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文獻翻譯 The understanding of heat treatment is embraced 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 of 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 stresses may be removed, grain size reduced, toughness increased, or a hard surface produced on a ductile interior. The analysis of the steel must be known because small percentages of certain elements, notably carbon, greatly affect the physical properties.Alloy steel 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 steels known as plain carbon steels. With this process the rate of cooling is the controlling factor, rapid cooling from above the critical range results in hard structure, whereas very slow cooling produces the opposite effect.To begin to understand these processes, consider a 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), several changes occur simultaneously.The iron wants to change from the FCC austenite structure to the BCC ferrite structure, but the ferrite can only contain 0.02% carbon in solid solution. The rejected carbon forms the carbon-rich cementite intermetallic with composition Fe3C. In essence, the net reaction at the eutectoid is austenite 0.77%Cferrite 0.02%C+cementite 6.67%C.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. Specimens prepared by polishing and etching in a weak solution of nitric acid and alcohol reveal the lamellar structure of 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.HardeningHardening is the process of heating a 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 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 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. 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 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.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.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. 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. 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 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. 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.TemperingSteel 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 conditionsAs 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. 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.Tempering is possible because of the instability of the martensite, the principal constituent of hardened steel. Low-temperature draws, from 300 to 400 (150205), 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(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. 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.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.AnnealingThe 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 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. 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 carbide equilibrium diagram. When the annealing temperature has been reached, the steel should be held there until it is uniform throughout. 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. 