The Influence of Thermal Cycles on the Performance and Spheroidization Process of SAE 8640 Steels

Variations in the microstructure of steel materials might change the alloy’s properties and characteristics and influence the performance of manufacturing processes. The literature on the materials characterization in this area has been limited to examining the effects of the spheroidization of microstructure in certain materials. Taking this into account, this study aims to assess the influence of the spheroidization rate on the microstructure and mechanical properties in a SAE 8640 steel subjected to different annealing cycles. Three different thermal cycles were tested, varying the time, temperature, and type parameters. The practical results of the annealing cycles were evaluated through optical microscopy, scanning electron microscopy, microhardness tests, and x-ray diffraction. Findings showed hardness reduction for all the cycles examined, particularly for Cycle III (isothermal annealing), whose reduction was approximately 98 Hv compared to the samples without heat treatment. The findings showed that hardness values below the average at 0.10 mm under the tested surfaces were observed in the three cycles examined, suggesting the occurrence of the decarburization process. A spheroidization rate of 79.93% was achieved in Cycle II (pendulum temperature behavior). The study’s findings make a positive contribution to the research that investigates the mapping of the metallurgical and mechanical aspects of spherical low-alloy and medium-carbon steels.