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Reading Material 9
Heat Treatment of Steel
Types of Heat Treating OperationsFive operations are detailed in this leon as the basis of heat treatment.Explanations of these
Operations follow.Stre RelievingWhen a metal Is heated，expansion occurs which is more or le proportional to the temperature rise.Upon cooling a metal,the reverse reaction takes place.That is, a contraction is observed．When a steel bar or plate is heated at one point more than at another，as in welding or during forging，Internal strees are set up．During heating, expansion of the heated area cannot take place unhindered，and it tends to deform．On cooling，contraction is prevented from taking place by the unyielding cold metal surrounding the heated area．The forces attempting to contract the metal are not relieved，and when the metal is cold again，the forces remain as internal strees．strees also result from volume changes, which accompany metal transformations and precipitation．Internal or residual strees are bad because they may cause warping of steel parts when they are machined．To
0relieve these strees，steel is heated to around 595C，auming that the entire
part is heated uniformly, then cooled slowly back to room temperature．This procedure is called stre relief annealing, or merely stre relieving.Because of characteristics inherent in cast steel, the normalizing treatment is more frequently applied to ingots prior to working，and to steel castings and forgings prior to hardening.NormalizingThe proce of normalizing consists of heating to a temperature above the third transformation temperature and allowing the part to cool in still air.The actual temperature required for this depends on the composition of the steel,0but is usually around 870C.Actually, the term normalize does not describe the
purpose.The proce might be more accurately described as a homogenizing or grain-refining treatment.Within any piece of steel, the composition is usually not uniform throughout.That is, one area may have more carbon than the area adjacent to it.These compositional differences affect the way in which the steel will respond to heat treatment.If it is heated to a high temperature, the carbon can readily diffuse throughout, and the result is a reasonably uniform composition from one area to the next.The steel is then more homogeneous and will respond to the heat treatment in a more uniform way.During cold deformation, steel has a tendency to harden in deformed areas, making it more difficult to bend and liable to breakage.Alternate deforming and annealing operations are performed on most manufactured steel products.Full annealingFull annealing, where steel is heated 50 to 100C above the third transformation temperature for hypoeutectoid steels, and above the lowest transformation temperature for hypereutectoid steels, and slow cooled, makes the steel much easier to cut, as well as bend.In full annealing, cooling must take place very slowly so that a coarse pearlite is formed.Slow cooling is not eential for
