooling is done at a rate slightly below the critical rate, the austenite will undergo only a partial transformation in the upper temperature range and the structure will consist of the products of transformation in the upper temperature range (troostite) and martensite.
The critical rate of hardening can be determined from the diagram of isothermal decomposion of austenite. This analysis shows that a simple superposition of cooling curve on the isothermal diagram of austenite decomposition can give only an approximate quantitative estimation of a transformation occurring in continuous cooling.
鋼的熱處理原則
在現(xiàn)代機(jī)械工程中，熱處理的作用不能被過高估計(jì)，但是，熱處理使金屬性能發(fā)生變化仍是具有重要意義的。
1、溫度和時(shí)間
所有熱處理工藝的目的都是通過加熱溫度和后期冷卻獲得所期望的金屬結(jié)構(gòu)變化。
因此，影響熱處理的主要因素是溫度和時(shí)間。所以，熱處理過程可以用溫度—時(shí)間坐標(biāo)系來描述。
熱處理?xiàng)l件可以用以下條件來描述：熱處理時(shí)間、合金加熱的最高溫度及保溫時(shí)間、加熱速度和冷卻速度。
如果加熱或冷卻連續(xù)進(jìn)行，那么溫度—時(shí)間曲線可以用各自的傾斜直線來描述。
在變化的加熱或冷卻速度下，實(shí)際的速度應(yīng)歸因于給定的溫度，更嚴(yán)格的說是溫度和時(shí)間的無限變化，也就是溫度關(guān)于時(shí)間的一階導(dǎo)數(shù)。
熱處理可能是一個(gè)很復(fù)雜的過程，包括多個(gè)加熱階段或階梯加熱（或冷卻），冷卻至零度以下等。任何一種熱處理工藝都可以用溫度—時(shí)間坐標(biāo)描述。
2、 奧氏體的構(gòu)成
根據(jù)鐵碳相圖，在很慢的加熱速度下，珠光體向奧氏體轉(zhuǎn)變，在通常條件下，轉(zhuǎn)變被延遲，導(dǎo)致過熱，即轉(zhuǎn)變發(fā)生在比鐵碳相圖所示的溫度較高一點(diǎn)的溫度下。
當(dāng)過熱超過臨界溫度時(shí)，珠光體向奧氏體轉(zhuǎn)變，轉(zhuǎn)變的速度取決于過熱度。
在各種溫度下的轉(zhuǎn)變時(shí)間顯示，在較高的溫度下轉(zhuǎn)變的速度較快，而在較高的加熱速度下發(fā)生轉(zhuǎn)變的溫度較高。
例如，快速加熱并保溫在780℃時(shí)，珠光體倒奧氏體的轉(zhuǎn)變需2分鐘，保溫在740℃時(shí)，轉(zhuǎn)變需要8分鐘。
轉(zhuǎn)變結(jié)束以奧氏體的形成和珠光體的消失為標(biāo)準(zhǔn)。然而，奧氏體時(shí)不均勻的，甚至含有大量的單晶體顆粒，在珠光體滲碳體的晶粒和片層占據(jù)的地方碳含量比鐵素體片層的高，這就是為什么奧氏體的形成時(shí)不均勻的。
