【正文】 ot 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。 (e) the use of materials about which 3 there was insufficient or uncertain data, particularly to its longterm properties。 (c) production design. During the conceptualdesign stage, the designer is concerned primarily with the function the product is to fulfill, Usually several concepts are visual Jails and considered, and a decision is made either that the idea is not practical or that the idea is sound and one or more of the conceptual designs should be developed further. Here, the only concern for materials is that materials exist that can provide the desired properties, If no such materials are available, consideration is given as to whether them is a reasonable prospect that new ones could be developed within cost and time limitations. At the functional or engineeringdesign stage, a practical, workable deign is developed. Fairly plete drawings are made, and materials are selected and specified for the various ponents. 0ften a prototype or working model is made that can be tested to permit evaluation of the product as to function, re