Spheroidizing annealing is an annealing process performed by spheroidizing carbides in steel balls. The steel balls are heated to 750 degrees, held for a while, and then slowly cooled to obtain a spherical or granular carbide structure uniformly distributed on the ferrite matrix.
The structure obtained by air cooling of the steel ball is lamellar pearlite and reticulated cementite. This kind of structure is hard and brittle, which is not only difficult to be machined, but also easily deformed and cracked during the subsequent quenching process. The spheroidized annealing results in a spherical pearlite structure in which the cementite is spherical particles dispersed on the ferrite matrix. Compared with the flaky pearlite, it has low hardness, is easy to be cut, and is quenched. When heated, the austenite grains are not easily grown, and the workpiece is deformed and cracked little during cooling.
The spheroidizing annealing heating temperature is 727 degrees or more, and the isothermal cooling or the direct slow cooling after the heat preservation. Austenization is "incomplete" during spheroidizing annealing, except that the flaky pearlite is transformed into austenite and a small amount of excess carbide is dissolved. Therefore, it is impossible to eliminate the network carbide. If the hypereutectoid steel has the reticulated carbide, it is necessary to carry out normalizing before the spheroidizing annealing to eliminate the spheroidal annealing.
Ordinary spheroidizing annealing is to heat the steel to a temperature of 20 to 30 ° C above Ac1, heat for a suitable time, and then slowly cool with the furnace, and then cool to about 500 ° C to be air-cooled. The isothermal spheroidizing annealing is the same heating and heat preservation as the ordinary spheroidizing annealing process, and is isothermally cooled with the furnace to a temperature slightly lower than Ar1, and the isothermal time is 1.5 times of the heating and holding time. After isothermal, the furnace is cooled to about 500 °C and air-cooled. Compared with ordinary spheroidizing annealing, spheroidizing annealing can not only shorten the cycle, but also make the spheroidized structure uniform and strictly control the hardness after annealing.