翻译_钢的热处理.doc

Heat Treatment of SteelsHeat treating refers to the heating and cooling operations performed on a metal for the purpose of altering such characteristics as hardness, strength, or ductility. A tool steel intended to be machined into a punch may first be softened so that it can be machined. After being machined to shape, it must be hardened so that it can sustain the punishment that punches receive. Most heating operations for hardening leave a scale on the surface, or contribute other surface defects. The final operation must, therefore, be grinding to remove surface defects and provide a suitable surface finish.When a steel part is to be either hardened or softened, its temperature must be taken above the critical temperature line; that is, the steel must be austenitized. Usually a temperature of 50 to 100 degree above the critical temperature is selected, to ensure that the steel part reaches a high enough temperature to be completely austenitized, and also because furnace temperature control is always a little uncertain.The steel must be held at furnace temperature for sufficient time to dissolve the carbides in the austenite, after which the steel can be cooled. How much residence time in the furnace is required is to some degree a matter of experience with any particular steel.Usually, for a 3/4 in. bar Clin=O. 0254m), 20 minutes or slightly more will do. Double the time for twice the diameter. Alloy steels may require a longer furnace time; many of these steels are best preheated in a lower-temperature furnace before being charged into the hardening furnace.When the heating time is completed, the steel must be cooled down to room temperature. The cooling method determines whether the steel will be hardened or softened. If the steel is quickly removed from the furnace and quenched into cold water, it will be hardened. If it is left in the furnace to cool slowly with the heat turned off, or cooled in air (small pieces of plain carbon steel cannot be air-softened, however), it will be softened. High-alloy steels may be hardened by aircooling , but plain carbon steels must have a more severe quench, almost always water.There are several softening methods for steels , and the word softening therefore does not indicate what softening process or purpose was used. The method of softening by slow cooling from austenite is called annealing, not softening, Annealing leaves the steel in the softest possible condition (dead soft) .To conclude, the difference between hardening and annealing is not in the heating process but in the cooling process.The definition for the term powder metallurgy, as provided by the committee for Powder Metallurgy of the American Society for Metals, isThe art of producing metal powders and objects shaped from individual, mixed, or alloyed metal powders, with or without the inclusion of nonmetallic constituents, by pressing or molding objects which may be simultaneously or subsequently heated to produce a coherent mass, either without fusion, or with the fusion of a low melting constituent only.Originally Developed as a Step in Refining. References to the granulation of gold and silver and subsequent shaping into solid shapes go back as far as 1574. It is also noteworthy that in the nineteenth century more metallic elements were produced in powder form than in any other form. For the most part, however, these were all precious or rare metals for which powder metallurgy was only practical method of manufacture, and it has only been within more recent year that this process has become competitive with more conventional processes in the manufacture of articles from iron, copper, aluminum, and the other more common metals.Two Unique Advantages. Early developments in powder metallurgy were based on two factors. During the production of platinum, tantalum, osmium, tungsten, and similar refractory metals, reduction was purely a chemical process from which the reduced metal was obtained as a precipitate in flake or powder form. Because furnaces and techniques were not available for complete melting of these materials, the only procedure for producing them in solid form was to press them into coherent masses and sinter at temperatures below the melting point. This procedure still