Heat treatments for obtaining spheroidal cementite in ductile cast iron

The purpose of the present work consists in obtaining ductile cast iron that will be used in the shaft production, with the following mechanical characteristics: minimum tensile strength of 700 N/mm², minimum yield strength of 450 N/mm², minimum elongation of 5%, and hardness in the range of 235 HB to 285 HB. The most adequate microstructure for obtaining these desired mechanical characteristics consists of spheroidal cementite dispersed in a ferritic matrix, and this microstructure can be obtained by annealing a pearlitic microstructure or tempering a martensitic microstructure. All types of heat treatments used in this work, depending on the treatment time, led to the formation of spheroidal cementite. The material, normalised and annealed for two hours at 700°C, yielded the desired mechanical characteristics: ultimate tensile strength and yield strength values of 827 N/N/mm² and 547 N/N/mm², respectively, elongation of 7% and hardness value of 277 HB.