Effect of Bainitic Packet Size Distribution on Impact Toughness and its Scattering in the Ductile-Brittle Transition Temperature Region of Q&T Mn-Ni-Mo Bainitic Steels

Abstract

The impact toughness of Q&T Mn-Ni-Mo bainitic steel in the ductile–brittle transition temperature (DBTT) region was measured. The fracture micromechanism associated with the microstructural features was studied, by statistically analyzing the grain sizes of various structures. The results reveal that the cleavage fracture behavior is controlled by bainitic packets. There exists a critical bainitic packet size. When a microcrack formed in the bainitic packet with a size exceeding the critical size propagates into adjacent packets, brittle cleavage fracture occurs. The probability of finding the bainitic packets larger than the critical size dominates the impact toughness and its scattering. It is suggested that refining the bainitic packets and homogenizing their sizes is an effective method of improving the impact toughness and reducing its scattering in the DBTT region.