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回火的种类及应用根据工件性能要求的不同，按其回火温度的不同，可将回火分为以下几种：
（一）低温回火（150－250度）
低温回火所得组织为回火马氏体。其目的是在保持淬火钢的高硬度和高耐磨性的前提下，降低其淬火内应力和脆性，以免使用时崩裂或过早损坏。它主要用于各种高碳的切削刃具，量具，冷冲模具，滚动轴承rolling bearing以及渗碳件carburization等，回火后硬度一般为HRC58－64。
（二）中温回火（350－500度）
中温回火所得组织为回火屈氏体。其目的是获得高的屈服强度，弹性极限和较高的韧性。因此，它主要用于各种弹簧和热作模具的处理，回火后硬度一般为HRC35－50。
（三）高温回火（500－650度）
高温回火所得组织为回火索氏体。习惯上将淬火加高温回火相结合的热处理称为调质处理，其目的是获得强度，硬度和塑性，韧性都较好的综合机械性能。因此，广泛用于汽车，拖拉机，机床等的重要结构零件，如连杆，螺栓，齿轮及轴类。回火后硬度一般为HB200－330。
热处理
（ 1）：退火：指金属材料加热到适当的温度，保持一定的时间，然后缓慢冷却的热处理工 艺。常见的退火工艺有：再结晶退火，去应力退火，球化退火，完全退火等。退火的目的： 主要是降低金属材料的硬度，提高塑性，以利切削加工或压力加工，减少残余应力，提高组 织和成分的均匀化，或为后道热处理作好组织准备等。
（2 ）：正火：指将钢材或钢件加热到 或 （钢的上临界点温度）以上，30～50℃ 保持适当时间后，在静止的空气中冷却的热处理的工艺。正火的目的：主要是提高低碳钢的 力学性能，改善切削加工性，细化晶粒，消除组织缺陷，为后道热处理作好组织准备等。
（3）：淬火：指将钢件加热到 Ac3 或 Ac1（钢的下临界点温度）以上某一温度，保持一 定的时间，然后以适当的冷却速度，获得马氏体（或贝氏体）组织的热处理工艺。常见的淬 火工艺有盐浴淬火，马氏体分级淬火，贝氏体等温淬火，表面淬火和局部淬火等。淬火的目 的：使钢件获得所需的马氏体组织，提高工件workpiece的硬度，强度strength和耐磨性，为后道热处理作好组 织准备等。
（4）：回火：指钢件经淬硬后，再加热到 以下的某一温度，保温一定时间，然后冷 却到室温的热处理工艺。常见的回火工艺有：低温回火，中温回火，高温回火和多次回火等。
回火的目的：主要是消除钢件在淬火时所产生的应力，使钢件具有高的硬度和耐磨性wear resistance外，并 具有所需要的塑性plastic property和韧性等。
（5）：调质：指将钢材或钢件进行淬火及高温回火的复合热处理工艺。使用于调质处理的钢称调质钢。它一般是指中碳结构钢和中碳合金结构钢。
（6）：渗碳：渗碳是指使碳原子渗入到钢表面层的过程。也是使低碳钢的工件具有高碳钢的表面层，再经过淬火和低温回火，使工件的表面层具有高硬度和耐磨性，而工件的中心部分仍然保持着低碳钢的韧性和塑性。
金属热处理是机械制造中的重要工艺之一，与其他加工工艺相比，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。
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---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。
Tempering（回火） is a process done subsequent （后来的）to quench（淬火冷却）hardening. Quench-hardened parts are often too brittle（易碎的）. This brittleness is caused by a predominance（优势）of Martensite硬化铁炭. This brittleness is removed by tempering. Tempering results in a desired combination of hardness, ductility延性, toughness韧性, strength, and structural stability. Tempering is not to be confused with tempers on rolled stock-these tempers are an indication of the degree of cold work performed.
Heat Treatment is the controlled heating and cooling of metals to alter改变their physical and mechanical机械的 properties属性without changing the product shape. Heat treatment is sometimes done inadvertently不注意地 | 疏忽地due to manufacturing processes that either heat or cool the metal such as welding焊接or forming形成(成形法,模锻)|.
Heat Treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability可制造性,工艺性objectives具体目标 | 目标 | 目的 | 目標 such as improve machining, improve formability成形性, restore恢复ductility 延展性 after a cold working operation. Thus it is a very enabling manufacturing process that can not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics.
Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the commercial use of steels exceeds that of any other material. Steels are heat treated for one of the following reasons:
1. Softening
2. Hardening
3. Material Modification
Full annealing完全退火 is the process of slowly raising the temperature about 50 ºC (90 ºF) above the Austenitic奥氏体的temperature line A3 or line ACM in the case of Hypoeutectoid低易融质steels (steels with < 0.77% Carbon) and 50 ºC (90 ºF) into the Austenite-Cementite region in the case of Hypereutectoid steels (steels with > 0.77% Carbon).
It is held at this temperature for sufficient time for all the material to transform into Austenite or Austenite-Cementite as the case may be. It is then slowly cooled at the rate of about 20 ºC/hr (36 ºF/hr) in a furnace to about 50 ºC (90 ºF) into the Ferrite-Cementite range. At this point, it can be cooled in room temperature air with natural convection.
The grain structure has coarse Pearlite with ferrite or Cementite (depending on whether hypo or hyper eutectoid). The steel becomes soft and ductile.
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Normalizing
Normalizing is the process of raising the temperature to over 60 º C (108 ºF), above line A3 or line ACM fully into the Austenite range. It is held at this temperature to fully convert the structure into Austenite, and then removed form the furnace and cooled at room temperature under natural convection. This results in a grain structure of fine Pearlite with excess of Ferrite or Cementite. The resulting material is soft; the degree of softness depends on the actual ambient conditions of cooling. This process is considerably cheaper than full annealing since there is not the added cost of controlled furnace cooling.
The main difference between full annealing and normalizing is that fully annealed parts are uniform in softness (and machinablilty) throughout the entire part; since the entire part is exposed to the controlled furnace cooling. In the case of the normalized part, depending on the part geometry, the cooling is non-uniform resulting in non-uniform material properties across the part. This may not be desirable if further machining is desired, since it makes the machining job somewhat unpredictable. In such a case it is better to do full annealing.
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Process Annealing
Process Annealing is used to treat work-hardened parts made out of low-Carbon steels (< 0.25% Carbon). This allows the parts to be soft enough to undergo further cold working without fracturing. Process annealing is done by raising the temperature to just below the Ferrite-Austenite region, line A1on the diagram. This temperature is about 727 ºC (1341 ºF) so heating it to about 700 ºC (1292 ºF) should suffice. This is held long enough to allow recrystallization of the ferrite phase, and then cooled in still air. Since the material stays in the same phase through out the process, the only change that occurs is the size, shape and distribution of the grain structure. This process is cheaper than either full annealing or normalizing since the material is not heated to a very high temperature or cooled in a furnace.
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Stress Relief Annealing
Stress Relief Anneal is used to reduce residual stresses in large castings, welded parts and cold-formed parts. Such parts tend to have stresses due to thermal cycling or work hardening. Parts are heated to temperatures of up to 600 - 650 ºC (1112 - 1202 ºF), and held for an extended time (about 1 hour or more) and then slowly cooled in still air.
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Spheroidization
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沙发，俺自己做了，(*^__^*) 嘻嘻……
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