机械专业英文翻译

机械专业英文翻译内容摘要：

to crystal. This process occurs with time and can be represented in the form of so called kiic 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 kiic 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, . a deeper undercooling, the rate of transformation is greater, and the time of the incubation period and of the transformation is shorter. TTT Diagram or CCurve 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, . 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℃ , . the transformation begins in a shortest time at that temperature. The right hand curve shows the time needed to plete 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). 5 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 deposition depends on the temperature at which the transformation has taken place. At high temperatures, . 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 t。