Effect of Bain Unit Size on Low-temperature Fracture Toughness in Medium-carbon Martensitic and Bainitic Steels

This study investigated the low-temperature fracture toughness of martensite and bainite with various Bain unit sizes. The three-point bending tests revealed that the apparent fracture toughness increased with decreasing the Bain unit size. We also found that even when the carbide size and distribution were almost the same, the apparent fracture toughness of tempered martensite with Bain unit size of 2.5 µm was much higher than that of bainite with Bain unit size of 16.2 µm. The propagation of micro-crack stopped at the Bain unit boundaries when the Bain unit size was small. The additional load was necessary for further propagation of crack which stopped at the Bain unit boundaries, leading to the improvement of fracture toughness. The critical local fracture toughness corresponding to the propagation of crack across the Bain unit boundaries was estimated at 1.04 MPa m1/2 by finite element simulations. Based on this value, we proposed that the Bain unit boundary whose interval was less than 9.4 µm could become obstacle for the crack propagation after penetrating matrix/carbide boundary.