為了獲得均勻的奧氏體組織，不僅要加熱到珠光體到奧氏體轉(zhuǎn)變結(jié)束點(diǎn)，還要過熱并保溫到奧氏體晶粒擴(kuò)散過程結(jié)束。
奧氏體的均勻度取決于鋼的原始結(jié)構(gòu)，特別是滲碳體顆粒的形狀和彌散度，滲碳體顆粒越細(xì)，接觸面積就越大，轉(zhuǎn)變發(fā)生的就越快。
3、奧氏體晶粒的長大
在珠光體到奧氏體轉(zhuǎn)變之初，奧氏體首先是在鐵素體和滲碳體的邊界處產(chǎn)生的（這兩種物質(zhì)是珠光體的結(jié)構(gòu)成分），因?yàn)檫@些邊界對(duì)轉(zhuǎn)變非常有利，轉(zhuǎn)變以細(xì)小晶粒的形成開始，因此，在轉(zhuǎn)變之末，奧氏體由大量細(xì)晶粒構(gòu)成，這些細(xì)晶粒的尺寸叫做原始奧氏體晶粒尺寸。
在轉(zhuǎn)變過程中進(jìn)一步加熱或保溫會(huì)引起奧氏體晶粒的粗大，這些晶粒粗化過程是自發(fā)的，由于晶粒表面積的減小和高溫條件能夠加快這一過程的速度。
在那種條件下，兩種鋼的主要區(qū)別為：內(nèi)部細(xì)晶粒和外部粗晶粒。前者晶粒粗大的傾向比后者小，這種通過加熱處理在鋼中形成的晶粒尺寸被叫做實(shí)際晶粒尺寸。
因此，他們的區(qū)別在于：（1）原始晶粒，即在珠光體向奧氏體轉(zhuǎn)變剛結(jié)束時(shí)奧氏體的尺寸；（2）固有（天然）晶粒，即奧氏體晶粒粗化的可能性（傾向）；和（3）實(shí)際晶粒，即在一定的珠光體轉(zhuǎn)變條件下奧氏體晶粒之間的尺寸。
在奧氏體向珠光體轉(zhuǎn)變的相同溫度下，珠光體晶粒的尺寸取決于所形成的奧氏體晶粒。奧氏體晶粒的長大只發(fā)生在加熱過程中（但沒有在隨后的冷卻中被提純），這是由于鋼被加熱到最高溫度時(shí)，鋼處在奧氏體狀態(tài)，并且鋼的本質(zhì)晶粒度決定最終晶粒尺寸。
鋼的性能只受實(shí)際晶粒度的影響，而不是本質(zhì)晶粒度。如果兩種相同品級(jí)（一種是固有的晶粒，另一種是細(xì)致紋理）的鋼在不同的加熱溫度下，有相同的實(shí)際晶粒尺寸，他們也具有相同的性能；如果在其他條件下，這兩種鋼的性能將會(huì)不同。
4、奧氏體的分解
奧氏體到珠光體的轉(zhuǎn)變，實(shí)際上是奧氏體分解為純凈的鐵素體和滲碳體的過程。
在平衡溫度下，轉(zhuǎn)變是不可能發(fā)生的，因?yàn)樵紛W氏體的自由能等于最終產(chǎn)物——珠光體的能量。只有當(dāng)鐵素體碳化物的混合物（珠光體）的自由能比奧氏體低，即具有一定過冷度時(shí)轉(zhuǎn)變才會(huì)發(fā)生。當(dāng)較低的轉(zhuǎn)變溫度，較高的過冷度和較大的自由能差時(shí)轉(zhuǎn)變會(huì)在一個(gè)較高的速率下進(jìn)行。
在珠光體轉(zhuǎn)變中，新相與初始相有不同的成分；新相中大多是不含碳的鐵素體，和含6.67%碳的滲碳體?；谶@個(gè)原因，奧氏體到珠光體的轉(zhuǎn)變，伴隨著碳的擴(kuò)散和再分配。擴(kuò)散隨著溫度的降低而減小，因此，轉(zhuǎn)變?cè)谝粋€(gè)較大過冷度下被延遲。
