It is the process of converting the carbides into spherical shape by slow cooling (kiln cooling) after keeping the steels around the Ac1 temperature line for a long time and oscillating annealed in this region. It is used to facilitate the processing of steels and to increase their ductility.

Spheroidizing annealing can be done in 3 methods.

  1. The steel material is annealed to a temperature just below the Ac1(723 ˚C) line for a long time (15-25 hours).
  2. The steel material oscillates between temperatures just below and above the low critical temperature line (Ac1).
  3. After the material is annealed at a temperature above the Ac1 critical temperature line, it is either cooled very slowly in the furnace or kept for a long time at a temperature just below the Ac1 line. The high temperature annealing process causes the pearlitic structure in the steel and the cementite phase to break up and disperse. As a result of spheroidizing annealing, an internal structure consisting of a ferritic matrix and spherical carbides dispersed in it is obtained. At the end of the spheroidizing annealing, the hardness of the steel decreases, as a result of this process, supra-eutectoid steels become suitable for processing.

Usage Areas

Spheroidizing annealing is mostly applied to high carbon steels. Low carbon steels are rarely subjected to spheroidizing annealing. Because such steels become very soft at the end of spheroidizing annealing and this excessive softening creates some difficulties during machining. Medium carbon steels, on the other hand, are sometimes subjected to spheroidizing annealing before plastic forming to gain sufficient ductility. During spheroidizing annealing, the annealing time must be well adjusted. If the steel is annealed longer than necessary, the cementite particles coalesce to elongate, which adversely affects the machinability of the steel.