proce annealing, since any cooling rate from temperatures below the lowest transformation temperature will result in the same microstructure and hardne.Proce annealingProce annealing consists of heating steel to a temperature just below the lowest transformation temperature for a short time.This makes the steel easier to form.This heat treatment is commonly applied in the sheet and wire industries, and the temperatures generally used are from 550 to 650C.Annealing The two--stage heat treating proce of quenching and tempering is designed to produce high strength steel capable of resisting shock and deformation without breaking.On the other hand, the annealing proce is intended to make steel easier to deform or machine.1n manufacturing steel products, machining and severe bending operations are often employed.Even tempered steel may not cut or bend very easi1y and annealing is often neceary.The effect of tempering may be il1ustrated as follows.If the head of a hammer were quenched to a fully martensitic structure, it probably would crack after the first few blows.Tempering during manufacture of the hammer imparts shock resistance with only a slight decrease in hardne.Tempering is accomplished by heating a quenched part to some point below the transformation temperature, and holding it at this temperature for an hour or more, depending on its size.Most steels are tempered between 205°C and 595°C.As higher temperatures are employed, toughne or shock resistance of the steel is increased, but the hardne and strength decrease.TemperingDuctility is the ability of a metal to change shape before it breaks.Fleshly quenched martensite is hard but not ductile;in fact, it is very brittle.Tempering is needed to impart ductility to the martensite, usually at a small sacrifice in strength.In addition, tempering greatly increases the resistance of martensite to shock loading.Heat TreatmentThe hardest condition for any given steel is obtained by quenching to a fully martensitic structure.Since hardne is directly related to strength, a steel composed of 100% martensite is at its strongest poible condition.However, strength is not the only property that must be considered in the application of steel parts.Ductility may be equally important.Change or modify the magnetic properties of steel.Improve the electrical properties;
Improve the machinability;
Increase the toughne;that is, to produce a steel having both a high tensile strength and good ductility, enabling it to withstand high impact;
Increase the hardne so as to increase resistance to wear or to enable the steel to withstand more service conditions;
Decrease the hardne and increase the ductility;
Secure the proper grain structure;
Refine the grain structure of hot worked steels which may have developed coarse grain size;
Remove strees induced by cold working or to remove strees set up by nonuniform cooling of hot metal objects;
Reasons for Heat TreatingHeat treatment of steel is usually intended to accomplish any one of the following objectives:
