机械毕业设计英文外文翻译50材料的热处理

外文资料

HEAT TREATMENT OF METALS

The understanding of heat treatment is embrace by the broader study of metallurgy .Metallurgy is the physics, chemistry , and engineering related to metals from ore extraction to the final product . Heat treatment is the operation do heating and cooling a metal in its solid state to change its physical properties. According to the procedure used, steel can be hardened to resist cutting action and abrasion , or it can be softened to permit machining .With the proper heat treatment internal ductile interior . The analysis of the steel must be known because small percentages of certain elements,notably carbon , greatly affect the physical properties .

Alloy steels owe their properties to the presence of one or more elements other than carbon, namely nickel, chromium , manganese , molybdenum , tungsten ,silicon , vanadium , and copper . Because of their improved physical properties they are used commercially in many ways not possible with carbon steels.

The following discussion applies principally to the heat treatment of ordinary commercial steel known as plain-carbon steels .With this proves the rate of cooling is the controlling factor, produces the opposite effect .

A SIMPLIFIED IRON-CARBON DAGRAM

If we focus only on the materials normally known as steels, a simplified diagram is often used . Those portions of the iron-carbon diagram near the delta region and those above 2% carbon content are of little importance to the engineer and are deleted. A simplified diagram, such as the one in Fig . 2.1 focuses on the eutectoid region and is quite useful in understanding the properties and processing of steel.

The key transition described in this diagram is the decomposition of single-phase austenite (γ)to the two-phase ferrite plus carbide structure as temperature drop . Control of this reaction ,which arises due to the drastically different carbon solubilities of austenite and ferrite , enables a wide range of properties to be achieved through heat treatment .

To begin to understand these processes , consider s steel of the eutectoid composition , 0.77% carbon , being slow cooled along line X X '- in Fig .2.1 At the upper temperatures , only austenite is present , the 0.77% carbon being dissolved in solid solution with the iron . When the steel cools to 727(1341)C F ??, several changes occur simultaneously . The iron wants to change from the bcc austenite structure to the bcc ferrite Structure , but the ferrite san only contain 0.02% carbon in solid solution . The rejected carbon forms the carbon-rich cementite intermetallic with composition 3Fe C .In essence , the net reaction at the eutectoid is:

Austenite →ferrite +cementite

Since this chemical separation of the carbon component occurs entirely in the solid state, the resulting structure is a fine mechanical mixture of ferrite and cementite . Speciments prepared by plolishing and etching in a weak solution lf nitric acid and alcohol reveal the lamellar structure lf alternating plates that forms on slow cooling . This structure is composed of two distinct phases, but has its own set of characteristic properties and goes by the name pearlite , because of its resemblance to mother-of-pearl at low magnification.

Steels having less than the eutectoid amount of carbon(less than 0.77%)are known as hypoeutectoid steels . Consider now the transformation of such a material represented by cooling along line y-y ′ in Fig .2.1.At high temperatures , the material is entrirely austenite, but upon cooling

enters a region where the stable phases are ferrite and austenite . Tie-line and lever-law calculations show that low-carbon ferrite nucleates and grows, leaving the remaining austenite richer in carbon . At 727° C (1341°F),the austenite is of eutectoid compositon(0.77%carbon)and further cooling transforms the remaining austenite to pearlite. The resulting structure is a mixture lf primary or proeutectoid ferrite (ferrite that formed above the eutectoid reaction )and regions of pearlite.

Hypereutectoid steels are steels that contain greater than the eutectoid amount of carbon. When such a steel cools, as in z-z′of Fig .2.1 the process is similar to the hypoeutectoid case, except that the primary or proeutectoid phase is now cementite instead lf ferrite . As the carbon-rich phase forms, the remaining austenite decreases in carbon content, reaching the eutectoid composition at 727°C(1341°F).As before, any remaining austenite transforms to pearlite upon slow cooling through this temperature.

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 revertse manner . 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.

HARDENING

Hardening is the process of heating p piece of steel to a temperature within or above its critical range and then cooling it rapidly . 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 t steel is unknown, a little preliminary experimentation may be necessary to

determine the range. A good procedure to follow is to heat-quench a number lf small specimens lf the steel at various temperatures lf the steel at various temperatures and observe the results, either by hardness testing or by microscopic examination. When then correct temperature is obtained ,there will be marked change in hardness and other properties.

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. 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. Even after the correct remperature 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.

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 lf alloying element influences only the harden ability (the ability lf the workpiece to be hardened to depths ) lf the steel and does not affect the hardness except in unhardened or partially hardened steels .

Steel with low carbon content will not respond appreciably to hardening treatments. As the carbon content in steel increases up to around 0.60%,the possible 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; any steel composed mostly of pearlite can be transformed into a hard steel .

