Abstract: As-cast HK40 steel was aged at 700, 800, or 900℃ for times as long as 2000 h. Microstructural characterization showed that the primary M₇C₃ carbide network contained a substantial content of manganese, in agreement with the microsegregation of manganese calculated by Thermo-Calc using the Scheil-Gulliver module. The dissolution of primary carbides caused the solute supersaturation of austenite and subsequent precipitation of fine M₂₃C₆ carbides in the austenite matrix for aged specimens. During prolonged aging, the carbide size increased with increasing time because of the coarsening process. A time-temperature-precipitation diagram for M₂₃C₆ carbides was calculated using the Thermo-Calc PRISMA software; this diagram showed good agreement with the experimental growth kinetics of precipitation. The fine carbide precipitation caused an increase in hardness; however, the coarsening process of carbides promoted a decrease in hardness. Nanoindentation tests of the austenite matrix indicated an increase in ductility with increasing aging time.