Browsing by Author "Prabhu, K.N."

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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.
A Computer Aided Cooling Curve Analysis method to study phase change materials for thermal energy storage applications
(Elsevier Ltd, 2016) Sudheer, R.; Prabhu, K.N.
The suitability of a simple Computer Aided Cooling Curve Analysis (CACCA) technique for characterizing thermal energy storage phase change materials (PCM) was proposed in the present work. Two modes of CACCA, namely, Newtonian and Fourier techniques were used to predict the phase transition temperatures, the latent heat of fusion and thermal diffusivities of PCMs. Solidification of potassium nitrate and zinc-8% aluminium alloy (ZA8) was studied using CACCA method. These PCMs were chosen to demonstrate the ability of the proposed technique to characterize PCMs freezing at a single temperature as well as over a range of temperatures. CACCA method showed that potassium nitrate and ZA8 are suitable candidate materials for TES applications operating at 300-350 °C and 350-450 °C respectively. © 2015 Elsevier Ltd.
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
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.
A quantitative approach for thermal characterization of phase change materials
(ASTM International, 2021) Agarwala, S.; Prabhu, K.N.
A quantitative method for the calculation of phase change parameters of salt-based phase change materials (PCMs) has been proposed. This technique involves the estimation of mold-salt interfacial heat flux by solving Fourier's law of heat conduction within the salt and using it for the calculation of phase change enthalpy of salt PCMs. Radial heat transfer was ensured by keeping the length to diameter (L/D) ratio of the mold equal to 5. The proposed method eliminates any drawbacks involved with sample size, reference material, the baseline fitting calculations, and the errors introduced due to the selection of solidification points. Pure salt PCMs such as potassium nitrate (KNO3), sodium nitrate (NaNO3), and solar salt mixture (60 wt. % NaNO3 + 40 wt. % KNO3) were used for validation of this technique. The thermal behaviors of the salt and the mold during solidification of the salt sample were analyzed, and solidification characteristics such as cooling rate, solidification time, and phase change enthalpy of PCMs were determined. © © 2021 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959
An experimental approach based on inverse heat conduction analysis for thermal characterization of phase change materials
(Elsevier B.V., 2020) Agarwala, S.; Prabhu, K.N.
A new method based on solution to inverse heat conduction problem for the assessment of solidification parameters of PCM salts has been proposed. The method estimates the mold -salt interfacial heat flux and it is used to calculate the latent heat of salt PCMs using calorimetry based energy balance equations. This method is more accurate compared to Computer Aided Cooling Curve Analysis (CACCA) techniques as it eliminates the drawbacks involved with base line fitting calculations and errors introduced due to the improper selection of solidification points. Pure salt PCMs such as KNO3 and solar salt were used for the validation of this technique. Both air and furnace cooling were adopted to demonstrate the effect of cooling rate on solidification characteristics. The wettability of salt samples on mild steel surface was analyzed to account for the difference in the thermal behavior of salts. © 2020 Elsevier B.V.
Assessment of Cooling Performance of Neem Oil for Distortion Control in Heat Treatment of Steel
(Springer, 2020) Pranesh Rao, K.M.; Prabhu, K.N.
Growing concerns over the hazardous impact of mineral oil-based industrial quench media on human health and the environment have forced researchers to seek renewable and non-hazardous alternatives. Non-edible vegetable oil-based quench media are perceived to be a potential replacement for mineral-based industrial quench media. The present work focuses on assessing the cooling performance of neem oil as compared to commercial hot oil quench media. Inconel and steel probes were used to characterize the cooling performance of these quench media maintained at bath temperatures 100 °C, 150 °C and 200 °C. The heat extraction rates and uniformity of heat extraction in Inconel probes quenched in neem oil were observed to be substantially higher at all bath temperatures. The hardness of AISI 52100 steel probe quenched in neem oil at all bath temperatures was observed to be higher. The pearlitic microstructure was observed in the steel probe quenched in hot oil maintained at 200 °C bath temperature. In contrast to this, a mixture of bainite, martensite and carbide was observed in case of steel probes quenched in neem oil maintained at 200 °C. Oxidation experiments revealed that neem oil is susceptible to an increase in viscosity due to oxidation. An increase in the viscosity by about 15% was observed in the case of neem oil as compared to only 4% increase in viscosity of hot oil. However, after an initial increase, the viscosity of neem oil stabilized and further no significant change in viscosity due to oxidation were observed. Oxidation had no significant effect on the cooling performance hot neem oil quench medium, and thus, it can be considered as an effective replacement for hot oil. © 2020, ASM International.