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.Normalizing and SpheroidizingThe process of normalizing consists of heating the steel about 50 to 100 (1040) 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 steel is heated slowly to a temperature just below the critical range and held there for a prolonged period of time, this structure will be obtained. The globular structure obtained gives improved machinability to the steel. This treatment is particularly useful for hypereutectoid steels that must be machined.Surface Hardening CarburizingThe oldest known method of producing a hard surface on steel is case hardening or carburizing. Iron at temperatures close to and above its critical temperature has an affinity for carbon. The carbon is absorbed into the metal to form a solid solution with iron and converts the outer surface into high-carbon steel. The carbon is gradually diffused to the interior of the part. The depth of the case depends on the time and temperature of the treatment. Pack carburizing consists of placing the parts to be treated in a closed container with some carbonaceous material such as charcoal or coke. It is a long process and used to produce fairly thick cases of from 0.03 to 0.16 in.(0.764.06mm) in depth.Steel for carburizing is usually a low-carbon steel of about 0.15% carbon that would not in itself responds appreciably to heat treatment. In the course of the process the outer layer is converted into high-carbon steel with a content ranging from 0.9% to 1.2% carbon.A steel with varying carbon content and, consequently, different critical temperatures requires a special heat treatment. Because there is some grain growth in the steel during the prolonged carburizing treatment, the work should be heated to the critical temperature of the core and then cooled, thus refining the core structure. The steel should then be reheated to a point above the transformation range of the case and quenched to produce a hard, fine structure.The lower heat-treating temperature of the case results from the fact that hypereutectoid steels are normally austenitized for hardening just above the lower critical point. A third tempering treatment may be used to reduce strains.CarbonitridingCarbonitriding, sometimes known as dry cyaniding or nicarbing, is a case-hardening process in which the steel is held at a temperature above the critical range in a gaseous atmosphere from which it absorbs carbon and nitrogen. Any carbon-rich gas with ammonia can be used. The wear-resistant case produced ranges from 0.003 to 0.030 inch(0.08 0.76mm) in thickness. An advantage of carbonitriding is that the hardenability of the case is significantly increased when nitrogen is added, permitting the use of low-cost steels.CyanidingCyaniding, or liquid carbonitriding as it is sometimes called, is also a process that combines the absorption of carbon and nitrogen to obtain surface hardness in low-carbon steels that do not respond to ordinary heat treatment. The part to be case hardened is immersed in a bath of fused sodium cyanide salts at a temperature slightly above the Ac1 range, the duration of soaking depending on the depth of the case. The part is then quenched in water or oil to obtain a hard surface. Case depths of 0.005 to 0.015in. (0.130.38mm) may be readily obtained by this process. Cyaniding is used principally for the treatment of small parts.NitridingNitriding is