Department of Metallurgical & Materials Engineering

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Heavy fuel oil (HFO) as a fuel in industrial and power generation plants ensures the availability of energy at economy. Coke and cenosphere emissions from HFO combustion need to be controlled by particulate control equipment such as electrostatic precipitators, and collection effectiveness is impacted by the properties of these particulates. The cenosphere formation is a function of HFO composition, which varies depending on the source of the HFO. Numerical modelling of the cenosphere formation mechanism presented in this paper is an economical method of characterising cenosphere formation potential for HFO in comparison to experimental analysis of individual HFO samples, leading to better control and collection. In the present work, a novel numerical model is developed for understanding the global cenosphere formation mechanism. The critical diameter of the cenosphere is modelled based on the balance between two pressures developed in an HFO droplet. First is the pressure (Prpf) developed at the interface of the liquid surface and the inner surface of the accumulated coke due to the flow restriction of volatile components from the interior of the droplet. Second is the pressure due to the outer shell strength (PrC) gained from van der Walls energy of the coke layers and surface energy. In this present study it is considered that when PrC ≥ Prpf the outer shell starts to harden. The internal motion in the shell layer ceases and the outer diameter (DSOut) of the shell is then fixed. The entire process of cenosphere formation in this study is analysed in three phases: regression, shell formation and hardening, and post shell hardening. Variations in pressures during shell formation are analysed. Shell (cenosphere) dimensions are evaluated at the completion of droplet evaporation. The rate of fuel evaporation, rate of coke formation and coke accumulation are analysed. The model predicts shell outer diameters of 650, 860 and 1040 µm, and inner diameters are 360, 410 and 430 µm respectively, for 700, 900 and 1100 µm HFO droplets. The present numerical model is validated with experimental results available from the literature. Total variation between computational and experimental results is in the range of 3–7%. © 2016 Taylor & Francis.

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Combustion Theory and Modelling

Cenosphere formation from heavy fuel oil: a numerical analysis accounting for the balance between porous shells and internal pressure

The microstructural features present in titanium alloys not only vary over a wide range of length scale but are also highly interdependent. It is not possible to experimentally study the variation effect of only one microstructural feature while maintaining others at a constant value. Due to this complexity, there are no physical models available to relate the microstructural features to the mechanical properties. This limitation can be overcome by using artificial neural networks (ANN) which are capable of establishing highly nonlinear and complex relationship that exists between input and output variables. A variety of architectural and training parameters affect the performance of a neural network. Currently, there are no guidelines available to select an appropriate neural network that can be used to solve a particular class of problem. An attempt is made here to circumvent this problem by using Taguchi based design of experiments (DOE) approach. A total of 8 neural network architectural and training parameters have been identified to study (1) their effect on ANN performance and (2) establish a suitable neural network that can be used to establish the complex relationship that exists between various microstructural features—mechanical properties in a near-α titanium alloy. Results obtained indicate that architectural parameters contribute 58 % while the training parameters contribute 35 % to the network performance and the network so identified can establish the complex non-linear relationship between the microstructural features and mechanical properties. © 2016, The Indian Institute of Metals - IIM.

Transactions of the Indian Institute of Metals

Taguchi Based Optimisation of Artificial Neural Network to Establish a Direct Microstructure: Mechanical Property Correlation in a near-α Titanium Alloy

Creep behavior is one of the most critical factors determining the failure life of components at elevated temperature. Turbine rotor shaft is subjected to such elevated conditions. The creep behavior of rotor grade steel 28CrNiMoV59 was studied at elevated temperatures from 550 to 650 °C and at stress levels varying from 120 to 300 MPa. Tensile tests were conducted to determine the basic properties. Microstructural and chemical analyses were performed to characterize the as received material. Creep rupture data were determined from experimental results. These results were used to calculate the creep constants at elevated temperature which are used in time–temperature parametric (TTP) methods. Different TTP methods like, Larson–Miller, Orr–Sherby–Dorn and Manson–Haferd, were analyzed to check their suitability for extrapolation of creep rupture data of (28CrNiMoV59) steel. © 2015, The Indian Institute of Metals - IIM.

Estimation of Creep Failure Life of Rotor Grade Steel by Using Time–Temperature Parametric Methods

In the present study, precipitation of sigma (σ) phase was investigated over the temperature range of 700-850 °C in undeformed and deformed (60% cold rolling) samples of 2507 super duplex stainless steel. The fraction of sigma phase formed as a result of the transformation α → σ + γ2 increases with increasing time and temperature. The increase in sigma phase leads to increase in yield strength and decrease in ductility. Preaging deformation leads to accelerated precipitation of sigma phase. The activation energy for sigma phase precipitation in deformed sample is found to be lower than that in undeformed sample. © 2016, ASM International.

Journal of Materials Engineering and Performance

Effect of Preaging Deformation on Aging Characteristics of 2507 Super Duplex Stainless Steel

The orthopaedic implants for human body are generally made of different biomaterials like stainless steels or Ti based alloys. However, it has been found that from surface properties point of view, none of these materials is attractive for fast tissue or cell growth on the surface of implant. This is one of the most important criteria to assure quick bonding between implant and body tissues vis-à-vis minimum recovery time for the patient. Keeping in view of the above facts, this work involves the pulsed electro-deposition coating of biocompatible hydroxyapatite and its group compounds from a diluted bath of calcium and phosphate salt at various current densities over the biomaterial sheet of SS316. SEM study confirms different morphologies of the coatings at different current densities. Characterization techniques like X-ray diffraction, SEM with EDX and FTIR have been used to confirm the phase and percentage quantity of hydroxyapatite compound in the depositions. This coating can serve as a medium for faster tissue growth over the metallic implants. © 2016 Elsevier B.V.

