![Direct extrusion (a) versus indirect extrusion (b) [2]](https://www.researchgate.net/profile/Hanghang-An/publication/338474060/figure/fig12/AS:867089321558018@1583741612410/Direct-extrusion-a-versus-indirect-extrusion-b-2_Q320.jpg)
Content uploaded by Hanghang An
Author content
All content in this area was uploaded by Hanghang An on Mar 09, 2020
Content may be subject to copyright.
A preview of the PDF is not available
Control Center Segregation in Continuously Cast GCr15 Bloom by Optimization of Solidification Structure
Figures
Figures - uploaded by Hanghang An
Author content
All figure content in this area was uploaded by Hanghang An
Content may be subject to copyright.
ResearchGate has not been able to resolve any citations for this publication.
A novel flash ironmaking technology (FIT) based on the direct reduction of iron ore concentrate by a reducing gas such as hydrogen, natural gas or coal gas has been developed at the University of Utah. A proper choice of refractories is expected to play an important role in the development of FIT and its scale-up. In this work, the interaction of iron with alumina refractory under flash ironmaking conditions has been investigated. A thermodynamic basis for the interaction phenomena has been developed by considering the Fe-Al-O system and a kinetic model based on solid-state diffusion is proposed to describe the growth of the hercynite (FeAl2O4) spinel formed as a result of the interaction between Fe, O (from H2O and CO2) and Al2O3. Experiments were conducted with Fe powders and pure Al2O3 in the temperature range 1200 - 1500°C under gas atmospheres relevant to flash ironmaking and the reacted samples were analyzed using XRD, SEM-EDX and EPMA techniques. The analyses of reacted samples confirmed the formation of the hercynite (FeAl2O4) spinel phase and the thickness of the FeAl2O4 spinel as measured using elemental line-scans in SEM-EDX and EPMA varied in the range of 0.5 - 1.3 mm. Results showed that the proposed model suitably describes the growth of the hercynite spinel layer, which obeyed the parabolic rate law at all temperatures. The parabolic rate-constants were obtained and the diffusion-controlled growth of the FeAl2O4 layer strongly depended on temperature. Furthermore, using the kinetic model, an expression for the effective diffusivity ((D_eff ) ̅) was obtained and its values at the experimental temperatures were determined. The solid-state diffusion was an activated process, as expected, with an activation energy value of 231 kJ/mol.
In the present work, solidification structure and centre segregation in continuously cast bearing steel GCr15 blooms with 220mm×260 mm were obtained by different casting conditions (M-EMS, M + F-EMS, M + F-EMS + MSR), which were systematically optimized by numerical simulation and experiments. The relationship between centre segregation in continuously cast GCr15 bloom, heredity in hot-rolled steel and carbide inhomogeneity of hot-rolled steel with was investigated comprehensively. Results showed that compared with M-EMS, centre carbon segregation ratio in the bloom decreased from 1.1∼1.4 to 1.04∼1.22 by M + F-EMS, and could be reduced to 0.96∼1.08 by MSR in combination with M + F-EMS. Moreover, centre segregation in hot-rolled steel decreased with the decrease of centre segregation in bloom. Grade of network and banded carbide in hot-rolled steel decreased accordingly, especially for centre carbon segregation ratio with 0.96∼1.08, grade of network and banded carbide could be significantly reduced to 1. It was important to control centre segregation in as-cast steel to improve the resulting carbide homogeneity of as-rolled steel. According to the experimental results, higher casting speed in bloom with M + F-EMS and MSR integrated process was preferred to effectively reduce centre segregation ratio and further decreased the occurrence of network and banded carbide in as-rolled bearing steel GC15.
Internal cracks could be healed under the process of hot plastic deformation. In this study, mechanical properties recovery after crack healing in SA 508–3 steel were investigated. Microstructures of the crack healing zones were observed using an optical microscope (OM) and electron back scattered diffraction (EBSD) technology, and the recovery degrees of mechanical properties in the crack healing zones with the healing temperature and a reduction ratio were tested systematically. The results showed that the internal cracks in SA 508–3 steel disappeared and were replaced by newly formed grains, achieved by recrystallization and abnormal grain growth. The tensile properties of crack healing zones could be fully restored, while their impact and low cycle fatigue properties could only be partially achieved. The recovery degrees of mechanical properties in crack healing zones increased with increasing the healing temperature and reduction ratio in the temperature range of 950–1050℃. When the temperature was above 1150℃, the impact properties began to deteriorate because of grain coarsening and larger MA (martensite–austenite) constituents. The microstructural evolution of the crack zone in the SA 508–3 steel was sketched.
