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Metal Heat Treatment basic knowledge
Metal is metal heat treatment on certain parts of the heating medium to the appropriate temperature, and maintained at this temperature in a certain time, on a different speed cooling process.
Machinery Manufacturing metal heat treatment is an important process in one, compared with other processing, heat treatment does not generally change the shape of the workpiece and the overall chemical composition, but by changing the internal microstructure of the workpiece, or to change the chemical composition of the workpiece surface, to the use of the workpiece or improve performance. Its characteristics are inherent to improve workpiece quality, and this is not generally the naked eye can see.
To make metal parts with the required mechanical properties, physical properties and chemical properties, in addition to reasonable use of various materials and forming process, the heat treatment process is often essential. Steel Machinery Industry of the most widely used materials, iron and steel complex microstructure can be controlled by heat treatment, heat treatment of steel is the main contents of metal heat treatment. In addition, aluminium, copper, magnesium, titanium and its alloys, such as through heat treatment can also change its mechanical, physical and chemical properties in order to obtain the use of different properties.
Metal Heat Treatment Process
Heat Treatment Process generally include heating, insulation, cooling process of the three, sometimes only two heating and cooling process. interface between these processes can not be stopped.
Heating is an important process of heat treatment. Metal Heat Treatment many of the heating method, is the first use of charcoal and coal as a heat source, thus application of liquid and gaseous fuels. Application of heating, easy to control, and pollution-free. These heat can be used directly for heating can also melt salt or metal, as well as floating particles indirect heating.
Heating metal workpiece exposed in the air, often oxidation, decarbonization (that is, the surface of carbon steel components decreased), which is heat-treated parts of the surface have a negative impact. Metal should normally result in the controlled atmosphere, or the protection of the atmosphere, melting salt and vacuum heating, paint or packaging methods can be used to protect heating.
Heating temperature of the heat treatment process is an important process parameters of choice and control temperature, heat treatment is to ensure that the quality of the main issues. With the heating temperature of the metal being processed materials and the purpose of heat treatment vary, but are generally heated to phase transition temperature above to obtain high-temperature organizations. Also changing needs a certain period of time, so when metal surface meet the required temperature, but also maintain a certain temperature in this time, the temperature inside and outside the same, microstructure changes completely, this time as holding time. Heating by high-energy density and surface heat treatment, heat fast, the general will be no holding time, and the chemical heat treatment often longer holding time.
Cooling heat treatment process is an indispensable step, because of cooling methods vary, mainly for controlling the cooling rate. Annealing generally the slowest rate of cooling, the cooling normalizing faster, quenching cooling faster. But also because different types of steel have different requirements, such as empty steel hardware can be used as normalizing the cooling speed of hardened.
Metal heat treatment process can be roughly divided into the overall heat treatment, heat treatment and surface chemical heat treatment three categories. According heating medium temperature heating and cooling methods of each major categories can be divided into a number of different heat treatment process. With a different metal heat treatment process, the organization will be different, with different performance. Application of the iron and steel industry is the most widely metals, iron and steel microstructure and also the most complex, a wide range of steel heat treatment process.
Heat Treatment for the overall workpiece heating, and then cooling the appropriate speed in order to change the overall mechanical properties of metal heat treatment process. Steel Heat Treatment generally annealing, normalizing, quenching and tempering four basic technology.
Annealing workpiece is heated to a suitable temperature, according to the workpiece material and size of different soaking time, and then slowly cooled to ensure that the internal organization of metal at or close to a state of equilibrium, good performance and the use of performance, or for further hardening the readiness of the Organization for. Normalizing the workpiece is heated to a suitable temperature in the air after cooling, with the effect of annealing normalizing similar, but the organization received more detailed, the material used to improve the cutting performance, but also sometimes used in some parts of the main requirement The final heat treatment.
Quenching the workpiece is heating insulation, water, oil or other inorganic salts, organic solution such as quenching medium fast cooling. Quenched steel stiffen, but. In order to reduce the brittleness of steel parts will be quenched steel pieces in the above room temperature lower than 650 ℃ and the proper temperature for a long period of maintenance, and further cooling, this process called tempering. Annealing, normalizing, quenching and tempering heat treatment is the overall "Four fire", which closely related to the quenching and tempering, often used in conjunction with, and neither is dispensable.
