Browsing by Author "Prasad Rao, P."

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A ductile iron was austempered at 302 and 385°C for various times to get lower and upper ausferrite microstructures respectively. The microstructures were characterised by optical microscopy and X-ray diffraction. Plane strain fracture toughness was determined under all heat treatment conditions. While the austempered ductile iron with lower ausferrite microstructure showed higher fracture toughness, the one with upper ausferrite microstructure exhibited higher tensile toughness and strain hardening coefficient. A model was developed relating fracture toughness to the yield strength (?y) volume fraction of retained austenite (X?) and the carbon content of the retained austenite (C?). Experimental results showed excellent agreement with the prediction of the model that K21C is proportional to ?y(X?C?)1/2.
(Maney Publishing, Comparative study of fracture toughness of austempered ductile irons with upper and lower ausferrite microstructures) Prasad Rao, P.; Putatunda, S.K.
1998
Abstract Studies have been conducted on the effect of overstressing in rotary bending fatigue on the fatigue properties of an annealed and austempered ductile iron containing 1.5 Ni–0.3 Mo. For various R ratios S–N curves were determined and the fatigue limit estimated. It was found that the fatigue limit was a function of the level of overstressing and cycle ratio. In the case of austempered samples a beneficial effect of overstressing was observed at a certain level of overstressing. This was related to the work hardening behaviour of the austenite phase. In annealed samples, a reduction in the fatigue limit was observed at all levels of overstressing. Copyright © 1995, Wiley Blackwell. All rights reserved
(EFFECT OF OVERSTRESSING ON FATIGUE BEHAVIOUR OF AUSTEMPERED DUCTILE IRON) Prasad Rao, P.; Padmaprabha
1995
An alloy containing 49 at.% aluminium was heat treated at different temperatures within the ? + ? two-phase field to get five different volume fractions of lamellar constituent. Creep studies were carried out on these samples at five different temperatures in the interval between 1023 and 1223 K, by impression creep technique using four levels of stresses at each temperature. For a given temperature and stress, steady state impression velocity decreased with increasing volume fraction of lamellar constituent. Activation energy for steady state creep increased linearly with lamellar content from 185 kJ/mol at 22 vol.% to 362 kJ/mol at 100 vol.%. The stress exponent was found to be around 1.2 in all the cases. The results show that a fully lamellar structure has a superior behaviour where creep is an important factor.
(Carl Hanser Verlag, Effect of microstructure on the impression creep of two-phase titanium aluminide) Prasad Rao, P.; Swamy, K.Shivananda
1995
An investigation was carried out to examine the influence of chromium content on the plane strain fracture toughness of austempered ductile iron (ADI). ADIs containing 0, 0.3 and 0.5 wt.% chromium were austempered over a range of temperatures to produce different microstructures. The microstructures were characterized by optical microscopy and X-ray diffraction. Plane strain fracture toughness of all these materials was determined and correlated with microstructure and chromium content. The chromium content was found to influence the fracture toughness through its influence on the processing window. Since the chromium addition shifts the processing window to shorter durations, the higher chromium alloys at higher austempering temperatures tend to fall outside of the processing window, resulting in less than optimum microstructure and inferior fracture toughness. A small chromium addition of 0.3 wt.% was found to be beneficial for the fracture toughness of ADI. © 2002 Elsevier Science B.V. All rights reserved.
(Elsevier BV, Investigations on the fracture toughness of austempered ductile iron alloyed with chromium) Prasad Rao, P.; Putatunda, S.K.
2003
Austempered ductile iron containing 0.9 Ni and 0.3 Mo and subject to low stress dry sand abrasive testing shows that wear behaviour is very sensitive to heat treatment conditions. It is concluded that large retained austenite content promotes wear resistance.
(Inst of Engineers (India), Low stress dry sand abrasive wear behaviour of austempered ductile iron) Prasad Rao, P.
1995
Cu-44Ni-5Cr alloy has been subjected to thermomechanical treatment which consisted of plastic deformation of as-quenched material by 50, 65 and 80% reduction in thickness followed by ageing in the interval of 500 to 650 °C for various durations of time. Progress in age-hardening was studied by means of hardness measurement and X-ray diffraction studies. The wavelength of composition modulation and strain amplitude were measured. It was found that age-hardening was a result of interaction between spinodal decomposition and recovery processes. Prior deformation was found to enhance the kinetics of both spinodal decomposition and coarsening. It was concluded that this resulted from increased vacancy concentration and increased coherency strain in the cold-worked material. © 1994 Chapman & Hall.
(Kluwer Academic Publishers, Effect of thermomechanical treatment on the phase transformation in Cu-44Ni-5Cr alloy) Raghavendra Bhat, R.; Prasad Rao, P.
1994
Ductile cast iron samples were austenitized at 927 °C and subsequently austempered for 30 minutes, 1 hour, and 2 hours at 260 °C, 288 °C, 316 °C, 343 °C, 371 °C, and 399 °C. These were subjected to a plane strain fracture toughness test. Fracture toughness was found to initially increase with austempering temperature, reach a maximum, and then decrease with further rise in temperature. The results of the fracture toughness study and fractographic examination were correlated with microstructural features such as bainite morphology, the volume fraction of retained austenite, and its carbon content. It was found that fracture toughness was maximized when the microstructure consisted of lower bainite with about 30 vol pet retained austenite containing more than 1.8 wt pet carbon. A theoretical model was developed, which could explain the observed variation in fracture toughness with austempering temperature in terms of microstructural features such as the width of the ferrite blades and retained austenite content. A plot of KIC2 against ?? (X?C?)1/2 resulted in a straight line, as predicted by the model.