The aluminium extrusion process is based on simple principles, but allows profiles with the most complex cross-sectional shapes to be produced at a very high rate. One of the greatest challenges of aluminium extrusion is to control material flow and dimensional variability of thin-walled high-strength profiles, for which the demand is growing. The die outlet geometry and the temperatures of the billet and tools must be carefully tuned in order to secure satisfactory material flow conditions. Due to the high pressures in the container, the deformation of the extrusion dies and the distortion of the die outlet may be significant and must be compensated for. The thermal conditions in the extrusion press must also be controlled. Even in the age of numerical modelling much trial and error is needed to make certain that the product satisfies the customer requirements. If simple and effective process control could be implemented, the cost and dimensional variability of extruded profiles could be significantly reduced. This would not only secure the continued use of extruded profiles in old markets but also open new ones.
The main objective of this study has been to establish useful and simple methods for measuring the pressure at the interface between the die and the billet and the deformation of the die during hot extrusion of aluminium. Pressure measurement data may be used to establish a better understanding of the extrusion process and to carefully evaluate the many numerical extrusion models that are presently being developed and refined for the purpose of predicting profile shape and properties. A most important task is the evaluation of constitutive models used to describe bulk material and friction behaviour. The requirements for such models should be viewed in relation to common flow instability phenomena such as buckling. Sensors may in the future be integrated in intelligent extrusion dies in order to make certain that temperature and flow condition changes are as small as possible and to prevent overloading of extrusion dies during production. It is of the utmost importance that dimensional variability is detected early.
This study has consisted of many parts. The first step was a careful evaluation of sensor designs using the Capacitec HPC-75 high-temperature capacitive displacement probes and the Capacitec 4004 amplifier series. The feasibility of high- and low-temperature pressure measurement was demonstrated through various types of compression testing. The capacitive sensors were repeatedly tested in a hot air furnace to 650 ºC, and results were satisfactory for all but one of the sensors. The sensor sensitivity to temperature changes that occur during extrusion is usually less than 10 % of the full sensor response.
The pressure sensors have been repeatedly tested in several dies for aluminium rod extrusion. The feasibility of, and a method for, performing useful measurements in the high-temperature extrusion environment have been demonstrated. The measurement accuracy is better than ± 10 % of full scale of 200 MPa when the effects of temperature changes are compensated for. The measurement repeatability is of a similar magnitude for genuinely replicated measurements. The measurement resolution is better than 1 %. It is firmly believed that the measurement and calibration technology may be further improved, and that the measurement accuracy may be better than 5 % of full scale.
Rod extrusion experiments allowed the quality of the finite element modelling approach to be evaluated. The code ALMA2π was used, and material data of the Zener-Hollomon flow rule have been obtained by compression testing. Simulated and measured results of the ram force, die face pressure and liner force generally differed less than 10 %. The estimated die outlet temperature change was systematically 10 ºC too high. As there are significant differences between extrusion and compression testing, the use of material data from compression testing amounts to an extrapolation of data. Experiments did not demonstrate that the approach is unacceptable, but plots showing the deviation between experimental and estimated ram force and outlet temperature data indicated that there are a number of parameter combinations that are equally good as or even better than those obtained through compression testing. Very high accuracy determination of flow parameters may be difficult. Measurement errors significantly complicate matters.
The pressure sensors may be used to study practical extrusion problems and to establish a better understanding of metal flow and the significance of die deformation. Thin strip extrusion experiments were performed to gain insight into the thermo-mechanics of flow instability (buckling). The feasibility of performing pressure measurement during the extrusion of thin strips was demonstrated, but sensors were not properly calibrated. The first round of the experiments was run with a die outlet 78.5 mm wide and 1.7 mm thick, and a container diameter of 100 mm. During extrusion of AA6060 flow instability phenomena were not encountered. A second round of experiments was performed with non-instrumented dies and somewhat thinner profiles (1.1 and 1.4 mm). Flow instability in the form of buckling was provoked for high outlet temperatures, and many replicate experiments were performed. The shape of the buckled thin strip was also measured continuously with a laser triangulation technique during extrusion at high speed. Due to limitations related to the experimental set-up, neither the resolution nor the accuracy of the approach was entirely satisfactory. Nonetheless, the feasibility of the approach was demonstrated, and it is quite possible to improve the measurement technique.
