Post-processing of laser powder bed fusion-produced H11 tool steel: transferability of conventional heat treatment and long-term thermal stability

Laser powder bed fusion (LPBF) enables conformal cooling in hot-work tooling, but heat treatments developed for wrought H11 may not be optimal for LPBF material. This study compared LPBF-produced and conventionally manufactured (CM) H11 in their initial states and after soft annealing, austenitisation at 1030 °C followed by air cooling, and double tempering. For LPBF H11, direct quenching and tempering were compared with a route that included preliminary annealing. The as-built LPBF steel contained a fine cellular/cellular-dendritic martensitic–austenitic microstructure with 31 vol% retained austenite, whereas CM H11 consisted of a soft ferrite–carbide mixture. After austenitisation and cooling, CM H11 exhibited higher ultimate tensile strength (UTS) and hardness, but the difference narrowed markedly after double tempering. LPBF H11 reached 2030 MPa UTS and 620 HV10 after tempering at 550 °C, while 540 °C provided the best tested strength–ductility balance, with 1994 MPa UTS and 9% total elongation. Preliminary annealing did not improve the final tensile properties or hardness, although it may remain useful when intermediate machining is required. Exposure at 75 and 350 °C for up to 2880 h caused no systematic loss of tensile strength, and hardness after 2880 h did not decrease. Instead, heat-treated LPBF H11 hardened from 609 to 674 HV10 at 350 °C, while no major microstructural transformation was resolved by scanning electron microscopy. These results support a simplified quenching-and-tempering route for near-net-shape LPBF H11 when intermediate soft machining is unnecessary. Reduced ductility and defect sensitivity remain important limitations.