钢的热处理原则[中文.3967字] 【中英文WORD】
钢的热处理原则[中文.3967字] 【中英文WORD】,中文.3967字,中英文WORD,钢的热处理原则[中文.3967字],【中英文WORD】,热处理,原则,中文,3967,中英文,WORD
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.
钢的热处理原则
在现代机械工程中，热处理的作用不能被过高估计，但是，热处理使金属性能发生变化仍是具有重要意义的。
1、温度和时间
所有热处理工艺的目的都是通过加热温度和后期冷却获得所期望的金属结构变化。
因此，影响热处理的主要因素是温度和时间。所以，热处理过程可以用温度—时间坐标系来描述。
热处理条件可以用以下条件来描述：热处理时间、合金加热的最高温度及保温时间、加热速度和冷却速度。
如果加热或冷却连续进行，那么温度—时间曲线可以用各自的倾斜直线来描述。
在变化的加热或冷却速度下，实际的速度应归因于给定的温度，更严格的说是温度和时间的无限变化，也就是温度关于时间的一阶导数。
热处理可能是一个很复杂的过程，包括多个加热阶段或阶梯加热（或冷却），冷却至零度以下等。任何一种热处理工艺都可以用温度—时间坐标描述。
2、 奥氏体的构成
根据铁碳相图，在很慢的加热速度下，珠光体向奥氏体转变，在通常条件下，转变被延迟，导致过热，即转变发生在比铁碳相图所示的温度较高一点的温度下。
当过热超过临界温度时，珠光体向奥氏体转变，转变的速度取决于过热度。
在各种温度下的转变时间显示，在较高的温度下转变的速度较快，而在较高的加热速度下发生转变的温度较高。
例如，快速加热并保温在780℃时，珠光体倒奥氏体的转变需2分钟，保温在740℃时，转变需要8分钟。
转变结束以奥氏体的形成和珠光体的消失为标准。然而，奥氏体时不均匀的，甚至含有大量的单晶体颗粒，在珠光体渗碳体的晶粒和片层占据的地方碳含量比铁素体片层的高，这就是为什么奥氏体的形成时不均匀的。
为了获得均匀的奥氏体组织，不仅要加热到珠光体到奥氏体转变结束点，还要过热并保温到奥氏体晶粒扩散过程结束。
奥氏体的均匀度取决于钢的原始结构，特别是渗碳体颗粒的形状和弥散度，渗碳体颗粒越细，接触面积就越大，转变发生的就越快。
3、奥氏体晶粒的长大
在珠光体到奥氏体转变之初，奥氏体首先是在铁素体和渗碳体的边界处产生的（这两种物质是珠光体的结构成分），因为这些边界对转变非常有利，转变以细小晶粒的形成开始，因此，在转变之末，奥氏体由大量细晶粒构成，这些细晶粒的尺寸叫做原始奥氏体晶粒尺寸。
在转变过程中进一步加热或保温会引起奥氏体晶粒的粗大，这些晶粒粗化过程是自发的，由于晶粒表面积的减小和高温条件能够加快这一过程的速度。
在那种条件下，两种钢的主要区别为：内部细晶粒和外部粗晶粒。前者晶粒粗大的倾向比后者小，这种通过加热处理在钢中形成的晶粒尺寸被叫做实际晶粒尺寸。
因此，他们的区别在于：（1）原始晶粒，即在珠光体向奥氏体转变刚结束时奥氏体的尺寸；（2）固有（天然）晶粒，即奥氏体晶粒粗化的可能性（倾向）；和（3）实际晶粒，即在一定的珠光体转变条件下奥氏体晶粒之间的尺寸。
在奥氏体向珠光体转变的相同温度下，珠光体晶粒的尺寸取决于所形成的奥氏体晶粒。奥氏体晶粒的长大只发生在加热过程中（但没有在随后的冷却中被提纯），这是由于钢被加热到最高温度时，钢处在奥氏体状态，并且钢的本质晶粒度决定最终晶粒尺寸。
