The development and application of Spheroidizing annealing for carbon steel

Spheroidizing annealing process is made and spheroidization of carbide in steel. Will steel heated to AC1 above 20 to 30 DEG C, insulation for a period of time, and then slowly cooling to obtain in the ferrite matrix uniformly distributed spherical or granular carbides. Spheroidizing is mainly applicable to the eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc.. These steel after rolling and forging after air cooling, the organization income is piece of lamellar pearlite and reticular cementite, the hard and brittle, not only difficult to machining and later in quenching process also easy to deformation and cracking. The spheroidizing annealing is globular pearlite, the cementite is spherical particles dispersed in the ferrite matrix, and lamellar pearlite compared, not only low hardness, easy machining and in heating and quenching, austenitic grain not easy to grow, cooling workpiece deformation and cracking tendency. In addition, for some need to improve the cold plastic deformation, such as stamping, cold heading of hypoeutectoid steel can also sometimes using spheroidizing annealing. Spheroidizing heating temperature is Ac1+ (20 ~ 40) or Acm- (20 ~ 30 DEG C), heat preservation after isothermal cooling or slow cooling. In spheroidizing annealing austenitic is "incomplete", just flake pearlite to austenite, and a small amount of surplus carbide dissolution. Therefore, it is not possible to eliminate carbides, such as hypereutectoid steel with carbide network exists, in front of the spheroidizing annealing shall first normalizing, to eliminate it, in order to ensure the normal spheroidizing annealing. Spheroidizing annealing process of many methods, two kinds of process is the most commonly used common ball annealing and isothermal spheroidizing annealing. Common ball annealing is the steel is heated to AC1 above 20 to 30 DEG C, holding the appropriate time, and with the furnace slow cooling, cold to 500 DEG C released air cooling. And common ball annealing process of the same heat insulation, with the furnace cooling to slightly below AR1 temperature isothermal isothermal spheroidizing annealing, isothermal time for heating the holding time of 1.5 times. After isothermal furnace cooling to 500 DEG C followed by air cooling. Compared with common ball annealing, spheroidization annealing can not only shorten the cycle and the spheroidizing microstructure is uniform, and strictly control the annealing hardness.