外文翻译--金属热处理.doc

沈阳理工大学学士学位论文英文原文HEAT TREATMENT OF METALAnnealing The word anneal has been used before to describe heat-treating processes for softening and regaining ductility in connection with cold working of material. It has a similar meaning when used in connection with the heat treating of allotropic materials. The purpose of full annealing is o decrease hardness, increase ductility, and sometimes improve machinability of high carbon steels that might otherwise be difficult to cut. The treatment is also used to relieve stresses,refine grain size, and promote uniformity of structure throughout the material. Machinability is not always improved by annealing. The word machinability is used to describe several interrelated factors, including the ability of a material to be cut with a good surface finish. Plain low carbon steels, when fully annealed, are soft and relatively weak , offering little resistance to cutting, but udually having sufficient ductility and toughness that acut chip tends to pull and tear the surface from which it is removed, leaving a comparatively poor quality surface, which results in a poor machinability rating.1 For such steels annealing may not be the most suitable treatment. The machinability of many of the higher plain carbon and most of the alloy steels can usually be greatly improyed by annealing, as they are often too hard and strong to be easily cut at any but their softest condition.2The procedure for annealing hypoeutectoid steel is to heat slowly to approximately 60 above the Ac3 line,3 to soak for a long enough period that the temperature equalizes throughout the material and homogeneous austenite is formed, and then to allow the steel to cool very slowly by cooling it in the fumace or burying it in lime ot some other insulating material. The slow cooling is easential to the precipitation of the maximum ferrite and the coarsest pearlite to place the steel in its softest, most ductile, and least strained condition.Normalizing The purpose of normalizing is somewhat similar to that of annealing with the exceptions that the steel is not reduced to its softest condition and the pearlite is left rather fine instead of coarse. Refinement of grain size, relief of internal stresses, and improvement of structural uniformity together with recovery of some ductility provide high toughness qualities in normalized steel. The process is frequently used for improvement of machinability and for stress relief to reduce distortion that might occur with partial machining or aging. The procedure for normalizing is to austenitize by slowly heating to approximately 80 above the Ao3 or Accm3 temperature for hypoeutectoid or hyereutectoid sreels, respectively.Providing soaking time for the formation of austenite; and cooling slowly in still air, Note that the steels with more carbon than the eutectoid composition are heated abou the Accm instead of the Ac13 used for annealing. The purpose of normalizing is to attempt to dissolve all the cementite during austenitization to eliminate, as far as possible, the settling of hard, brittle iron carbide in the grain boundaries. The desired decomposition products are smallgrained, fine pearlite with a minimum of free ferrite and free cementite1SpheroidizingMinimum hardness and maximum ductility of steel can be produced by a process called spheroidizing, which causes the iron carbide to form in small spheres or nodules in a ferrite matrix. In order to start with small grains that spheroidize more readily, the process is usually performed on normalized steel. Several variations ofprocessing are used, but all require the holding of the steel near the A1 temperature usually slightly below for a number of hours to allow, the iron carbide to form on its more stable and lower energy state of small, rounded globules.The main need for the process is to improve the machinability quality of high carbon steel and to pretreat hardened steel to help produce greater structural uniformity after quenching. Because of the lengthy treatment time and therefore rather high cost, spheroidizing is not performed nearly as much as annealing or normalizing.Hardening of Steel Most of the heat treatment hardening processes for steel ate based on the production of high percebtages of martensite.The first step,therefore, is that used for most of the other heat-treating processestreatmentto produce austenite. Hypoeutectoid steels ate heated to approximately 60above the Ac3 temperature and allowed to soak to obtain temperature uniformity and austenite homogeneity. Hypereutectoid steels ate soaked at about 60above the Ac1 temperature,which leaves some iron carbide present in the material.The second step involves cooling rapidly in an attempt to avoid pearlite transformation by missing the nose of the ITcurve.The cooling rate is determined by the temperature and ability of the quenching media to carry heat away from the surface of the material being quenched and by the conduction of heat through the material itself.Table 111 shows some of the commonly used media and the method of application to remove heat, arranged in order of decreasing cooling ability.High temperature gradients contribute to high stresser that cause distortion and cracking, so the quench should only as extreme as is necessary to produce the desired structure. Care must be exercised in quenching that heat