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The standard ISO 1.2738 medium-carbon low-alloy steel have long been used tofabricate plastic molds for large automotive components (bumpers anddashboards) by machining large previously quenched and tempered steel blooms.Due to the bloom size, the heat treatment yields mechanical properties and mixedmicrostructures continuously varying from surface to core. Alternative steelgrades, including both non-standard microalloyed steels, designed for the sameproduction cycle, and precipitation hardening steels, were recently proposed.Results of a large experimental effort concerning the fracture toughness andfatigue properties (as well as other mechanical properties) of plastic mold steelblooms are presented and commented, also on the basis of microstructural andfractographic analysis. These steels generally exhibit low fracture toughnessvalues; in the traditional quench and temper production cycle this characteristicarises from the presence of mixed microstructures, whereas in the precipitationhardened steel the brittleness probably stems from the carbide precipitationstrengthening mechanism.