applies in the production of some metals, especially tungsten. A second major advantage of the process, which led to early use and is still applied today, is in the production of porous shapes obtained with lighter pressing pressures or lower sintering temperatures. Materials in this form are useful as chemical catalysts, filtering elements, and bearings.Process Involves a Series of Steps. Figure 12-13 shows the steps ordinarily required in the production of a part by the powder metallurgy process. Suitable powder must first be produced. While theoretically any crystalline material may be fabricated by powder metallurgy, the production of suitable has presented restrictions in many cases, either because of difficulty in obtaining adequate purity or because of economic reasons. After selection and blending of the powder and manufacture of a die for the shape to be produce, the powder is pressed to size and shape. The application of heat results in crystalline growth and the production of a homogeneous body.Properties Influcenced by Heat-Pressure Cycle. Various combinations of heat and pressure may be used. Some sintering takes place under high pressure at room temperature. However, cold pressing is usually followed by sintering at a temperature. Somewhat below the lowest melting point of any of the constituents. An intermediate elevated temperature may be used during pressing, then the shape removed from the press and subjected to higher temperature. In hot pressing, the final sintering temperature is applied simultaneously with the pressure.Application for Powdered Metal ProductsPowder metallurgy occupied tow rather distinct areas. It is a basic shape-producing method for practically all metals, in direct competition with other methods. In addition, for many refractory (high melting point) materials, both metals and nonmetals, powder metallurgy is the only practical means of shape production, tungsten is typical means of shape production. Tungsten is typical of the refractory metals; it has a melting point of 3400 C, and no satisfactory mold or crucible materials exist for using conventional casting techniques at this temperature. Tantalum and moly bdenum are similar. For some other metals, possible to melt, impurities picked up by the liquid from the containers would be undesirable, and powder metallurgy offers the most economical means of obtaining solid shapes.Cemented Carbides an Important Powder Product. Cemented carbides form one of the most important groups of materials that can be fabricated into solid shapes by powder metallurgy only. These materials will be discussed as cutting tools in later chapter, but their method of manufacture may be noted. The principal material used is tungsten carbide, although titanium carbide and tantalum carbide are also used. While it is possible to press and sinter these metal carbides in pure form, the resulting solid material is too brittle for most practical use. The addition of 3% to 20% cobalt or nickel powder yields a product with somewhat reduced hardness but with sufficient shock resistance to be useful for many applications in which high hardness and wear resistance at high temperatures are of importance.The final combination of hardness and ductility may be controlled by the percentage of nickel or cobalt added, with the smaller amounts yielding the hardest but most brittle products.Carbides have a high modulus of elasticity of 350 G (50 million Psi) or more, compared to 200 G (30 million Psi) for steel. The quality of rigidity indicated by this value is important in applications where minimum deflection under load is desired. Cemented carbides have high damping qualities, giving them an additional value m many machining operations where vibration might otherwise be a problem. Compressive strength varies from 3.5 to 6 G (500,000-9000,000 Psi). Hardness values of the different grades run from approximately Rockwell C 65 to 90.Cemented carbide was first used as a cutting-tool material prior to 1930. The earliest tools consisted only of tungsten carbide and cobalt, and their use was restricted to machining nonferrous materials and treatment has since developed a wide range of hardness and toughness properties that are applicable to most machining