外文翻譯--材料選擇與熱處理-wenkub
2022-11-14 02:07:48 本頁面
【正文】 hat they are specified fully, that they are patible with, and can be processed economically by, existing equipment, and that they are readily available in the needed quantities. As manufacturing progresses, it is inevitable that situations will arise that may require modifications of the materials being used. Experience may reveal that substitution of cheaper materials can be made. In most cases, however, changes are much more costly to make after ma nufacturing is in progress than before it starts. Good selection during the productiondesign phase will eliminate the necessity for most of this type of change. The more mon type of change the occurs after manufacturing starts is the result of the availability of new materials. These, of course, present possibilities for cost reduction and improved performance. However, new materials must be evaluated very carefully to make sure that all their characteristics are well established. One should always remember that it is indeed rare that as much is known about the properties and reliability of a new material as about those of an existing one. A large proportion of product failure and productliability cases have resulted from new materials being substituted before their longterm properties were really known. Product liability actions have made it imperative that designers and panies employ the very best procedures in selecting materials. The five most mon faults in material selection have been: (a) failure to know and use the latest and best information available about the materials utilized。 and (e) material selection made by people who are pletely unqualified to do so. An examination of the faults above will lead one to conclude that thee is no good reason why they should exist. Consideration of them provides guidance as to how they can be eliminated. While following the very best methods in material selection may not eliminate all product liability claims, the use of proper procedures by designers and industries can greatly reduce their numbers. From the previous discussion, it is apparent that those who select materials should have a broad, basic understanding of the nature and properties of materials and their processing. The generally accepted definition for heat treating metals and metal alloys is heating and cooling a solid mental or alloy in a way so as to obtain specific conditions and/or properties. Heating for the sole purpose of hot working (as in fing operations) is excluded from this definition. Likewise, the types of heat treatment that are sometimes used for products such as glass or plastics are also excluded from coverage by this definition. Transformation Curves The basis is the timetemperaturetransformation curves or TTT curves where, in a single diagram all the three parameters are plotted. Because of the 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 tim
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