Intrinsic brittleness of laser powder bed fusion processed H13 hot work tool steel

Laser powder bed fusion processed H13 hot work tool steels (L-H13) show promising tensile and hardness properties compared to their commercial counterpart, C-H13. However, L-H13 consistently exhibits lower impact toughness than C-H13, necessitating a deeper understanding of its brittleness and the optimization of tempering conditions. Initial comparisons reveal similar tensile and elastic properties between L-H13 and C-H13 before tempering. However, after 24 h tempering at 550 ℃, the bulk modulus of L-H13 increased by 5.8 %, while the bulk modulus of C-H13 increased by 20.3 %. In addition, C-H13 exhibits an improvement in impact toughness after tempering, accompanied by a transition from brittle to ductile fracture mode. In contrast, L-H13 did not change its impact toughness and fracture mode despite tempering. The observed precipitation behavior and decrease in solute carbon with tempering time indicated slower tempering kinetics for L-H13 than for C-H13. Considering the amount of solute carbon and carbide phase fraction, bulk modulus simulation confirmed that the 0.083 wt% C remaining in L-H13 after 24 h tempering and low carbide fraction resulted in a bulk modulus 20 % lower than that of C-H13. In addition, using the Pugh and Pettifor ductility and brittleness criteria, the ratio of shear modulus to bulk modulus of L-H13 exceeded 0.57, indicating its intrinsic brittleness.