Description of a new concept for the development of adapted hot-work tool steels for laser-powder bed fusion

Abstract

This work presents a new approach for developing carbon-martensitic hardenable hot work tool steel, which can be processed to defect-free components by laser powder bed fusion (L-PBF). The alloy approach is based on low transition temperature steels (LTT), which are characterized by a low martensite start temperature at which the austenite transforms into martensite. With this transformation, a volume expansion occurs, which causes a reduction in the tensile residual stresses previously formed in the austenite phase during cooling. At the same time, low martensite start temperature counteracts the further formation of high residual stresses as the material continues to cool down to room temperature. A modified Schaeffler diagram was used to find an appropriate alloy composition as starting point for alloy development. This diagram was adapted to the L-PBF process by laser-remelting the surfaces of cast specimens with different Ni- and Cr-equivalents. The resulting retained austenite content and the associated residual stresses of the remelted surfaces were determined using x-ray diffraction. Knowing the interaction between the chemical composition, the retained austenite volume fraction, the martensite start temperature, and the residual stress state, an optimal alloy window for L-PBF-processable hot work tool steels could be determined with the help of the adjusted Schaeffler diagram. Finally, a suitable commercially available alloy X38CrMo7–2 was adapted that can be processed using L-PBF without preheating of the build platform. The microstructure and the properties associated (hardness, strength) of the L-PBF-processed and heat-treated steel X38CrMo7–2 were compared with the frequently used tool steel X40CrMoV5–1 (L-PBF-processed with preheating of the build platform of 300 °C).