钢的性能只受实际晶粒度的影响，而不是本质晶粒度。如果两种相同品级（一种是固有的晶粒，另一种是细致纹理）的钢在不同的加热温度下，有相同的实际晶粒尺寸，他们也具有相同的性能；如果在其他条件下，这两种钢的性能将会不同。
4、奥氏体的分解
奥氏体到珠光体的转变，实际上是奥氏体分解为纯净的铁素体和渗碳体的过程。
在平衡温度下，转变是不可能发生的，因为原始奥氏体的自由能等于最终产物——珠光体的能量。只有当铁素体碳化物的混合物（珠光体）的自由能比奥氏体低，即具有一定过冷度时转变才会发生。当较低的转变温度，较高的过冷度和较大的自由能差时转变会在一个较高的速率下进行。
在珠光体转变中，新相与初始相有不同的成分；新相中大多是不含碳的铁素体，和含6.67%碳的渗碳体。基于这个原因，奥氏体到珠光体的转变，伴随着碳的扩散和再分配。扩散随着温度的降低而减小，因此，转变在一个较大过冷度下被延迟。
因此，我们得到一个重要结论，就是过冷度（低于转变温度）对转变速率具有双面的影响。一方面，较低的温度（较高的过冷度）使奥氏体的过冷度和珠光体有较大的不同；另一方面，随着碳扩散速率的降低，转变速率减慢。综上所述，转变速率由于过冷度增加到一个特定值而增加，然后，随着过冷度的降低而降低。
在727℃（A1线）到200℃之间，转变速率是0，因为727℃时的自由能是0，在200℃以下时的碳扩散速率是0（更严格的说，对转变来说太低）。
根据1939年Mirkin首先提出而后又在1941年被R.F. Mehl 发展的理论来看，珠光体的形成过程是珠光体形核及其晶粒的长大过程。因此，珠光体在不同的过冷度下有不同的转变速率，实际上是在A1线和200℃之间，不同的过冷度对形核速率和晶粒长大速度的影响，形核结晶化参数和晶粒长大速率都等于0，并且在150~200℃之间具有一个最大过冷度。
根据前面所说的，一旦条件具备，即奥氏体冷却至A1以下，碳的扩散速率将不再是0，并且结晶中心还会出现晶体，这一过程随时间的变化关系可以用转变动力曲线来描述，这条曲线显示的是珠光体数量和时间的关系。
起始阶段的特点是转变速率很低，这一阶段被称为孕育期。转变速率随着转变过程的进行而增加。他的最大值大约相当于50%的奥氏体转变成珠光体。然后，转变速率逐渐减小并最终停止。
转变速率依据过冷度而定。在较低或较高的过冷度下转变过程发生的很缓慢，因为结晶速率和晶粒长大速度低，前者是由于自由能的不同，而后者是由于原子的流动扩散速率低。在动力曲线的顶点是转变速率的最大值，并且转变在一个很短的时间内完成。
在高温（略低于过冷度）的条件下，转变进行的缓慢，并且孕育期和转变时间很长。在较低的转变温度，即较大的过冷度下，转变速率很大，。并且孕育期和转变时间很短。
5、TTT图表或C曲线
在不同过冷度下奥氏体到珠光体的转变开始时间（孕育期）和转变的结束时间都已经确定。我们能够会出一个曲线图，曲线图的左支用来代表转变的起始时间，即在过冷状态下依然存在奥氏体，并且从纵坐标的轴心到曲线这一部分表示成分的稳定程度，这部分温度最少在500~600℃之间，即在那个温度下转变时间最短。
右支曲线描绘的是在给定的过冷度下转变完成所需要的时间，这个时间在相同温度（500~600℃）下是最短的。曲线以对数为横坐标。这样做较为方便，因为珠光体的转变略微有些差异（临界点在A1线上占据数千秒，而在曲线的最后只有一到两秒）。
