Tailoring microstructure and mechanical properties of additively manufactured H13 tool Steel: Influence of build orientation and tempering treatments

In this study, H13 hot work tool steel (TS) was additively manufactured (AM) using laser powder bed fusion (LPBF) technology to investigate the effects of various tempering treatments on its microstructural evolution and mechanical properties. AM-H13 TS specimens were produced with a volumetric energy density of 57.3 J/mm3 in two distinct build orientations (BOs): vertical and diagonal (45 degrees). Three heat treatment protocols were applied: solution hardening at 1040 degrees C followed by double tempering at 550 degrees C (SH-DT550), and two separate double tempering at 550 degrees C (DT550) and 650 degrees C (DT650) for 4 h, without solution hardening. Detailed microstructural analysis was conducted using scanning electron microscopy (SEM) with secondary electron (SE) imaging and electron backscatter diffraction (EBSD) techniques. Mechanical properties were evaluated through uniaxial tensile testing and hardness measurements. The as-built samples mainly exhibited a finely layered martensitic microstructure with a moderate fraction of retained austenite (about 18.6 %). Following tempering treatments at DT550 and DT650, significant features from the as-built state, such as melt pool boundaries and cellular structures, remained intact. Although both treatments encouraged microstructural recovery and carbide formation, distinct differences were apparent. DT550 promoted the development of nanoscale carbide precipitates, enhancing hardness and strength. In contrast, DT650 led to carbide coarsening, resulting in a decrease in material hardness. In the as-built state, the tensile properties exhibited slight variation across different BOs, with the vertical direction showing slightly enhanced ductility. The tensile strengths for the as-built samples were 1897 MPa (vertical) and 1882 MPa (diagonal). Tempering significantly influenced mechanical performance, with DT550 increasing tensile strength to approximately 2100 MPa through secondary carbide strengthening. In contrast, DT650 and SH-DT550 treatments decreased strengths of about 1450 MPa and 1480 MPa, respectively.