Definition：
Spheroidizing annealing is an annealing process to spheroidize carbide (cementite Fe3C) in steel.The steel is heated to 20 ~ 30℃ above Ac1, held for a period of time, and then slowly cooled to a temperature slightly lower than Ac1, and stays for a period of time, so that the organizational transformation is completed, and the organization of spheroidal or granular carbide is uniformly distributed on the ferrite matrix.
Range of application：
Spheroidizing annealing is mainly applied to eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel and so on. The steel is air-cooled after rolling and forging, and the resulting structure is lamellar pearlite and mesh cementite, which is hard and brittle, not only difficult to cut, but also easy to deform and crack during the quenching process. The spheroidizing annealing obtained is a spherical pearlite structure, in which the cementite is spherical particles, dispersed in the ferritic matrix, compared with the flake pearlite, not only low hardness, easy to cut, and in the quenching heating, austenite grains are not easy to grow, cooling workpiece deformation and cracking tendency is small. In addition, spheroidizing annealing can also be used for some hypoeutectoid steels that need to improve cold plastic deformation (such as stamping, cold heading, etc.).
Spheroidizing annealing heating temperature is Ac1+(20 ~ 40)℃ or Acm-(20 ~ 30)℃, and isothermal cooling or direct slow cooling after heat preservation. In spheroidizing annealing, austenization is "incomplete", only the flake pearlite is transformed into austenite, and a small amount of excess carbide is dissolved. Therefore, it is impossible to eliminate the network carbide, such as hypereutectoid steel network carbide exists, it must be normalized before the spheroidizing annealing, to eliminate it, in order to ensure the normal process of spheroidizing annealing.
Technique：
There are many nodular annealing processes, and the two most commonly used processes are ordinary nodular annealing and isothermal nodular annealing. Ordinary spheroidizing annealing is to heat the steel to 20 ~ 30℃ above Ac1, hold the heat for an appropriate time, and then slowly cool with the furnace, cold to about 500℃ out of the oven air cooling. Isothermal spheroidizing annealing is the same as the ordinary spheroidizing annealing process after heating and holding, with the furnace cooling to a temperature slightly lower than Ar1 isothermal time is 1.5 times of its heating and holding time. After isothermal cooling with the furnace to about 500℃ out of the air cooling. Compared with ordinary nodularization annealing, isothermal nodularization annealing not only can shorten the cycle, but also can make the nodularization structure uniform, and can strictly control the hardness after annealing.
Spheroidizing annealing is mainly used for over-eutectoid carbon steel and alloy tool steel (such as steel used in manufacturing cutting tools, measuring tools, molds). Its main purpose is to reduce the hardness, improve the machinability, and prepare for later quenching. This process is conducive to plastic processing and cutting processing, but also to improve mechanical toughness. Especially for bearing steel, tool steel and other steel, such as the implementation of spheroidizing annealing before quenching, you can obtain the following effects:
Bearing steel: uniform quenching effect; Reduce quenching deformation; Increase quenching hardness; Improve workpiece cutting performance; Improve bearing properties such as wear resistance and pitting resistance.
Tool steel: uniform quenching effect; Inhibition of quenching cracking, quenching bending and other phenomena; Improve wear resistance, blade sharpness and service life.
According to the steel grade and the purpose of annealing, nodular annealing can be divided into the following types:
(1) Ordinary spheroidizing annealing, that is, the steel is heated to 730 ~ 740 ° C for sufficient time, and then slowly cooled to 650 ° C at a speed of less than 20 ° C /h. This annealing process is suitable for carbon tool steels near the eutectoid composition.
(2) Periodic spheroidizing annealing, also known as cyclic annealing. It is repeatedly heated and cooled at the temperature near point A, generally for 3 to 4 cycles, so that the flake pearlite is dissolved and precipitated several times during the repeated process, and the carbide is nodulated. The process has long production cycle, inconvenient operation and difficult control, and is suitable for steel with serious pearlite.
(3) isothermal spheroidizing annealing. Generally, it is heated to 800±10 ° C, and then cooled to 700±10 ° C (near A1) for a long time after insulation, and then cooled to 600 ° C at a speed of 30 ~ 50 ° C /h. General bearing steel mostly uses this process.
(4) Deformation - spheroidizing annealing. When plastic deformation is combined with spheroidizing annealing process, the density and distortion of dislocation in steel are increased due to plastic deformation, which can accelerate the dissolution and spheroidization of flake carbide during annealing, thus speeding up the rate of spheroidization and shortening the time of spheroidization.
