外文翻译--普通碳钢的淬火

河南科技大学毕业论文 1 翻 译 文 献 原文： HARDDENING OF PLAIN CARBON STEELS Heat treatment is used to soften metal and relieve internal stresses (annealing), harden metal, and temper metal (to toughen certain parts). 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 strength. Hardening increases the strength of pieces after they are temperature. It is accomplishd by heating the steel to some in air, oil, water, or brine. Only medium, high, and very high carbon steels can be hardened by this method. The temperature at which the steel must be heated varies with the steel used. The tendency of a steel to harden may or may not be desirable depending upon how it is going to be processed. For example, if it is to be welded, a strong tendency to harden will make a steel brittle and susceptible to cracking during the welding process. Special precautions such as preheating and a very careful control of heat input and cooling will be necessary to minimize this condition. During welding, an extremely high localized temperature differential exists between the molten metal of the weld and the soild, much colder metal being welded. The resulting structure if these areas is hard, brittle martensite. The greater the hardenability of a steel, the less severe the rate of heat extraction necessary to cause it to harden. This is one of the reasons that alloy and high carbon steels have to be welded with greater care than ordinary low carbon steels. The phase changes in plain carbon steels as the carbon content and the temperature vary. For a pure iron, the phase change from 河南科技大学毕业论文 2 body-centered cubic at lower temperatures to face-centered cubic austenite occurs at 1666F. The bcc state is termed ferrite. This ferrite phase is commonly found in carbon and alloy steels and cast irons. It can be considered to be pure iron, but in plain carbon steels it is actually a solution of 0.008% carbon in iron at room temperature (though iron can dissolve somewhat more carbon than this at higher temperatures).The carbon atoms are small enough (0.77A) to fit between the iron atoms in the body-centered cubic crystal lattice. But even an extra low-carbon stainless steel contains much more carbon than 0.008%, so that the carbon in steels must be in the steel in some form other than this. The irresistible chemical attraction between most metals and carbon has been a recurrent theme in this book: virtually all the carbon in steels denum, vanadium, titanium, and other metals. Here we come to the heart of the matter. We can heat-treat any steel that contains sufficient carbides. Pure iron cannot be heat-treated. Carbon steels with less than about 0.3% carbon have carbides insufficient for any significant heat treating to be possible. Thus the 300-series stainless steels, in which carbon is generally less than 0.15%, cannot be heat-treated. The martensitic stainless steels are enough carbon to produce sufficient carbides of iron and chromium for heat treating. The non-heat-treatable ferritic stainless steels must be discussed later. To heat-treat then, we need carbides plus one other steel characteristic-a phase transformation from austenite at higher the 300-series austenitic stainless steels do not have this second characteristic, but most steels do. Iron carbide, Fe3C, is called cementite. Like other carbides, it is hard, strong, and brittle, the hardest constituent in carbon steels. Cementite has a carbon content of 6.6%. At room temperature, all carbon steels are mixtures of ferrite and