Faculty Publications
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Item Effect of Bath Temperature on Cooling Performance of Molten Eutectic NaNO3-KNO3 Quench Medium for Martempering of Steels(Springer Boston, 2017) Pranesh Rao, K.M.; Prabhu, K.N.Martempering is an industrial heat treatment process that requires a quench bath that can operate without undergoing degradation in the temperature range of 423 K to 873 K (150 °C to 600 °C). The quench bath is expected to cool the steel part from the austenizing temperature to quench bath temperature rapidly and uniformly. Molten eutectic NaNO3-KNO3 mixture has been widely used in industry to martemper steel parts. In the present work, the effect of quench bath temperature on the cooling performance of a molten eutectic NaNO3-KNO3 mixture has been studied. An Inconel ASTM D-6200 probe was heated to 1133 K (860 °C) and subsequently quenched in the quench bath maintained at different temperatures. Spatially dependent transient heat flux at the metal–quenchant interface for each bath temperature was calculated using inverse heat conduction technique. Heat transfer occurred only in two stages, namely, nucleate boiling and convective cooling. The mean peak heat flux (qmax) decreased with increase in quench bath temperature, whereas the mean surface temperature corresponding to qmax and mean surface temperature at the start of convective cooling stage increased with increase in quench bath temperature. The variation in normalized cooling parameter t85 along the length of the probe increased with increase in quench bath temperature. © 2017, The Minerals, Metals & Materials Society and ASM International.Item A Comparative Study on Cooling Performance of Hot Oil and Molten Salt Quench Media for Industrial Heat Treatment(Springer, 2020) Pranesh Rao, K.M.; Prabhu, K.N.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. © 2020, ASM International.Item Polymer/mold interfacial heat transfer during injection molding(John Wiley and Sons Inc, 2024) Kamala Nathan, D.K.; Prabhu, K.N.An experimental injection molding setup was designed and fabricated. The purpose of the setup is to cast polymer components and estimate the polymer/mold interfacial heat flux transients during injection molding. The mold plate is instrumented with K-type thermocouples to record its thermal history continuously during the cyclic process. Experiments were performed at a melt injection temperature of 280°C. Velocity and shear rate profiles were determined to assess the flow behavior of the melt. The spatiotemporal heat flux transients at the interface and the mold surface temperature were estimated using measured temperature data inside the mold as input to an inverse heat conduction problem. The estimated boundary heat flux transients were used to numerically simulate the polymer melt's cooling behavior. From the estimated heat flux and surface temperatures, heat transfer coefficients (HTC) were determined. The peak value of the HTC was 5775 W/m2K and occurred at a mold surface temperature of 35.7°C and polymer surface temperature of 47.4°C. The evolution of the air gap at the interface was quantified using an exponential fit. The estimated air gap width corresponding to peak HTC was about 4 μm and increased to about 100 μm towards the end of the solidification. While the peak heat flux is associated with the start of the formation of polymer skin on the mold surface, the peak HTC corresponds to the onset of nucleation of the air gap or a nonconforming contact. Highlights: An experimental setup to study heat transfer during injection molding. Spatiotemporal heat flux transients (q) were estimated during injection molding. Polymer temperatures were simulated using q, and HTC was determined. The peak HTC indicated the onset of nucleation of an air gap. Evolution of air gap at the interface was modeled using an exponential fit. © 2023 Society of Plastics Engineers.