因此，我們得到一個(gè)重要結(jié)論，就是過冷度（低于轉(zhuǎn)變溫度）對(duì)轉(zhuǎn)變速率具有雙面的影響。一方面，較低的溫度（較高的過冷度）使奧氏體的過冷度和珠光體有較大的不同；另一方面，隨著碳擴(kuò)散速率的降低，轉(zhuǎn)變速率減慢。綜上所述，轉(zhuǎn)變速率由于過冷度增加到一個(gè)特定值而增加，然后，隨著過冷度的降低而降低。
在727℃（A1線）到200℃之間，轉(zhuǎn)變速率是0，因?yàn)?27℃時(shí)的自由能是0，在200℃以下時(shí)的碳擴(kuò)散速率是0（更嚴(yán)格的說，對(duì)轉(zhuǎn)變來說太低）。
根據(jù)1939年Mirkin首先提出而后又在1941年被R.F. Mehl 發(fā)展的理論來看，珠光體的形成過程是珠光體形核及其晶粒的長大過程。因此，珠光體在不同的過冷度下有不同的轉(zhuǎn)變速率，實(shí)際上是在A1線和200℃之間，不同的過冷度對(duì)形核速率和晶粒長大速度的影響，形核結(jié)晶化參數(shù)和晶粒長大速率都等于0，并且在150~200℃之間具有一個(gè)最大過冷度。
根據(jù)前面所說的，一旦條件具備，即奧氏體冷卻至A1以下，碳的擴(kuò)散速率將不再是0，并且結(jié)晶中心還會(huì)出現(xiàn)晶體，這一過程隨時(shí)間的變化關(guān)系可以用轉(zhuǎn)變動(dòng)力曲線來描述，這條曲線顯示的是珠光體數(shù)量和時(shí)間的關(guān)系。
起始階段的特點(diǎn)是轉(zhuǎn)變速率很低，這一階段被稱為孕育期。轉(zhuǎn)變速率隨著轉(zhuǎn)變過程的進(jìn)行而增加。他的最大值大約相當(dāng)于50%的奧氏體轉(zhuǎn)變成珠光體。然后，轉(zhuǎn)變速率逐漸減小并最終停止。
轉(zhuǎn)變速率依據(jù)過冷度而定。在較低或較高的過冷度下轉(zhuǎn)變過程發(fā)生的很緩慢，因?yàn)榻Y(jié)晶速率和晶粒長大速度低，前者是由于自由能的不同，而后者是由于原子的流動(dòng)擴(kuò)散速率低。在動(dòng)力曲線的頂點(diǎn)是轉(zhuǎn)變速率的最大值，并且轉(zhuǎn)變?cè)谝粋€(gè)很短的時(shí)間內(nèi)完成。
在高溫（略低于過冷度）的條件下，轉(zhuǎn)變進(jìn)行的緩慢，并且孕育期和轉(zhuǎn)變時(shí)間很長。在較低的轉(zhuǎn)變溫度，即較大的過冷度下，轉(zhuǎn)變速率很大，。并且孕育期和轉(zhuǎn)變時(shí)間很短。
5、TTT圖表或C曲線
在不同過冷度下奧氏體到珠光體的轉(zhuǎn)變開始時(shí)間（孕育期）和轉(zhuǎn)變的結(jié)束時(shí)間都已經(jīng)確定。我們能夠會(huì)出一個(gè)曲線圖，曲線圖的左支用來代表轉(zhuǎn)變的起始時(shí)間，即在過冷狀態(tài)下依然存在奧氏體，并且從縱坐標(biāo)的軸心到曲線這一部分表示成分的穩(wěn)定程度，這部分溫度最少在500~600℃之間，即在那個(gè)溫度下轉(zhuǎn)變時(shí)間最短。
右支曲線描繪的是在給定的過冷度下轉(zhuǎn)變完成所需要的時(shí)間，這個(gè)時(shí)間在相同溫度（500~600℃）下是最短的。曲線以對(duì)數(shù)為橫坐標(biāo)。這樣做較為方便，因?yàn)橹楣怏w的轉(zhuǎn)變略微有些差異（臨界點(diǎn)在A1線上占據(jù)數(shù)千秒，而在曲線的最后只有一到兩秒）。
