① For hypoeutectoid steel, normalizing is used to eliminate the overheated coarse grain structure and Widmanstatten structure of castings, forgings, and weldments, and the banded structure in rolled materials; to refine the grains; and as a preheat treatment before quenching.
② For hypereutectoid steel, normalizing can eliminate the reticular secondary cementite and refine the pearlite, which not only improves the mechanical properties, but also facilitates subsequent spheroidizing annealing.
③ For low carbon deep drawing thin steel plates, normalizing can eliminate free cementite at the grain boundaries to improve its deep drawing performance.
④ For low carbon steel and low carbon low alloy steel, normalizing can be used to obtain more fine flake pearlite structures, increasing the hardness to HB140-190, avoiding the “sticking” phenomenon during cutting, and improving the cutting processability. . For medium carbon steel, when both normalizing and annealing can be used, normalizing is more economical and convenient.
⑤ For ordinary medium-carbon structural steel, when the mechanical properties are not high, normalizing can be used instead of quenching and high-temperature tempering. This is not only easy to operate, but also stabilizes the structure and size of the steel.
⑥ High-temperature normalizing (150~200℃ above Ac3) can reduce component segregation of castings and forgings due to the high diffusion rate at high temperatures. Coarse grains after normalizing at high temperature can be refined by subsequent 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 tempered at high temperature to have good creep resistance when used at 400 to 550°C.
⑧ In addition to steel parts and steel products, normalizing is also widely used in the heat treatment of ductile iron to obtain a pearlite matrix and improve the strength of ductile iron.
Since normalizing is characterized by air cooling, the ambient temperature, stacking method, air flow and workpiece size all have an impact on the structure and performance after normalizing. Normalized structure can also be used as a classification method for alloy steel. Usually, alloy steel is divided into pearlitic steel, bainitic steel, martensite steel and austenitic steel based on the structure obtained by air cooling after heating a 25 mm diameter sample to 900°C.
Annealing is a metal heat treatment process that slowly heats metal to a certain temperature, maintains it for a sufficient time, and then cools it at an appropriate speed. Annealing heat treatment is divided into complete annealing, incomplete annealing and stress relief annealing. The mechanical properties of annealed materials can be tested by tensile testing or hardness testing. Many steels are supplied in an annealed and heat-treated state. The hardness of steel can be tested using a Rockwell hardness tester to test the HRB hardness. For thinner steel plates, steel strips and thin-walled steel pipes, a surface Rockwell hardness tester can be used to test the HRT hardness.