"Four fire" With the heating temperature and cooling of the different ways, but it turns out different heat treatment process. In order to obtain a certain strength and toughness, the quenching and tempering temperature combined with the technology, known as tuned mass. Quenching a certain alloy supersaturated solid solution, it will be placed at room temperature or slightly higher to maintain the proper temperature longer time, in order to improve the alloy hardness, strength or electrical properties such as magnetic. This heat treatment process called aging.
Deformation and the pressure heat treatment processing effective and closer together, so that part was good strength, toughness with the method known as thermomechanical treatment in the atmosphere or a vacuum suction of the vacuum heat treatment known as heat treatment, which not only enables the workpiece no oxidation, decarbonization, and maintain smooth surface treatment to enhance the performance of the workpiece can also infiltration of chemical-heat treatment.
Surface Heat Treatment is only heating workpiece surface to change its surface mechanical properties of metal heat treatment process. In order to heat only the surface layer of the workpiece without excessive internal heat into the workpiece, the use of heat required to have a high energy density, that is, in part on the size of units to larger heat to the workpiece surface or in part to achieve short-term or transient high temperature. Surface heat treatment is the primary means of quenching the flame and induction heating heat treatment, heat is used oxyacetylene flame, such as propane or oxygen, the induced currents, such as laser and electron beam.
Chemical heat treatment is the workpiece surface by changing the chemical composition, structure and properties of metal heat treatment process. Chemical heat treatment and surface heat treatment difference is the latter changed the chemical composition of the workpiece surface. Chemical heat treatment is the workpiece on the carbon, nitrogen or other alloying elements medium (gas, liquid and solid) in the heating, insulation longer period of time, so that the workpiece surface infiltration of carbon, nitrogen, boron and chromium and other elements. After the infiltration of elements, and sometimes engage in other heat treatment process such as quenching and tempering. The main chemical heat treatment methods are carburizing, nitriding, seepage metal.
Heat treatment and mechanical parts manufacturing workers die in the course of one of the important processes. Broadly speaking, it can guarantee and improve the performance of the various parts, such as wear resistance, corrosion resistance, etc.. Blank also can improve the organization and stress state to facilitate a variety of cold and hot processing.
For example, white cast iron after long annealing can be malleable cast iron, increase ductility; gear using the correct heat treatment process, life can be without heat treatment than doubled or gear several times to improve addition, inexpensive carbon steel through infiltration some alloying elements on a certain price Leon steel properties, can replace certain heat-resistant steel, stainless steel; Die almost all need to go through heat treatment before use.
The mechanical properties of metallic materials
Metal performance generally consists of two types of properties and performance. Process performance is that the so-called mechanical parts in the manufacturing process, the metal material, as set out in the cold and hot under the conditions of processing performance from the performance. Metal Performance of the quality of its decision in the manufacturing process of forming processing ability to adapt. As different processing conditions, the performance requirements of the process is different, such as casting properties, weldability, forging, heat treatment performance, cutting and so on. Performance refers to the use of the so-called mechanical parts under the conditions of use for metals, demonstrated performance, which includes mechanical properties, physical properties, chemical properties. Metal Performance is good or bad, has determined with the use of the service life.
In the machinery manufacturing industry, the general mechanical parts are at normal temperature and pressure, and strong non-corrosive used in the media and in the use of the mechanical parts in the process will be under different loads role. Metallic Materials in damage resistance under load performance, as mechanical properties.
The mechanical properties of metallic materials in the design and components is the main basis for selection. Plus load different in nature, the metal material requirements of the mechanical properties will be different. Commonly used mechanical properties include: strength, ductility, hardness, impact toughness, and fatigue resistance of several attacks, such as the limit. Below will be a discussion of mechanical properties.
