Journal Articles
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Item Weldments of AISI grade 316 stainless steel, having a ferrite content of 4-6% and a variety of nitrogen concentrations were prepared using a modified element implant technique. Charpy impact specimens prepared from these weldments were subjected to a variety of aging treatments. Impact toughness decreases with aging time at all aging temperatures. Nitrogen is found to be beneficial to toughness. An empirical relation connecting the aging temperature, aging time and nitrogen content with toughness has been developed which can be used to estimate the time for embrittlement.(Indian Academy of Sciences, Estimation of embrittlement during aging of AISI 316 stainless steel TIG welds) Nayak, J.; Udupa, K.R.; Hebbar, K.R.; Nayak, H.V.S.2004Item Simulated weldments of AISI grade 304 stainless steel having a ferrite content of 4-6% with three levels of nitrogen (0.03, 0.08, and 0.11 wt-%) were prepared using a modified elemental implant technique. From these weldments, subsize Charpy impact specimens were prepared and subjected to aging treatment at different temperatures, 623-748 K, and for different times, 1000-5000 h. Impact toughness curves for these aged samples were generated by testing at various temperatures from 77 K to 300 K. From the impact curves the upper shelf energy (USE) and lower shelf energy (LSE) were determined. It was observed that both USE and LSE decreased with aging time at all temperatures. Nitrogen seems to offer a beneficial effect as far as impact toughness is concerned, as both USE and LSE values increased with increasing nitrogen content. The worst aging conditions were identified as 748 K, 2000 h at the lowest nitrogen level of 0.03 wt-%. An empirical relation connecting the aging temperature, aging time, and nitrogen content to the LSE was developed, which can be used to predict the time for embrittlement at a given nitrogen level and aging temperature. © 2005 Institute of Materials, Minerals and Mining.(Prediction of embrittlement during aging of nuclear grade AISI 304 stainless steel TIG welds) Nayak, J.; Udupa, K.R.; Hebbar, K.R.; Nayak, H.V.S.2005Item Effect of surface roughness on metal/quenchant interfacial heat transfer and evolution of microstructure(Elsevier Ltd, 2007) Prabhu, K.N.; Fernandes, P.In the present work, the effect of surface roughness on heat transfer rates in various quenchants was determined. The heat flux transients at the probe/quenchant interface were estimated by inverse modeling of heat conduction during end quenching of stainless steel probes with three different surface roughness (grooved, Ra = 3.0 and 1 ?m). Heat transfer during quenching was correlated with the hardness obtained for medium carbon AISI 1060 steel specimens. The effect of surface roughness on heat transfer rate during quenching in water and brine was significant for rough surface whereas its effect on heat transfer rate is only marginal in high viscosity oil quenchants. A fully martensitic structure was observed with grooved surface subjected to water quenching. With a smooth surface a mixed microstructure was obtained. The oil quenched specimens were found to be less sensitive to surface roughness. © 2005 Elsevier Ltd. All rights reserved.Item N and p doped poly(3,4-ethylenedioxythiophene) electrode materials for symmetric redox supercapacitors(2007) Bhat, D.K.; Muthu, M.S.A symmetric redox supercapacitor has been fabricated based on n and p doped Poly(3,4-ethylenedioxythiophene)(PEDOT) coated on stainless steel (SS) electrodes. The characterization and performance of the supercapacitor has been studied by FTIR, Cyclic Voltammetry and AC Impedance spectroscopy. The supercapacitor showed a maximum specific capacitance of 121 F g-1 at a scan rate of 10 mV s-1. The time constant calculated for the supercapacitor through the active-reactive power behavior measurement was 12 milliseconds indicating the suitability of the system for efficient use at low frequency range. © 2007 Springer Science+Business Media, LLC.Item Effect of substrate surface roughness on wetting behaviour of vegetable oils(2009) Prabhu, K.N.; Fernades, P.; Kumar, G.Vegetable oils are mainly used in the heat treating industry due to their environmental friendliness. In the present work the effect of surface roughness on spreading of vegetable oils on stainless steel substrates was investigated. Spreading phenomenon was digitally recorded and analyzed. All of the oils under investigation exhibited power law spreading behaviour of the type: A = ktn, where A, t, k and n represent the drop base contact area, spreading time, constant and exponent, respectively. The coconut and sunflower oils exhibited accelerated kinetics owing to their lower viscosity as compared to palm and mineral oils while peanut oil showed intermediate behaviour. Viscous regime was dominant during spreading of mineral and palm oils as compared to that of coconut oil. All the oils took longer period of time on rough surfaces than on smooth surfaces to relax to the same degree of wetting. Oils spreading on rough surfaces had to overcome the additional barrier due to asperities of the rough surface. Contact angle decreased with increase in roughness supporting the Wenzel's proposition. The decrease was significant for increase in roughness from 0.25 ?m to 0.50 ?m for all oils. However, the effect was negligible with further increase in roughness particularly for high viscosity oils. A spread parameter (?) is