Measurement of bainite packet size and its influence on cleavage fracture in a medium carbon bainitic steel

摘要

For a bainitic steel of C=0.38%, the bainitic packet size measured by OM is about 3.5 times larger than that measured by EBSD with a 15° grain boundary misorientation. This ratio is used to calculate the effective surface energy given by Griffith's model for cleavage fracture. Average ferrite lath width and cementite carbide width values are also measured. Following the general form of Griffith's equation, the effective surface area of cleavage fracture γ_p is determined for each microstructural unit. It is concluded that the bainite packet size determined by EBSD with a 15° misorientation angle criterion is the microstructural parameter that controls cleavage crack propagation. Given the relationship between the average unit crack path (UCP) and the bainitic packet size, it is concluded that the effective surface energy of cleavage fracture (γ_p) is between 71.6 and 82.6Jm~(-2).