1.Strength
Strength refers to the metal materials under static load resistance undermine performance. As the role of load mode tensile, compression, bending, shearing and other forms, are also divided into tensile strength and compressive strength, flexural strength, shear strength. Often a certain intensity of the various links to more general use of tensile strength as the fundamental strength indicators.
2. Plastic
Plastic is metallic materials in the load, a plastic deformation without undermining the capacity.
2.Hardness
Hardness is a measure of the extent of soft and hard metal materials indicators. At present the production of the most commonly used methods of hardness is pressed into hardness, it is with certain geometry pressure head in a certain load to be tested under the pressure of the metal surface, was under pressure to the extent of its hardness value.
Commonly used method Brinell hardness, Rockwell hardness and Vickers hardness, and other methods.
3.Fatigue
Discussed earlier strength, ductility, hardness are in the static load of metal mechanical properties under the action of indicators. In fact, many machine parts are under cyclic loading work, in which parts will be produced under the conditions of fatigue.
5. Toughness
A great role in the speed of the load on the device known as impact loading, the metal under impact loading resistance ability to destroy called toughness.
Annealing quenching --- Tempering
1. Complete annealing and isothermal annealing
Complete recrystallization annealing also known as annealing, the general referred to as annealing, which mainly used for annealing eutectoid composition of the various carbon steel and alloy steel casting, forging and hot-rolled profiles, and sometimes also for welded structure. General often are not as heavy workpiece the final heat treatment, or as a part of some pre-heat treatment.
2. Spheroidizing
Spheroidizing mainly used for the Analysis of the total carbon steel and alloy tool steel (such as the manufacture of cutting tools, measuring tool, used by the steel mold species). Its main purpose is to reduce hardness and improve machining, and to prepare for the subsequent hardening.
3. To stress annealing
Also known as low-temperature annealing to stress annealing (or high-temperature tempering), which are mainly used for annealing eliminate casting, forging, welding parts, pieces of hot-rolled, cold-drawn parts of the residual stress. If the stress is not eliminated, will cause steel parts after a certain period of time, or in the subsequent process of machining the deformation or cracks.
2. Quenching most commonly used when the cooling medium is saline, water and oil.
Quenching of the workpiece brine, easy access to high hardness and clean the surface, it is not easy does not produce hard quenching soft, but easy to serious deformation of the workpiece, or even cracking. The oil for quenchant apply only to the stability of the Austenitic relatively large number of small-sized steel or carbon steel workpiece quenching.
3. The purpose of tempering steel
1. Brittle reduce, eliminate or reduce stress, steel quenched with stress and is very brittle, if not promptly tempering steel took often cause deformation or even cracking.
2. Workpiece was requested by the mechanical properties of the workpiece by quenching high hardness and brittleness, in order to meet a variety of different workpiece performance requirements can be tempered with through appropriate to adjust the hardness, reducing the brittleness, which are required for the toughness, plastic.
3. Stability workpiece size
4. Annealing difficult for the softening of certain alloy steel, in the quenching (or normalizing) often using high temperature tempering, so that appropriate gathered carbide steel, the hardness will be lowered to cut processing Liqi.
Defects and heating control
First, overheating
We know that the heat treatment process led to the most vulnerable heating overheating austenitic coarse grains, the mechanical properties of components decline.
1. Overheated in general: the temperature is too high or high temperature insulation for too long, coarse grains from austenite as overheating. Coarse grain austenitic steel will lead to lower the strength and toughness, brittle transition temperature rise, increasing the deformation hardening tendency to crack. Lead to overheating and the reason is that temperature control instrumentation or mixture (often do not know how to process). The overheated organizations annealing, tempering temperature normalizing or more, under normal circumstances re Albright of the grain refinement.
2. Fracture genetic: Organization overheated steel, re-heating quenched, although enable austenite grain refinement, but sometimes there are still coarse granular fracture. Genetic fracture of a more controversial theory is generally believed that excessive heating temperature had MnS like debris into austenite grain and enriched in the interface, and when the cooling of these inclusions will be along the crystal interface precipitated by the attacks on the vulnerable along the coarse austenite grain boundary fracture.
