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What is the difference between fully annealed and incompletely annealed

Quenching may have been heard in daily life, and we may know about annealing, but it may be awkward for incomplete annealing. Then talk about the meaning of not completely annealing.

What is the difference between fully annealed and incompletely annealed?

Take the hypoeutectoid steel as an example:

Fully annealed

The hypoeutectic steel is heated to a temperature of 20 to 30 ° C above Ac3 for complete austenitization. When the heat is sufficient, the furnace is slowly cooled to obtain a nearly balanced structure. This heat treatment process is called complete annealing. After the hypoeutectic steel is completely annealed, the integrated structure of ferrite + pearlite is obtained, and the eutectoid steel is annealed to obtain a sheet-like pearlite structure.

Ingots and cast steels that have been cast and cooled in steel ingot molds, or forged steels with too high aging temperature, tend to have coarse grains, easy to obtain Wei's structure, and have residual internal stress. Through complete annealing to refine the grains, uniform structure, eliminate internal stress, reduce hardness, facilitate cutting, and prepare the quenched structure for some machined parts.

Hypereutectoid carbon steel or hypereutectoid alloy steel should not be completely annealed because the hypereutectoid steel is heated to a single-phase austenite region above Accm, completely austenitized, and precipitated twice after slow cooling. Cementite or alloy carbides reduce the strength, norm and toughness of steel.

Incomplete annealing

The hypoeutectic steel is heated between Ac1 and Ac3 or in the two-phase region of the hypereutectoid steel between Ac1 and Accm, and is kept for a sufficient time to perform a slow cooling heat treatment process, which is called incomplete annealing.

Incomplete annealing, also called incomplete crystallization annealing, refers to the process of heating the steel to 30 to 50 ° C above Ac1, holding it for a sufficient time, and then slowly cooling with the furnace. Since the incomplete annealing is heated in the two-phase region, the microstructure cannot be completely recrystallized, the morphology, size and distribution of the ferrite cannot be changed, and the grain refining effect is not as good as the complete annealing. Therefore, incomplete annealing is mainly used for the grain is not roughened, the ferrite distribution is normal, but the forging, rolling termination temperature is too low, or the sub-eutectoid steel is cooled too fast to reduce the hardness, fire internal stress, improve organization.

If the forging end temperature of the hypoeutectoid steel is appropriate, the grain coarsening is not caused, and the distribution of ferrite and pearlite is not abnormal. Partial recrystallization can be performed by incomplete annealing to refine the grain. Improve tissue, reduce hardness and eliminate internal stress. The incomplete annealing temperature of the hypoeutectoid steel is generally 740-780 ° C, which has the advantages of low heating temperature, easy operation, energy saving, reduced consumption, and improved productivity, and thus is more common than full annealing application.
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