Spheroidization

in physical metallurgy, a process consisting in the transition of excess-phase crystals into a globular (spheroidal) form. The transition occurs at relatively high temperatures and is associated with a decrease in the interfacial energy. Of particular importance is the spheroidization of the cementite plates contained in pearlite. In this process, the lamellar pearlite is converted into granular pearlite. As a result, the hardness and the strength of the metal are significantly decreased, but the ductility is increased.

Spheroidization is achieved by holding the metal for a prolonged period at temperatures near the lower critical point or by a cyclic heating and subsequent cooling close to these temperatures. The process can be accelerated by prior deformation or hardening. Spheroidize annealing to form granular pearlite, especially in the case of high-carbon tool steels and the high-carbon steels used for ball bearings, serves to improve machinability and prepare the metal structure for hardening.