utectoid amount of carbon. When such steel cools, as shown in z-z’ of Fig.
  2.1 the process is similar to the hypo-eutectoid case, except that the primary or pro-eutectoid phase is now cementite instead of ferrite. 过共析钢是含碳量大于共析量的钢。当这种钢冷却时,就像图
  2.1 的 z-z’线所示,除了初步的共析状态用渗碳体 取代铁素体外,其余类似亚共析钢的情况。 As the carbon-rich phase forms, the remaining austenite decreases in carbon content, reaching the eutectoid composition at 727℃ (1341?). As before, any remaining austenite transforms to pearlite upon slow cooling through this temperature. 随着富碳部分的形成,剩余奥氏体含碳量减少,在 727℃(1341?)时达到共析组织。就像以前说的一样,当缓慢冷却到这 温度时所有剩余奥氏体转化为珠光体。 It should be remembered that the transitions that have been described by the phase diagrams are for equilibrium conditions, which can be approximated by slow cooling. With slow heating, these transitions occur in the reverse manner. 应该记住由状态图描述的这种转化只适合于通过缓慢冷却的近似平衡条件。如果缓慢加热,则以相反的方式发生 这种转化。 However, when alloys are cooled rapidly, entirely different results may be obtained, because sufficient time is not provided for the normal phase reactions to occur, in such cases, the phase diagram is no longer a useful tool for engineering analysis. 然而,当快速冷却合金时,可能得到完全不同的结果。因为没有足够的时间让正常的状态反应发生,在这种情况下对工 程分析而言状态图不再是有用的工具。
? Hardening
淬火
Hardening is the process of heating a piece of steel to a temperature within or above its critical range and then cooling it rapidly. 淬火就是把钢件加热到或超过它的临界温度范围,然后使其快速冷却的过程。 If the carbon content of the steel is known, the proper temperature to which the steel should be heated may be obtained by reference to the iron-iron carbide phase diagram. However, if the composition of the steel is unknown, a little preliminary experimentation may be necessary to determine the range. 如果钢的含碳量已知,钢件合适的加热温度可参考铁碳合金状态图得到。然而当钢的成分不知道时,则需做一些预备试 验来确定其温度范围。 A good procedure to follow is to heat-quench a number of small specimens of the steel at various temperatures and observe the result, either by hardness testing or by microscopic examination. When the correct temperature is obtained, there will be a marked change in hardness and other properties. 要遵循的合适步骤是将这种钢的一些小试件加热到不同的温度后淬火,再通过硬度试验或显微镜检查观测结果。一旦获 得正确的温度,硬度和其它性能都将有明显的变化。 In any heat-treating operation the rate of heating is important. Heat flows from the exterior to the interior of steel at a definite rate. If the steel is heated too fast, the outside becomes hotter than the interior and uniform structure cannot be obtained. 在任何热处理作业中, 加热的速率都是重要的。 热量以一定的速率从钢的外部传导到内部。 如果钢被加热得太快, 其外部比内部热就不能得到均匀的组织结构。 If a piece is irregular in shape, a slow rate is all the more essential to eliminate warping and cracking. The heavier the section, the longer must be the heating time to achieve uniform results. 如果工件形状不规则,为了消除翘曲和开裂最根本的是加热速率要缓慢。截面越厚,加热的时间就要越长才能达到均匀 的结果。 Even after the correct temperature has been reached, the piece should be held at that temperature for a sufficient period of time to permit its thickest section to attain a uniform temperature. 即使加热到正确的温度后,工件也应在此温度下保持足够时间以让其最厚截面达到相同温度。 The hardness obtained from a given treatment depends on the quenching rate, the carbon content, and the work size. In alloy steels the kind and amount of alloying element influences only the hardenability (the ability of the workpiece to be hardened to depths) of the steel and does not affect the hardness except in unhardened or partially hardened steels. 通过给定的热处理所得到的硬度取决于淬火速率、含碳量和工件尺寸。除了非淬硬钢或部分淬硬钢外,合金钢中 合金元素的种类及含量仅影响钢的淬透性(工件被硬化到深层的能力)而不影响硬度。 Steel with low carbon content will not respond appreciably to hardening treatment. As the carbon content in steel increases up to around
  0.60%, the possible hardness obtainable also increases. 含碳量低的钢对淬火处理没有明显的反应。随着钢的含碳量增加到大约
  0.60%,可能得到的硬度也增加。 Above this point the hardness can be increased only slightly, because steels above the eutectoid point are made up entirely of pearlite and cementite in the annealed state. Pearlite responds best to heat-treating operations; and steel composed mostly of pearlite can be transformed into a hard steel. 高于此点,由于超过共析点钢完全由珠光体和退火状态的渗碳体组成,硬度增加并不多。珠光体对热处理作业响应最好; 基本由珠光体组成的钢能转化成硬质钢。 As the size of parts to be hardened increases, the surface hardness decreases somewhat even though all other conditions have remained the same. There is a limit to the rate of heat flow through steel. 即使所有其它条件保持不变,随着要淬火的零件尺寸的增加其表面硬度也会有所下降。热量在钢中的传导速率是 有限的。 No matter how cool the quenching medium may be, if the heat inside a large piece cannot escape faster than a certain critical rate,
there is a definite limit to the inside hardness. However, brine or water quenching is capable of rapidly bringing the surface of the quenched part to its own temperature and maintaining it at or close to this temperature. 无论淬火介质怎么冷,如果在大工件中的热量不能比特定的临界速率更快散发,那它内部硬度就会受到明确限制。然而 盐水或水淬火能够将被淬零件的表面迅速冷却至本身温度并将其保持或接近此温度。 Under these circumstances there would always be some finite depth of surface hardening regardless of size. This is not true in oil quenching, when the surface temperature may be high during the critical stages of quenching. 在这种情况下不管零件尺寸如何,其表面总归有一定深度被硬化。但油淬情况就不是如此,因为油淬时在淬火临界阶段 零件表面的温度可能仍然很高。
