Hot work tool steels are widely used as thermal and mechanical highly stressed tools, amongst others for pressure die casting. Advanced die-casting processes require further increase regarding the size of the molds, the complexity of the geometries, and the tool life of the dies, resulting in a multifaceted stress profile for the used tool steel. Thus, an optimum combination of certain properties, such as a high thermal stability and toughness as well as a particularly good through hardenability for large cross-sections, is hence necessary. The mechanical properties of engineering structural materials are highly dependent on their microstructure and are achieved after heat treatment, which is therefore crucial for the good performance of the tool steel. Depending on the size of the tools, the hardening process in these materials can lead to varying microstructures and properties through their cross-section resulting from the quenching step and the inconsistent cooling rates. Particularly slow cooling rates can lead to the formation of bainite, which is known to decline the toughness and hence degrade the mechanical performance.In this work large dimensioned samples of 5%Cr martensitic steel with a size of 810 x 510 x 350 mm and a well-defined geometry are prepared and heat treated under standard conditions for die casting molds. Subsequently impact energy and fracture toughness in different zones of the samples are determined. The corresponding microstructures are investigated using optical microscopy and scanning electron microscopy. The results are compared with those of numerical simulations and discussed in this presentation.
Keywords
Hot-work tool steels are exposed to complex interacting cyclic thermal and mechanical loadings. Due to the combination of strengthening via carbides and intermetallic precipitates, dual hardening steels achieve well-balanced mechanical properties in terms of fatigue strength and fracture toughness. Therefore, dual hardening steels have a great potential for hot-work applications. Herein, out-of-phase thermomechanical fatigue tests are used to simulate the loading conditions experienced in hot-work tool steel applications on a laboratory scale. The testing is conducted on Fe-C-Cr-Mo-V and Fe-C-Cr-Mo-V-Ni-Al alloys to compare common 5% Cr and dual hardening hot-work tool steels. The resistance to thermomechanical fatigue is therefore correlated with single or dual hardening. Both alloys experience softening during the fatigue testing. Atom probe tomography investigations reveal coarsening of the secondary hardening precipitates for both alloys. However, the number density of surface cracks is greater for the 5% Cr hot-work tool steel. The dual hardening steel possesses higher resistance to softening and reaches a higher lifetime.
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