《材料成型及控制工程专业英语- 2. HEAT TREATMENT OF STEEL》.ppt

- * - 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 diffusion, and thus slows down the transformation. 2.1.4 Decomposition of Austenite - * - 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 decreases with further undercooling. 2.1.4 Decomposition of Austenite - * - 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 200 ℃ the rate of carbon diffusion is zero (more strictly, too low for the transformation to proceed). 2.1.4 Decomposition of Austenite - * - 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 1939 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 - * - 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150~200℃时过冷度最大. 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 to zero and have a maximum at an undercooling of 150~200℃. 2.1.4 Decomposition of Austenite - * - 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 are favorable, i.e. austenite is undercolled below A1, the diffusion of carbon