Several Common Annealing Process Purposes and Applications (CHAPTER TWO)

 

3.

Isothermal annealing

Isothermal annealing is to heat the steel to a critical temperature (hypereutectoid Ac1 or hypoeutectoid Ac3) above austenitizing, and then move the steel into another temperature slightly lower than Ar1 in the furnace isothermal stay, not too high and not too low. Too high isothermal time is too long, and the hardness is low; Too low and high hardness. The principle is to choose a lower isothermal temperature as far as possible under the condition of ensuring qualified hardness, in order to shorten the isothermal time and improve labor productivity. When the transformation is complete, the oven is air-cooled to room temperature.

The transformation is easy to control during isothermal annealing, and it is more suitable for alloy steels with high stability of supercooled austenite, which can save the time of steel parts in the furnace and improve the turnover rate of annealing furnace.

4.

Spheroidizing annealing

Spheroidizing annealing is a heat treatment process to spheroidize carbides in steel and obtain granular pearlite, which is mainly used in hypereutectoid steels, such as carbon tool steel, low alloy tool steel and ball bearing steel.

The purpose of spheroidizing annealing is to reduce hardness, improve cutting performance, and obtain a uniform structure, and prepare the structure for the final quenching treatment. Its heating temperature range is generally 20-30℃ above Ac1 after spheroidizing annealing of the advantages of the structure:

(1) From sheet to granular pearlite, reduce hardness, improve cutting performance.

(2) When granular pearlite is heated, austenite grains are not easy to grow, allowing a wide quenching temperature range, and the deformation and cracking tendency are small during quenching, that is, the quenching process performance is good.

(3) The best quenched structure can be obtained, that is, the martensite sheet is small, the residual austenite volume is small, and a certain amount of uniformly distributed granular carbide is retained.

In addition, the steel with obvious network carbide structure must be normalized to eliminate the carbide network and then spheroidized annealing.

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