2024 Volume 64 Issue 2 Pages 389-400
In this study, we prepared hypereutectoid SUJ2 and SK3 steels with cementite (θ) particles of varying area fraction, particle size and circularity. The steels were then subjected to a specially-designed heat treatment aimed at eliminating coarse θ particles located on the grain boundaries. The effect of heat treatment on the characteristics of the θ particles and the resultant toughness were evaluated by Charpy impact tests. The designed heat treatment successfully achieved the spheroidization and annihilation of the θ particles on the grain boundaries, especially in the SUJ2 steels. Specifically, the heat-treated SUJ2 samples showed a ductile fracture surface with fine and homogeneously distributed dimples, whereas the SK3 samples showed a fracture surface with a mixture of cleavage and dimple surfaces, and grain boundary fracture was also partly observed. The impact values tended to be high when the size and area fraction of the θ particles on the grain boundaries were small. Furthermore, the circularity of the θ particles at the grain boundaries significantly influenced the impact value. It has been demonstrated that if the circularity of the θ particles is high, the impact values are less likely to deteriorate, even if there are large θ particles on the grain boundaries.
