HP40-Nb heat-resistant cast steels are widely used for reformer furnace tubes operating at elevated temperatures, where accidental overheating may induce microstructural degradation and alter mechanical properties. This study investigates the influence of overheating time on the microstructure and mechanical properties of centrifugally cast HP40-0.63Nb alloy. Samples were overheated at 950 and 1150 °C for 30, 120 and 480 min. Microstructural characterization was performed using light microscopy (LM), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), while tensile testing and hardness measurements were used to evaluate mechanical performance. The as-cast microstructure consisted of an austenitic dendritic matrix and eutectic chromium-rich and niobium-rich carbides. Overheating promoted the precipitation of secondary chromium-rich carbides, causing significant changes in carbide morphology and distribution. The highest yield strength (346 MPa) was achieved after overheating at 950 °C for 120 min due to intensive precipitation hardening. Prolonged overheating led to carbide coarsening and partial dissolution, reducing the strengthening effect. The highest tensile strength (582 MPa) and favorable ductility (8.76 %) were obtained after overheating at 1150 °C for 30 min. The results demonstrate that overheating time strongly influences carbide evolution and mechanical behavior of HP40-0.63Nb alloy, with 950 °C/120 min providing maximum strengthening and 1150 °C/30 min the best strength–ductility balance.