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Item MDN-250 maraging steel was electroslag-refined in a 350 kVA DC ESR unit using a slag of composition, CaF2-65, CaO-9, Al2O3-2O, TiO2-5, and MoO3-1 (wt % basis) in straight as well as reverse polarity modes of melting. The chemical homogeneity of the ingots was established by X-ray fluorescence, plasma scan and spectrovac methods. The dissolved gas content (O and N) in the ingots was determined by a LECO gas analyser. Volume fraction, density and size distribution of inclusions in the ingots and the electrode were determined in a Quantimet image analyser. These results were supplemented by EPMA and SEM studies on selected inclusions. Mechanical properties of the ingots in the heat-treated and aged conditions were assessed and compared with those of the electrode material. The present results confirm that the reverse polarity mode of melting is best suited for producing quality maraging steels for the ingots produced by this mode of melting were found to possess superior metallurgical characteristics in terms of cleanliness and mechanical properties compared to those produced by the straight polarity ESR route and other conventional methods. © 1990 Indian Academy of Sciences.(Springer India, Metallurgical characteristics of electroslag-refined MDN 250 maraging steel) Udupa, K.R.; Sastry, D.H.; Iyengar, G.N.K.1990Item Electroslag remelting experiments have been carried out on En 24 steel using two slag compositions in indigeneously constructed 350 KVA DC and 50 KVA AC ESR units. Detailed quantitative characterisation of the inclusions of different types has been carried out using Quantimet Image Analyser. The inclusion content in terms of volume fraction and density (No/mm2) and their size distribution in the refined ingot and the unrefined steel have been assessed. The chemical composition of selected inclusions has been established by electron probe microanalyser (EPMA). These studies are supplemented by scanning electron microscopy on typical fractured specimens. The present study shows that the DC reverse polarity mode of melting using a slag with higher silica content (10 wt%) offers the best conditions with particular reference to removal of inclusions and modification of inclusion morphology as compared to the ingots produced by DC straight polarity and AC modes of melting.(Studies on inclusion characterization in electroslag refined En24 steel) Udupa, K.R.; Subramanian, S.; Sastry, D.H.; Iyengar, G.N.K.1994Item 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 Investigation into creep behaviour of Sn-40%Pb alloy using impression creep method(2009) Udaya Prasanna, H.U.; Udupa, K.R.; Prabhu, K.N.The creep behaviour of Sn-40%Pb hypereutectic alloys cast in the molds made of different materials was investigated using impression creep technique in the temperature range from zero to 32 °C and under the punching stress of 50 MPa. The creep curves.ie, profiles of indentation depth against time are generated and steady state creep rates (SSCRs) are determined. Activation energy was calculated knowing creep rates at different temperature levels. Standard metallographic technique was used to determine the grain size of alloys which were poured into different molds. It was found that SSCR, at all the temperature levels of testing, is a function of grain size of the material. The activation energy being in the range of 10kJ/mol -12 kJ/mol, suggests that the probable creep mechanism is dislocation glide aided by vacancy diffusion. Results of the experiments are discussed.Item Improvement in fracture toughness of austempered ductile iron by two-step austempering process(2010) Ravishankar, K.S.; Prasad Rao, P.P.; Udupa, K.R.Ductile cast iron samples were austenitised at 900°C and subjected to two types of austempering called as conventional austempering and two-step austempering. Five different temperatures, 280, 300, 320, 350, 380 and 400°C, with an austempering time of 2 h, were chosen for conventional austempering. For two-step austempering process, the first step temperatures were 280, 300 and 320°C. The samples were austempered at each of these temperatures for different times, i.e. 10, 20, 30, 45 and 60 min, and then upquenched to higher temperature of 400°C for 2 h. Fracture toughness and tensile studies were carried out under all these austempering conditions. During conventional austempering, the fracture toughness initially increased with increasing austempering temperature, reached a peak value of 63 MPa m 1/2 and dropped with further increase in temperature. During the two-step austempering, fracture toughness was found to increase with increasing first step time. The curve shifted to higher values of fracture toughness as the first step temperature was decreased and the maximum value of 78 MPa m 1/2 was obtained. The results of the fracture toughness study and the fractographic examination were correlated with microstructural features such as bainitic morphology, the volume fraction of retained austenite, and its carbon content. Ferrite lath size and stability of the retained austenite were found to influence the fracture toughness. © 2010 W. S. Maney & Son Ltd.Item Influence of Ti, B and Sr on the microstructure and mechanical properties of A356 alloy(2011) Mallapur, D.G.; Kori, S.A.; Udupa, K.R.In the present investigation, the microstructural and mechanical properties study of A356 alloy have been discussed. The microstructural aspect of cast A356 alloy employed in the present study is strongly dependent on the grain refinement (Ti and B) and modification (Sr). The mechanical properties such as PS, UTS, %E, %R, YM and VHN have been investigated. This paper deals with the combined effect of grain refinement and modification, which improves the overall mechanical properties of the alloy. It is also a well-known fact that the mechanical properties of cast A356 alloy were improved by subjecting suitable melt treatment such as grain refinement, modification and mould vibration, etc. The quality of castings and their properties can be achieved by refining of ?