Effect of deep cryogenic treatment,tempering temperature and time on hardness of Nimonic-90

The present study has been done to determine the effect of deep cryogenic treatment (DCT) on the hardness of Nimonic-90. From the study, it has been found that the hardness of the material is significantly affected with cryogenic treatment which includes soaking the samples at subzero temperature (−196 °C) for 24 h and 48 h and tempering operation which includes heating the samples in temperature range of 100–200 °C. Also, it has been found that as the tempering temperature decreases from 200–100 °C, the hardness of the samples increases. The number of tempering cycles affected the hardness of the samples as well. The lowest hardness value came out for the untreated sample as 284 HV. Among all the cryo-treated samples, the highest value of hardness was for the single tempering at a tempering temperature of 100 °C, soaking period of 48 h, and tempering time of 1 h which was 435.76 HV. The minimum hardness in DCT samples was for the case of 24 h soaking period, tempering temperature of 200 °C and tempering time of 3 h. which was 376.9 HV. The tremendous improvement in hardness of Nimonic-90 is because of microstructural changes at different soaking period and tempering temperature which includes phase transformation and morphological changes that includes grain size which decreased from 43 to 4.418 µm, precipitation of Cr₂₃C₆, NiC, and Cr₂Ti at respective soaking period, tempering temperature, and the number of tempering cycles in each treatment.