Assessment of corrosion behavior of ductile irons by factorial experiments
(2009) Surendranathan, A.O.; Prabhu, K.N.; Sudhaker Nayak, H.V.
The corrosion behavior of unalloyed and alloyed ductile irons (as cast, annealed, and cold worked) in sea water, dilute sulfuric acid, and dilute sodium hydroxide solutions was assessed. Specimen history had a significant effect on the corrosion potential except in ductile iron alloyed with Ni. When the specimens were subjected to different levels of cold working, the corrosion rate was influenced by both the history and the medium. Temperature had a significant effect on the corrosion rate except in the case of unalloyed ductile iron. Factorial experiments indicated that the cold-worked samples were more sensitive to the effect of temperature and composition on the corrosion rate as compared to annealed and as-cast samples. The medium had a significant effect on the corrosion rate in all the cases. © 2009 ASM International.
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.
Assessment of heat transfer during solidification of Al-22% Si alloy by inverse analysis and surface roughness based predictive model
(2012) Jayananda; Prabhu, K.N.
Heat flux transients were estimated during unidirectional downward solidification of Al-22% Si alloy against copper, die steel and stainless steel chills. The chill instrumented with thermocouples was brought into contact with the liquid metal so as to avoid the effect of convection associated with the pouring of liquid metal. Heat flux transients were estimated by solving the inverse heat conduction problem. Higher thermal conductivity of chill material resulted in increased peak heat flux at the metal/ chill interface. Peak heat flux decreased when 100 lm thick alumina coating was applied on the chill surface. The lower thermal conductivity of alumina based coating and the presence of additional thermal resistance decreases the interfacial heat transfer. For uncoated chills, the ratio of the surface roughness (R a) of the casting to chill decreased from 6.5 to 0.5 with decrease in the thermal conductivity of the chill material. However when coating was applied on the chill, the surface roughness ratio was nearly constant at about 0.2 for all chill materials. The measured roughness data was used in a sum surface roughness model to estimate the heat transfer coefficient. The results of the model are in reasonable agreement with experimentally determined heat-transfer coefficients for coated chills.
Assessment of heat transfer during solidification of Al-22% Si alloy by inverse analysis and surface roughness based predictive model
(Springer India sanjiv.goswami@springer.co.in, 2012) Jayananda; Prabhu, K.N.
Heat flux transients were estimated during unidirectional downward solidification of Al-22% Si alloy against copper, die steel and stainless steel chills. The chill instrumented with thermocouples was brought into contact with the liquid metal so as to avoid the effect of convection associated with the pouring of liquid metal. Heat flux transients were estimated by solving the inverse heat conduction problem. Higher thermal conductivity of chill material resulted in increased peak heat flux at the metal/ chill interface. Peak heat flux decreased when 100 lm thick alumina coating was applied on the chill surface. The lower thermal conductivity of alumina based coating and the presence of additional thermal resistance decreases the interfacial heat transfer. For uncoated chills, the ratio of the surface roughness (R a) of the casting to chill decreased from 6.5 to 0.5 with decrease in the thermal conductivity of the chill material. However when coating was applied on the chill, the surface roughness ratio was nearly constant at about 0.2 for all chill materials. The measured roughness data was used in a sum surface roughness model to estimate the heat transfer coefficient. The results of the model are in reasonable agreement with experimentally determined heat-transfer coefficients for coated chills.
Assessment of Joint Reliability of Sn–2.5Ag–0.5Cu Solder/Cu as a Function of Reflow Time
(Springer India sanjiv.goswami@springer.co.in, 2016) Sona, M.; Prabhu, K.N.
Solder interconnect reliability is exceedingly influenced by the solder material properties, the intermetallics formed during soldering, and the environmentally imposed loads. The current study involves the study of the wetting behaviour, interfacial reactions and growth of intermetallic compounds (IMCs) in Sn–2.5Ag–0.5Cu lead free solder solidified on Cu substrate as function of reflow time. The reflow temperature was maintained at 270 °C and reflow time was varied from 10 to 10,000 s. The solder alloy showed better wettability on the substrate with the increase in reflow time. A growth in Cu6Sn5 IMC thickness was observed up to a reflow time of 500 s. IMC layer formed during a reflow time of 10 s was 3.09 ?m and its thickness increased to 4.40, 8.21, 10.96 ?m during 100, 300 and 500 s reflow time respectively. A decrease in the thickness of IMC was observed for samples reflowed for 1000 s. The IMC thickness increased with further increase in reflow time. The joint reliability of Sn–2.5Ag–0.5Cu solder solidified on copper substrate surfaces was assessed by performing bond shear test. The average shear strength decreased with an increase in reflow time after an optimum value. © 2015, The Indian Institute of Metals - IIM.