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. No matter how

cool the same . There is a limit to the rate lf heat flow through steel. No matter how cool the quenching medium many 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 lf rapidly bringing the surface lf the quenched part to it own temperature and maintaining it at or close to this temperature. Under these circumstances there would always be some finite depth of surface hardening regardless lf size. This is not true in oil quenching , when the surface temperature may be high during the critical stages of quenching.

TEMPERING

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 . 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 lf reheating quench-hardened steel to some temperature below the critical range followed by any rate lf cooling . Although this process softens steel , it differs considerably from annealing in that the process lends itself to close control lf 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 .

Tempering is possible because of the instability of the martensite ,the principal constituent of hardened steel. Low-temperature draws, from 300°to 400°F(150°-205°C), do not cause much decrease in hardness and are used principally to relieve internal strains. As the tempering temperatures are increased, the breakdown of the martensite takes place at a faster rate, and at about 600°F(315°C) the change to a structure called tempered martensite is very rapid.

The tempering operation may be described as one lf precipitation and agglomeration or coalescence of cementite. A substantial precipitation lf cementite begins at 600°F(315°C),which produces a decrease in hardness. Increasing the temperature causes coalescence lf the carbides with continued decrease in hardness.

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. Usual practice is to heat the steel to the desired temperature and hold it there only long enough to have it uniformly heated.

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.

ANNEALING

The primary purpose of annealing is to soften hard steel so that it may be machined or cold worked . This is usually accomplished by heating the steel to 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. 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.

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 the partial iron-iron carbide equilibrium diagram. 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 possible. When the annealing temperature has been reached, the steel should be held there until is uniform throughout. This usually takes about 45 min for each inch (25mm) lf thickness lf 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.

NORMALIZING AND SPHEROIDIZING

The process of normalizing consists of heating the steel about 50°to 100°F(10°-40°)above the upper critical range and cooling in still air to room temperature . this process is principally used with low-and medium-carbon steels as well as alloy steels to make the grain structure more uniform, to relieve internal stresses, or to achieve desired results in physical properties . Most commercial steels are normalized after being rolled or cast.

Spheroidizing is the process of producing a structure in which the cementite is in a spheroidal distribution. If a steel is heated slowly to a temperature just below the critical range and held there for a prolonged machinability to the steel. This treatment is particularly useful for hypereutectoid steels that must be machined.

中文翻译

材料的热处理

了解材料热处理是学习冶金技术的关键，冶金技术是金属通过物理学、化学、工程学，从矿石中提取，最终成为产品的过程，热处理是使固态金属加热的情况下改变它的物理特性的一种加热操作（根据程度不同使用）钢的坚硬能抵抗切割和擦伤，钢的韧性允许它加工，适当的热处理能消除内应力，颗粒减小、韧性增加，硬的表面导致内部的可塑性，分析钢时可以发现它有小百分比元素，特别是碳，它一般会影响它的物理性能；由于物理性能的提高，它们被用在了许多不可能碳钢的商业上。

接下来讨论的是，碳钢的热处理在普通商业上的应用，冷却速度的比率是它的控制因素，快速冷却的结果，使结构硬化，而很慢的冷却使工件产生相反的结

果。

铁碳合金图

如果我们研究的是普通材料的钢，对于工程人员来讲，铁碳合金图中的近铁素体区和含碳量大于2% 的部分不重要，所以这两部分被除数去掉。如图2所示，在懂的属性和钢的热处理方面，在共析混合物的地区结晶，是完全有用的，主要过渡在这张图表显示，叙述了单项的奥氏体的分解，随着温度下降，转变为双向铁素体和化合物，这是反应的控制，使大量的属性能够通过热处理来完成，其理由是奥氏体和铁素体的温度不同，碳的析出时间也不同。

开始分析这个过程，考虑到钢的共析成份为0.77%的碳，如图所示，沿x-x 线逐渐冷却，在温度线的上方，仅有一些奥氏体存在，0.77%的碳开始分解为含

Fe C，在共析处的反应式是：碳固熔体状态，拒绝碳形成是渗碳体的本质成份是

3

奥氏体=铁素体+渗碳体

因为碳化学成份在固态时分离，由此形成一种铁素体和渗碳体的机械混合物，准备好的工件在和缓慢冷却时，其内部结构为层状的结构形式，这种特殊的结构有两部分组成，而它本身也具有一系列的特性，这种结构称为珠光体。

含碳量比共析钢少的（少于0.77%）是亚共析钢，这种材料通过冷却时在图2.1 表现形式为Y-Y线。这种材料在高温时完全是奥氏体，但在冷却线上却是进入稳定期，这是铁素体和奥氏体的区域低碳的铁素体成核，并结晶，剩余的奥氏体较多，在727C

