The cooling rate of the annealing furnace from the self-annealing temperature to the isothermal temperature can be arbitrarily set, and two furnaces are commonly used in the production. The isothermal temperature of the annealing furnace is generally 10-30 degrees Celsius below Ar1, which is the pearlite transformation temperature. The cooling after the conversion can be arbitrary.
There are two ways to retreat the annealing furnace, one is the ordinary ball retreat, and the other is the retraction of the ball.
Ordinary ball retreat: The steel is heated to a slightly higher temperature with Ac1 (generally Ac1 + 20-30 degrees Celsius), slowly cooled to below 500 degrees Celsius at 20-50 degrees Celsius/h after insulation, and air cooled.
Isothermal ball retreat: After heating at the same heating temperature as the ordinary ball retreat, the furnace is cooled to a temperature slightly lower than Ar1 for isothermal, and the oven is air-cooled after the isothermal finish.
Cycle retreat: It is alternately heated and cooled above Ac1, which is equivalent to multiple cold isothermal retreats.
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