(Minerals, Metals and Materials Society, Dependence of Fracture Toughness of Austempered Ductile Iron on Austempering Temperature) Prasad Rao, P.; Putatunda, S.K.
1998
[No abstract available]
(Flow stress - grain size relationship in aluminum) Shiroor, V.S.; Kulkarni, A.G.; Prasad Rao, P.; Parameswaran, V.R.
1975
Progress in spinodal decomposition of a Cu-32Ni-2Cr alloy subjected to quench?age and quench?work?age treatments have been studied through hardness measurements as well as by X-ray diffraction techniques. It is found that recovery processes occur during early stages of aging which are interpreted in terms of the migration of matrix dislocations to interfaces. The kinetics of spinodal decomposition is found to be enhanced by prior deformation. Thermomechanical treatment results in a substantial increase in strength.
(Publ by Carl Hanser Verlag GmbH & Co, Thermomechanical treatment of spinodal Cu-32Ni-2Cr alloy) Bhat, R.Raghavendra; Prasad Rao, P.
1994
Rotating bending fatigue tests were carried out on austempered ductile iron containing 1.5 wt% nickel and 0.3 wt% molybdenum. The ductile iron was austenitized at 900 or 1050 °C and then austempered at 280 or 400 °C for different lengths of time to obtain different microstructures. The fatigue strength was correlated with the amount of retained austenite and its carbon content, which were both determined by X-ray diffraction technique. While the tensile strength decreased with increasing retained austenite content, the fatigue strength was found to increase. Carbide precipitation was found to be detrimental to fatigue strength. Lower austenitizing temperature resulted in better fatigue strength. © 1994 Chapman & Hall.
(Kluwer Academic Publishers, Effect of microstructure on the fatigue strength of an austempered ductile iron) Shanmugam, P.; Prasad Rao, P.; Rajendra Udupa, K.; Venkataraman, N.
1994
Spinodal decomposition in three alloys of nominal compositions 71Cu-27Ni-2Cr, 45Cu-45Ni-10Cr and 33Cu-52Ni-15Cr were studied by X-ray diffraction technique and transmission electron microscopy. It was found that the first and third alloys are asymmetric in nature while the second is symmetric. The symmetric alloy was found to decompose faster than the asymmetric ones. The asymmetry of the side bands was found to be related to the proportion of phases in the alloy. Electron microscopic studies revealed that during coarsening the major phase increases its connectivity by isolating the minor phase. © 1991 Chapman and Hall Ltd.
(Kluwer Academic Publishers, Comparative study of spinodal decomposition in symmetric and asymmetric Cu-Ni-Cr alloys) Prasad Rao, P.; Agrawal, B.K.; Rao, A.M.
1991
Spinodal decomposition in two CuNiCr alloys has been investigated. The variations in strain amplitude, wavelength of composition modulation and yield stress increment were studied with aging time. It was found that the variation in yield stress increment with aging time followed monotonically the variations in strain amplitude and wavelength of composition modulation. It was concluded that theories which relate amplitude to yeild stress best explain the strengthening mechanism. Experimental values were found to correlate well with the theory of Kato, Mori and Schwartz. © 1987.
(Hardening mechanism in spinodal CuNiCr alloys) Prasad Rao, P.; Agrawal, B.K.; Rao, A.M.
1987
The deformation behaviour of a series of alloys containing from 32 to 52 at.-%Al and containing various proportions of ?2 and ? has been studied through compression tests at room temperature. The results showed that strain hardening tendency increases with increasing amount of lamellar constituent in the two phase alloys, which could be attributed to increased slip activity in these alloys as compared with single phase ?2 or ? alloys. It is found that, in the two phase alloys, microcrack formation occurs by the mechanism of microvoid formation due to decohesion of the ?2/? interface and the coalescence of these microvoids. © 1992 The Institute of Materials.
(Deformation behaviour of titanium aluminides at room temperature) Prasad Rao, P.; Tangri, K.
1992
The effect of temperature and strain rate on the 0-2% yield strength, ultimate tensile strength, and percentage elongation of M250 maraging steel was investigated under uniaxial tensile conditions in the temperature range from 25 (room temperature) to 550°C and strain rate range 10-4-10-1 s-1. Up to 400°C the steel shows essentially strain rate insensitive behaviour with a gradual decrease in the 0-2% yield strength and ultimate tensile strength. The elongation remains constant at all strain rates up to 300°C. Fractographic analysis indicates that the increasing strain rate induces strain constraint resulting in an increased dimple size. An elongated structure was observed at temperatures above 400°C. X-ray diffraction reveals the presence of reverted austenite in the specimens tested at 550°C. © 1996 The Institute of Materials.
(Maney Publishing, Effect of temperature and strain rate on tensile behaviour of M250 maraging steel) Venkatanarayana, G.; Arumugham, S.; Lakshmanan, T.S.; Prasad Rao, P.
1996
Transformation characteristics of a Cu-27Ni-2Cr alloy were studied on ageing in the temperature range of 773 to 1073 K, by means of X-ray diffraction and transmission electron microscopy. It was concluded from morphological features that the alloy undergoes spinodal decomposition and the coherent spinodal was estimated to be 946 K. Yield stress increment on ageing was found to follow monotonically the strain amplitude and to be independent of the wavelength of composition modulation. © 1986 Chapman and Hall Ltd.
(Kluwer Academic Publishers, Studies on spinodal decomposition in Cu-27Ni-2Cr alloy) Prasad Rao, P.; Agrawal, B.K.; Rao, A.M.
1986