外文翻译--材料选择与热处理

外文翻译--材料选择与热处理内容摘要：

shape of the curves, they are also sometimes called Ccurves or Scurves. To plot TIT curves, the particular steel is held at a given temperature and the structure is examined at predetermined intervals to record the amount of transformation taken place. It is known that the eutectoid steel (C80) under equilibrium conditions contains, all austenite above 723 C .whereas below, it is pearlier. To form pearlier, the carbon atoms should diffuse to form mentate. The diffusion being a rate process , would require sufficient time for plete transformation of austenite to pearlier. From different samples, it is possible to note the amount of the transformation taking place at any temperature. These points are then plotted on a graph with time and temperature as the axes. Through these points, transformation curves can be plotted as shown in for eutectoid steel. The curve at excreta left represents the time required for the transformation of austenite to pearlier to start at any given temperature . Similarly, the curve at extreme right represents the time required for pleting the transformation. Between the two curves are the points representing partial transformation. The horizontal lines SM 4 and fM represent the start and finish of martens tic transformation. Classification of Heat Treating Processes In some instances, heat treatment procedures are clear cut in terms of technique and application, whereas in other instances, descriptions or simple explanations are insufficient because the same technique frequently may be used to obtain different objectives. For example, stress relieving and tempering are often acplished with the same equipment and by use of identical time and temperature cycles. The objectives, however, are different for the two processes. The following descriptions of the principal heat treating processes am generally arranged according to their interrelationships. Isothermal Deposition Diagram of C80 Steel Normalizing consists of heating a ferrous alloy to a suitable temperature (usually 50 F to 100 F or 28 C to 56 C ) above its specific upper transformation temperature. This is followed by cooling in still air to at least some temperature well below its transformation temperature range. For lowcarbon steels, the resulting structure and properties are the same as those achieved by full annealing。 for most ferrous alloys, normalizing and annealing are not synonymous. Normalizing usually is used as a conditioning treatment, notably for refining the grains of steels that have been subjected to high temperatures for fing or other hot working operations. The normalizing process usually is succeeded by another heat treating operation such as austenitic for hardening, annealing, or tempering. Annealing is a generic term denoting a heat treatment that consists of heating to and holding at a suitable temperature followed by cooling at a suitable rate. It is used primarily to soften metallic materials, but also to simultaneously produce desired changes in other properties or in microstructure. The purpose of such changes may be. 5 but is not confined to, improvement of mach inability, facilitation of cold work (known as inprocess annealing), improvement of mechanical or electrical properties, or to increase dimensional stability. When applied solely to relieve。