Materials Science and Engineering C

Synthesis of calcium hydrogen phosphate and hydroxyapatite coating on SS316 substrate through pulsed electrodeposition

There has been a paradigm shift in research foci toward elemental electrodes from the conventional intercalation compound-based electrochemical storage. Replacing intercalation transition metal (oxide) compounds with elemental cathodes (e.g., sulfur, oxygen) theoretically raises the storage capacities by more than one order in magnitude. The insulating nature and complexities of the redox reaction associated with electroactive elements necessitates their housing inside an electronic conductor, which has been mainly carbon. Efficiency of the electrochemical storage using such elemental electrodes, besides depending on factors related to the electrolyte, solid-state diffusion, mainly depends on characteristics of the carbon host. We report here a novel, simple, and efficient pressure-induced capillary encapsulation protocol for the confinement of chalcogens, sulfur (S) and selenium (Se), inside carbon nanotubes (CNTs). Confinement led to lowering of the surface tension of molten S/Se, resulting in superior wetting and ultrahigh loading of the CNTs. Higher than 95% of the CNTs is loaded, and very high loading, nearly 85% of S/Se inside the CNTs, is achieved. When assembled at a very high areal loading (∼10 mg cm-2) in the Li-S/Se battery, the S/Se-CNT cathodes exhibited very stable cyclability and high values of specific capacity at widely varying operating current densities (0.1-10 C-rates). (Graph Presented). © 2016 American Chemical Society.

The Journal of Physical Chemistry C

Pressure-induced capillary encapsulation protocol for ultrahigh loading of sulfur and selenium inside carbon nanotubes: application as high performance cathode in Li–S/se

Magnesium alloys have come to the fore as attractive lightweight structural materials owing to their high specific strength, whilst some of them are considered for bioresorbable implant applications. A new Mg-4Li-1Ca (LX41) alloy, processed by two-step rolling followed by annealing, showed extraordinary properties outperforming the existing Mg alloys in terms of specific strength, strain hardening ability, ductility, and cytocompatibility. The record-breaking superiority in mechanical properties combined with cytocompatibility qualifies the alloy for applications in lightweight engineering structures and bone implants. © 2016 Elsevier B.V. All rights reserved.

Materials Letters

New hot rolled Mg-4Li-1Ca alloy: A potential candidate for automotive and biodegradable implant applications

We show that the lattice contribution to the thermal conductivity of MoS2 strongly dominates the carrier contribution in a broad temperature range from 300 to 800 K. Since theoretical insight into the lattice contribution is largely missing, though it would be essential for materials design, we solve the Boltzmann transport equation for the phonons self-consistently in order to evaluate the phonon lifetimes. In addition, the length scale for transition between diffusive and ballistic transport is determined. The low out-of-plane thermal conductivity of bulk MoS2 (2.3 Wm-1K-1 at 300 K) is useful for thermoelectric applications. On the other hand, the thermal conductivity of monolayer MoS2 (131 Wm-1K-1 at 300 K) is comparable to that of Si. © Copyright EPLA, 2016.

Thermal conductivity of bulk and monolayer MoS2

Microstructural and mechanical characterization investigations on three variants of a through-hardened M50 bearing steel are presented to compare and contrast their performances under rolling contact fatigue (RCF) loading. Baseline (BL) variant of M50 steel bearing balls is subjected to: (i) a surface nitriding treatment and (ii) a surface mechanical processing treatment, to obtain distinct microstructures and mechanical properties. These balls are subjected to RCF loading for several hundred million cycles at two different test temperatures, and the subsequent changes in subsurface hardness and compressive stress-strain response are measured. It was found that the RCF-affected subsurface regions grow larger in size at higher temperature. Micro-indentation hardness measurements within the RCF-affected regions revealed an increase in hardness in all the three variants. The size of the RCF-affected region and intensity of hardening were the largest in the BL material and smallest in the mechanically processed (MP) material. Based on Goodman's diagram, it is shown that the compressive residual stress reduces the effective fully reversed alternating stress amplitude and thereby retards the initiation and evolution of subsurface plasticity within the material during RCF loading. It is quantitatively shown that high material hardness and compressive residual stress are greatly beneficial for enhancing the RCF life of bearings. Copyright © 2016 by ASME.

Journal of Engineering Materials and Technology, Transactions of the ASME

Influence of Residual Stress and Temperature on the Cyclic Hardening Response of M50 High-Strength Bearing Steel Subjected to Rolling Contact Fatigue

The microstructure and mechanical properties improve substantially by hot working. This aspect in as-cast Mg-7.7Al-0.4Zn (AZ80) alloy is investigated by compression tests over temperature range of 30-439°C and at strain rates of 5 × 10−2, 10−2, 5 × 10−4 and 10−4 s−1. The stress exponent (n) and activation energy (Q) were evaluated and analyzed for high-temperature deformation along with the microstructures. Upon deformation to a true strain of 0.80, which corresponds to the pseudo-steady-state condition, n and Q were found to be 5 and 151 kJ/mol, respectively. This suggests the dislocation climb-controlled mechanism for deformation. Prior to attaining the pseudo-steady-state condition, the stress-strain curves of AZ80 Mg alloy exhibit flow hardening followed by flow softening depending on the test temperature and strain rate. The microstructures obtained upon deformation revealed dissolution of Mg17Al12 particles with concurrent grain growth of α-matrix. The parameters like strain rate sensitivity and activation energy were analyzed for describing the microstructure evolution also as a function of strain rate and temperature. This exhibited similar trend as seen for deformation per se. Thus, the mechanisms for deformation and microstructure evolution are suggested to be interdependent. © 2016, ASM International.