PurposeIn this study, the Incoloy 825 alloy was hot-forged 90% by open die forging, and the Rockwell hardness was measured by change of solution treatment temperature and aging time. The Rockwell hardness was analyzed by the Weibull statistics.Design/methodology/approachThe hot forging process generally involved heating to 1,423 K, to make grain size smaller than required grain size of 50 µm of the Incoloy 825 alloy. Thereafter, after a certain time, a forging ratio of 90% was obtained with a 5,000 ton press. At this time, the finish forging temperature was 1,173 K. To investigate the effect of solution treatment temperature, the material was maintained at 973, 1,073, 1,173 and 1,273 K for 1 h and then water-cooled. To investigate the effect of aging time, the material was water-cooled after holding for 1, 5, 10 and 30 h at 973 K.FindingsThe amount of precipitate increased with aging time, and the size of precipitate became larger. The lower the solution treatment temperature, the longer the aging time, and the Rockwell hardness increased. Shape parameter of solution-treated material increased with increasing temperature, but decreased sharply at 1,273 K. The scale parameter decreased. Shape parameters and the scale parameters of the aging treatment material increased, according to increasing of aging time.Originality/valueThe effect of heat treatment on microstructure and Rockwell hardness, in Incoloy 825 alloy with hot forging of 90% grain size increased and precipitates decreased with increasing solution treatment temperature, precipitate increased and precipitates' size increased with increasing aging time at 973 K. Shape parameter increased but scale parameter decreased. Shape parameters and scale parameters of the aging-treated material increased.