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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For the carbide spheroidisation to be as complete as possible, the following schedules included austenitizing and subsequent austenite decomposition cycles which were repeated two and three times (H15-2, H15-3).
The basis of accelerated carbide spheroidisation is shortrange diffusion of carbon.
c1] was not sufficient for complete spheroidisation of carbides.
This can be explained by the compensation of the hardness decline due to carbide spheroidisation by strengthening due to ferrite grain refinement.
The second stage of thermomechanical treatment (schedules with consecutive deformation steps in perpendicular directions) did not result in significant differences in hardness and carbide spheroidisation as compared to specimen no.
c1] and slow cooling in a furnace were not sufficient for complete spheroidisation of carbides.
With increasing strain, carbide spheroidisation and ferrite grain recrystallization get closer to completion.
The results clearly indicate that from certain limit amount, further increase in plastic strain does not contribute to grain refinement and pearlite spheroidisation.
This paper includes results achieved within the project GACR P107/10/2272: Accelerated Carbide Spheroidisation and Grain Refinement in Steels.