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钢的淬火
淬火是亚共析刚的温度加热AC3线以上30到50℃或者是过共析钢的温度加热到ACm线以上30到50℃，保持直到相变完成，然后以一定的速度冷却，高于临界冷却速度。像这样的冷却被称为淬火冷却；碳钢通常在水中冷却，而合金钢在油或其他媒体中冷却。淬火并不是最终热处理。由于淬火后它必须通过回火去减少脆性和应力，并获得它所需要的力学性能。
工具钢的淬火和回火主要是增加其硬度，耐磨性和强度，以及钢的结构，去增加其强度和硬度，并获得足够高的塑性和韧性，以及为各种机械部件，提供高的耐磨性。
亚共析钢应加热到AC3线以上30到50℃。这种由珠光体和铁素体组成的原始组织转变成单一奥氏体，次奥氏体在超过临界冷却速度的条件下冷却，组织将转变成马氏体。
过共析钢是加热到ACM线以上的淬火温度。在这个温度下，奥氏体形成，但二次渗碳体有一定量的保留，因此。淬火后，钢的组织由马氏体和未溶碳化物颗粒构成，这是很难的。因此，淬火钢具有高硬度。
加热时间应该足够加热工作在整个横截面，使铁碳相图能够被完成，但不应时间过长从而导致钢表面层的晶粒生长和脱碳。
淬火临界冷却速度应该能够得到马氏体结构，并在所给命令的横截面的限度内，不应造成淬火缺陷：裂纹失真，扭曲和高的拉伸残余应力在表面层。为了抑制过冷奥氏体在珠光体区域及中间转变区域发生分解，最符合要求的冷却速度就是在A1到Ms点之间具有高的冷却速度，以及在Ms到Mf温度范围具有较低的马氏体转变速度。在马氏体的温度范围内的高的冷却速率是不可取的，因为它导致了残余应力水平的急剧增加，甚至裂纹的形成。同时在从MS到Mf的温度范围内过于缓慢的冷却，可能会导致粒子回火是的马氏体和残余奥氏体的大量增加，由于了它的稳定从而降低了钢的硬度。
Hardening of Steel
Hardening consists in heating to a temperature from 30 to 50℃above point AC3 for hypoeutectoid steels or above point Acm for hypereutectoid steels, holding until the phase transformations are completed and then cooling at a rate above the critical. Such cooling is called quenching; carbon steels are commonly quenched in water, alloy steels, in oil or other media. Hardening by quenching is not a final heat-treatment. It must be followed by tempering to reduce brittleness and stress due to hardening, and obtain the required mechanical properties.
Tool steel is hardened and tempered mainly to increase its hardness, wear resistance and strength and structural steel, to increase its strength and hardness, and to obtain sufficiently high ductility and toughness, and for many kinds of machine components, to provide high wear resistance as well
Hypoeutectoid steels should be heated to a temperature 30 to 50℃higher than point AC3.This transforms the initial structure, consisting of pearlite and ferrite, into one of austenite which, upon cooling at a rate above the critical value, is transformed into martensite
Hypereutectoid steels are heated in hardening to a temperature somewhat above Acm. At this temperature, austenite is formed, but a certain amount of secondary cementite is retained therefore. After quenching, the structure of the steel consists of martensite and undissolved carbon carbide particles, which are very hard. Thus, the hardened steel possesses high hardness.
The heating time should be sufficient to heat the work throughout its cross section and to enable the phase transformations to be completed, but should not be so long that it leads to grain growth and decarburization of the surface layers of steel.
Quenching in hardening should enable a martensite structure to be obtained .within the limits of the given cross section of the word and should not cause hardening defects: cracks distortion, warping and high tensile residual stresses in the surface layers. It is most desirable to provide a high cooling rate in the temperature range from Ai to Ms to suppress super-cooled austenite decomposition in the regions of the pearlite and intermediate transformations. and slower cooling in the temperature range of martensite transformation, from Ms to Mf. A high cooling rate in the martensite temperature range is undesirable because it leads to a drastic increase in the level of residual stress and even to crack formation. At the same time excessively slow cooling in the temperature range from Ms to Mf may lead to particle tempering of the martensite and to an increase in the amount of retained austenite ,owing to its stabilization. this lowers the hardness of the steel.