Magnetic phenomena at metal surfaces.

Issued as Technical progress report, and Final report, Project A-102

Spontaneous emulsification as a function of material exchange.

Direct visualization at 1873 K of 0% to 8% molten FeAl droplets suspended in a SiO2 enriched oxide medium was carried out to image the evolution of droplet morphology during reaction between Al and SiO2. Phenomena such as perturbation growth, necking and budding of offspring droplets from a bulk body are observed. The observations are used to discuss and inform a new approach to the nature of interfacial tension and the impact this has on concepts used to define interfacial tension for a two phase system with material exchange across the interface. The mapping of global interfacial tension coupled with free energy dissipation has been used to give an energetic reasoning as to the behaviour seen with respect to aluminium content in the metal phase

Hidden phenomena during transient reaction trajectories in liquid metals processing

The transient trajectory taken for a system striving toward equilibration has consequences on the rate of processes and on the chemical and physical state of products in metallurgical processes. A case study approach to recent advancements in liquid steel processing is given. A combination of techniques and knowledge developed is given as a targeted showcase of the authors’ contributions to the understanding of liquid metal droplet reactions and their contribution to the large-scale production processes within the steel industry. Examples relevant to novel ironmaking technologies, oxygen steelmaking, ladle metallurgy, and continuous casting are discussed, showing the range of processes that benefit from greater understanding in this area. This article considers specifically the reaction of liquid ferrous droplets, immersed in molten oxides, involving key alloying components, including phosphorus, aluminum, and carbon. The studies use high-temperature–confocal scanning laser microscopy (HT-CSLM), X-ray computed tomography (XCT), phase-field modeling, and in situ limited angle X-ray imaging. These techniques have seen significant development over recent years, and the combination of these powerful tools reveals the occurrence of spontaneous emulsification driven by chemical reaction (in the case of oxygen/phosphorus/aluminum reactions) and gas-phase formation (in the case of decarburization) both internally and externally to a steel droplet. A key finding is that the interfacial area pertinent for the heterogenous reactions to occur changes considerably (by up to an order of magnitude) depending on the chemical driving force. Additional key findings include the shift between preferential internal and external gas nucleation during decarburization, an inflection point of behavior as to whether or not spontaneous emulsification will occur (within the study discussed, this is between 3 and 4 wt pct Al) and the pathway of perturbation growth through which spontaneous emulsification occurs, including the physical maxima a perturbation will grow to before breaking away from the parent droplet

Control of intermetallic nano-particles through annealing in duplex low density steel

In high Al-low-density steels for future vehicle light weighting, it is vital to design a thermal profile to form and retain the uniformly dispersed nanosize B2-type intermetallic precipitates that are crucial for the material strength. In this paper, the influence of heating rate, during annealing to 1050◦C was simulated in a Au-image furnace. The post annealing structure was then characterized and two different morphologies of B2 particles were observed: triangle-like with a few micrometres and disk-like precipitates with a diameter of around a few hundred nanometres. It was found that a slower heating rate (2.5 ◦C/s) led to an increase in the volume fraction and to uniform distribution of particles within the microstructure and considerably affected the shape and size of the precipitates

Observation of the reversible stabilisation of liquid phase iron during nitriding

Confocal scanning laser microscopy has been used to observe in-situ cooling of Grade 1 iron whilst under a nitrogen atmosphere. Liquid iron has been stabilised to temperatures below 1400 °C through nitriding at high temperatures. This has been achieved in two ways. Firstly by exposing the melt to nitrogen and observing the onset of solidification whilst cooling. Secondly, under isothermal conditions by re-melting of initially nitrogen free iron at 1400 °C through the introduction of a nitrogen atmosphere. The latter process has been shown to be reversible with liquid iron being unstable once reverting back to a helium atmosphere

Quantifying the transient interfacial area during slag-metal reactions

The steel industry is facing significant competition on a global scale due to the drive for light-weighting and cheaper more sustainable construction. Not aided by oversupply in geographic sectors of the industry, there is significant competition within the slowly shrinking sector. The recent growth in developing countries through installation of modern plant technology has led to the reduction in unique selling points for mature steelmaking locations. As such, to compete with the equalling product capability and innate cheaper production costs within developing areas the industries in Europe and North America require significant improvements in productivity and agile resource management. To date the basic oxygen furnace has been somewhat treated as a black box within industry, where only control parameters are monitored, not the fundamental mechanisms within the converter. Studies over the past 30 years have shown the basic oxygen furnace is unable to attain the thermodynamic minimum phosphorus content within the output liquid steel. Coupled with the need to drive down resource cost, with a potential for high content phosphorus ores the internal dynamic system of the basic oxygen furnace requires more rigorous understanding. With the aid of in-situ sampling of a pilot scale basic oxygen furnace, and laboratory studies of individual metal droplets suspended in a slag medium (known to be a key driving environment for impurity removal) the present project aims to provide insight into the transient interfacial area between slag and liquid metal through basic oxygen steelmaking processing. Initially the macroscopic dynamics including the amount of metal suspended in the gas/slag/metal emulsion, the period of time it is suspended for, and the speed at which it moves, is investigated. It was found that these parameters vary greatly through the blow, with a normal peak in residence times near the beginning of the blow and a dramatic increase in metal circulation rates at the end of the blow, when foaming is reduced or collapsed. Further to this, a method of interrogating the size of metal droplets within the slag layer using X-ray computed tomography is introduced. The study then progresses into the microscopic environments that individual droplets are subjected to during steel processing. Initially the cause of spontaneous emulsification in basic oxygen furnace type slags is investigated through high temperature-confocal scanning laser microscopy/X-ray computed tomography led experimentation, with the addition of null experiments conducted to rationalize the experimental technique. It was found that the flux of oxygen across the interface was the cause and thus the confirmation of material transfer across the interface being the driving force. Furthermore the physical pathway of emulsification is interrogated and quantified, with in-situ observation of spontaneous emulsification in the high temperature-confocal scanning laser microscope enabled through use of optically transparent slags. The life cycle of perturbation growth, necking and budding is observed and quantified through high-resolution X-ray computed tomography. In addition a phase-field model is developed to interrogate slag/metal systems in 2D and 3D variations, giving rise to the ability to track the cause of emulsification and to predict its occurrence. Finally the project progresses with the in-situ investigation of spontaneous emulsification as a function of initial metal composition. The behaviour of droplet spontaneous emulsification is seen to reduce in severity and subsequently to decline into a non-emulsifying regime below a critical level. Free energy calculations coupled with a measure of the global interfacial tension increase give quantifiable reasoning as to the behaviour seen

