Journal Articles
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Item Estimation of spatially dependent heat flux transients during quenching of inconel probe in molten salt bath(ASTM International, 2017) Pranesh Rao, K.M.; Prabhu, K.N.Several industrial heat treatment processes, such as martempering and austempering, require a quench bath to be maintained at a temperature ranging between 150°C–600°C. Molten salts, molten alkali, and hot oils are the preferred quenchants for these processes. Molten salts and molten alkali are preferred over hot oil because they possess properties like wide operating temperature range, excellent thermal stability, and tolerance for contaminants. In the present work, the performance of a molten potassium nitrate (KNO3) quench bath was analyzed with an Inconel probe that measured 60 mm in height and 12.5 mm in diameter. The probe was heated to 850°C and subsequently quenched in a bath maintained at 450°C. Cooling curves at different locations of the probe were recorded using the K-type thermocouples inserted into the probe. Spatially dependent transient heat flux at the metal/quenchant interface was estimated using inverse heat conduction technique. The existence of two stages of quenching—boiling stage and convection stage—was confirmed by analyzing the heat flux. The heat transfer coefficient was calculated based on heat flux obtained by the inverse method. The nonuniformity in heat transfer along the length of the probe was quantified by calculating the range of surface temperatures at each instance. The hardness distribution in an AISI 4140 steel was predicted using the temperature distribution in the Inconel probe and obtained using inverse method. Uneven distribution of hardness predicted in the probe was attributed to the nonuniform cooling of the probe during quenching. © © 2017 by ASTM International.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 Critical Heat Transfer Coefficients for Selection of Quench Media during Heat Treatment of Steels(Springer, 2025) Samuel, A.; Pranesh Rao, K.M.; Prabhu, K.N.The depth of hardness in during quench hardening of steels depends on the steel composition, section thickness, and the boundary heat transfer coefficient. A simulation study is performed in the present work to optimize the heat transfer coefficients for selecting quenchants for a particular grade of steel and section thickness. The simulation study is performed by solving phase transformation coupled transient heat conduction equation using the finite element method. The finite element model adopted in this work uses the one-dimensional radially symmetric model with a constant heat transfer coefficient boundary condition at the surface. The variables in the simulation study are the carbon content, the diameter of steel, and the heat transfer coefficients. The effect of these variables on the martensite formation is studied. A critical heat transfer coefficient is defined corresponding to a 50 pct. martensite transformation at the core of the steel cylindrical specimens. The critical heat transfer coefficient increased with the increase in the diameter; whereas, it showed a parabolic relation with the carbon content. The usefulness of the study in selecting a suitable quenchant for quench hardening of plain carbon steels with varying carbon content is illustrated. © ASM International 2024.