Stre relievingStre relieving is the heating of steel to a temperature below the transformation temperature, as in tempering, but is done primarily to relieve internal stre and thus prevent distortion or cracking during machining.This is sometimes called proce annealing.Tempering Tempering consists of reheating a quenched steel to a suitable temperature below the transformation temperature for an appropriate time and cooling back to room temperature.How this proce makes steel tough will be discued later.Hardening Hardening is carried out by quenching a steel, that is, cooling it rapidly from a temperature above the transformation temperature.Steel is quenched in water or brine for the most rapid cooling, in oil for some alloy steels, and in air for certain higher alloy steels.After steel is quenched, it is usually very hard and brittle;it may even crack if dropped.To make the steel more ductile, it must be tempered.NormalizingNormalizing is identical with annealing, except that the steel is air cooled;this is much faster than cooling in a furnace.Steel is normalized to refine grain size, make its structure more uniform, or to improve machinability.Full annealingFull annealing is the proce of softening steel by a heating and cooling cycle, so that it may be bent or cut easily.In annealing, steel is heated above a transformation temperature and cooled very slowly after it has reached a suitable temperature.The distinguishing characteristics of full annealing are:(a)temperature above the critical temperature and(b)very slow cooling, usually in the furnace.阅读材料9
钢的热处理
各种不同的热处理操作 本单元介绍了五种热处理的基本方法。这些方法介绍如下。
完全退火完全退火是对钢进行反复的加热和冷却使钢软化的过程，这样就容易弯曲和切割。在退火中，使钢加热到转变温度以上的一个适宜温度后缓慢地冷却。完全退火的突出的特点是：（a）温度高于临界温度（b）缓慢冷却，通常是炉冷。
正火正火和退火相同，除了钢是被空冷的；这比在炉中冷却得更快。钢的正火是为了改善晶粒大小，使它的结构更加均匀，或者是提高机械性能。
淬火淬火就是通过冷浸钢，那就是使钢从转变温度以上的一个温度快速冷却。为了最快的冷却，钢被冷浸在水中或是盐水里，合金钢的是在油里，某些更高合金钢的要在空气中冷却。当钢被淬火之后，它通常是硬和易碎的；甚至落地会破碎，为了使钢更有韧性，它必须被回火。
回火回火是指重新加热已经被淬火的钢到转变温度以下的一个适当温度一段时间后再冷却到室温。至于该过程怎样使钢变得有韧性，我们将在以后讨论。
去应力是指加热钢到转变温度以下的适宜温度，正如回火一样，但这样做是为了减少内应力从而避免在加工过程中的变形和破裂。这有时也被称作退火过程。
热处理的原因钢的热处理通常是为了达到以下的任一目的：
消除冷却过程中产生的内应力和高温金属物体因冷却不均匀而产生的应力。
改善热处理钢可能产生的粗糙晶粒的晶粒结构。
得到适当的晶粒结构
降低硬度，提到塑性。
增加硬度，以提高到钢的抗耐磨性和加强钢使之能承受更多的使用条件。
增加韧性，这就是使钢同时拥有高的拉伸强度和好的延展性，使它能承受高的撞击。提高切削性能
提高导电性。
改变或修正钢的磁性。
热处理任何一种钢通过淬火而获得完全的马氏体是最难的。由于硬度直接关系到强度，一种钢由100%的马氏体组成是其处于最高强度的可能条件。但是，在钢的应用部分里，强度不是唯一的必需考虑的性能。延展性同样重要。
回火延展性是金属在破裂前改变形状的能力。淬火马氏体很硬但不能延展，事实上它是非常碎的。回火是用来使马氏体获得可延展性，通常强度会降低一些。另外，回火大大增加马氏体抵抗冲击负荷的能力。
回火的影响举例说明如下:如果一个锤头被淬火到完全马氏体结构，它可能在前几次敲击就会破碎。回火在锤头的制造中能增加抗敲击能力，而硬度只有一点点的下降。回火过程是这样达到的：把淬火后的部分加热到转变温度的某点，然后维持这温度一个小时或更多，这要根据部件的大小判断。大多的钢是被加热到205°C到595°C，更高的温度，钢的韧性和抗敲击能力会增加，但硬度和强度会下降。
退火淬火和回火两个热处理过程来生产高强度的钢以便抵抗冲击和变形而不受破坏。另一方面，退火过程的目的是为了使钢更容易变形和机器加工。在钢产品的制造中，机器加工和严格的弯曲操作经常被运用到。即使回火钢也不会被经意的切割和弯曲，退火就常常是不可缺少的。
退火过程退火过程就是加热钢到稍低于最低转变温度的一个温度后保持一会儿。这使钢更容易成形。这种热处理通常应用于薄板和电线工业，它的温度一般在550度到650 度之间。
完全退火完全退火对于亚共析钢要将温度加热到第三转变温度以上50~100℃，对于过共析钢，要加热到最低转变温度以上，然后缓慢冷却，使钢更容易切割和弯曲。在完全退火中，冷却必须要非常缓慢地进行从而形成粗糙的珠光体。退火过程不必需要缓慢冷却，因为最低转变温度下的任何冷却速度都会得到相同的微观结构和硬度。
在冷变形中，钢有一种在变形中变硬的趋势，使之更难于弯曲和被破坏。大多数的机械加工钢产品都需要交替变形和退火操作。
正火正火过程包括将温度加热到第三转变温度以上，然后让该加热部分在空气中冷却直到与空气温度相同。实际所需的温度要根据钢的组成部分来确定，但通常在870°C左右。其实正火这个术语不是描述目的，该过程被描述成均匀或细化晶粒处理会更准确。任何一块钢，它的组成部分通常是不统一的。那就是说，一块区域可能比周围的含有更多的碳。这种成分的不同会影响热处理的方法。如果加热到一个高的温度，其中的碳能扩散到四周，结果理所当然的刀均匀的成分。这样钢的成分就会更均匀从而使热处理方法更统一。
由于铸铁的固有性质，对工作前的铸铁块，硬化前的钢的铸件和锻件，正火处理会运用得更频繁。
去应力当一块金属被加热时就会发生膨胀，膨胀的多少跟温度的上升成比例。当冷
却一金属，就会发生相反的反应。那就是说，金属的收缩可以被观测到。当一根钢棒或钢板被加热到一个点高于另一个点，就像焊接或锻造一样，内应力就会产生。在加热过程中，被加热部分不能自由膨胀，就会转为变形。在冷却时，热部分周围的冷金属就会阻止收缩。打算使金属收缩的力并没有减少，当金属再次被冷却时，这个力与内应力一样不变。体积的变化同样能产生应力，这个应力随着金属转变和沉淀。内应力和残余应力是不好的因为它使钢的加工部分产生弯曲。为了减少这种应力，钢被加工到约595°C，假设所有的部分被均匀加热，然后缓慢冷却到室温。这步骤就叫做去应力退火，或仅仅叫去应力。
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