Mathematical model for predicting the thermal and mechanical behaviour of rebar during quenching and self tempering

A mathematical model has been developed to predict the thermal and phase transformation behaviour of plain carbon steel rebars having mass contents of carbon in the range of 0.10 to 0.18 %, during quenching and self tempering process. The model takes into account the operating parameters such as rebar diameter, rolling speed, finishing temperature of rebar leaving the last stand of hot rolling mills. The design features of cooling tube and water flow rate have also been included in the model. The phase transformation kinetics from austenite to martensite at the surface, bainite beneath the surface and ferrite and pearlite in the core has been incorporated in the model. An attempt has been made to predict the yield strength of rebars as a function of chemical composition and volume fractions of different phases. In-plant trials were conducted at Visakhapatnam Steel Plant for 10 and 18 mm diameter rebars. Tempering temperature was measured and volume fractions of different phases and mechanical properties were determined. A very good agreement was found between measured and predicted volume fractions of martensite, bainite and ferrite + pearlite. Comparison between predicted and measured yield strength values show a reasonably good agreement under industrial conditions.     

Synthesis and microwave dielectric properties of Sr3Zn1−xMgxNb2O9 phases

Oxides of the type, Sr₃Zn_1−xMg_xNb₂O₉ (0≤x≤1) have been obtained by the ceramic method. These oxides crystallize in the hexagonal cell corresponding to ordered triple perovskites. Sintered disks show nearly frequency-independent dielectric constant for all the compositions. Compositions sintered at 1425°C yield dielectric constant of 20-22 at ∼6 GHz, with quality factor ranging from 1300 to 1500. Sr₃Zn_0.5Mg_0.5Nb₂O₉ shows a very low temperature coefficient of resonant frequency (τ_f) of +4 ppm/°C.     

A Three-Phase Controlled-Current PWM Converter with Leading Power Factor

Experimental tests performed on a three-phase bipolartransistor controlled-current PWM power modulator show that it can operate with near-sinusoidal currents at 60 Hz with a 360-degree power angle range. Because of its capability to operate with leading power factor and good waveform, the PWM converter is a promising alternative to the thyristor Graetz bridge. A theoretical method based on the concept of ``local average' is presented.     

Ordered structures in ternary hcp alloys

A study of ordered structures in ternary hcp alloys has been undertaken. For this, the hcp structure has been divided into several sublattices and used to generate ordered structures with three types of atoms. Nine ground-state ordered structures have been identified on the basis of maximum or minimum number ofAB, BC andCA bonds. Complete crystallographic details about these structures have been worked out. In an alternate approach, ordered ternary structures were obtained by populating 8 types of sublattices (which generate Ti₃Al structure) with three types of atoms. Thus, fortyeight ordered structures were found. For each structure, complete structural details have also been worked out, some of which are reported. Configurational energy of each structure has been calculated using pairwise interactions up to third neighbour distances. To gain insight regarding low-energy structures, some assumptions were utilized to reduce the number of independent parameters in the energy expressions and their consequences explored. Two types of degenerate situations have been observed. One type of degeneracy occurs for ideal hcp alloys where only first neighbour interactions are considered. Another degenerate situation occurs for non-ideal hcp alloys where interactions are considered up to third neighbour distances.     

A study of coating properties of novel poly(amino-quinone)s prepared from 2-methylbenzoquinone and 2-phenylbenzoquinone

Novel poly(amino-quinone)s were quantitatively prepared from in situ produced 2-methylbenzoquinone and 2-phenylbenzoquinone with Jeffamine D-400^® in the presence of calcium hypochlorite as the oxidizing agent. These polymers were readily soluble in most of the organic solvents such as ethanol, acetone, tetrahydrofuran, chloroform, N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), and N,N-dimethylacetamide (DMAc). The coatings prepared from these polymers showed very good adhesion to metals, hardness, and flexibility even after curing at 110°C for 5 h. Particularly, coatings formulated with epoxy-novalac resin D.E.N-438 showed excellent corrosion resistance in artificial sea water and did not form any blisters. © 1994 John Wiley & Sons, Inc.     

Stochastic behaviour of a maintained system with protection system

This paper deals with the analysis of a single unit system backed by a protection system (.P.S.). Both are subject to failure with exponential failure time distribution. Repair time of working unit, fault detection and inspection time of protection system are assumed to follow general distribution. The system has been investigated in detail by the help of semi-Markov process and several parameters of interest are obtained.     

Finite size effect on Ni doped nanocrystalline NixZn1 − xFe2O4 (0.1 ≤ x ≤ 0.5)

Nanocrystalline nickel ferrite with different concentration of Ni and Zn (Ni_xZn_1 − xFe₂O₄ where x = 0.1, 0.3, 0.5) were synthesized using chemical co-precipitation method. The effect of doping ion concentration on physical properties like crystalline phase, crystallite size, particle size, and saturation magnetization are investigated. The X-ray diffraction pattern confirms the synthesis of single crystalline Ni_xZn_1 − xFe₂O₄ nanoparticles. The lattice parameter decreases with increase Ni content resulting in reduction of lattice strain. HRTEM images revealed that the as-prepared nanoparticles were crystalline with particle size distribution in 10-30 nm range. The saturation magnetization show the superparamagnetic nature of sample for x = 0.1 and x = 0.3 whereas for x = 0.5, the material is ferromagnetic. The saturation magnetization value is 23.95 emu/gm for Ni_0.1Zn_0.9Fe₂O₄ sample and it increases with increase in Ni content.     

Microstructural stability of copper with antimony dopants at grain boundaries: experiments and molecular dynamics simulations

This study presents evidence that the microstructural stability of fine-grained and nanocrystalline Cu is improved by alloying with Sb. Experimentally, Cu_100−x Sb _x alloys are cast in three compositions (Cu-0.0, 0.2, and 0.5 at.%Sb) and extruded into fine-grained form (with average grain diameter of 350 nm) by equal channel angular extrusion. Alloying the Cu specimens with Sb causes an increase in the temperature associated with microstructural evolution to 400 °C, compared to 250 °C for pure Cu. This is verified by measurements of microhardness, ultimate tensile strength, and grain size using transmission electron microscopy. Complementary molecular dynamics (MD) simulations are performed on nanocrystalline Cu–Sb alloy models (with average grain diameter of 10 nm). MD simulations show fundamentally that Sb atoms placed at random sites along the grain boundaries can stabilize the nanocrystalline Cu microstructure during an accelerated annealing process.     

Dynamics of bi-dispersed settling suspension of non-colloidal particles in rotating cylinder

Non-neutrally buoyant suspension of bi-dispersed non-colloidal particles in viscous fluid rotating in a horizontal cylinder displays in-homogeneities in particle distribution with alternate bands of high and low particle concentrations along the symmetric axis of the cylinder. Experiments were carried out to characterize the axial segregation in bi-dispersed suspension at various filling fraction and rotation speed of cylinder. The mixture of same particles in absence of any suspending fluid did not show any segregation. However, in case of particles suspended in water it was observed that the rate of segregation increases with increase in filling fraction. Once the particles get segregated along the full length of the cylinder, these bands start to migrate along the tube axis finally merging to give wider bands. For a given filling fraction the rate of segregation increases with the angular speed of the rotating cylinder. When the tube is partially filled the particle segregation is observed at higher angular speed, whereas in fully filled case the segregation starts at much lower rotation speed for the same concentration of particles. The segregation pattern changes as the rotation speed is increased. At higher speed the centrifugal force dominates over gravitational and viscous drag forces and this result into completely different segregation patterns. We have also analyzed the evolution of concentration profile from the image analysis of the particles.