The ductile to brittle transition for C–Mn steel with an ultrafine grain ferrite/cementite microstructure

The Charpy impact energy was measured as a function of temperature for plain C–Mn steel with an ultrafine grain ferrite/cementite (UGF/C) microstructure and with a coarse-grain ferrite/pearlite (CGF/P) microstructure. Instrumented Charpy impact tests were carried out at 273 K and 77 K for the UGF/C microstructure. The steel with the UGF/C microstructure exhibited an upper shelf energy lower than the steel with the CGF/P microstructure, a higher lower shelf energy and the transition in impact energy occurred more gradually at a lower temperature. The ductile to brittle transition in the steel with the UGF/C microstructure was caused by a decrease in energy absorbed by ductile fracture rather than a change of fracture mode from ductile fracture to cleavage as occurred for the CGF/P microstructure. The fracture surface of the UGF/C steel in the upper shelf region contained large dimples and numerous small dimples whereas the fracture surface in the lower shelf region contained small dimples and cleavage facets. A lower amount of energy was absorbed in the transition region and the lower shelf region due to the decrease of dimple diameter and depth. Nevertheless, some energy was absorbed in the lower shelf because there were some small dimples even at the liquid helium temperature. In contrast, essentially no energy was absorbed in the lower shelf for the CGF/P steel because of typical cleavage fracture. The UGF/C microstructure has a high cleavage fracture stress.