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For processing parts made from medium carbon steel, toughness and flexibility are of importance. Therefore, to achieve these properties, the cementite in the steels is spheroidized through heat treatment. Different parameters such as the time and temperature of spheroidizing and the initial microstructure of the steel affect the amount of spheroidized cementite. In the present work, the percent of contribution of two parameters, I. E. Initial microstructure and spheroidizing time, to the percent of spheroidization in CK60 steel was investigated using Taguchi robust design. The initial microstructures consisted of martensite, coarse pearlite, fine pearlite and bainite and the chosen spheroidization times were 4, 8, 12, and 16 h. Spheroidizing was done at the constant temperature 700 ℃ After spheroidizing was completed, the samples were prepared in order to observe their microstructure under an optical microscope and to determine the spheroidized percent using MIP(TM)(metallographic image processing) software. It was found that the spheroidizing time had the most influence (58.5%) on spheroidized percent and the initial microstructure only had a 31.1% contribution. Finally, the instantaneous growth rate of the carbide was also deduced.