Evaluation of microstructural parameters controlling cleavage fracture toughness in Mn–Mo–Ni low alloy steels

This study aims to investigate, from a microstructural viewpoint, the main factors controlling the cleavage fracture toughness of SA508 Gr.3 Mn–Mo–Ni low alloy steel. The microstructures of the tested material were independently controlled by modifying the conditions of austenitization and tempering. Cleavage fracture toughness was evaluated in the transition temperature region in accordance with the ASTM E1921 standard test method, and fracture toughness data were interpreted by a simple statistical model based on the size distribution of microstructural constituents. The T0 values decreased with a decrease of carbide size, but the cleavage fracture toughness did not show a significant dependence on the prior-austenite grain size. Statistical analysis to determine carbide size distribution showed a linear correlation between the T0 values and the probability of finding carbides larger than the critical size for cleavage initiation. As a result, cleavage fracture is found to be controlled by the initiation of micro-cracks in the carbides, and reducing the number of coarse carbides near the crack tip is an effective way to improve the cleavage fracture toughness in the transition region.