Assessment of latent heat and solid fraction of Al-22Si alloy using Newtonian and Fourier analysis techniques
(2015) Vijayan, V.; Prabhu, K.N.
Computer aided cooling curve analysis (CACCA) is an online prediction tool for the determination of solidification characteristics of metals or alloys. The results of CACCA can be used to accurately determine latent heat and solid fraction needed for modeling of the solidification process. Newtonian and Fourier analysis techniques adopt a data base line fitting technique to the first derivative curve for calculation of the solid fraction and latent heat of solidification. This paper describes the theoretical and experimental procedures involved Newtonian and Fourier analysis techniques with reference to an Al-22% Si alloy. The correlations between the solid fraction and temperature/time for the alloy were determined. � (2015) Trans Tech Publications, Switzerland.
Assessment of PCM-container interfacial heat transfer using a hot/cold probe technique
(John Wiley and Sons Inc. P.O.Box 18667 Newark NJ 07191-8667, 2019) Sudheer, R.; Prabhu, K.N.
A novel technique for assessing heat transfer characteristics of salt-based phase change materials (PCM) was proposed here. The method is based on solution to inverse heat conduction problem. Nanoparticles (Graphite, Graphene, and multi wall carbon nanotube [MWCNT]) were dispersed in the PCM (KNO3) to assess their respective influence on heat transfer in the PCM. Graphite added PCM offered highest heat flow values and heating rates, while the pure salt-PCM offered the least. The probe material had a significant influence on the heat transfer rates at the PCM-probe interface. © 2018 Wiley Periodicals, Inc.
Assessment of Solidification Parameters of Salts and Metals for Thermal Energy Storage Applications Using IHCP-Energy Balance Combined Technique
(Springer, 2018) Agarwala, S.; Prabhu, K.N.
Solar energy storage technologies have proved to be promising in terms of providing uninterrupted power supply. The phase change materials (PCMs) with their higher heat storage capacity are more efficient than sensible heat storage materials. In this study, a new method for thermal analysis of PCM salts was proposed. The method was based on the estimation of heat flux at the mold–salt interface using solution to inverse heat conduction problem and characterizing the salt using a simplified energy balance method. It was advantageous over other computer-aided cooling curve analysis methods as it eliminated the use of curve fitting approach involved in baseline calculations. KNO3 and NaNO3 salts were used to validate this method. The solidification parameters like cooling rate, liquidus and solidus temperatures, solidification time and latent heat were assessed. The results of the analysis were in agreement with the data reported in the literature. © 2018, The Indian Institute of Metals - IIM.
Assessment of spatiotemporal heat flux during quenching in TiO2 and AlN nanofluids
(ASTM International, 2017) Nayak, U.V.; Ramesh, G.; Prabhu, K.N.
In the present work, spatiotemporal heat flux transients were estimated during quenching of an Inconel 600 alloy probe in water-based titanium dioxide (TiO2) and aluminum nitride (AlN) nanofluids that have nanoparticle concentrations varying from 0.001 to 0.5 vol. %. The results showed reduced peak heat flux and a longer vapor phase stage during quenching with nanofluids compared to quenching with water. The peak heat flux for quenching in nanofluids was lowered with increase in the nanoparticle concentration. Quenching with TiO2 nanofluids resulted in slower heat extraction compared to quenching in AlN nanofluids at higher concentrations. Quenching with nanofluids resulted in a more uniform quench compared to quenching with water because of the reduction in the rewetting period. © © 2017 by ASTM International.
Assessment of the Performance of Sn–3.5Ag/Cu Solder Joint Under Multiple Reflows, Thermal Cycling and Corrosive Environment
(Springer, 2018) Samuel, A.; Tikale, S.; Prabhu, K.N.