在曲线下方的水平线确定的是马氏体转变的无扩散相变温度。下面将讨论使马氏体通过不同的方法发生转变。
我们讨论的图表通常被称为TTT图表（即时间、温度和转变的 关系曲线）或曲线，是由于曲线的特殊形状。奥氏体分解的产物的结构和性能依据转变发生时的温度而定。
在高温条件下，即低的过冷度下，铁素体和渗碳体晶粒组成的混合物在显微镜下很容易被发现，这种结构被叫做珠光体。
因此，在较低温度下分散而质地较硬的产物将会形成，珠光体的这种细致型结构被叫做索氏体。
依旧在较低的温度下（接近C曲线的结尾），转变产物甚至会更分散，形成的铁素体的层片状结构和转变产物在电子显微镜下是可以分辨出来的，这种结构被称作屈氏体。
这样，珠光体、索氏体和屈氏体除了铁素体和渗碳体的弥散度不同外，就具有了相同的结构（铁素体+渗碳体）。
珠光体结构可能有两种类型：颗粒状（在晶粒的形成过程中渗碳体一直是存在的）和片层状（渗碳体层）。
均匀的奥氏体总是转变成层状珠光体，因此，加热到一个很高温度来为更多均匀结构的形成建立条件，也借此来提高片状结构的外观。不均匀的奥氏体在任何一个过冷度下都会产生颗粒状的珠光体，因此，加热到一个较低温度（Accm线以下）时，会导致在冷却时颗粒状珠光体的形成。通过控制奥氏体中未溶解碎颗粒的成分可能提高这种颗粒状珠光体的结构，而未溶解碎颗粒本身可以充当晶核。
6、伪共析钢
我们已经讨论过钢中奥氏体向珠光体的转变，产物的成分接近于共析结构。如果钢中碳含量与共析结构的不同，珠光体的转变将会发生在铁素体和渗碳体析出之前（从铁碳相图中可以知道）。
在低碳钢中，奥氏体的转变开始于铁素体的形成和碳元素随着溶解而达到饱和情况下，而在过共析钢中，是随着渗碳体的析出和奥氏体中碳的溶解而发生的。在平衡条件下，当奥氏体中剩余碳的含量超过析出的铁素体和渗碳体时，大概相当于总含碳量的0.8%时，奥氏体分解成铁素体和渗碳体的过程才开始。
这类由过冷奥氏体组成的共熔体与真正意义上的共熔体有所不同，它在过共析钢中被称为伪共析，这种钢的含碳量超过0.8%，而在低碳钢中，含碳量低于0.8%，从这一层面讲在较低的转变温下意义就很重大了。因此，较低的转变温度下，多余的铁素体（或渗碳体）析出物较珠光体转变开始前的要少。在铁碳相图拐点附近的温度以及较低温度下，奥氏体的分解开始时并没有出现过多的析出物。
如果我们用过共析钢代替亚共析钢，奥氏体在小过冷度下的分解将会发生在渗碳体析出之前。
7、马氏体转变
如果冷却速率比较高，没有足够的时间在高温范围内发生转变，奥氏体将会过冷到一个较低的温度，并且转变成马氏体。这样的冷却将会导致硬化，因此，高硬度钢，将会以一个较高的速度冷却，以使奥氏体在高温范围内没有足够的时间发生分解。
最低的冷却速率使奥氏体发生马氏体转变，被称为硬化速率。如果钢发生硬化，冷却速度将会比临界速度高。因为钢的转变开始时的曲线延伸到右边，使得临界速度变得更低。换句话说，随着奥氏体到珠光体转变的速度越来越低，更容易使奥氏体过冷到向马氏体转变的温度，并且硬化临界速度更低。
如果冷却速度略低于临界速度，奥氏体将会经历在较高温度范围内发生局部转变，并且产物的结构将会由屈氏体和马氏体组成。
硬化临界速度可以由奥氏体转变的等温图来确定，这个分析结果只说明冷却曲线和奥氏体分解的等温图有一定的重合，也只能告诉我们在连续冷却时转变发生的近似的数量上的
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