Effects of Cementite Particles on Impact Properties in High-hardness Hypereutectoid Steels

Abstract

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.