Spheroidizing annealing is a heat treatment process for spheroidizing carbides in steel to obtain granular pearlite.
Process: Heating to 20 ~ 30 ℃ above Ac1, the holding time should not be too long, generally 2 ~ 4h is suitable, and the cooling method is usually furnace cooling, or a long time isothermal at about 20 ℃ below Ar1.
It is mainly used for eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc. The microstructure of hypereutectoid steel after rolling and forging is lamellar pearlite and reticulated cementite. This structure is hard and brittle, not only difficult to cut, but also easy to deform and crack in the subsequent quenching process. . Spheroidized annealing results in spherical pearlite. In spherical pearlite, fine cementite particles are spherical and dispersed on the ferrite substrate. Compared with lamellar pearlite, spherical pearlite is not only harder and easier to cut, but also harder to austenite grains during quenching and heating, and has less tendency to deform and crack during cooling. If there is a network of cementite in the hypereutectoid steel, it must be eliminated by normalizing process before spheroidizing annealing to ensure that the spheroidizing annealing proceeds normally.
Objective: To reduce hardness, uniform structure, and improve machinability to prepare the structure for quenching. There are many methods of spheroidizing annealing, mainly including:
a) One-time spheroidizing annealing process: The steel is heated to above Ac1 at 20 ~ 30 ° C, held for an appropriate time, and then slowly cooled with the furnace. Before annealing, it is required that the original structure is fine flaky pearlite, and no cementite network is allowed to exist.
b) Isothermal spheroidizing annealing process: After heating and holding the steel, the furnace is cooled to a temperature slightly lower than Ar1 for isothermal (generally 10 ~ 30 ℃ below Ar1). After the isothermal process is completed, the furnace is gradually cooled to about 500 ° C and then air-cooled. It has the advantages of short cycle, uniform spheroidization and easy quality control.
c) reciprocating spheroidizing annealing process.