3. Genetic large organizations: large martensite, bainite, Ngai Organization of the steel parts of the re-Albright, to slow the hardening of conventional heating to the temperature, or even lower, its austenite grain tablets is still large, this phenomenon is known as hereditary organizations. To remove the genetic large organizations, may be among many high-temperature tempering or annealing treatment.
Second, the phenomenon of burning
Heating temperature is too high, not only attracted large austenite grain, and grain boundaries in partial oxidation or melt, leading to weakening of the grain boundary, known as the burn. Steel serious deterioration in performance after the burn, was formed in quenching cracking. Over-organization could not be resumed, only abandoned. Therefore, in order to avoid the work of the fire occurred.
Third, decarbonization and oxidation
Steel heating, the carbon surface with medium (or atmosphere) of oxygen, hydrogen, carbon dioxide and water vapor, such as the reaction and reduce the surface concentration of carbon called decarbonization, quenched from carbon steel surface hardness, resistance and fatigue strength grinding lower, and formed on the surface of the residual tensile stress cracks formed easily mesh surface.
Heating, the surface of iron and steel and alloy elements and medium (or atmosphere) of oxygen, carbon dioxide, water vapor, such as in the reaction of the oxide film is called oxidation. High temperature (570 degrees above normal) of the workpiece after the dimensional accuracy and surface brightness deterioration of the oxide film hardenability poor quenched steel in the soft easy points.
In order to prevent the oxidation and reduction of CO2 removal measures: surface coatings, stainless steel foil packaging sealing heating, heating furnace used bath, a heated atmosphere (such as the inert gas purification, furnace control carbon potential), flame burner (to a reduction of the gas furnace)
Fourth, hydrogen embrittlement phenomenon
High-strength steel in hydrogen-rich atmosphere, heating and toughness in plastic to reduce the phenomenon known as hydrogen embrittlement. Hydrogen Embrittlement of a workpiece by Hydrogen (such as tempering, aging, etc.) can also eliminate hydrogen embrittlement, a vacuum or inert atmosphere low hydrogen atmosphere heating hydrogen embrittlement can be avoided.
1. Normalizing: steel or steel heated to the critical point AC3 ACM or above the appropriate time to maintain a certain temperature in the air cooling, the types of organizations are pearlite heat treatment process.
2. Annealing: Asia eutectoid steel workpiece is heated to 20-40 degrees above AC3, insulation after a period of time, with the slow cooling furnace (or buried in the sand or lime in the cooling) to 500 degrees in the air-cooled heat treatment process
3. Solution heat treatment: alloy heated to a high temperature thermostat to maintain single-phase area, so that excess phase solid solution to fully dissolved, and then quickly cooling, in order to get a supersaturated solid solution of the heat treatment process
4. Aging: alloy solution heat treatment or cold plastic deformation, at room temperature or slightly above room temperature maintained at the time of his performance change over time phenomenon.
5. Solution Treatment: all of the alloy fully dissolved solid solution strengthening and improving toughness and corrosion resistance, eliminating stress and softening to continue Forming
6. Aging: strengthening phase precipitation in the temperature of heating and insulation, the strengthening phase precipitates can be hardening, strength
7. Quenching: austenitic steel after an appropriate cooling of the cooling rate, the cross-section of the workpiece in whole or in certain areas, such as instability within the organizational structure of martensite changing Heat Treatment Process
8. Tempering: After hardening of the workpiece will be heated to the critical point AC1 following the appropriate time to maintain a certain temperature, and then use to meet the requirements of the cooling method to obtain the necessary structure and properties of heat treatment process
9. Steel carbonitriding: carbonitriding to the steel surface at the same time the carbon and nitrogen into the process. Customary carbonitriding also known as cyanide, the gas temperature in carbonitriding and cryogenic gas carbonitriding applications more widely. Temperature Gas carbonitriding The main purpose is to improve the steel hardness, wear resistance and fatigue strength. Low-temperature gas carbonitriding mainly to nitriding, and its main purpose is to improve the wear resistance of steel and anti-occlusion of.