somewhat similar to ordinary case hardening, but it uses a different material and treatment to create the hard surface constituents. In this process the metal is heated to a temperature of around 950(510) and held there for a period of time in contact with ammonia gas. Nitrogen from the gas is introduced into the steel, forming very hard nitrides that are finely dispersed through the surface metal.Nitrogen has greater hardening ability with certain elements than with others, hence, special nitriding alloy steels have been developed. Aluminum in the range of 1% to 1.5% has proved to be especially suitable in steel, in that it combines with the gas to form a very stable and hard constituent. The temperature of heating ranges from 925 to 1,050(495565).Liquid nitriding utilizes molten cyanide salts and, as in gas nitriding, the temperature is held below the transformation range. Liquid nitriding adds more nitrogen and less carbon than either cyaniding or carburizing in cyanide baths. Case thickness of 0.001 to 0.012in.(0.030.30mm) is obtained, whereas for gas nitriding the case may be as thick as 0.025 in.(0.64mm). In general the uses of the two-nitriding processes are similar.Nitriding develops extreme hardness in the surface of steel. This hardness ranges from 900 to 1,100 Brinell, which is considerably higher than that obtained by ordinary case hardening. Nitriding steels, by virtue of their alloying content, are stronger than ordinary steels and respond readily to heat treatment. It is recommended that these steels be machined and heat-treated before nitriding, because there is no scale or further work necessary after this process. Fortunately, the interior structure and properties are not affected appreciably by the nitriding treatment and, because no quenching is necessary, there is little tendency to warp, develop cracks, or change condition in any way. The surface effectively resists corrosive action of water, saltwater spray, alkalies, crude oil, and natural gas.對熱處理的理解包含于對冶金學較廣泛的研究。冶金學是物理學、化學和涉及金屬從礦石提煉到最后產(chǎn)物的工程學。熱處理是將金屬在固態(tài)加熱和冷卻以改變其物理性能的操作。按所采用的步驟，鋼可以通過硬化來抵抗切削和磨損，也可以通過軟化來允許機加工。使用合適的熱處理可以去除內(nèi)應(yīng)力、細化晶粒、增加韌性或在柔軟材料上覆蓋堅硬的表面。因為某些元素(尤其是碳)的微小百分比極大地影響物理性能，所以必須知道對鋼的分析。合金鋼的性質(zhì)取決于其所含有的除碳以外的一種或多種元素，如鎳、鉻、錳、鉬、鎢、硅、釩和銅。由于合金鋼改善的物理性能，它們被大量使用在許多碳鋼不適用的地方。下列討論主要針對被稱為普通碳鋼的工業(yè)用鋼而言。熱處理時冷卻速率是控制要素，從高于臨界溫度快速冷卻導致堅硬的組織結(jié)構(gòu)，而緩慢冷卻則產(chǎn)生相反效果。為了理解這些過程，考慮含碳量為0.77%的共析鋼，沿著圖2.1的x-x線慢慢冷卻。在較高溫度時，只存在奧氏體，0.77%的碳溶解在鐵里形成固溶體。當鋼冷卻到727 (1341)時，將同時發(fā)生若干變化。鐵需要從面心立方體奧氏體結(jié)構(gòu)轉(zhuǎn)變?yōu)轶w心立方體鐵素體結(jié)構(gòu)，但是鐵素體只能容納固溶體狀態(tài)的0.02%的碳。被析出的碳與金屬化合物Fe3C形成富碳的滲碳體。本質(zhì)上，共析體的基本反應(yīng)是奧氏體0.77%的碳鐵素體0.02%的碳+滲碳體6.67%的碳由于這種碳成分的化學分離完全發(fā)生在固態(tài)中，產(chǎn)生的組織結(jié)構(gòu)是一種細致的鐵素體與滲碳體的機械混合物。通過打磨并在弱硝酸酒精溶液中蝕刻制備的樣本顯示出由緩慢冷卻形成的交互層狀的薄片結(jié)構(gòu)。這種結(jié)構(gòu)由兩種截然不同的狀態(tài)組成，但它本身具有一系列特性，且因與低倍數(shù)放大時的珠母層有類同之處而被稱為珠光體。淬火淬火就是把鋼件加熱到或超過它的臨界溫度范圍，然后使其快速冷卻的過程。如果鋼的含碳量已知，鋼件合適的加熱溫度可參考鐵碳合金狀態(tài)圖得到。然而當鋼的成分不知道時，則需做一些預備試驗來確定其溫度范圍。要遵循的合適步驟是將這種鋼的一些小試件加熱到不同的溫度后淬火，再通過硬度試驗或顯微鏡檢查觀測結(jié)果。一旦獲得正確的溫度，硬度和其它性能都將有明顯的變化。 在任何熱處理作業(yè)中，加熱的速率都是重要的。熱量以一定的速率從鋼的外部傳導到內(nèi)部。如果鋼被加熱得太快，其外部比內(nèi)部熱就不能得到均勻的組織結(jié)構(gòu)。如果工件形狀不規(guī)則，為了消除翹曲和開裂最根本的是加熱速率要緩慢。截面越厚，加熱的時間就要越長才能達到均勻的結(jié)果。即使加熱到正確的溫度后，工件也應(yīng)在此溫度下保持足夠時間以讓其最厚截面達到相同溫度。