Fatigue and fracture toughness properties of large-bloom mixed-microstructure heat-treated steels
Russo Spena, P
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The standard ISO 1.2738 medium-carbon low-alloy steel have long been used to fabricate plastic molds for large automotive components (bumpers and dashboards) by machining large previously quenched and tempered steel blooms. Due to the bloom size, the heat treatment yields mechanical properties and mixed microstructures continuously varying from surface to core. Alternative steel grades, including both non-standard microalloyed steels, designed for the same production cycle, and precipitation hardening steels, were recently proposed. Results of a large experimental effort concerning the fracture toughness and fatigue properties (as well as other mechanical properties) of plastic mold steel blooms are presented and commented, also on the basis of microstructural and fractographic analysis. These steels generally exhibit low fracture toughness values; in the traditional quench and temper production cycle this characteristic arises from the presence of mixed microstructures, whereas in the precipitation hardened steel the brittleness probably stems from the carbide precipitation strengthening mechanism.