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。
电热管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。
Heat treatment process generally includes three heating, heat preservation, cooling process, sometimes only two heating and cooling process. The process to connect to each other, completely. Heating is one of the important process of heat treatment. Heating of metal heat treatment method are many, the first is to use charcoal and coal as a heat source, and application of liquid and gas fuel. The application of electric heating is easy to control, and no environmental pollution. The heat source can be used to direct heating, but can be by molten salt or metal, and floating particles with indirect heating.
Metal heating workpiece when it is exposed to the air, often occur oxidation and decarburization (that is, the surface carbon content lower) steel parts, the performance on the surface of the parts after heat treatment have a bad effect. Thus metals usually should be in a controlled atmosphere or protective atmosphere, molten salt and a vacuum heating, coating or packing method to protect the heating is also available.
The heating temperature is one of the important process parameters of heat treatment process, selection and control of heating temperature, is a major problem to ensure the quality of heat treatment. Heating temperature is processed and varies the purpose of the metal material and heat treatment, but is usually heated to above phase transition temperature, in order to obtain high temperature. Another change will take time, so when the heating temperature of metal surface is achieved, still must in the temperature keeping time, make internal and external temperature, the microstructure change completely, this paragraph of time is called the heat preservation time. Heating with high energy density and surface heat treatment and heating speed, generally there is no heat preservation time, and the chemical heat treatment of heat preservation time tend to be long.
Cooling is indispensable in the process of heat treatment process steps, cooling method for craft different and different, main is to control the cooling rate. General annealing cooling speed of the slowest, normalizing the cooling speed faster, quenching cooling speed is faster. But also with different steel grade have different requirements, such as empty hard steel can be hardened by normalizing the same cooling rate.
Metal heat treatment process can be divided into the overall heat treatment, surface heat treatment and chemical heat treatment three categories. According to the heating medium, heating temperature and cooling method is different, each categories and can be divided into a number of different heat treatment process. The same kind of metal in different heat treatment process, can obtain different organizations, which have different performance. Steel is one of the most widely used industrial metals, and the microstructure of iron and steel is the most complex, therefore there are many different kinds of steel heat treatment process.
The overall heat treatment is heated for artifacts as a whole, and then cooling at the appropriate speed, to change its overall mechanical properties of metal heat treatment process. Steel overall roughly annealing heat treatment, the four basic normalizing, quenching and tempering process.
Annealing is the workpiece of heating to proper temperature, according to the material and workpiece size in different heat preservation time, and then slowly cooling, the purpose is to make the metal internal organization at or near equilibrium state and obtain good process performance and use of performance, or for further quenching organization. Normalizing is after the workpiece heated to proper temperature cooling in the air, the effect of normalizing and annealing are similar, just get the organization of a finer, commonly used to improve the cutting performance of material, also sometimes used for low requirements for some parts as the final heat treatment.
After quenching is the workpiece heating temperature, water, oil or other inorganic salts, organic aqueous solution fast cooling quenching cooling medium. Hardened steel after quenching, but at the same time become fragile. In order to reduce the brittleness of steel after quenching of steel is higher than the room temperature and under 650 ℃ at some stage in the proper temperature for a long time of heat preservation, again for cooling, this process is called tempering. Annealing, normalizing, quenching and tempering is the overall heat treatment of the "four" fire, closely related to the quenching and tempering, often, be short of one cannot.
"Four fire" as the heating temperature and cooling methods, and developed the different heat treatment process. In order to obtain a certain amount of strength and toughness, combine quenching and high temperature tempering process, known as conditioning. Some alloy after quenching formation of supersaturated solid solution, put it at room temperature or slightly higher under proper temperature to keep a long time, in order to improve the hardness and strength of the alloy or electrical magnetic, etc. The heat treatment process called aging treatment.
The pressure deformation and heat treatment processing effective and combined closely, obtain good strength and toughness of components with method called thermomechanical treatment; In negative pressure or vacuum atmosphere heat treatment is called vacuum heat treatment, it can not only make the workpiece no oxidation, no decarburization, keeping the surface is bright and clean after processing, improve the performance of the workpiece can also lead to infiltration agent for chemical heat treatment.
Surface heat treatment is the only heating workpiece surface to change the surface mechanical properties of metal heat treatment process. For heating the workpiece surface and not only make excessive internal heat into the workpiece, using the heat source must have a high energy density, the larger on unit area of the workpiece and the heat, or local can short the surface layers of components or instantaneous at high temperature. The main method of surface heat treatment has the flame hardening and induction heating heat treatment, the common source of heat oxygen acetylene and oxygen propane flame, induced current, laser and electron beam, etc.
Chemical heat treatment is by changing the workpiece surface chemical composition, microstructure and mechanical properties of metal heat treatment process. The chemical heat treatment and surface heat treatment difference is that the latter has changed the chemical composition of workpiece surface. Chemical heat treatment is artifacts in a medium containing carbon, nitrogen and other alloying elements (gas, liquid and solid) in heating, heat preservation for a long time, so that the workpiece surface infiltration of carbon, nitrogen, boron and chromium element. After the infiltration of elements, sometimes even on other heat treatment such as quenching and tempering. Chemical heat treatment of the main methods of carburizing, nitriding, metallic cementation.
Heat treatment is the mechanical parts and one of the important working procedure in process of mould manufacturing. In general, it can ensure and improve the properties of the workpiece, such as wear resistance, corrosion resistance, etc. Can also improve the organization and the stress state of blank, for all kinds of cold and hot working.
Such as white cast iron after annealing treatment for a long time can get the malleable iron, improve plasticity; The service life of the gear are the correct heat treatment process, can be better than without heat treatment of gear exponentially or dozens of times to improve; In addition, the low price of carbon steel through the infiltration of some alloying elements will have a certain price, performance of alloy steel, can replace some heat resistant steel, stainless steel mold is almost all need after heat treatment before use.
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