通過給定的熱處理所得到的硬度取決于淬火速率、含碳量和工件尺寸。除了非淬硬鋼或部分淬硬鋼外，合金鋼中合金元素的種類及含量僅影響鋼的淬透性(工件被硬化到深層的能力)而不影響硬度。含碳量低的鋼對淬火處理沒有明顯的反應(yīng)。隨著鋼的含碳量增加到大約0.60%，可能得到的硬度也增加。高于此點，由于超過共析點鋼完全由珠光體和退火狀態(tài)的滲碳體組成，硬度增加并不多。珠光體對熱處理作業(yè)響應(yīng)最好；基本由珠光體組成的鋼能轉(zhuǎn)化成硬質(zhì)鋼。即使所有其它條件保持不變，隨著要淬火的零件尺寸的增加其表面硬度也會有所下降。熱量在鋼中的傳導速率是有限的。無論淬火介質(zhì)怎么冷，如果在大工件中的熱量不能比特定的臨界速率更快散發(fā)，那它內(nèi)部硬度就會受到明確限制。然而鹽水或水淬火能夠?qū)⒈淮懔慵谋砻嫜杆倮鋮s至本身溫度并將其保持或接近此溫度。在這種情況下不管零件尺寸如何，其表面總歸有一定深度被硬化。但油淬情況就不是如此，因為油淬時在淬火臨界階段零件表面的溫度可能仍然很高.回火快速淬火硬化的鋼是硬而易碎的，不適合大多數(shù)場合使用。通過回火，硬度和脆性可以降低到使用條件所需要的程度。隨著這些性能的降低，拉伸強度也降低而鋼的延展性和韌性則會提高。回火作業(yè)包括將淬硬鋼重新加熱到低于臨界范圍的某一溫度然后以任意速率冷卻。雖然這過程使鋼軟化，但它與退火是大不相同的，因為回火適合于嚴格控制物理性能并在大多數(shù)情況下不會把鋼軟化到退火那種程度。回火完全淬硬鋼得到的最終組織結(jié)構(gòu)被稱為回火馬氏體。由于馬氏體這一淬硬鋼主要成分的不穩(wěn)定性，使得回火成為可能。低溫回火， 300到400(150205)，不會引起硬度下降很多，主要用于減少內(nèi)部應(yīng)變。隨著回火溫度的提高，馬氏體以較快的速率分解，并在大約600(315)迅速轉(zhuǎn)變?yōu)楸环Q為回火馬氏體的結(jié)構(gòu)?；鼗鹱鳂I(yè)可以描述為滲碳體析出和凝聚或聚結(jié)的過程。滲碳體的大量析出開始于600(315)，這使硬度下降。溫度的上升會使碳化物聚結(jié)而硬度繼續(xù)降低。在回火過程中，不但要考慮溫度而且要考慮時間。雖然大多數(shù)軟化作用發(fā)生在達到所需溫度后的最初幾分鐘，但如果此溫度維持一段延長時間，仍會有些額外的硬度下降。通常的做法是將鋼加熱到所需溫度并且僅保溫到正好使其均勻受熱。兩種采用中斷淬火的特殊工藝也是回火的形式。這兩種工藝中，淬硬鋼在其被允許冷卻前先在一選定的較低溫度鹽浴淬火。這兩種分別被稱為奧氏體回火和馬氏體回火的工藝，能使產(chǎn)品具有特定所需的物理性能。 退火退火的主要目的是使堅硬的鋼軟化以便機加工或冷作。通常是非常緩慢地將鋼加熱到臨界溫度以上，并將其在此溫度下保持到工件全部均勻受熱，然后以受控的速率慢慢地冷卻，這樣使得工件表面和內(nèi)部的溫度近似相同。這過程被稱為完全退火，因為它去除了以前組織結(jié)構(gòu)的所有痕跡、細化晶粒并軟化金屬。退火也釋放了先前在金屬中的內(nèi)應(yīng)力。給定的鋼其退火溫度取決于它的成分；對碳鋼而言可容易地從局部的鐵碳合金平衡圖得到。達到退火溫度后，鋼應(yīng)當保持在此溫度等到全部均勻受熱。加熱時間一般以工件的最大截面厚度計每英寸(25mm )大約需45min。為了得到最大柔軟性和延展性冷卻速率應(yīng)該很慢，比如讓零件與爐子一起冷下來。含碳量越高，冷卻的速率必須越慢。加熱的速率也應(yīng)與截面的尺寸及均勻程度相協(xié)調(diào)，這樣才能使整個零件盡可能均勻地加熱。正火和球化正火處理包括先將鋼加熱到高于上臨界區(qū)50到100(1040)然后在靜止的空氣中冷卻到室溫。退火主要用于低碳鋼、中碳鋼及合金鋼，使晶粒結(jié)構(gòu)更均勻、釋放內(nèi)應(yīng)力或獲得所需的物理特性。大多數(shù)商業(yè)鋼材在軋制或鑄造后都要退火。球化是使?jié)B碳體產(chǎn)生成類似球狀分布結(jié)構(gòu)的工藝。如果把鋼緩慢加熱到恰好低于臨界溫度并且保持較長一段時間，就能得到這種組織結(jié)構(gòu)。所獲得的球狀結(jié)構(gòu)改善了鋼的可切削性。此處理方法對必須機加工的過共析鋼特別有用。表面硬化 滲碳最早的硬化鋼表面的方法是表面淬火或滲碳。鐵在靠近并高于其臨界溫度時對碳具有親合力。碳被吸收進金屬與鐵形成固溶體使外表面轉(zhuǎn)變成高碳鋼。碳逐漸擴散到零件內(nèi)部。滲碳層的深度取決于熱處理的時間和溫度。固體滲碳的方法是將要處理的零件與木炭或焦炭這些含碳的材料一起放入密閉容器。這是一個較長的過程，用于產(chǎn)生深度為0.03到0.16 英寸(0.764.06mm)這么厚的硬化層。 用于滲碳的一般是含碳量約為0.15%、本身不太適合熱處理的低碳鋼。在處理過程中外層轉(zhuǎn)化為含碳量從0.9%到1.2%的高碳鋼。含碳量變化的鋼具有不同的臨界溫度，因此需要特殊的熱處理。由于在較長的滲碳過程中鋼內(nèi)部會有些晶粒生長，所以工件應(yīng)該加熱到核心部分的臨界溫度再冷卻以細化核心部分的組織結(jié)構(gòu)。然后重新加熱到高于外層轉(zhuǎn)變溫度再淬火以生成堅硬、細致的組織結(jié)構(gòu)。由于恰好高于低臨界溫度通常使過共析鋼奧氏體化而硬化，所以對外層采用較低的熱處理溫度。第三次回火處理可用于減少應(yīng)變。碳氮共滲碳氮共滲，有時也稱為干法氰化或滲碳氮化，是一種表面硬化工藝。通過把鋼放在高于臨界溫度的氣體中，讓它吸收碳和氮?？梢允褂萌魏胃惶細怏w加氨氣，能生成厚度從0.003到0.030英寸(0.08 0.76mm)的耐磨外層。碳氮共滲的優(yōu)點之一是加入氮后外層的淬透性極大增加，為使用低價鋼提供條件。氰化氰化，有時稱為液體碳氮共滲，也是一種結(jié)合了吸收碳和氮來獲得表面硬度的工藝，它主要用于不適合通常熱處理的低碳鋼。需表面硬化的零件浸沒在略高于Ac1溫度熔化的氰化鈉鹽溶液中，浸泡的持續(xù)時間取決于硬化層的深度。然后將零件在水或油中淬火。滲氮滲氮有些類似普通表面硬化，但它采用不同的材料和處理方法來產(chǎn)生堅硬表面成分。這種工藝中金屬加熱到約950(510)，然后與氨氣接觸一段時間。氨氣中的氮進入鋼內(nèi)，形成細微分布于金屬表面又十分堅固的氮化物。氮與某些元素的硬化能力比其它元素大，因此開發(fā)了專用的滲氮合金鋼。在鋼中含鋁1%到1.5%被證明特別合適，它能與氨氣結(jié)合形成很穩(wěn)定堅固的成分。其加熱溫度范圍為925到1,050 (495565)。液體滲氮利用熔化的氰化物鹽，就像氣體滲氮，溫度保持在低于轉(zhuǎn)化范圍內(nèi)。液體滲氮時在氰化物溶液中加入比氰化及滲碳都較多的氮和較少的碳。液體滲氮可以獲得厚度為0.001到0.012英寸 (0.030.30mm)的硬化層，然而氣體滲氮則能獲得厚0.025英寸(0.64mm)的硬化層。一般而言兩種滲氮方法的用途是類似的。滲氮在鋼表面獲得遠遠超出正常標準的硬度。其硬度范圍為900到1,100布氏硬度，這遠高于普通表面硬化所獲得的硬度。由于滲氮鋼的合金比例，它們比普通鋼更強，也容易熱處理。建議對這種鋼在滲氮前先機加工和熱處理，因為滲氮后沒有剝落并不需要更多的加工。值得慶幸的是由于滲氮處理一點都不影響內(nèi)部結(jié)構(gòu)和性能，也無需淬火，所以幾乎沒有任何產(chǎn)生翹曲、裂縫及變化條件的趨勢。這種表面能有效地抵御水、鹽霧、堿、原油和天然氣的腐蝕反應(yīng)。