Faculty Publications
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Item Compositional and Bath Temperature Effects on Heat Transfer During Quenching in Molten NaNO3–KNO3 Salt Mixtures(Springer, 2020) Pranesh Rao, K.M.P.; Prabhu, K.N.The present study involved the assessment of cooling severity of molten NaNO3–KNO3 mixtures which are widely used as quench media for austempering and martempering operations. An Inconel probe instrumented with thermocouples was quenched in molten NaNO3–KNO3 binary mixtures of varying concentration maintained at different quench bath temperatures. The temperature data acquired at various locations in the Inconel probe during quenching was used to calculate the spatially dependent transient heat flux at the metal–quenchant interface. Two critical points corresponding to peak heat extraction rates during the nucleate boiling stage and transition from boiling to convection stage were identified for each quench medium. The variation of average heat flux and average surface temperature corresponding to these critical points was mapped with variation in bath temperature and composition of the quench medium. AISI 4140 steel probes were quenched in these quench media maintained at 300 and 350 °C. The average hardness values measured in steel probes agreed with the cooling performance of these quench media determined using Inconel probe. The degree of uniformity in heat transfer as indicated by the spatial variation of normalized heat energy decreased with the increase in the concentration of KNO3 in the quench medium. A mechanism of boiling heat transfer during quenching based on thermochemical decomposition of the salt was proposed. © 2020, ASM International.Item Numerical Simulation to Predict the Effect of Process Parameters on Hardness during Martempering of AISI4140 Steel(Springer, 2021) Pranesh Rao, K.M.P.; Prabhu, K.N.Martempering is a widely practiced industrial heat treatment process to harden steel parts with minimum distortion. A numerical experiment to predict hardness distribution in AISI 4140 steel cylinders of various diameters during martempering is presented in this work. Apart from the diameter (D), the effect of other process variables such as heat transfer coefficient (h), bath temperature (Tb), and residence time (tr) was also studied. The relationship between hardness distribution and the aforementioned process variables was highly nonlinear. An artificial neural network (ANN) model with a single hidden layer and 30 hidden layer neurons was thus developed to predict the hardness distribution in martempered AISI 4140 steel cylinders. The increase in bath temperature, diameter, and residence time decreased the average hardness, and an increase in the heat transfer coefficient increased the average hardness of martempered AISI 4140 cylinders. The weights of the ANN model were used to calculate the relative importance of all input variables and they followed a decreasing order of Tb>D>tr>h. © 2021, ASM International.Item A Novel LiNO3-Based Eutectic Salt Mixture for Industrial Heat Treatment(ASTM International, 2022) Pranesh Rao, K.M.P.; Prabhu, K.N.A potassium nitrate-lithium nitrate-sodium nitrate (KNO3-LiNO3-NaNO3) eutectic mixture having a low melting point has been proposed as an alternative high-temperature quench medium. Inconel and steel probes were used to compare the quench performance of a conventional sodium nitrite (NaNO2) eutectic mixture and the proposed alternative medium at different bath temperatures. For the Inconel probe, the heat extraction rate was higher in the eutectic LiNO3 mixture maintained at 150°C. At elevated bath temperatures of 200°C, 250°C, and 300°C, the heat extraction rate was higher in the eutectic NaNO2 mixture. AISI 52100 steel probes quenched in eutectic LiNO3 quench medium at 150°C and 200°C showed higher hardness. At bath temperatures of 250°C and 300°C, the hardness of AISI 4140 steel probes quenched in both media was comparable. Wettability studies on Inconel and steel surfaces revealed the occurrence of nonuniform dilation of a LiNO3 eutectic mixture droplet. On the steel surface, the phenomenon occurred at lower temperatures, which resulted in an extended boiling stage and increased hardness. © 2022 by ASTM International