cementite. To harden a carbon steel, the steel is first heated to just above its critical temperature into 河南科技大学毕业论文 3 the austenite phase. It is held at this temperature long enough for cementite and other carbides to dissolve in the austenite. The steel in then cooled at a rapid rate. This fast cooling is obtained by quenching the hot steel in water or oil, or, in the case of a weld, the cooling is fast. Because of the rapid cooling, the austenite does not have time to dissociate into the usual ferrite and cementite. What comes down with drastic cooling is a supersaturated solution of carbon trapped in a body-centered tetragonal (i.e., rectangular) crystal structure, this frozen solution being given the name martensite. The transformation from austenite to martensite does not occur at the transformation temperature between ferrite and austenite. Instead, the martensite transformation occurs over a range of temperature. Austenite may begin to transform to martensite at 80 F in a low-alloy steel. As the temperature contiues to fall, more martensite is formed , until at room temperature the structure of the steel may be 99% martensite. With added alloy ingredients the mertensite transformation begins at a lower temperature, and the transformation is also less complete. In a high-speed steel, martensite may not begin to appear until a temperature of 600 F is reached, and perhaps only 80% of the austenite will have transformed when room temperature is reached. The untransformed fraction will still be austenite. All these changes teke place only during a fast quench of an alloy steel or a carbon steel of 0.3% carbon or more. Martensite is hard, brittle, and nonductile, so that the denger of cracking due to thermal stresses is ever present. Worse still, there is a vonlume expansion when martensite appears. The part of the steel that is merely cooling is contracting, while the fraction that is transforming is expanding. This makes the cracking possibilities even greater. In carbon steels, the brittle martensitie condition is obtainable only with a very rapid cooling rate. Additions of any alloying 河南科技大学毕业论文 4 ingredients affect this cooling rate. The greater the proportion of these ingredients in a steel, the slower the cooling rate that will still give a martensite condition. This statement holds true whether the alloying metals are carbideformers. Like tungsten and molybdenum or those that dissolve in ferrite, such as nickel and manganese. 翻译 ： 普通碳钢的淬火 热处理是使金属变软，消除内应力（退火）和使金属淬硬及回火（使某些部分变韧）。 淬火是把钢加热然后冷却以提高其硬度得，抗拉强度， 降低其韧性并获得细晶粒组织的方法。 在工件制好以后进行淬火能提高其硬度淬火是把钢加热到再结临界温度以上的某个温度然后使之在空气中，油中，水中或盐水中迅速冷却来完成的。只有中碳钢，高碳钢以及很高的碳钢才能用这种方法来硬化。钢加热到的温度因钢种的不同而不同。 钢淬硬这一特定，是不是合乎需要，取决于何种加工。例如：如果进行焊接加工过程中，强烈的淬硬趋势是使钢容易变脆或者开裂。必须采用专门的措施把这种情况降低到最低限度，例如进行预热，非常小心的控制输入热量以及冷却。在焊接过程中当焊逢熔化了的金属和被 焊接的，固态的，温度低的多的金属存在着极大的局部温差。凉的母体金属对焊逢金属及其附近已经加热到的临界温度上限以上的金属起着一种淬火剂的作用。这些区域最后产生的结构是脆硬的马氏体。钢的可淬性越高，使钢淬火所必须的吸热率的剧烈程度就越低。这是高碳钢和合金钢焊接时必须比低碳钢更加小心的原因之一。 普通碳钢的相，随含碳量和温度的变化而变化。对于纯铁，其相变从较低温度的体心立方结构到 1666 F 出现面心立方奥氏体。体心立方状态时叫做铁素体。通常在碳钢，合金钢和铸铁种都有这种铁素体相。可以把铁素体河南科技大学毕业论文 5 相看成是纯铁，但是在普 通碳钢种铁素体相实际上是在室温下溶有 0.008%的碳的铁（虽然在温度更高的情况下铁所能溶解的碳还要多些）。碳原子是很小的（ 0.77），足以嵌在体心立方晶格种的铁原子之间。 然而，就是超低碳不锈钢的含碳量也大大超过 0.008%，因此，钢种的碳必然以不同于溶入铁素体的某种其他形式存在于钢种。关于大多数金属和碳之间不可抗拒的亲和力已经是本书种多次重复谈到的问题。实际上钢中所有的碳都与铁 .钨 .铬 .钼 .钒 .钛和其他金属化合成这些元素的碳化物。这里谈到了问题的核心。我们能对含有足量碳化物的任何一种钢进行热处理 。纯铁是不能热处理的。含碳量低于 0.3%左右的碳钢所含有的碳化物都不足以进行任何有效的热处理。因此含碳量通常低于 0.15%的 300 系列的不锈钢是不能热处理的。马氏体不锈钢含有足量的碳以形成足以进行热处理的碳化铁和碳化铬。不能热处理的铁素体不锈钢将在以后讨论。 因此为了进行热处理，我们需要碳化物再加上钢的另一种特性即从较高温度下的奥氏体转变到较低温度下的其他相的相变。而 300