?的时候奥氏体是共析合成并进一步冷却转换成为珠光体，剩余铁素体的结构是珠光体的再结晶或先共析铁素体区域的一个混合物。

比共析钢含碳量高的是过共析钢，这种钢冷却的时候，在图z-z中所示，其热处理与亚共析钢类似，除此之外是渗碳体而不是铁素体，剩余的奥氏体的含碳量在减少，在727C

?成为共析结构，在这个温度下缓慢冷却，保留下来的奥氏体转变成珠光体。

它应该已经是图中描述的转换为近似缓慢冷却的平衡状态，低温加热时，会发生相反的变化，然而合金加热时，可能得到完全不同的结果，因为得时间为常态相，没有被提供反作用力，在这种情况下，从工程上分析相图确实是一个有用的工具。

淬火

淬火是将一个工件加热到某一个温度或临界范围以上，然后快速冷却的过程，如果知道钢的含碳量的话，到达某一温度可能通过铁碳合金图来获得数据，然而如果要是不知道钢的结构的话，做一个初步试验来决定温度范围也是非常必要的，在各种温度下，用许多细小的的材料来试温，并且通过显微镜观察结果，或硬度测试，钢在硬度和其他特性上会有标志性的改变。

在一些热处理中，速度也是非常重要的，热扩散从钢的外部到内部具有一定的速度，如果钢加热过快，外部变热会比内部快，就不能得到预期的结构，如果工件是不规则形状则估算变弯和砸碎缓慢时的速度更为重要，断面较为严重，必须要达到一样的结果，加热时间相应较长即温度到达要求定值后工件不应再加热，一段时间后使它厚截面达到一样的温度。

通过热处理得到硬度，取决于淬火的速度及碳的含量和工件的尺寸，在合金中，只有合金元素的类型和数量决定。

低碳钢不能进行热处理，当碳的含量增加到0.6%时硬度也可能增加，但只会硬度少量的增加，因为共析钢在退火是被莱氏体和珠光体完全保围，莱氏体易被加热，所以莱氏体组成的任何一种钢材可以热处理变为硬钢。

当增大零件尺寸时表面硬度的增加或减低的情况仍然一样，无论是哪种介质的回火，如果在工件尺寸较大的部分散不其它部分热较慢的话，就应该有一个明确的回火极限。然而盐水和水淬火能够快速带走一部分表面温度，而且可以维持到这个温度结束。在这些环境中，不会管表面硬度的深度大小，当淬火时表面温度可能在危险的范围内，此时用油淬火是错误的。

回火

钢被快速淬火，是易碎的，不被广泛使用，回火使硬度和脆性状态可以被达到所需要的点，这些性质的减低（也有抗拉强度和延展性的减小）和被冷却后的任何继临界范围韧性增加。虽然这个过程使钢变软，但却不同于退火过程，在大多数情形下借助本身物理特性的控制不使钢变软。在那个范围，最后的结构从回火获得完全增加硬度的钢叫回火马氏体，因为马氏体可能是不稳定性的，增加硬度的钢的主要组织成份，低温从300-400F

?）主要不引起硬度反

?（150-205F

面的减少和减轻内部应力，回火是增加温度，此时马氏体的结构变化非常快，回火操作可描述为珠氏体的析出和聚结，珠氏体的析出从600F

?）使硬

?（315F

度降低，温度的增加会继续使碳化合物的结合减少。

在回火工程中，有些过程应该考虑到时间和温度，虽然温度达到了，但大部分软化处理是在前几分钟内发生，在有硬度的附加还原，如果加热温度过长的话，平常的热处理要使钢被加热的温度一样。使用分段淬火的三个过程是回火形成的，在两者中，增加钢的温度在允许范围之内，在较低的温度下，在冷却的盐浴中淬火，这些过程即是奥氏体回火法和马氏体回火法，使钢获得我们想要的物理特性。

退火

退火的最初目的是使用于制造机器的钢变软，否则影响工件的刚度。通常用稍高于临界温度的温度加热，直到工件获得均匀的温度。然后以慢速冷却时表面温度与中心温度大体一致。这个过程就是完全退火，因为它去话了先前结构中所有痕迹细化了晶粒，而且使金属变软，减少了内应力。

在钢的稳定的退火工程中，被加热的温度决定于它的合成，因为碳使它们坚如钢，可能性从铁碳合金图中获得一部分因素，对断面的加热速度应该一致，所以要使整个部分尽可能的获得一样的增高温度，但退火温度达到定值时，钢的温度应该在此处被保持。其中厚度为（25mm）的最大断面大约需加热40分钟，最大的柔性和延展性冷却速度应该非常慢。应该允许零件一炉冷却，含碳量越高，这个比率会越慢。

正火和球化处理

正火处理在临界范围以上加热到华氏50-100F

?），并且在室温

?（10-40F

内冷却。这个过程用与低碳钢，就如合金钢的晶粒细化一样，来减少内应力以获得更好的物理特性。大多数的普通钢在液压之后被卷起。

球化处理是一个类似于球体的分布结构中产生珠光体的过程。如果这种钢在它的临界温度下加热并保持一段时间，这种处理在加工过共析钢尤为重要。