Strengthening mechanisms in a heatvar hot work tool steel fabricated by laser powder bed fusion

Abstract

Abstract The Heatvar alloy was specifically designed to address the need for new alloys compatible with the Laser Powder Bed Fusion (LPBF) process that can withstand prolonged exposure to heat in the tool and die industries. For this reason, the microstructural evolution, mechanical properties, and texture development of the LPBF-produced Heatvar after heat treatment were studied using multi-scale characterization techniques. In the as-printed condition, epitaxial grain growth with cellular dendritic structure was detected together with the eutectic Laves phase due to molybdenum segregation. Two heat treatment temperatures were initially employed: i) 1223 K for 0.5 h, and ii) 903 K for 4 h. The former is considered a solutionizing treatment that allows the eutectics to be re-melted to form Mo-rich M6C, leading to a slight increase in hardness and tensile strength compared to the as-printed condition. The latter is categorized as a direct aging process that results in the dissolution of eutectics while the Fe2Mo precipitates were formed in the martensite matrix. The formation of precipitates leads to a significant increase in the hardness, tensile strength, and yield strength. These experimental observations are in agreement with the simulated hardness of the material as a function of Fe2Mo precipitate size during aging heat treatment using Kurz-Giovanolva and Trivedi (KGT) and Lifshitz-Slyozov-Wanger (LSW) modeling. The tensile strengths of as-printed, solutionized, and directly aged Heatvar samples are 1190 ± 16, 1369 ± 18, and 1980 ± 20 MPa, respectively. It has been determined that the aged condition maintains its superior strength during prolonged heat treatment at 893 K for 100 h. Therefore, based on the results, the benefits of additive manufacturing in the tool and die industries can be realized by utilizing the new Heatvar alloy.