Additive manufacturing of a crack-free tool steel with high C and Cr contents: Microstructure and mechanical properties

Abstract

Martensitic Cr steels are valued in tool steel applications for their polishability and balanced mechanical properties at high temperatures. Traditionally produced using conventional methods, these steels can benefit from additive manufacturing (AM), which allows for adjustments in component shape and microstructure. However, AM techniques have rarely been used for martensitic Cr steels due to their susceptibility to cracking caused by the elevated carbon contents. Hence, this study aimed to produce a crack-free, fully dense martensitic Cr steel with up to 20 wt% Cr via laser beam powder bed fusion (LB-PBF), using a platform preheated to 200 °C. A design-of-experiments model was implemented for 18 cubic samples, varying laser power, scanning speed, and hatch distance. Metallographic analysis revealed a crack-free microstructure for all samples. The best parameter set achieved a remarkable relative density of 99.99 %, printed at a volumetric energy density of 72 J/mm³. Using this parameter set, larger samples were printed for detailed material characterization in both the as-built (AB) and the heat-treated (HT) condition, after austenitization, quenching and three tempering steps. Mechanical properties were evaluated through tensile and hardness tests, while microstructure analysis employed scanning electron microscopy, electron backscatter diffraction, and X-ray diffraction. Results revealed a homogeneous microstructure, primarily consisting of fcc-Fe phase in the AB condition and bcc-Fe phase in the HT condition, with mechanical properties comparable to conventionally produced martensitic Cr steels. This comprehensive characterization provided a thorough understanding of the microstructure and mechanical properties of the 3D-printed martensitic Cr steel in both the AB and HT conditions.