1. What is normalizing?
Normalizing is a heat treatment that improves the toughness of steel.After heating the steel components to 30 ~ 50℃ above the Ac3 temperature, the steel components shall be kept warm for a period of time to be air-cooled.The main characteristic is that the cooling speed is faster than annealing but lower than quenching. During normalizing, the crystallized grain of steel can be refined in slightly faster cooling, which not only achieves satisfactory strength, but also significantly improves toughness (AKV value) and reduces cracking tendency of components.After normalizing some low-alloy hot rolled steel and low-alloy steel forgings and castings, the comprehensive mechanical properties of the materials can be greatly improved and the cutting properties can also be improved.
2. Normalizing has the following purposes and USES:
(1) on hypoeutectoid steel, normalizing to eliminate casting, forging, welding parts of the superheated coarse crystal structure and weijin-structure, rolling strip structure;Fine grain;And can be used as pre-heat treatment before quenching.
(2) on hypereutectoid steel, normalizing can eliminate the network secondary cementite, and make pearlite refining, not only improve the mechanical properties, and is beneficial to the spheroidization annealing.
(3) on the low carbon deep drawing sheet, normalizing can eliminate the grain boundary free cementite, in order to improve its deep drawing performance.
(4) on low carbon steel and low carbon low alloy steel, the use of normalizing, can get more thin sheet pearlite structure, hardness increased to hb140-190, to avoid cutting “stick knife” phenomenon, improve the machinability.For medium carbon steel, normalizing is more economical and convenient when both normalizing and annealing can be used.
(5) for ordinary medium carbon structural steel, where the mechanical properties are not high, normalizing can be used instead of quenching and tempering at high temperature, which is not only easy to operate, but also makes the structure and size of the steel stable.
6 high temperature normalizing (Ac3 above 150 ~ 200℃) due to high temperature diffusion speed is higher, can reduce the composition of the casting and forging segregation.The 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 steel used for steam turbine and boiler, often used normalizing to obtain bainite structure, and then by high temperature tempering, used for 400 ~ 550℃ has a good creep resistance.
8 in addition to steel and steel, normalizing is also widely used in ductile iron heat treatment, make it obtain pearlite matrix, improve the strength of ductile iron.
Since normalizing is characterized by air cooling, ambient temperature, stacking mode, 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 steels.Alloy steels are usually divided into pearlite, bainite, martensite and austenite steels according to the structure obtained by air cooling after heating 25 mm diameter specimens to 900℃.
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