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The main application and precautions of ultrafine mill and normalizing

Ultrafine mill normalizing is to heat the steel to Ar3 (hypoeutectoid steel) or rrm (hyper-eutectoid steel) 30-500C, and then heat it in air to obtain a heat treatment process of Soxite-based microstructure. . Compared with annealing, the normalizing cooling rate is faster and the transition temperature is lower, so the pseudoeutectoid structure transformation occurs, the amount of pearlite in the structure is increased, the pearlite type structure obtained is fine, and the strength and hardness of the steel are also high. The structure after normalizing is usually a sorbite. For carbon steel with a carbon content of less than 0.6%, there is still some ferrite, and a hypereutectoid carbon steel with a high carbon content precipitates a certain amount of carbide.
The main applications of ultrafine mill normalizing are:
(1) As a pre-heat treatment of low and medium carbon steel, a suitable hardness can be obtained, the machinability can be improved, and the structure for quenching can be prepared.
(2) Eliminating the network of secondary cementite of hypereutectoid steel, preparing for spheroidizing annealing.
(3) Eliminating the defects of superheated microstructures such as hot-rolled Weiss's structure, grain coarseness, etc. in the casting, forging and welding of medium carbon structural steel, refining the grains, uniformizing the structure and eliminating internal stress.
(4) As the final heat treatment of ordinary structural parts, to achieve a certain mechanical properties, in some cases can be used instead of quenching and tempering treatment.
In summary, the ultrafine mill annealing and normalizing purposes are similar, and the choice between them can be considered from the following aspects.
(1) Machinability. In general, the hardness of steel is HB170-230, and when there is no large piece of ferrite in the structure, the machinability is good. Therefore, it is preferable to use normalizing for low- and medium-carbon steels; high-carbon structural steels and tool steels, and medium-carbon alloy steels containing more alloying elements, and annealing is preferred.
(2) Performance. For ordinary structural parts that are not required for performance and then quenched and tempered, it is often possible to use normalizing to improve mechanical properties. However, if a part with a complicated shape or a large casting is used, there is a risk of normalizing deformation and cracking, and annealing is used. Just as considering the tendency to reduce quenching and cracking, normalizing is not as good as annealing.
(3) Economics. Normalizing has a shorter production cycle than annealing, high equipment utilization, energy saving, time saving, and easy operation. Therefore, if possible, normalizing is preferred.

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