Fracture behavior of two ultrahigh carbon steels

Two ultrahigh carbon steels containing 1.3 and 1.5 pct carbon have been tes ted for their plane-strain (Che,ron-notch) fracture toughness using Barker tests. The microstructure of these two materials consisted of fine carbides dispersed in a ferrite matrix. Fracture toughness of UHC-1.3 C was twice t hat of UHC-1.5 C, This result can be readily explained by means of the diff erence in size of the carbide particles found in each microstructure. The c rack growth mechanism consisted of ductile tearing at the notch tip region followed by nucleation of a brittle crack at carbides. Crack propagation in the UHC-1.3 C specimen occurred unstably and smooth crack growth behavior was observed in UHC-1.5 C. The difference in crack growth behavior was attr ibuted to the elastic energy stored in the grips, which was higher for the UHC-1.3 C test than for the UHC-1.5 C test.