Quantifying the pathway and predicting spontaneous emulsification during material exchange in a two phase liquid system

Kinetic restriction of a thermodynamically favourable equilibrium is a common theme in materials processing. The interfacial instability in systems where rate of material exchange is far greater than the mass transfer through respective bulk phases is of specific interest when tracking the transient interfacial area, a parameter integral to short processing times for productivity streamlining in all manufacturing where interfacial reaction occurs. This is even more pertinent in high-temperature systems for energy and cost savings. Here the quantified physical pathway of interfacial area change due to material exchange in liquid metal-molten oxide systems is presented. In addition the predicted growth regime and emulsification behaviour in relation to interfacial tension as modelled using phase-field methodology is shown. The observed in-situ emulsification behaviour links quantitatively the geometry of perturbations as a validation method for the development of simulating the phenomena. Thus a method is presented to both predict and engineer the formation of micro emulsions to a desired specification

Basic oxygen steelmaking slag : formation, reaction, and energy and material recovery

Basic oxygen steelmaking (BOS) slag, a product of hot metal element (e.g. Si, Mn, Fe, P) oxidation and flux (e.g. lime, dolomite) dissolution, plays a critical role in the production of high quality crude steel while its behavior inside the BOS vessel (formation and reaction with metal droplets and gas) is still not clear and its recycling has always been challenging. This paper introduces BOS slag-related research examples carried out by the present authors, in addition to critical review of relevant research reported in literature. This includes four important topics: (1) slag formation, i.e. dissolution of lime in the BOS slag, (2) in-situ observation of high temperature behavior of BOS slags, (3) transient phenomena in BOS slags due to metal droplet-slag-gas reactions, and (4) energy and materials recovery from molten BOS slags. The paper aims to provide the state-of-the-art understanding and new research ideas in these research topics. An emphasis is placed on methods which seek to in-situ probe the evolution of the slag and to document its physical and chemical changes

Assessing the Ability of Simulated Laboratory Scenes to Predict the Image Quality Performance of HDR Captures (and Rendering) of Exterior Scenes Using Mobile Phone Cameras

With the advent of computational photography, most cellphones include High Dynamic Range (HDR) modes or “apps” that capture and render high contrast scenes in-camera using techniques such as multiple exposures and subsequent “addition” of those exposures to render a properly exposed image. The results from different cameras vary. Testing the image quality of different cameras involves field-testing under dynamic lighting conditions that may involve moving objects. Such testing often becomes a cumbersome and time-consuming task. It would be more efficient to conduct such testing in a controlled, laboratory environment. This study investigates the feasibility of such testing. Natural exterior scenes, at day and night, some of which include “motion”, were captured with a range of cellphone cameras using their native HDR modes. The luminance ratios of these scenes were accurately measured using various spectro-radiometers and luminance meters. Artificial scenes, which include characteristics of the natural exterior scenes and have similar luminance ratios, were created in a laboratory environment. These simulated scenes were captured using the same modes as the natural exterior scenes. A subjective image quality evaluation was conducted using some 20 observers to establish an observer preference scale separately for each scene. For each natural exterior scene, the correlation coefficients between its preference scale and the preference scale obtained for each laboratory scene were calculated, and the laboratory scene with the highest correlation was identified. It was determined that while it was difficult to accurately quantify the actual dynamic range of a natural exterior scene, especially at night, we could still simulate the luminance ratios of a wide range of natural exterior HDR scenes, from 266:1 to 15120:1, within a laboratory environment. Preliminary results of the subjective study indicated that reasonably good correlation (0.8 or higher on average) was obtained between the natural exterior and laboratory simulated scenes. However, such correlations were determined to be specific to the type of scene studied. The scope of this study needs to be narrowed. Another consideration, how moving objects in the scene would affect the results, needs further investigation

BOS slag : formation, reaction, and energy and materials recovery

This paper summarises some BOS slag-related researches carried out by the authors. This includes (1) slag formation (i.e. dissolution of lime in the BOS slag), (2) real time in-situ observation of high temperature behaviour of synthetic BOS slags, (3) transient phenomena in BOS slags due to metal droplet-slag reactions, and (4) a novel approach to recover energy and materials from molten BOS slags