在曲線下方的水平線確定的是馬氏體轉(zhuǎn)變的無擴(kuò)散相變溫度。下面將討論使馬氏體通過不同的方法發(fā)生轉(zhuǎn)變。
我們討論的圖表通常被稱為TTT圖表（即時(shí)間、溫度和轉(zhuǎn)變的 關(guān)系曲線）或曲線，是由于曲線的特殊形狀。奧氏體分解的產(chǎn)物的結(jié)構(gòu)和性能依據(jù)轉(zhuǎn)變發(fā)生時(shí)的溫度而定。
在高溫條件下，即低的過冷度下，鐵素體和滲碳體晶粒組成的混合物在顯微鏡下很容易被發(fā)現(xiàn)，這種結(jié)構(gòu)被叫做珠光體。
因此，在較低溫度下分散而質(zhì)地較硬的產(chǎn)物將會(huì)形成，珠光體的這種細(xì)致型結(jié)構(gòu)被叫做索氏體。
依舊在較低的溫度下（接近C曲線的結(jié)尾），轉(zhuǎn)變產(chǎn)物甚至?xí)稚?，形成的鐵素體的層片狀結(jié)構(gòu)和轉(zhuǎn)變產(chǎn)物在電子顯微鏡下是可以分辨出來的，這種結(jié)構(gòu)被稱作屈氏體。
這樣，珠光體、索氏體和屈氏體除了鐵素體和滲碳體的彌散度不同外，就具有了相同的結(jié)構(gòu)（鐵素體+滲碳體）。
珠光體結(jié)構(gòu)可能有兩種類型：顆粒狀（在晶粒的形成過程中滲碳體一直是存在的）和片層狀（滲碳體層）。
均勻的奧氏體總是轉(zhuǎn)變成層狀珠光體，因此，加熱到一個(gè)很高溫度來為更多均勻結(jié)構(gòu)的形成建立條件，也借此來提高片狀結(jié)構(gòu)的外觀。不均勻的奧氏體在任何一個(gè)過冷度下都會(huì)產(chǎn)生顆粒狀的珠光體，因此，加熱到一個(gè)較低溫度（Accm線以下）時(shí)，會(huì)導(dǎo)致在冷卻時(shí)顆粒狀珠光體的形成。通過控制奧氏體中未溶解碎顆粒的成分可能提高這種顆粒狀珠光體的結(jié)構(gòu)，而未溶解碎顆粒本身可以充當(dāng)晶核。
6、偽共析鋼
我們已經(jīng)討論過鋼中奧氏體向珠光體的轉(zhuǎn)變，產(chǎn)物的成分接近于共析結(jié)構(gòu)。如果鋼中碳含量與共析結(jié)構(gòu)的不同，珠光體的轉(zhuǎn)變將會(huì)發(fā)生在鐵素體和滲碳體析出之前（從鐵碳相圖中可以知道）。
在低碳鋼中，奧氏體的轉(zhuǎn)變開始于鐵素體的形成和碳元素隨著溶解而達(dá)到飽和情況下，而在過共析鋼中，是隨著滲碳體的析出和奧氏體中碳的溶解而發(fā)生的。在平衡條件下，當(dāng)奧氏體中剩余碳的含量超過析出的鐵素體和滲碳體時(shí)，大概相當(dāng)于總含碳量的0.8%時(shí)，奧氏體分解成鐵素體和滲碳體的過程才開始。
這類由過冷奧氏體組成的共熔體與真正意義上的共熔體有所不同，它在過共析鋼中被稱為偽共析，這種鋼的含碳量超過0.8%，而在低碳鋼中，含碳量低于0.8%，從這一層面講在較低的轉(zhuǎn)變溫下意義就很重大了。因此，較低的轉(zhuǎn)變溫度下，多余的鐵素體（或滲碳體）析出物較珠光體轉(zhuǎn)變開始前的要少。在鐵碳相圖拐點(diǎn)附近的溫度以及較低溫度下，奧氏體的分解開始時(shí)并沒有出現(xiàn)過多的析出物。
如果我們用過共析鋼代替亞共析鋼，奧氏體在小過冷度下的分解將會(huì)發(fā)生在滲碳體析出之前。
7、馬氏體轉(zhuǎn)變
如果冷卻速率比較高，沒有足夠的時(shí)間在高溫范圍內(nèi)發(fā)生轉(zhuǎn)變，奧氏體將會(huì)過冷到一個(gè)較低的溫