Influence of the Cooling Rate on the Microstructure and Mechanical Properties in 5% Cr Martensitic Steel

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.