proposed to account for the variation of contact angle with surface roughness of the substrate and momentum diffusivity of the spreading liquid. The result suggested that low viscosity liquids exhibit improved wetting characteristics during spreading on rough surfaces. A model is proposed to estimate dynamic contact angles on substrates having varying surface roughness. © 2008 Elsevier Ltd. All rights reserved.Item Enhancement of heat transfer characteristics of transformer oil by addition of aluminium nanoparticles(2011) Rajesh, E.; Prabhu, K.N.A two step approach involving the synthesis of Al nanoparticles by mechanical milling followed by dispersion of the nanoparticles in the base fluid is adopted in the present work to prepare transformer oil based nanofluids. Stainless steel (AISI 304) probes of diameter 15 mm and height 70 mm were used to determine the cooling rate intensities of nanofluids. Heat transfer coefficients were determined using Kobasko's method. A dynamic contact angle analyzer was used to determine the contact angle of the droplet on the substrate. The addition of Al nanoparticles to the base fluid decreases the wettability and improves its heat transfer capability. The vapour phase stage existed for a longer period of time for transformer oil than for Al-transformer oil based nanofluids. The dispersion of nanoparticles in the base fluid is believed to disrupt the vapour blanket stage in the early stage of the cooling process. The peak heat transfer coefficient increases with an increase in the Al nanoparticle content in the oil. The addition of 0.5 vol % nanoparticles enhances the peak heat transfer coefficient by about 70 %. Copyright © 2011 by ASTM International.Item Comparison of Grossmann and lumped heat capacitance methods for assessment of heat transfer characteristics of quench media(2011) Prabhu, K.N.; Ali, I.The suitability of Grossmann and lumped heat capacitance (LHC) methods for estimation of quench severity of nanofluids, brine solutions and a laboratory detergent based medium was investigated. The study involved the assessment of the effect of section thickness of the probe on heat transfer coefficients for different quench media. Computer aided cooling curve analysis during quenching of type 304 stainless steel probes was carried out. The measured thermal history data were used to estimate heat transfer coefficients by Grossmann and LHC methods. The LHC method based on a 10 mm diameter stainless steel probe was found to be applicable for characterising media having quench severity less than 20 m-1. Although Grossmann method is based on the concept of average heat transfer coefficient, it can be used for quench media having varying severity of quenching. Grossmann method is found to be more sensitive to the effect of section thickness on heat transfer. © 2011 IHTSE Partnership.Item Heat transfer at the casting/chill interface during solidification of commercially pure Zn and Zn base alloy (ZA8)(2012) Ramesh, G.; Prabhu, K.N.Casting/chill interfacial heat transfer during solidification of commercially pure zinc and ZA8 alloy against copper, hot die steel, stainless steel and aluminiuminstrumented chills was investigated. The peak heat flux strongly depends on the thermophysical properties of chill, chill surface condition and superheat of the castingmaterial. Contact angles of alumina coatingmeasured on various substrates suggested that the adhesion of the coating material on copper chill was significantly better as compared to other chill materials. The heat flux curve in the case of coated chills is characterised by a double peak indicating remelting of the solidified casting shell. The second peak in the HTC curve is lower for high conductivity and higher for low conductivity chills as compared to the first peak. It is possible that solid shell formation and remelting occurred in the case of high thermal conductivity chills, whereas shell remelting did not happen in lower thermal conductivity chills. © 2012 W. S. Maney & Son Ltd.Item Effect of surfactant on high capacitance of galvanostatically deposited MnO2(Elsevier B.V., 2012) Suhasini; Hegde, A.Manganese dioxide has been galvanostatically deposited on stainless-steel substrate from an aqueous acidic solution of manganese sulphate (1 M) in presence of a surface active agent (surfactant), namely, sodium lauryl sulphate (SLS), for supercapacitor applications. The deposits have been developed under different conditions of SLS and their specific capacitance is measured by cyclic voltammetry (CV) and also by galvanostatic charge/discharge cycle. The oxide film (?0.1 mg cm-2) anodized from the manganese solution at 2.0 mA cm-2 has shown the highest specific capacitance of 255.8 F g -1, at scan rate of 10 mV s-1. It is observed that the capacitance increased by about 40% compared to the oxide prepared in the absence of SLS. Improved specific capacitance is due to the effect of the surfactant molecules in the deposit, causing high surface area of the deposit. The deposit is found to display good cycleability, even up to 1500 cycles. The structure and surface morphology of the deposits have been studied by means of X-ray diffraction (XRD) analysis and Scanning Electron Microscopy (SEM). XRD study reveals that crystallinity of the deposit with SLS remains unchanged, both are amorphous in nature. The surface area of the deposit is found to increase considerably due to the effect of SLS, as evident by SEM study.© 2012 Elsevier B.V. All rights reserved.