10. Quenching and tempering: general habits will be quenched and tempered combination of high temperature heat treatment known as quenching and tempering.
Quenching and tempering wide range of important structural parts, especially those in the alternating load the work of the connecting rod, bolts, and other gear and shaft. After quenched and tempered by tempering sorbite organization, and its mechanical properties than the same hardness of normalizing sorbite organizations gifted. It depends on the hardness of the tempering temperature and high temperature and stability and tempered steel workpiece section size, generally in HB200-350 between.
11. Brazing: solder bonding together two types of workpiece Heat Treatment Process
金属热处理基础知识
金属热处理是将金属工件放在一定的介质中加热到适宜的温度，并在此温度中保持一定时间后，然后以不同速度冷却的一种工艺。
金属热处理是机械制造中的重要工艺之一，与其它加工工艺相比，热处理一般不改变工件的形状和整体的化学成分，而是通过改变工件内部的显微组织，或改变工件表面的化学成分，赋予或改善工件的使用性能。其特点是改善工件的内在质量，而这一般不是肉眼所能看到的。
为使金属工件具有所需要的力学性能、物理性能和化学性能，除合理选用材料和各种成形工艺外，热处理工艺往往是必不可少的。钢铁是机械工业中应用最广的材料，钢铁显微组织复杂，可以通过热处理予以控制，所以钢铁的热处理是金属热处理的主要内容。另外，铝、铜、镁、钛等及其合金也都可以通过热处理改变其力学、物理和化学性能，以获得不同的使用性能。
金属热处理工艺
热处理工艺一般包括加热、保温、冷却三个过程，有时只有加热和冷却两个过程。这些过程互相衔接，不可间断。
加热是热处理的重要工序之一。金属热处理的加热方法很多，最早是采用木炭和煤作为热源，进而应用液体和气体燃料。电的应用使加热易于控制，且无环境污染。利用这些热源可以直接加热，也可以通过熔融的盐或金属，以至浮动粒子进行间接加热。
金属加热时，工件暴露在空气中，常常发生氧化、脱碳(即钢铁零件表面碳含量降低)，这对于热处理后零件的表面性能有很不利的影响。因而金属通常应在可控气氛或保护气氛中、熔融盐中和真空中加热，也可用涂料或包装方法进行保护加热。
加热温度是热处理工艺的重要工艺参数之一，选择和控制加热温度 ，是保证热处理质量的主要问题。加热温度随被处理的金属材料和热处理的目的不同而异，但一般都是加热到相变温度以上，以获得高温组织。另外转变需要一定的时间，因此当金属工件表面达到要求的加热温度时，还须在此温度保持一定时间，使内外温度一致，使显微组织转变完全，这段时间称为保温时间。采用高能密度加热和表面热处理时，加热速度极快，一般就没有保温时间，而化学热处理的保温时间往往较长。