? Tempering
回火
Steel that has been hardened by rapid quenching is brittle and not suitable for most uses. By tempering or drawing, the hardness and brittleness may be reduced to the desired point for service conditions. 快速淬火硬化的钢是硬而易碎的,不适合大多数场合使用。通过回火,硬度和脆性可以降低到使用条件所需要的 程度。 As these properties are reduced there is also a decrease in tensile strength and an increase in the ductility and toughness of the steel. The operation consists of reheating quench-hardened steel to some temperature below the critical range followed by any rate of cooling. 随着这些性能的降低,拉伸强度也降低而钢的延展性和韧性则会提高。回火作业包括将淬硬钢重新加热到低于临界范围 的某一温度然后以任意速率冷却。 Although this process softens steel, it differs considerably from annealing in that the process lends itself to close control of the physical properties and in most cases does not soften the steel to the extent that annealing would. The final structure obtained from tempering a fully hardened steel is called tempered martensite. 虽然这过程使钢软化,但它与退火是大不相同的,因为回火适合于严格控制物理性能并在大多数情况下不会把钢软化到 退火那种程度。回火完全淬硬钢得到的最终组织结构被称为回火马氏体。 Tempering is possible because of the instability of the martensite, the principal constituent of hardened steel. Low-temperature draws, from 300? to 400? (150℃~205℃), do not cause much decrease in hardness and are used principally to relieve internal strains. 由于马氏体这一淬硬钢主要成分的不稳定性,使得回火成为可能。低温回火, 300?到 400?(150℃~205℃),不 会引起硬度下降很多,主要用于减少内部应变。 As the tempering temperatures are increased, the breakdown of the martensite takes place at a faster rate, and at about 600? (315℃) the change to a structure called tempered martensite is very rapid. The tempering operation may be described as one of precipitation and agglomeration or coalescence of cementite. 随着回火温度的提高,马氏体以较快的速率分解,并在大约 600?(315℃)迅速转变为被称为回火马氏体的结构。回火作 业可以描述为渗碳体析出和凝聚或聚结的过程。 A substantial precipitation of cementite begins at 600?(315℃), which produces a decrease in hardness. Increasing the temperature causes coalescence of the carbides with continued decrease in hardness. 渗碳体的大量析出开始于 600?(315℃),这使硬度下降。温度的上升会使碳化物聚结而硬度继续降低。 In the process of tempering, some consideration should be given to time as well as to temperature. Although most of the softening action occurs in the first few minutes after the temperature is reached, there is some additional reduction in hardness if the temperature is maintained for a prolonged time. 在回火过程中,不但要考虑温度而且要考虑时间。虽然大多数软化作用发生在达到所需温度后的最初几分钟,但 如果此温度维持一段延长时间,仍会有些额外的硬度下降。 Usual practice is to heat the steel to the desired temperature and hold it there only long enough to have it uniformly heated. 通常的做法是将钢加热到所需温度并且仅保温到正好使其均匀受热。
Two special processes using interrupted quenching are a form of tempering. In both, the hardened steel is quenched in a salt bath held at a selected lower temperature before being allowed to cool. These processes, known as austempering and martempering, result in products having certain desirable physical properties. 两种采用中断淬火的特殊工艺也是回火的形式。这两种工艺中,淬硬钢在其被允许冷却前先在一选定的较低温 度盐浴淬火。这两种分别被称为奥氏体回火和马氏体回火的工艺,能使产品具有特定所需的物理性能。
? Annealing
退火
The primary purpose of annealing is to soften hard steel so that it may be machined or cold worked. 退火的主要目的是使坚硬的钢软化以便机加工或冷作。 This is usually accomplished by heating the steel too slightly above the critical temperature, holding it there until the temperature of the piece is uniform throughout, and then cooling at a slowly controlled rate so that the temperature of the surface and that of the center of the piece are approximately the same. 通常是非常缓慢地将钢加热到临界温度以上,并将其在此温度下保持到工件全部均匀受热,然后以受控的速率慢慢地冷 却,这样使得工件表面和内部的温度近似相同。 This process is known as full annealing because it wipes out all trace of previous structure, refines the crystalline structure, and softens the metal. Annealing also relieves internal stresses previously set up in the metal. 这过程被称为完全退火,因为它去除了以前组织结构的所有痕迹、细化晶粒并软化金属。退火也释放了先前在金属中的 内应力。 The temperature to which a given steel should be heated in annealing depends on its composition; for carbon steels it can be obtained readily from the partial iron-iron carbide equilibrium diagram. When the annealing temperature has been reached, the steel should be held there until it is uniform throughout. 给定的钢其退火温度取决于它的成分;对碳钢而言可容易地从局部的铁碳合金平衡图得到。达到退火温度后,钢 应当保持在此温度等到全部均匀受热。 This usually takes about 45min for each inch(25mm) of thickness of the largest section. For maximum softness and ductility the cooling rate should be very slow, such as allowing the parts to cool down with the furnace. The higher the carbon content, the slower this rate must be. 加热时间一般以工件的最大截面厚度计每英寸(25mm )大约需 45min。为了得到最大柔软性和延展性冷却速率应该很慢, 比如让零件与炉子一起冷下来。含碳量越高,冷却的速率必须越慢。 The heating rate should be consistent with the size and uniformity of sections, so that the entire part is brought up to temperature as uniformly as poss
 

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