Tempering behavior of a direct laser deposited hot work tool steel: Influence of quenching on secondary hardening and microstructure

Direct Laser Deposition (DLD) opens new ways for the fabrication of tools with intricate designs as well as for dies and molds repairing. However, non-equilibrium, and highly dynamic characteristics of the DLD process, generally result in non-equilibrium microstructures, inhomogeneities across the build height, and unwanted phase transformations. This might lead to non-uniform and highly scattered mechanical properties. This highlights the need for a proper thermal post-processing aimed at achieving application-specific mechanical properties. AISI H13, which is commonly used in hot-forming applications (e.g., die casting tooling, extrusion dies), shows high potential in the rapid manufacturing of the tools by additive manufacturing (AM). In this study, the influence of the as-built microstructure on direct tempering response as well as tempering after austenitization and quenching of DLD H13 tool steel is evaluated. The factors governing hardness and tempering behavior are discussed in detail with the aid of microstructural analysis and isochronal tempering studies. As-built microstructure comprised martensite, inter-cellular/inter-dendritic retained austenite (RA), and solidification carbides. Decomposition of retained austenite by direct tempering of the as-built samples led to a stronger secondary hardening peak and a shift of this peak to higher temperature. Austenitized and quenched samples contained ≤2 vol% retained austenite. Increasing the austenitization temperature and time led to the dissolution of a larger vol% of solidification carbides, recovery of the micro-segregation, recrystallization and grain growth. In quenched and tempered parts, hardness increased by increasing austenitization temperature from 1020 °C to 1060 °C as a result of higher supersaturation of quenched martensite leading to larger vol% of secondary carbide precipitation. Within the whole technically significant tempering range, highest hardness was achieved by direct tempering of the as-built material.