A Comparative Study on Cooling Performance of Hot Oil and Molten Salt Quench Media for Industrial Heat Treatment

The present work presents a comprehensive comparative study on the cooling performance of hot oil and molten 54%KNO₃-7%NaNO₃-39%NaNO₂ eutectic mixture quench media. The study was conducted using a cylindrical Inconel probe of 12.5 mm diameter and 60 mm length. Cooling curves at different locations in the probe were acquired and were subsequently used to calculate spatially dependent transient heat flux at the metal/quenchant interface. The heat extraction mechanism in hot oil and NaNO₂ eutectic mixture was found to be different. Heat transfer occurred in two stages, namely boiling stage and convective cooling stage during quenching in molten NaNO₂ eutectic mixture. In the case of hot oil, apart from these two stages, third stage of cooling, namely vapor blanket stage, was observed. A detailed study was conducted to compare the magnitude and uniformity of heat extraction during each stage of quenching. Molten salt offered a higher cooling rate and more spatial uniform cooling as compared to that obtained in hot oil quench medium. The non-uniformity in surface temperature during boiling stage in Inconel probe was ten times lower in molten salt medium as compared to that observed in the hot oil medium. However, the non-uniformity in surface temperature during convective cooling stage in both the media were comparable. Based on the distribution of characteristic cooling time (t₈₅) calculated in quenched Inconel probe, higher and uniform hardness distribution is predicted in steel parts quenched in molten NaNO₂ eutectic mixture media as compared to that in hot oil.