operations. Many of the variations have been classified into standard grades suitable for various cutting uses.At present there are four general classed of carides available for cutting tool use straight tungsten carbide (WC); crater resistant steelcutting (We+Tic+Tac); straight titanium carbide (Tic); and coated carbides.As noted above, straight tungsten carbide is used principally for nonferrous metals and cast iron, both for roughing and finishing operations.The addition of titanium carbide and/or tantalum carbide to tungsten carbide increase the resistance to cratering and reduces the tendency for welding between the tool and work when machining steel.Straight titanium carbide with cobalt as a binder produces a very hard, wear resistant tool, suitable only for finishing operations on steel because of high brittleness and a tendency to chip under shock loading. Cemented carbide cutting tools are often made as composites by using two different grades of carbide or other materials. A tough, shock-resistant grade is used as the core of main body of the tool. This core is coated by vapor deposition with a very thin layer of titanium carbide, titanium nitride, hafnium nitride or aluminum oxide without cobalt or other binder. The core provides the toughness needed for shock loads and the coating provides a highly wear resistant surface, making the tools suitable for both roughing and finishing operations. Coated carbides are produced only as indexable inserts.Sintered Bearings. A further area in which powder metallurgy produces products not practical by other means is in the manufacture of materials with controlled low density. One of the first massproduced powder metallurgy products was sintered porous bronze bearings. After cold pressing, sintering, and sizing, the bearings are impregnated with oil, which in service is made available for lubrication. Although not true fluid film bearings, they provide long service with low maintenance. Porous materials are also useful as filters.Unusual Alloys Formed by Powder Metallurgy. Composite electrical materials form a group similar to the cemented carbides Tungsten and other refractory metals in combination with silver, nickel, graphite, or copper find wide application as electrical contacts and commutator brushes; powder metallurgy not only provides a means for producing the combination but also provides the finished shape for the parts. Many of the currently used permanent magnet materials are produced by powder metallutgy.钢的热处理热处理指对金属进行加热或冷却操作 , 以改变其硬度、强度或延性等特性。要想把一块工具钢加工成一只冲头 , 可先使其变软 , 以便能进行机械加工。在加工成一定的形状之后必须使其硬化 , 以便能承受冲压时受到的猛烈冲击。淬硬时大多数的加热操作会在金属表面上留下一层氧化皮或产生其他表面缺陷。因此 , 最后一道工序必须采用磨削 , 以消除表面陷和获得适当的表面粗糙度。要使钢件变硬或变软 , 必须使其温度超过临界温度线 , 也就是说 , 必须使钢件奥氏体化。通常 , 所选择的温度要比临界温度高出 50_ 100 C, 以确保钢铁达到足够高的温度 . 而完全奥氏体化 , 同时也由于炉温控制总会有点偏差。钢铁一定要在炉温下保持足够长的时间 , 以便碳化物溶解到奥氏体中 , 然后才可使钢件冷却。钢件在炉中究竟需要停留多长时间 , 在某种程度上是个经验问题 , 随具体钢材而异。一般说来 , 对直径为 3/4ino 的棒料 , 20min 或再稍长一点的时间就够了。若是直径增加一倍 , 时间也要延长一倍。合金钢在炉内的时间可能还要长一些 , 其中有许多最好先在 度较低的炉中预热 , 然后再放人淬硬炉中加热。加热过程结束后 , 就需使钢件冷却到室温。冷却的方法决定钢件是变硬还是变软。如果把钢件迅速从炉中取出 , 并立即放入冷水淬火 , 钢件就会变硬。如果让钢件留在炉中 , 停止加热 , 随炉慢慢冷却或在空气中冷却 , 钢件就会变软 ( 然而小块的普通碳钢不能采用放在气中冷却的方法 ) 。对高合金钢可以采用空气冷却法硬化 , 但是对于普通碳钢必须采用比较 剧烈的淬火方法 , 并且大多采用水淬法。使钢变软的方法有好几种 , 所以 软化 这个词并不能表明软化的方法和目的。从奥氏体状态缓慢地冷却的软化方法称为退火 , 而不称为软化。退火使钢处于尽可能软的状态 ( 极软 ) 。总之 , 洋火与退火之间的区别不在于加热的方法不同 , 而在于冷却方法不同。由美国社会在金属研究方面的委员会定义的粉末冶金学，是 制造制造金属粉末并将单一的混合的或合金化的粉末用成型的方法制成成品的技术，这一制造过程可不添加或添加非金属成分，可通过加压或模压成型，可在压制同时加热或在压制后再进行加热，能使金属粉末形成一个粘结牢固的整体，加热过程中粉末可不熔化或只有低熔点成分熔化。根据提纯技术的同步发展，有关金和银的冶炼以及随后把它们加工成固体形状的历史使从1574年开始的。更值得一提的是，在19世纪起其他形状来说更多金属元素被加工成粉末状。然而很大一部分上说，金属冶金只是一种价值很高的和稀有金属进行人工加工的实际方法。而且就在近些年，这种加工过程比起普通的对铁、铜、铝和其它常规金属的加工过程显得更有竞争力。它包括两个方面独特的优势，早期粉末冶金的发展建立在两种因素的基础上。在制造铂、钨、钽、锇和类似的的金属提炼的期间，还原作用纯粹是一个化学过程，在这个过程种被还原的金属以碎片或粉末状的沉淀物形式被获取。由于熔炉技术还不能完全利用这些被融化的金属，唯一把它们生成固体状的方法是把它们压成互相凝聚的团并且温度控制在它们熔点以下。这种方法还用在很多金属的生产中，特别是钨。第二个主要的优势是，这种方法早早被应用而且到现在仍在使用。只需在轻压和低控温度的情况下就能生产多孔形产品，这种形状的产品作为化学催化剂、过滤器零件和轴承是很有用的。过程包括一系列步骤。在一部分粉末冶金过程中，13-13步是要求的。首先制造合适的粉末。尽管在理论上可以用粉末冶金的方法加工任何晶体材料，其实粉末冶金的过程受到很多条件的限制，要么是由于得到足够纯度很难，要么是经济上得原因。在把粉末挑选提纯制造成生产所需要得西装得模具后，粉末已经被压成各种形状。热处理得应用导致晶体变大和其大小均匀。受热-压循环得影响。加热和加压得多种联合方式被用到。许多控制发生在室温下并加以高压。然而，冷压需要把温度控制在任何要素得最低熔点以下。所以在压制时一般把温度控制在中等温度。物体在压力作用下形状发生改变，但最终得成型取决于高温，在热压方法中，最终的控制温度和压力需要同时达到。粉末冶金产品的运用粉末冶金占据着两个截然不同的领域。与其它基本成型方法直接形成对比。另外，对许多有难度（高熔点）的物质，包括金属