What’s the difference between normalizing and tempering in steel heat treatment?

　　Difference between normalizing and tempering in steel heat treatment

　　The purpose of normalizing is similar to complete annealing for hypoeutectoid steel, but due to the rapid cooling rate of double annealing, the number of pearlite in the steel after normalizing is slightly more than that of annealing, and the thickness of the sheet is thin. The strength and hardness are higher than those of annealing. For hypereutectoid steels, normalizing is only to eliminate reticulated cementite and improve the mechanical properties of the steel. Tempering is mainly to eliminate stress, stabilize the structure and size of steel parts, and achieve the required performance of parts. For large-section parts: It has the following effects-and it is very important:

　　1) For parts such as motor shafts, press containers, etc. (the material is generally carbon steel or low alloy structural steel), normalizing + high temperature tempering is the final heat treatment. Therefore, timely tempering after normalization can effectively stabilize the organization And size to further eliminate the internal stress of the workpiece, which has a significant effect on improving the comprehensive mechanical properties;

　　2) For large-scale forgings, high-temperature tempering in time after normalizing (usually adding a subcooling in the middle) can not only achieve the purpose described above, but also play an important role in hydrogen diffusion, especially for high hardenability steel (Hydrogen is a factor that produces white spots), and large white forgings are generally scrapped if white spots are generated inside. Therefore, normalizing + high temperature tempering of important parts (or special materials) is very necessary. When materials with good hardenability are normalized, high temperature tempering is required, because after normalizing, materials with good hardenability have higher hardness, such as: 20Cr2Ni4, 18Cr2Ni4W, etc. A high temperature tempering is required. Main steel grades: T2 T11 T12 T22 T23 T91 2Cr13, 15CrMo, 15CrMoG, 12Cr1MoVG, R102 T92

　　Boiler steel requires normalizing and tempering. Tempering is mainly to stabilize the structure so that it can have certain high-temperature durability, creep strength, and durability at high temperature and pressure. In fact, tempering after normalizing is a very common thing in foreign countries. On the contrary, not tempering after normalizing is a special case. In order to reduce production costs in China, half of the performance is not tempered under normalizing conditions.

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