is removed uniformly to minimize thermal stresses. For example, a long slender bar should be end-quenched, that is, inserted into the qudenching medium vertically so that the entire section is subjected to temperature change at one time. If a shape of this kind were to be quenched in a way that caused one side to drop in tempeiature before the other, change of dimensions would likely cause high stresses producing plastic flow and permanent distortion.Seyeral special types of quench are conducted to minimize quenching stresses and decrease the tendency for distortion and cracking. One of these is called martemoering and consists of quenching an austenitized steel in a salt at a temperature above that needed for the start of martensite formation (Ms).The steel being quenched is held in this bath until it is of uniform temperature but is removed before there is time for formation of bainite to start.Completion of the cooling in air then causes the same hard martensite that would have formed with quenching from the high temperature,but the high thermal or “quench” stresses that are the primary source of cracks and warping will have been eliminated.A similar process performed at a slightly higher temperature is called austempering.In this case the steel is held at the bath temperature for a longer period,and the result of the formation of bainite.The bainite structure is not as hard as the martensite that could be formed from the same composition,but in addition to reducing the thermal shock to which the steel would be subjected under normal hardening procedures,it is unnecessary to perform any further treatment to develop good impact resistance in the high hardness range.4TemperingA third step usually required to condition a hardened steel for swevice is tempering,or as it is sometimes referred to,drawing. With the exception of austempered steel,which is frequently used in the ashardened condition,most steel are not serviceable “as quenched”.The drastic cooling to produce martensite causes the steel to be very hard and to contain both macroscopic internal stresses with the result that the material this little ductility and extreme brittleness. Reduction pg these faults is accomplished by reheating the steel to sometimes referred to, drawing. With the exception of austempered steel, which is frequently used in the as-hardened cognition, most steels are not serviceable “as quenched”, The drastic cooling to produce martensite causes the steel to be very hard and to contain both macroscopic and microscopic internal stresses with the result that the material has little ductility and extreme brittleness. Reduction of these faults is accomplished by reheating the steel to some point below the A1 (lower transformation) temperature.The structural changes caused by tempering of hardened steel are functions of both time and temperature, with temperature being the most important. It should be emphasized that tempering is not a hardening process, but is ,instead, the reverse. A tempered steel is one that has been hardened by heat treatment and then stress relieved, softened, and provided with increased ductility by reheating in the tempering or drawing procedure. The magnitude of the structural changes and the change of properties caused bytempering depend upon the temperature to which the steel is reheated. The higher the temperature, the greater the effect, so the choice of temperature will generally depend on willingness to sacrifice hardenss and strength to gain ductility and toughness. Reheating to below 100has little noticeable effect on hardened plain carbon steel. Between 100and 200,there is evidence of some structural changes. Above 200marked changes in structure and properties appear . Prolonged heating at just under the A1 temperature will result in a spheroidized structure similar to that produced by the spheroidizing process.In commercial tempering the temperature range of 250425C is usually avoided because of an unexplained embrittlement,or loss of ductility, that often occurs with steels tempered in this range of 425600C,particularly when cooled slowly from or through this range of temperature.when high temperature remperature tempering is necessary for these steels,they are usually headed to above600 C and quenched for rapid cooling. Quenches from this temperature, of course ,do not cause hardening because austenitization has not been accomplished.附录B 汉语翻译金属热处理一 退火在前面描述冷拔加工材料的软化并重新获得塑性的热处理方法时，就已使用退火这个词。当用于同素异晶材料的热处理时，该词具有相似的意义。完全退火的目的是降低塑性，有时也提高高碳钢的切削加工性能，否则这种钢很难加工。这种热处理方法也用于减少应力，细化晶粒，提高整个材料的结构均匀性。退火不总是能提高切削加工性，切削加工性一词用来描述几个相关因素，包括材料切削时获得好的表面光洁度（即较小的表面粗糙度值）的能力。当完全退火时普通低碳钢硬度较低，强度较小，对切削的阻力较小，但通常由于塑性和韧性太大以至切削力开工件表面时会划伤表面，工件表面质量比较差，导致较差的切削加工性。对这类钢，退火可能不是最适合的处理方法。许多高碳钢和大多数合金钢加工性通常可经退火大大改善，因为除在最软条件下，它们的硬度和强度太高而不易加工。亚共析钢的退火方法是将钢缓慢加热到Ac3线以上大约60C，保温一段时间，使整个材料温度相同，形成均匀奥氏体，然后岁炉或埋在石灰或其他绝缘材料中缓慢冷却。要析出粗大铁素体和珠光体，使钢处于最软最韧和应变最小的状态，必须缓慢冷却。二 正火正火的目的多少类似于退火，但钢不是最软状态且珠光体是细匀而不粗大。钢的正火能细化晶粒，释放内应力，改善结构均匀性同时恢复一些塑性，得到高的韧性。这种方法经常用于改进切削加工性，减少应力，减少因部分切削加工或时效产生的变形。正火的方法是将亚析钢或过共析钢分别缓慢加热到Ac3线或Accm线上约80C，保温一段时间以便形成奥氏体，并静止空气中缓冷。要注意，含碳量超过共析成分的钢要加热到Accm线以上，而不是退火时的Ac1线以上。正火的目的是在奥氏体化过程中试图溶解所有渗碳体，从而经可能减少晶界上的脆硬铁碳化合物，从而得到小晶粒的细珠光体最小自由铁素体和自由渗碳体。三 球化退火通过球化退火可使钢得到最小的硬度和最大的塑性，它可使铁碳化合物以小球状分布在铁素体基体上，为了使小颗粒球化更容易，通常对正火钢进行球化退火。球化退火可用几种不同的方法，但所有的方法都需要在A1 线温度附近（通常略低）保温很长时间，使铁碳化合物形成更稳定能级较低的小圆球。球化退火的方法的主要目的是改进高碳钢的切削加工性，并对脆硬钢进行热处理，使其淬火后结构更均匀。因为热处理时间长，因