1. What is normalizing?
Normalizing is a heat treatment to improve the toughness of steel. After heating the steel component to 30~50℃ above the Ac3 temperature, keep it warm for a period of time and then air cool it out of the furnace. The main feature is that the cooling speed is faster than annealing but lower than quenching. During normalizing, the crystal grains of the steel can be refined in a slightly faster cooling, which can not only obtain satisfactory strength, but also significantly improve the toughness (AKV value) and reduce the cracking tendency of the component. After normalizing, the comprehensive mechanical properties of some low-alloy hot-rolled steel plates, low-alloy steel forgings and castings can be greatly improved, and the cutting performance is also improved.

2. Normalizing has the following purposes and uses:
① For hypoeutectoid steel, normalizing is used to eliminate the overheated coarse-grained structure and Widmanstatten structure of castings, forgings and weldments, and the banded structure in rolled materials; refine the grains; and can be used as a pre-heat treatment before quenching.
② For hypereutectoid steel, normalizing can eliminate the network secondary cementite and refine the pearlite, which not only improves the mechanical properties, but also facilitates the subsequent spheroidizing annealing.
③ For low-carbon deep-drawn thin steel plates, normalizing can eliminate the free cementite at the grain boundary to improve its deep-drawing performance.
④ For low-carbon steel and low-carbon low-alloy steel, normalizing can obtain more fine-flaky pearlite structure, increase the hardness to HB140-190, avoid the “sticking knife” phenomenon during cutting, and improve the machinability. For medium-carbon steel, in situations where both normalizing and annealing can be used, normalizing is more economical and convenient.
⑤ For ordinary medium-carbon structural steel, in situations where the mechanical properties are not required to be high, normalizing can be used instead of quenching and high-temperature tempering, which is not only easy to operate, but also makes the structure and size of the steel stable.
⑥ High temperature normalizing (150-200℃ above Ac3) can reduce the composition segregation of castings and forgings due to the high diffusion rate at high temperature. The coarse grains after high temperature normalizing can be refined by the subsequent second normalizing at a lower temperature.
⑦ For some low and medium carbon alloy steels used in steam turbines and boilers, normalizing is often used to obtain bainite structure, and then high temperature tempering, which has good creep resistance when used at 400-550℃.
⑧ In addition to steel parts and steel products, normalizing is also widely used in the heat treatment of ductile iron to obtain pearlite matrix and improve the strength of ductile iron.
Since the characteristic of normalizing is air cooling, the ambient temperature, stacking method, airflow and workpiece size have an impact on the structure and performance after normalizing. Normalizing structure can also be used as a classification method for alloy steel. Usually, according to the structure obtained by heating a sample with a diameter of 25 mm to 900℃ and air cooling, alloy steel is divided into pearlite steel, bainite steel, martensite steel and austenite steel.