The strength and toughness of a newly developed 0.8C-8Cr cold work tool steel whose composition was controlled to suppress the precipitation of massive eutectic M7C3 carbides were investigated with reference to microstructure and were compared with conventional SKD11. The toughness was evaluated by the area under the stress-strain curve. The tensile strength of the newly developed steel (designated as Mod.SKD11) is about 400MPa higher and the toughness is 1.8 times larger than that of SKD11 throughout a wide range of tempering temperatures. It is revealed from microstructure examination, the fracture surface observation and the estimation of the tensile stress required to generate cleavage crack within M7C3 carbide based on Stroh's theory that superior mechanical properties of Mod.SKD11 are attributed to the refinement and homogeneous distribution of M7C3 carbides that retards the crack generation and prevents the crack growth, respectively.
