《冶金专业英语Unit 18.ppt》由会员分享，可在线阅读，更多相关《冶金专业英语Unit 18.ppt（13页珍藏版）》请在taowenge.com淘文阁网|工程机械CAD图纸|机械工程制图|CAD装配图下载|SolidWorks_CaTia_CAD_UG_PROE_设计图分享下载上搜索。
1、Textn Heat treatment is a term applied to a variety of procedures for changing the characteristics of metal by heating and cooling.By proper heat treatment,it is possible to obtain certain characteristics in metal such as hardness,tensile strength(ability to resist stretching),and ductility.The proc
2、edures of heat treatment of steel include hardening,tempering,annealing,and case hardening.Text1.Hardening Hardening is a process of heating and cooling steel to increase its hardness and tensile strength,to reduce its ductility,and to obtain a fine grain structure.The simple experiments described b
3、elow may be enlightening.Two old-fashioned steel knitting needles,a gas ring,a bowl of water,a piece of sandpaper and a pair of pliers are needed.Experiment 1:Bend one needle slightly to feel how tough and springy it is.Now hold the needle in the flame,using the pliers,the bowl of water being close
4、at hand.When it is bright red,dip the end of the needle as quickly as possible into the water.The needle should still be red hot as it is being quenched.Then try to bend the quenched end;it is hard and brittle and will snap off.Text2.Tempering Tempering is done by reheating the metal to low or moder
5、ate temperature,followed by quenching or by cooling in air.It is a process that follows the hardening procedure and makes the metal as hard and tough as possible and removes the brittleness from a hardened piece.Two important points are to be noted in connection with the quenching and tempering of s
6、teel:(1)To get a fully hardened structure,the steel should be heated to the appropriate temperature.And then rapidly cooled.The appropriate temperature before quenching depends on the carbon content of the steel concerned.(2)Tempering does not restore the pearlitic structure.Before that can be done,
7、the steel has to be heated to the appropriate temperature and then slowly cooled.Text3.Annealing Annealing is the process of softening steel to relieve internal strain.This makes the steel easier to machine.The metal is heated above the critical temperature and cooled slowly.The most common method i
8、s to place the steel in the furnace and heat it thoroughly.Then turn off the furnace,allowing the metal to cool slowly.Another method is to pack the metal in clay,heat it to the critical temperature,remove it from the furnace,and allow it to cool slowly.Experiment 2:Take the second needle and heat i
9、t until it is red-hot;maintain it at this temperature for about a quarter of a minute.Then withdraw it very slowly,so that it cools gradually.If you now test this end(which you have just annealed)it will bend,like a piece of soft wire,and furthermore it will remain bent.Text4.Case hardening Case har
10、dening is a process of hardening the outer surface or case of ferrous metal.There are many methods of case hardening now.In the simplest method of case-hardening the steel articles are packed in boxes containing charcoal and other substances,they are then heated gradually to about 925C,and maintaine
11、d at that temperature for several hours.The steel is quenched in oil,which is much preferable to water,since too rapid quenching may be lead to craking or peeling of the case.In industry,cyaniding is also used for case hardening,because a thinner carbon case can be applied by immersing the steel in
12、a bath of molten sodium cyanide.Text Another method of hardening the surface of steels is known as“nitriding”.Special alloy steels containing aluminium and other elements are heated in ammonia at about 500C;the gas decomposes to a certain extent into hydrogen and nitrogen;the latter combines with th
13、e iron,forming particles of iron nitride which impart great hardness to the surface of the steel.In addition,other hardening processes have been developed,under the general term“nitrocarburizing”;with these methods carbon and nitrogen are diffused into the surface of ferrous components at relatively
14、 low temperatures.These methods promote a tough compound layer of iron carbides and nitrides.Text This process can be done on such items as crankshafts,gears,hammer heads,piston pins,and other items that must stand a good deal of shock and wear.It can never be used on anything that must be sharpened
15、 by grinding.One of the very latest surface treatment techniques is laser hardening which has the big advantage that selected areas can be hardening without treating the whole part.As with many other aspects of metallurgy,the current trends of heat-treatment are towards higher throughouts,lower ener
16、gy use and reduced environmental pollution.Translation 热处理这一术语指的是各种各样通过加热和冷却以改变金属特性的过程。通过适当的热处理，能够使金属获得某些性质，例如：硬度、抗拉强度（抗拉伸能力）和韧性。钢的热处理方法包括淬火、回火、退火和表面硬化。Translation淬火淬火 淬火是把钢加热然后冷却以提高其硬度、抗强度，降低韧性并获得细晶粒组织的方法。下面的简单实验可能对人有所启发。取两根老式的钢制毛衣针，一个煤气炉，一碗水、一块砂纸和一把钳子。实验1：轻轻弯曲一根毛衣针，试一试它的韧性和弹性，然后用钳子夹住一端放在火上烧，手旁放一碗水
17、。当织针达到红热状态时，将其一端迅速浸入水中（织针在淬火时应当还是红热的），然后，弯曲淬过火的一端，即可发现它硬而脆，而且会折断。Translation回火 回火是把金属再加热到较低或适中的温度，然后急冷或在空气中冷却。回火是在淬火之后采用的使金属尽可能变硬变韧的方法，它能消除淬火工件的脆性。关于钢的淬火和回火有两个要点需要注意：（1）为了达到完全的硬化组织，必须把钢加热到合适的温度以上，然后进行急冷。淬火之前的合适温度取决于钢的碳含量。（2）回火不能恢复珠光体组织，要恢复珠光体，必须把钢加热到合适的温度，然后缓冷。Translation3、退火、退火 退火是把钢软化使之消除内应力的方法。这可
18、以使钢更易于机加工。把钢加热到临界温度之上然后慢慢冷却。最常用的方法是把钢放到炉子里热透，然后把炉子熄灭，让金属慢慢冷却。另一种方法，是把金属用粘土包起来，把它加热到临界温度，然后从炉中取出，让它慢慢冷却。实验2：取第二根织针烧至红热状态，维持这一温度约15秒钟，然后从火中慢慢取出，让它逐渐冷却。如果你试验这刚刚退过火的一端，它象一根软金属丝一样很容易弯曲，而且会保持弯曲状态。Translation4、表面硬化、表面硬化 表面硬化是使钢的外表面或壳体硬化的方法。目前，有多种表面硬化的方法。表面硬化的最简单方法是把钢件装入带有木炭和其它物质的渗碳箱中，逐渐加热到摄氏925度左右，并在这一温度下保
19、持几个小时。然后，把钢放到油中淬火，用油比用水好，因为过急淬火可能引起裂纹或外皮剥落。在工业上还可以采用氰化法进行表面硬化。因为把钢浸入熔融的氰化钠槽中会产生较薄的渗碳层。钢表面硬化的另一种方法就是“渗氮”。含铝合其它元素的特殊合金钢在氨气氛中加热到500左右，相当一部分氨分解成氢和氮，氮和铁结合生成氮化铁颗粒，使钢的表面具有很强的硬度。Translation 此外还开发了一些其它硬化方法，比如：“碳氮共渗；用这种方法可以在较低的温度下，把氮和碳扩散到钢部件的表层中去，促使碳化铁和氮化铁组成韧性化合物层。曲轴、齿轮、锤头、活塞销和其它必须承受大量振动和磨损的零件可以用该法进行处理。但该法决不能用于任何必须磨利的工件。最先进的表面技术之一是激光硬化，它的最大优点是能进行选择区域硬化，而不必硬化整个部件。和许多其它冶金部门一样，热处理工艺的发展趋势是提高产量、降低能源消耗，减少环境污染。