Spheroidization


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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.

REFERENCES

Rauzin, Ia. R. Termicheskaia obrabotka khromistoi stali, 3rd ed. Moscow, 1963.
Bunin, K. P., and A. A. Baranov. Metallografiia. Moscow, 1970.

R. I. ENTIN

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Key words: carbide morphology, spheroidization, thermomechanical treatment
The conventional after-treatment consists of a long time soft annealing to ensure spheroidization of pearlite and thus also better ductility.
Recently investigated spheroidization processes can be divided into four groups: isothermal annealing at a temperature slightly below [A.
It was furthermore necessary to estimate the effect of diffusion on carbide spheroidization.
However, none of them attempts to describe the particular details of the microstructure, especially the lamellae length and width during the spheroidization.
The evolution of the microstructure during the spheroidization has been considered.
It may happen, especially at an early stage of spheroidization, that two distinct lamellae touch each other by just a few pixels.
Different morphological parameters were estimated to characterize the microstructure of the cementite phase during the cementite spheroidization.
Solution temperatures of 540C, used in most foundries, may be reduced following recent observations that small amounts of liquid during the solution treatment leads to extremely high spheroidization and coarsening rates.
Precipitation of carbides from bainite or martensite and their redistribution is much quicker than carbide spheroidization from lamellar pearlite.
Spheroidization can occur according to one of the following methods (Kamyabi-Gol & Sheikh-Amiri, 2010):
Shorter annealing times are typical for this process and resulting spheroidization of microstructure with more homogeneous distribution of carbides.