A comparative study on cooling performance of hot oil and molten salt media for industrial heat treatment

Abstract

The present work presents a comprehensive comparative study on the cooling performance of hot oil and molten KNO3-NaNO2-NaNO3 eutectic mixture quench media. The study was conducted using a cylindrical Inconel probe of 16φ and 60mm length. Cooling curves at different locations in the probe were acquired using thermocouples-DAQ system. The temperature data was recorded in PC and was 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 different. Quench 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 magnitude and uniformity of heat extraction during each stage of quenching. Molten salt offered higher cooling rate and more spatial uniform cooling as compared to hot oil quench media. The non-uniformity in surface temperature during boiling stage in Inconel probe was 10 times lower in molten salt medium as compared to that observed in 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. Copyright © 2019 ASM International® All rights reserved.