The work focuses on deep cryogenic treatment (DCT) of Nimonic-90 super alloy in order to improve its hardness. The impact of post-tempered deep cryogenic treatment (DCT) on the microstructure and hardness of Nimonic-90 super alloy is studied. Deep cryogenic treatment leads to change in the microstructure by new grain formation, change in grain size, movement of dislocations, alteration of crystal structure and new phase formation which provides better material properties like tensile strength, hardness, wear resistance, corrosion resistance, fatigue strength and toughness when compared with conventional heat treatment. In our study, the samples were soaked for 36 h at − 196 °C and thereafter tempering the samples at 100 and 150 °C for 1 and 2 h. When DCT4 sample was post-tempered at 100 °C for 1 h using single tempering, a maximum hardness of 424.64 HV was attained, which was higher than the untreated and DCT1, DCT2 and DCT3 samples. DCT1 sample which was post-tempered at 150 °C for 2 h with double tempering resulted in a minimum hardness of 406.28 HV among all deep cryogenically treated samples. In order to characterize and track the alterations in the microstructure, scanning electron microscopy (SEM) was employed. The Nimonic-90 super alloy's increased hardness was due to drop in average grain size and increase in grain boundaries.