冷却也是热处理工艺过程中不可缺少的步骤，冷却方法因工艺不同而不同，主要是控制冷却速度。一般退火的冷却速度最慢，正火的冷却速度较快，淬火的冷却速度更快。但还因钢种不同而有不同的要求，例如空硬钢就可以用正火一样的冷却速度进行淬硬。
金属热处理工艺大体可分为整体热处理、表面热处理和化学热处理三大类。根据加热介质、加热温度和冷却方法的不同，每一大类又可区分为若干不同的热处理工艺。同一种金属采用不同的热处理工艺，可获得不同的组织，从而具有不同的性能。钢铁是工业上应用最广的金属，而且钢铁显微组织也最为复杂，因此钢铁热处理工艺种类繁多。
整体热处理是对工件整体加热，然后以适当的速度冷却，以改变其整体力学性能的金属热处理工艺。钢铁整体热处理大致有退火、正火、淬火和回火四种基本工艺。
退火是将工件加热到适当温度，根据材料和工件尺寸采用不同的保温时间，然后进行缓慢冷却，目的是使金属内部组织达到或接近平衡状态，获得良好的工艺性能和使用性能，或者为进一步淬火作组织准备。正火是将工件加热到适宜的温度后在空气中冷却，正火的效果同退火相似，只是得到的组织更细，常用于改善材料的切削性能，也有时用于对一些要求不高的零件作为最终热处理。
淬火是将工件加热保温后，在水、油或其它无机盐、有机水溶液等淬冷介质中快速冷却。淬火后钢件变硬，但同时变脆。为了降低钢件的脆性，将淬火后的钢件在高于室温而低于650℃的某一适当温度进行长时间的保温，再进行冷却，这种工艺称为回火。退火、正火、淬火、回火是整体热处理中的“四把火”，其中的淬火与回火关系密切，常常配合使用，缺一不可。
“四把火”随着加热温度和冷却方式的不同，又演变出不同的热处理工艺 。为了获得一定的强度和韧性，把淬火和高温回火结合起来的工艺，称为调质。某些合金淬火形成过饱和固溶体后，将其置于室温或稍高的适当温度下保持较长时间，以提高合金的硬度、强度或电性磁性等。这样的热处理工艺称为时效处理。
把压力加工形变与热处理有效而紧密地结合起来进行，使工件获得很好的强度、韧性配合的方法称为形变热处理；在负压气氛或真空中进行的热处理称为真空热处理，它不仅能使工件不氧化，不脱碳，保持处理后工件表面光洁，提高工件的性能，还可以通入渗剂进行化学热处理。
表面热处理是只加热工件表层，以改变其表层力学性能的金属热处理工艺。为了只加热工件表层而不使过多的热量传入工件内部，使用的热源须具有高的能量密度，即在单位面积的工件上给予较大的热能，使工件表层或局部能短时或瞬时达到高温。表面热处理的主要方法有火焰淬火和感应加热热处理，常用的热源有氧乙炔或氧丙烷等火焰、感应电流、激光和电子束等。
化学热处理是通过改变工件表层化学成分、组织和性能的金属热处理工艺。化学热处理与表面热处理不同之处是后者改变了工件表层的化学成分。化学热处理是将工件放在含碳、氮或其它合金元素的介质(气体、液体、固体)中加热，保温较长时间，从而使工件表层渗入碳、氮、硼和铬等元素。渗入元素后，有时还要进行其它热处理工艺如淬火及回火。化学热处理的主要方法有渗碳、渗氮、渗金属。
热处理是机械零件和工模具制造过程中的重要工序之一。大体来说，它可以保证和提高工件的各种性能 ，如耐磨、耐腐蚀等。还可以改善毛坯的组织和应力状态，以利于进行各种冷、热加工。
例如白口铸铁经过长时间退火处理可以获得可锻铸铁，提高塑性 ；齿轮采用正确的热处理工艺，使用寿命可以比不经热处理的齿轮成倍或几十倍地提高；另外，价廉的碳钢通过渗入某些合金元素就具有某些价昂的合金钢性能，可以代替某些耐热钢、不锈钢；工模具则几乎全部需要经过热处理方可使用。
金属材料的机械性能
