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

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 γp 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 (γp) is between 71.6 and 82.6 J m−2.

► Relationship between bainitic packet size measured by LM and EBSD with grain boundary misorientation of 15° was found.
► The average values of ferrite lath width and cementite carbide width were also measured.
► According to Griffith's equation, the effective work of cleavage fracture γp was determined for each microstructural unit.
► Bainite packet size determined by EBSD is the microstructural parameter that controls cleavage crack propagation.