Faculty Publications
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Item Assessment of Heat Transfer Characteristics of Transesterified Waste Sunflower Cooking Oil Blends for Quench Hardening(Springer, 2022) Samuel, A.; Prabhu, K.N.Mineral oils used in the heat treatment industry are derivatives of non-renewable petroleum fuel and are toxic and non-biodegradable. Vegetable oils are an ideal substitute for mineral oil due to their superior heat transfer characteristics and eco-friendliness. However, the initial cost of vegetable oils is very high. In addition, the maintenance cost of vegetable oils would be higher due to their poor thermal and oxidative stability than mineral oil. In this context, recycling and reusing waste cooking oil could be a cheaper and eco-friendly alternative. In this study, the fatty acid methyl ester (FAME) produced from the waste sunflower cooking oil through transesterification was blended with sunflower and mineral oils at various proportions. The cooling characteristics of the FAME/oil blends were assessed using the cooling curve analysis according to ASTM D6200 and ISO9950 standards. A solution to the inverse heat conduction problem was used to estimate the spatiotemporal metal/quenchant interfacial heat flux. The uniformity of heat flux was analyzed. The results indicated that blending waste cooking oil-derived FAME in sunflower oil up to 60 vol.% and mineral oil up to 50 vol.% provided comparable cooling characteristics to pure oils. The estimated heat flux transients showed a marginal decrease in peak heat flux for FAME blends in sunflower oil, whereas an increased peak heat flux with mineral oil. The FAME blends less than 60 vol.% in sunflower oil showed higher cooling uniformity. With mineral oil, the blend proportion of up to 50 vol.% increased cooling uniformities compared to pure oil. The characteristic cooling time (t85) increased with the increase in FAME blends in oils. However, the distribution of t85 in the quench probe was uniform for FAME/oil blends. © 2022, ASM International.Item Experimental investigation of heat transfer characteristics of polyethylene glycol (PEG) based quench media for industrial heat treatment(Elsevier Inc., 2023) Soni, A.; Samuel, A.; Prabhu, K.Aqueous polymer quenchants are now increasingly used in the quench hardening of steels. The inverse solubility property of polymer media leads to polymer film encapsulation of the quenched component, followed by an instantaneous rupture of the polymer film. The film boiling stage is absent, thus improving heat transfer uniformity. In the present investigation, the effect of molecular weight of Polyethylene glycol (PEG) on heat transfer characteristics of PEG/water quenchants with concentrations of 5, 10, and 20 vol% was studied. The cooling curve analysis is performed to assess the cooling characteristics. Spatially dependent surface heat flux transients are estimated using the inverse heat conduction method. The rewetting kinematics is analyzed by videography and acoustic analysis of polymer film rupture during quenching. The results indicated that an increase in the molecular weight of PEG from 200 to 6000 changed the rewetting kinematics from a local wetting front movement to an instantaneous rupture of the polymer film. The change in the rewetting kinematics is reflected in the surface heat flux, indicating an increased uniformity of heat transfer. The film rupture acoustics showed that the polymer film's instantaneous breakup had a higher sound intensity than the one showing wetting front motion. © 2023 Elsevier Inc.Item A Phase Transformation Enthalpy Parameter for Modeling Quench Hardening of Steels(Springer, 2024) Samuel, A.; Pranesh Rao, K.M.P.; Prabhu, K.N.The effect of phase transformations on the steel/quenchant interfacial heat flux during quench hardening heat treatment is investigated in the present work. Experimental and modeling approaches comprising the inverse heat conduction problem (IHCP) were employed to analyze the thermal behavior of different steel grades with varying section thicknesses. The results revealed that phase transformation led to a distinctive pattern of the interfacial heat flux, characterized by a dip and subsequent rise. We observed that increasing the section thickness increases the surface heat flux for stainless steel probes without phase transformation. In contrast, the surface heat flux decreased with thicker sections in phase transformation. The increased heat evolved due to the latent heat liberation during phase transformation, and a reduction in thermal diffusivity due to increased specific heat caused a fall in the heat flow rates. Furthermore, the study proposed a phase transformation enthalpy parameter (ΔQ) to access the enthalpy change during quenching. ΔQ was consistent for a specific steel grade and independent of section thickness but varied with the cooling rate or quench media. The incorporation of phase transformation in the quenching heat transfer model is complex due to the required material data, including TTT/CCT diagrams and thermophysical properties that vary with steel grade. The study suggests directly incorporating the ΔQ values into the heat conduction equation or the IHCP model with phase transformation, simplifying the simulation process and minimizing data inputs. A database on ΔQ as a function of temperature and cooling rate would facilitate heat transfer modeling during quench hardening. © 2023, The Minerals, Metals & Materials Society and ASM International.Item Thermal conformance parameters for assessment of heat transfer between similar and dissimilar metal contacts(John Wiley and Sons Inc, 2024) Pathumudy, R.D.; Samuel, A.; Prabhu, K.N.A novel approach to assess the thermal conformance between two metallic materials under transient conditions was proposed in the present investigation. Thermal conformance parameters (ƞ, ϴ, tg) were defined to quantify the contact condition between a metal–metal interface. To assess the effect of load and thermophysical properties of the sink and source materials on the degree of thermal conformance, a thermal conformance assessment parameter (TCAP) was proposed. Heat flux transients at the thermal interface was estimated by using an inverse heat conduction approach for various similar and dissimilar metallic surfaces in contact such as Cu─Cu, Al─Al, Al─Cu, and Cu─Al under both load and no load conditions. Commercially available silicone grease (SG) and thermal grease (CTG) were used as thermal interface materials (TIMs). The thermal conformance parameters increased with the increase in load for all the combinations of interfaces with and without TIMs. It was observed that, except for the copper–copper combination, thermal conformance parameters showed a linear relation with the TCAP. The enhancement in the heat transfer due to the application of load and TIM was validated by determining the maximum temperature difference (∆Tmax) across the interface. The experimental study revealed that the ∆Tmax decreases with the application of load and application of TIM leading to enhanced heat transfer. For the copper–copper combination, the thermal conformance depended solely on the load applied. Due to the lower thermal resistance offered by copper source/sink materials, the interfacial resistance between them becomes a dominant factor. The effect of TIM on heat absorbed by the sink was significant for the Cu/Cu interface. © 2024 Wiley Periodicals LLC.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.