金属材料的性能一般分为工艺性能和使用性能两类。所谓工艺性能是指机械零件在加工制造过程中，金属材料在所定的冷、热加工条件下表现出来的性能。金属材料工艺性能的好坏，决定了它在制造过程中加工成形的适应能力。由于加工条件不同，要求的工艺性能也就不同，如铸造性能、可焊性、可锻性、热处理性能、切削加工性等。所谓使用性能是指机械零件在使用条件下，金属材料表现出来的性能，它包括机械性能、物理性能、化学性能等。金属材料使用性能的好坏，决定了它的使用范围与使用寿命。
在机械制造业中，一般机械零件都是在常温、常压和非强烈腐蚀性介质中使用的，且在使用过程中各机械零件都将承受不同载荷的作用。金属材料在载荷作用下抵抗破坏的性能，称为机械性能。
金属材料的机械性能是零件的设计和选材时的主要依据。外加载荷性质不同，对金属材料要求的机械性能也将不同。常用的机械性能包括：强度、塑性、硬度、冲击韧性、多次冲击抗力和疲劳极限等。下面将分别讨论各种机械性能。
1． 强度
强度是指金属材料在静荷作用下抵抗破坏的性能。由于载荷的作用方式有拉伸、压缩、弯曲、剪切等形式，所以强度也分为抗拉强度、抗压强度、抗弯强度、抗剪强度等。各种强度间常有一定的联系，使用中一般较多以抗拉强度作为最基本的强度指针。
2． 塑性
塑性是指金属材料在载荷作用下，产生塑性变形而不破坏的能力。
3． 硬度
硬度是衡量金属材料软硬程度的指针。目前生产中测定硬度方法最常用的是压入硬度法，它是用一定几何形状的压头在一定载荷下压入被测试的金属材料表面，根据被压入程度来测定其硬度值。
常用的方法有布氏硬度、洛氏硬度和维氏硬度等方法。
4． 疲劳
前面所讨论的强度、塑性、硬度都是金属在静载荷作用下的机械性能指针。实际上，许多机器零件都是在循环载荷下工作的，在这种条件下零件会产生疲劳。
5． 冲击韧性
以很大速度作用于机件上的载荷称为冲击载荷，金属在冲击载荷作用下抵抗破坏的能力叫做冲击韧性。
退火---淬火---回火
一．退火的种类
1． 完全退火和等温退火
完全退火又称重结晶退火，一般简称为退火，这种退火主要用于亚共析成分的各种碳钢和合金钢的铸，锻件及热轧型材，有时也用于焊接结构。一般常作为一些不重工件的最终热处理，或作为某些工件的预先热处理。
2． 球化退火
球化退火主要用于过共析的碳钢及合金工具钢（如制造刃具，量具，模具所用的钢种）。其主要目的在于降低硬度，改善切削加工性，并为以后淬火作好准备。
3． 去应力退火
去应力退火又称低温退火（或高温回火），这种退火主要用来消除铸件，锻件，焊接件，热轧件，冷拉件等的残余应力。如果这些应力不予消除，将会引起钢件在一定时间以后，或在随后的切削加工过程中产生变形或裂纹。
二．淬火时最常用的冷却介质是盐水，水和油。盐水淬火的工件，容易得到高的硬度和光洁的表面，不容易产生淬不硬的软点，但却易使工件变形严重，甚至发生开裂。而用油作淬火介质只适用于过冷奥氏体的稳定性比较大的一些合金钢或小尺寸的碳钢工件的淬火。
三．钢回火的目的
1． 降低脆性，消除或减少内应力，钢件淬火后存在很大内应力和脆性，如不及时回火往往会使钢件发生变形甚至开裂。
2． 获得工件所要求的机械性能，工件经淬火后硬度高而脆性大，为了满足各种工件的不同性能的要求，可以通过适当回火的配合来调整硬度，减小脆性，得到所需要的韧性，塑性。
3． 稳定工件尺寸
4． 对于退火难以软化的某些合金钢，在淬火（或正火）后常采用高温回火，使钢中碳化物适当聚集，将硬度降低，以利切削加工。
加热缺陷及控制
一、过热现象
我们知道热处理过程中加热过热最易导致奥氏体晶粒的粗大，使零件的机械性能下降。
1.一般过热：加热温度过高或在高温下保温时间过长，引起奥氏体晶粒粗化称为过热。粗大的奥氏体晶粒会导致钢的强韧性降低，脆性转变温度升高，增加淬火时的变形开裂倾向。而导致过热的原因是炉温仪表失控或混料（常为不懂工艺发生的）。过热组织可经退火、正火或多次高温回火后，在正常情况下重新奥氏化使晶粒细化。
