How does a annealing furnace perform low temperature annealing?

Low temperature annealing

The heat treatment process in which the steel is heated to a temperature slightly lower than that of Acl, kept for a certain time, and then cooled slowly is called low temperature annealing. Low temperature annealing, because there is no crystallization process, cannot refine the grain and structure of the steel, but it can eliminate or reduce the internal stress in the steel and reduce the hardness, thus improving the cutting performance. Low temperature annealing has short heating time, low cost and less loss of oxidation and decarburization on steel surface, so it can replace complete annealing or incomplete annealing in some cases.

Recrystallization annealing in annealing furnace, intermediate annealing (softening annealing) and stress relieving annealing all belong to the category of low temperature annealing.

Low temperature annealing of steel ingot

Low temperature annealing can reduce the hardness of steel ingot, eliminate or reduce the as-cast stress, especially suitable for high hardenability, using ordinary (full annealing and full annealing) not easy to make its softening annealing steel mouth high hardenability steel grade in annealing in full, although very slow cooling, the surface of the steel ingot is inevitably occurred bainite and martensite transformation, not only can't eliminate stress, more is not easy to surface cleaning. Low temperature annealing is easy to take effect.

The low temperature annealing heating temperature of steel ingot is Acl, below 20 -- 30C. Theoretically, the closer the selected temperature is to THE Acl, the better the effect will be. However, in actual production, due to the fluctuation of temperature, the temperature is slightly higher than the Acl, resulting in partial austenite, which makes the ingot surface hardness high and fails to achieve the purpose of softening. Therefore, the ingot lengths of all kinds of structural steel, spring steel and die steel are 680 -- 710C, and those of high alloy steel are 640 -- 680C. The heating rate of ingot in low temperature annealing is 100-200c /h. The cooling rate of annealing furnace can not be controlled generally, but it should not be too fast, so as not to produce new internal stress.