不锈钢精密铸造的回火主要有两个目的:一是消除淬火过程中产生的应力;二足促进淬火后马氏体和残余奥氏体亚稳态结构的分解和转变。
There are two main purposes of tempering in stainless steel investment casting: one is to eliminate the stress produced during quenching; the other is to promote the decomposition and transformation of metastable structure of martensite and retained austenite after quenching.
回火温度低于共析转变温度,通常约为650℃,保持时间不少于2小时。随着回火温度的升高，马氏体将碳从过饱和状态析出，形成分散的细碳化物。马氏体最终会形成块状铁氧体。因此，“不锈钢精密铸造”钢的硬度会随着回火温度的升高而逐渐下降。
Tempering temperature is lower than eutectoid transition temperature, usually about 650 C, and holding time is not less than 2 hours. With the increase of tempering temperature, martensite precipitates carbon from supersaturated state to form dispersed fine carbides. Martensite eventually forms massive ferrite. Therefore, the hardness of "stainless steel precision casting" steel will gradually decrease with the increase of tempering temperature.
回火一个不容忽视的问题是:回火过程中的脆性问题,在300 ℃ ~ 400℃低温区回火脆性,在500 ℃~ 650℃高温回火脆性。当碳原子析出时，在晶界处形成一薄层碳化物时，会产生一种名为低温回火脆性的效应。这种碳化物称为e-碳化物，其结构和组成与碳化物不同，导致冲击韧性降低，称为低温回火脆性。当凹形火持续升温，形成稳定的回火结构时，回火脆性经过二次回火处理后不再出现，故又称“不可逆回火脆性”。在450 ℃ ~ 650℃高温回火脆性的速度与冷,有很多缓慢冷却回火脆性,无回火脆性的快速冷却。高温回火脆性是可逆的，只要去掉回火脆性的钢再加热到600 ℃”(:以上，然后缓慢冷却，仍然出现回火脆性。因此，回火脆性可能与晶界低熔点脆性化合物的析出有关。在某些合金钢中加入钼可以消除回火脆性，即低温回火脆性。专家普遍认为可能是钼与某些合金元素形成复合碳化物，抵消了碳化物的不利影响。
One of the problems that can not be ignored is the brittleness during tempering, tempering brittleness in low temperature range of 300 400 and tempering brittleness in high temperature range of 500 650. When carbon atoms precipitate, a thin layer of carbide is formed at the grain boundary, which produces an effect called low temperature tempering brittleness. This kind of carbide is called e-carbide. Its structure and composition are different from that of carbide, which leads to the decrease of impact toughness, called low temperature tempering brittleness. When concave fire continues to heat up and forms a stable tempering structure, tempering brittleness no longer appears after secondary tempering treatment, so it is also called "irreversible tempering brittleness". The tempering brittleness speed and cooling at 450 ~650 (?) C have many slow cooling tempering brittleness and fast cooling without tempering brittleness. Tempering brittleness at high temperature is reversible. As long as tempering brittleness is removed and reheated to 600 (: above, then slowly cooled, tempering brittleness still occurs. Therefore, tempering brittleness may be related to the precipitation of brittle compounds with low melting point at grain boundaries. Tempering brittleness, i.e. low temperature tempering brittleness, can be eliminated by adding molybdenum to some alloy steels. Experts generally believe that molybdenum and some alloying elements may form composite carbides, which offset the adverse effects of carbides.
根据铸造性能的不同要求，回火温度又分为低温回火、中温回火和高温回火。
According to the different requirements of casting properties, tempering temperature can be divided into low temperature tempering, medium temperature tempering and high temperature tempering.
(1)低温回火回火温度100 ℃ ~ 250℃,用火马氏体的组织,和过饱和析出碳马氏体的一部分,硬质合金马氏体形成。这种钢的含碳量非常小，马氏体中的过饱和碳也很小。因此回火马氏体本身具有良好的韧性，碳化物的析出和稳定进一步提高了钢的强度和硬度。
(1) The cemented carbide martensite is formed by tempering at low temperature from 100 250 and precipitating a part of carbon martensite with the structure of fire martensite and supersaturation. The carbon content of this steel is very small, and the supersaturated carbon in martensite is very small. Therefore, tempered martensite itself has good toughness, and the precipitation and stabilization of carbides further improve the strength and hardness of steel.
(2)中间温度回火温度300 ℃ ~ 500℃回火,回火组织回火屈氏体,具有良好的强度和韧性。该回火工艺广泛应用于低合金钢中。
(2) Tempering at intermediate temperature from 300 ~500 (?) and tempering at tempered structure of Troostite have good strength and toughness. The tempering process is widely used in low alloy steels.
(3)高温回火温度550 ℃ ~ 650℃回火,回火组织分散,细粒度的索氏体。淬火+高温回火工艺称为回火淬炼。适用于低合金钢，多元素强碳化物成型，具有较高的强度和韧性。
(3) The tempered structure is dispersed and the fine-grained sorbite is tempered at 550 650 C at high temperature. Quenching + high temperature tempering process is called tempering quenching. Suitable for low alloy steel, multi-element strong carbide forming, with high strength and toughness.