20xx-20xx年铁碳合金第二章

20xx-20xx年铁碳合金第二章内容摘要：

iously austenitized alloy. The structures formed during the continuous cooling of steel from above Ac3 can be understood best by studying the constanttemperature (isothermal) transformation of austenite, thus separating the two variables: time and temperature. One method consists of heating small specimens above Ac3 to form austenite, then quenching into a suitable bath (. liquid tin) at some constant subcritical temperature. After holding for selected periods of time, the specimens are withdrawn from the bath and rapidly quenched in cold water. This converts any untransformed austenite into martensite the volume of which can be estimated microscopically. Another method consists in measuring length changes caused by the deposition of austenite at the constant temperature by means of a dilatometer. Figure 4: Timetemperaturetransformation diagram. It is obvious that the equilibrium phase diagram (Fig. 1) does not contain any information about phases such as bainite, martensite etc. This is because it represents equilibrium whereas the varieties of transformation products have a range of deviations from the equilibrium state. Pg: 11/ 69 The phase diagram for obvious reasons does not feature time. The kiics of transformation is better illustrated using a timetemperaturetransformation (TTT) diagram as illustrated in Fig. 4. There are two C curves, the top one for reconstructive transformations and the lower one for displacive transformations. Also illustrated are schematic microstructures within individual austenite grains. FCC : Face center cubic austenite (a) Transmission electron micrograph of asquenched martensite in a wt% steel. The mottled contrast within the plates is due to a high density of dislocations. (b) Corresponding darkfield image showing the distribution of retained austenite. When carbon steel is quenched in the baths at constant temperatures, the velocity of austenite transformation is found to depend on temperature. The time for the beginning and pletion of the transformation of the austenite is plotted against the temperature to give the Bain Scurve, shown in Fig. 1 below, now called TTTcurve (timetemperaturetransformation). Pg: 12/ 69 Figure 1. Ideal TTTcurve for 0,65% carbon steel depicting time interval required for beginning, 50% and 100% transformation of austenite at a constant temperature A= Austenite F= Ferrite P = Pearlite B = Bainite Pg: 13/ 69 The logarithmic scale of time is used to condense results into a small space. Ae1 and Ae3 lines represent the equilibrium transformation temperatures.  Austenite is pletely stable above Ae3  Partially unstable between Ae3 and Ae1.  Below Ae1 austenite is pletely unstable and transforms in time. Two regions of rapid transformation occur about 550176。 and 250176。 C. The form of each of the curves and their positions with respect to the time axis depend on the position and grain size of the austenite which is transforming. The TTTcurve is most useful in presenting an overall picture of the transformation behaviour of austenite. This enables the metallurgist to interpret the response of steel to any specified heattreatment, to plan practical heattreatment operations and to control limited hardening or softening and the time of soaking. The deposition of austenite occurs according to three separate but sometimes overlapping mechanisms and results in three different reaction products:  pearlitic,  bainitic,  martensitic. When austenite is cooled slowly to a temperature below LCT (Lower Critical Temperature Ae3), the structure that is formed is Pearlite. As the cooling rate increases, the pearlite transformation temperature gets lower. The microstructure of the material is significantly altered as the cooling rate increases. By heating and cooling a series of samples, the history of the austenite transformation may be recorded. TTT diagram indicates when a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved. Depending on the cooling rate different phases may be formed. Cooling rates in the order of increasing severity are achieved by quenching from elevated temperatures as follows:  Furnace cooling,  Air cooling,  Oil quenching,  Liquid salts,  Water quenching,  Brine quenching. If these cooling curves are superimposed on the TTT diagram, the end product structure and the time required to plete the transformation may be found. Pg: 14/ 69 Below is an Isothermal Transformation (IT) diagram, also called a TTT (Time, Temperature, Transformation) curve for a eutectoid steel test piece which has been rapidly cooled in a bath at a set temperature, held for a time and then water quenched.  It can be seen that if the transformation is allowed to take place at a higher temperature then, as above coarse pearlite is formed.  If the test piece is cooled to a lower temperature in a bath a finer pearlitic structure results.  If the test piece is rapidly cooled to a temperature below a value Ms (which varies with the carbon content then a new metastable phase is produced called Martensite. Martensite is a supersaturated solid solution of carbon in ferrite.  If the test piece is cooled rapidly at a temperature between 220oC and 525oC a phase structure between pearlite and martensite is formed. This is called Bainite In Figure 1 the area on the left of the transformation curve represents the austenite region. Austenite is partially stable at temperatures above LCT but unstable below LCT. Left curve indicates the start of a transformation and right curve represents the finish of a transformation. The area between the two curves indicates the tr。