-Al dendrites in A356 alloy by means of the addition of elements such as Ti and B which reduces the size of a-Al dendrites, which otherwise solidifies with coarse columnar a-Al dendritic structure. In addition, modification is normally adopted to achieve improved mechanical properties. Metallographic studies reveal that the structure changes from coarse columnar dendrites to fine equiaxed ones on the addition of grain refiner and further, plate like eutectic silicon to fine particles on addition of 0.20% of Al-10Sr modifier. The present result shows that a reduction in the size of a-Al dendrites, modification of eutectic Si and improvement in the mechanical properties were observed with the addition of grain refiner Al-3Ti, Al-3B and modifier Al-10Sr either individual addition or in combination. The change in the microstructure from coarse columnar ?-Al dendrites to fine equiaxed dendrites and plate like eutectic silicon to rounded particles leads to improved mechanical properties. © Springer Science+Business Media, LLC 2010.Item Influence of Ti, B and Sr on tribological properties of A356 alloy(2011) Mallapur, D.G.; Udupa, K.R.; Kori, S.A.The wear behaviour of an A356 alloy has been investigated in this paper. To understand the wear behaviour of the materials, the experiments were carried out using a pin on disc testing machine at various combinations of normal pressure, sliding speed and sliding distances. Tribological results reveal that weight loss of A356 alloy increases with increasing normal pressure and decreases with increasing sliding speed. Also, the results at microlevel revealed a structural change from coarse columnar dendrites to fine equiaxed ones on the addition of grain refiner (Al and B) and furthermore, plate-like eutectic silicon to fine particles on addition of modifier (Sr). It is further noted in the present study that addition of modifier does not disturb the influence of grain refiner and vice versa. Abrasive wear mechanism was interrupted by the formation of microwelds and later by oxidation of the Al matrix. © 2011 W. S. Maney & Son Ltd.Item Studies on the influence of grain refining and modification on microstructure and mechanical properties of forged A356 alloy(Elsevier Ltd, 2011) Mallapur, D.G.; Udupa, K.R.; Kori, S.A.Microstructure and mechanical properties of the forged A356 alloy have been investigated in this paper. Results reveals that at micro level forged structure was more refined than as in the as cast conditions. This is due to the work hardening effect, where the original structure is destroyed during the forging and recrystallization helped in creating large number of nucleating sites leading to fine grain structure. From the investigations on the mechanical properties, we deduce that the PS, UTS and hardness of forged materials are obviously higher than those of the ones treated with as cast condition without and with the addition of grain refiners and modifier. © 2011 Elsevier B.V.Item Corrosion stability of electrodeposited cyclic multilayer Zn-Ni alloy coatings(2011) Bhat, R.S.; Udupa, K.R.; Hegde, A.C.This paper reports on a study of electrodeposition and characterisation of cyclic multilayer coatings of Zn-Ni alloy from a sulphate bath. Cyclic multilayer alloy coatings were deposited on mild steel through the single bath technique by appropriate manipulation of cathode current densities. The thickness and composition of the individual layers of the CMA deposits were altered precisely and conveniently by cyclic modulation of the cathode current during electrodeposition. Multilayer deposits with sharp change in composition were developed using square current pulses, using thiamine hydrochloride and citric acid as additives. Laminar deposits with different configurations were produced and their corrosion behaviours were studied by AC and DC methods in 5%NaCl solution. It was observed that the corrosion resistance of the CMA coating increased progressively with the number of layers (up to certain optimal numbers) and then decreased. The decrease in corrosion resistance at high degree of layering was attributed to interlayer diffusion due to less relaxation time for redistribution of metal ions at cathode during deposition. The coating configurations have been optimised for peak performance of the coatings against corrosion. It was found that CMA coating developed at cyclic cathode current densities of 3.0/5.0 A dm-2 with 300 layers showed the lowest corrosion rate (0.112×10-2 mm/year) which is ?54 times better than that of monolithic Zn-Ni alloy, deposited from the same bath. The protection efficacy of CMA coatings is attributed to the difference in phase structure of the alloys in successive layers, deposited at different current densities, evidenced by X-ray diffraction analysis. The formation of multilayers and corrosion mechanism were examined by scanning electron microscopy. © 2011 Institute of Metal Finishing.