2.断口遗传：有过热组织的钢材，重新加热淬火后，虽能使奥氏体晶粒细化，但有时仍出现粗大颗粒状断口。产生断口遗传的理论争议较多，一般认为曾因加热温度过高而使MnS之类的杂物溶入奥氏体并富集于晶接口，而冷却时这些夹杂物又会沿晶接口析出，受冲击时易沿粗大奥氏体晶界断裂。
3.粗大组织的遗传：有粗大马氏体、贝氏体、魏氏体组织的钢件重新奥氏化时，以慢速加热到常规的淬火温度，甚至再低一些，其奥氏体晶粒仍然是粗大的，这种现象称为组织遗传性。要消除粗大组织的遗传性，可采用中间退火或多次高温回火处理。
二、过烧现象
加热温度过高，不仅引起奥氏体晶粒粗大，而且晶界局部出现氧化或熔化，导致晶界弱化，称为过烧。钢过烧后性能严重恶化，淬火时形成龟裂。过烧组织无法恢复，只能报废。因此在工作中要避免过烧的发生。
三、脱碳和氧化
钢在加热时，表层的碳与介质（或气氛）中的氧、氢、二氧化碳及水蒸气等发生反应，降低了表层碳浓度称为脱碳，脱碳钢淬火后表面硬度、疲劳强度及耐磨性降低，而且表面形成残余拉应力易形成表面网状裂纹。
加热时，钢表层的铁及合金与元素与介质（或气氛）中的氧、二氧化碳、水蒸气等发生反应生成氧化物膜的现象称为氧化。高温（一般570度以上）工件氧化后尺寸精度和表面光亮度恶化，具有氧化膜的淬透性差的钢件易出现淬火软点。
为了防止氧化和减少脱碳的措施有：工件表面涂料，用不锈钢箔包装密封加热、采用盐浴炉加热、采用保护气氛加热（如净化后的惰性气体、控制炉内碳势）、火焰燃烧炉（使炉气呈还原性） 。
四、氢脆现象
高强度钢在富氢气氛中加热时出现塑性和韧性降低的现象称为氢脆。出现氢脆的工件通过除氢处理（如回火、时效等）也能消除氢脆，采用真空、低氢气氛或惰性气氛加热可避免氢脆。
1． 正火：将钢材或钢件加热到临界点AC3或ACM以上的适当温度保持一定时间后在空气中冷却，得到珠光体类组织的热处理工艺。
2． 退火：将亚共析钢工件加热至AC3以上20—40度，保温一段时间后，随炉缓慢冷却（或埋在砂中或石灰中冷却）至500度以下在空气中冷却的热处理工艺。
3． 固溶热处理：将合金加热至高温单相区恒温保持，使过剩相充分溶解到固溶体中，然后快速冷却，以得到过饱和固溶体的热处理工艺。
4． 时效：合金经固溶热处理或冷塑性形变后，在室温放置或稍高于室温保持时，其性能随时间而变化的现象。
5． 固溶处理：使合金中各种相充分溶解，强化固溶体并提高韧性及抗蚀性能，消除应力与软化，以便继续加工成型 。
6． 时效处理：在强化相析出的温度加热并保温，使强化相沉淀析出，得以硬化，提高强度 。
7． 淬火：将钢奥氏体化后以适当的冷却速度冷却，使工件在横截面内全部或一定的范围内发生马氏体等不稳定组织结构转变的热处理工艺。
8． 回火：将经过淬火的工件加热到临界点AC1以下的适当温度保持一定时间，随后用符合要求的方法冷却，以获得所需要的组织和性能的热处理工艺
9． 钢的碳氮共渗：碳氮共渗是向钢的表层同时渗入碳和氮的过程。习惯上碳氮共渗又称为氰化，目前以中温气体碳氮共渗和低温气体碳氮共渗应用较为广泛。中温气体碳氮共渗的主要目的是提高钢的硬度，耐磨性和疲劳强度。低温气体碳氮共渗以渗氮为主，其主要目的是提高钢的耐磨性和抗咬合性。
10． 调质处理：一般习惯将淬火加高温回火相结合的热处理称为调质处理。调质处理广泛应用于各种重要的结构零件，特别是那些在交变负荷下工作的连杆、螺栓、齿轮及轴类等。调质处理后得到回火索氏体组织，它的机械性能均比相同硬度的正火索氏体组织为优。它的硬度取决于高温回火温度并与钢的回火稳定性和工件截面尺寸有关，一般在HB200—350之间。
11． 钎焊：用钎料将两种工件粘合在一起的热处理工艺 。