The solder joint performance of Sn–3.5Ag/Cu combination was studied under multiple reflows, thermal cycling and exposure to the corrosive environment. Factorial experiment was carried out to assess the effect of individual parameters and the interaction of parameters on the shear strength of the solder joint. The results showed that the combination of thermal cycling and immersion in corrosive media resulted in the maximum decrease in the shear strength followed by the combination of multiple reflows and corrosive media. The shear strength reduced with the increase in immersion duration in corrosion medium. Factorial experiment was analyzed using analyis of variance (ANOVA). The results indicated that the individual parameters had a significant effect, whereas the effect of interaction of these parameters was less significant on the performance of the solder joint. Fracture surface indicated mixed mode of failure and the occurrence of fracture predominantly in the bulk solder. © 2018, The Indian Institute of Metals - IIM.
Bond shear strength of Al2O3 nanoparticles reinforced 2220-capacitor/SAC305 solder interconnects reflowed on bare and Ni-coated copper substrate
(Springer, 2021) Tikale, S.; Prabhu, K.N.
The influence of Al2O3 nanoparticles addition in trace amounts and electroless Ni–P substrate coating on the microstructure development and bond shear strength of Sn-3.0Ag–0.5Cu (SAC305) solder joint were investigated. The performance and reliability of the 2220-capacitor joints with Al2O3 nanoparticle reinforced nanocomposites reflowed on Cu and Ni–P coated substrate were analyzed under varying high-temperature environments. The addition of nanoparticles enhanced the wettability and microhardness of the solder and considerably refined the joint microstructure. The dispersion and adsorption of Al2O3 nanoparticles resulted in the suppression of intermetallic (IMC) growth at the interface and refinement of the ?-Sn grains as well as IMC precipitates into the matrix. The Ni–P coating on the substrate significantly retarded the IMC growth kinetics resulting in the formation of a thin and uniform IMC layer at the joint interface. The thermal stability and performance of the joint under high-temperature environments were enhanced due to the Ni–P coating on the substrate. Compared to the unreinforced SAC305 solder joint with bare Cu substrate, joints with SAC305 + 0.05Al2O3 composite showed about 17% higher shear strength with bare Cu substrate and about 27% higher strength with Ni–P coated substrate. The Weibull distribution analysis indicates a significant improvement in joint reliability of the 2220-capacitor/SAC305 solder assembly using SAC305 + 0.05Al2O3 nanocomposite and Ni-coated substrate. The ANOVA study suggests that the solder joint performance majorly depends on the operating environment, solder composition, and the substrate finish. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature.
Carbonated aqueous media for quench heat treatment of steels
(Springer New York LLC barbara.b.bertram@gsk.com, 2016) Nayak, U.V.; Pranesh Rao, K.M.P.; Pai, M.A.; Prabhu, K.N.
Distilled water and polyalkylene glycol (PAG)-based aqueous quenchants of 5 and 10 vol.% with and without carbonation were prepared and used as heat transfer media during immersion quenching. Cooling curves were recorded during quenching of an inconel 600 cylindrical probe instrumented with multiple thermocouples. It was observed that the vapor stage duration was prolonged and the wetting front ascended uniformly for quenching with carbonated media. The cooling data were analyzed by determining the critical cooling parameters and by estimating the spatially dependent probe/quenchant interfacial heat flux transients. The study showed significantly reduced values of heat transfer rate for carbonated quenchants compared to quenchants without carbonation. Further, the reduction was more pronounced in the case of PAG-based carbonated quenchants than carbonated distilled water. The results also showed the dependence of heat transfer characteristics of the carbonated media on polymer concentration. The effect of quench uniformity on the microstructure of the material was assessed. © 2016, ASM International.
Characterisation of Sn–3.5Ag solder/Cu joint under various reflow conditions
(Taylor and Francis Ltd., 2022) Georgy, K.; Tikale, S.; Prabhu, K.N.
The effect of reflow time and reflow temperatures on wettability and bond shear strength of Sn–3.5Ag solder alloy on a Cu substrate is assessed for reflow times of 10 s, 100 s, 300 s, 500 s and reflow temperatures of 250 °C, 270 °C, 290 °C, 320 °C. The wetting regime was found to have capillary, gravity, and viscous regimes. A microstructural study using a scanning electron microscope (SEM) with energydispersive spectroscopy revealed the presence of intermetallic compounds at the interface between the substrate and solder droplet. The bond strength of the solder joint is maximum at a reflow temperature of 250 °C and a reflow time of 10 s. Bond shear strength decreased with an increase in reflow time from 10 s to 500 s. The effect of different cooling conditions on the solder–substrate joint is also investigated. © 2022 Institute of Materials, Minerals and Mining.