Toughness of Fe-base materials containing hard phases

In the present work the toughness of powder metallurgical tool steels and Metal Matrix Composites (MMC) is being investigated, specifically the influences of heat treatment and the addition of hard phases on the fracture toughness K-Ic. The experimental materials are cold work tool steel X230CrVMo13-4 as well as hot work tool steel X40CrMoV5-1 with coarse hard phases of the type titanium carbide (TiC), and fused tungsten carbide (FTC) in contents of 0, 10 and 30 vol.-%. The materials were produced by hot isostatic pressing and subsequently heat treated by quenching and tempering in two essentially different treatments. The fracture toughness K-Ic was measured at precracked three point bending specimens. Fractographical investigations should characterize the fracture formation, progression and mechanism and give information about the influences of microstructure components on the fracture toughness. The experiments showed that generally an addition of coarse hard phases leads to a large decrease in fracture toughness. However, in case of powder metallurgically produced hot work tool steel X40CrMoV5-1, an increase in fracture toughness was achieved by adding 10 vol.-% of fused tungsten carbide. Even an addition of 30 vol.-% FTC exhibits K,, values in the region of a pure cold work matrix. Furthermore it is shown that retained austenite positively affects the fracture toughness.