Capacitive pressure measurement techniques have been combined with methods for measuring the deformation of the mandrel and the straining of the bridges to study the behaviour of dies during tube extrusion. Capacitec capacitive probes were used to measure bridge strains. High temperature Kyowa strain gauges were also used for the same purpose. The die mandrel deflection was measured by conventional displacement transducers connected to the mandrel and the die cap by rods. Only measurements by the capacitive sensors proved sufficiently reliable during measurements. The study revealed that the state of stress in dies for hollow profiles may be very close to critical. The die face pressure at the top face of the mandrel exceeded 500 MPa.
Two rounds of industrial experiments were performed with a U-profile that proved most difficult to extrude. In the first round of experiments, the flow stability was not satisfactory, and plugging of the outlet ruined experiments. The second round was more successful, and sensors were used to record the die face pressure on-line. The experiments demonstrated the feasibility of industrial experiments, but clearly indicated that further development of the sensors should be performed. It is important that sensors are made more durable, and that calibration techniques are further developed. Practical die designs that allow simple integration of sensors in the press should be developed.
A novel flash ironmaking technology (FIT) has been developed at the University of Utah based on the direct reduction of iron ore concentrate by a reducing gas such as hydrogen, natural gas, coal gas or a mixture thereof. In this work, the interaction of ferrous oxide (FeO) with alumina (Al 2 O 3) refractory under flash ironmaking conditions has been studied. A thermodynamic basis for the interaction process has been developed by considering the Fe-Al-O system and a kinetic model based on solid-state diffusion is formulated to describe the growth of the hercynite (FeAl 2 O 4) spinel formed as a result of the interaction between FeO and Al 2 O 3. Experiments were conducted with FeO powders and pure Al 2 O 3 refractory in the temperature range 1200-1400 °C under gas atmospheres relevant to FIT. The analyses of reacted samples using XRD, SEM-EDX and EPMA techniques confirmed the formation of the hercynite (FeAl 2 O 4) spinel under FIT conditions and results showed that the proposed kinetic model appropriately describes the growth of the hercynite spinel layer, which obeyed the parabolic rate law at all the three temperatures. Furthermore, using the kinetic model, expressions for the parabolic rate-constants (k 1) and the effective diffusivity (D eff) were obtained and their respective values at the experimental temperatures were determined. The values of D eff calculated in this study agreed closely with those obtained independently by the authors from experiments on Fe-Al 2 O 3 interaction thereby corroborating the interaction mechanism and the associated reaction scheme. The solid-state diffusion was temperature dependent , with an activation energy value of 268 kJ/mol.
In thermal energy conversion of biomass, the char conversion rate influences the design of industrial systems. Yet there is no consensus on the relative influences of O2, CO2 and H2O on the char conversion rate. The present study aims at clarifying the roles of these oxidizers on biomass char conversion. Single particles of different particles sizes were oxidized in mixtures of O2/N2, CO2/N2, H2O/N2 and O2/CO2/H2O/N2 at various temperatures. The results show that for large particles and high temperatures, CO2 and H2O gasification reactions play a dominant role, while for sufficiently small particles and low temperatures, the O2 oxidation reaction plays a dominant role in mixtures of O2/CO2/H2O/N2. Two different particle models were used to compute char conversions: a detailed single particle model, and a conventional particle model assuming that rates in mixtures of oxidizers can be predicted based on rates of single oxidizers. The results show that the char oxidation rates are strongly overestimated when predicting rates based on single oxidizers, while the detailed model shows a good agreement to the experimental measurements.
A two-dimensional volume of fluid (VOF) model was developed to simulate the behavior of bubbles in a physical model of the aluminum degassing unit employing the compound technique of rotary blowing and ultrasonic. The effect of rotation speed on bubble behavior under rotating flow field, and the effects of ultrasonic parameters including the amplitude, length, and position of ultrasonic end-face on bubble behavior under compound field were analyzed according to the simulation results, respectively. The experiment shows the simulation results for the bubble shape and trajectory agree well with the experimental observations. Under single rotating flow field, the rotation speed has a strong impact upon bubble trajectory and the optimal speed is 450 rpm. Under compound field, ultrasonic can increase the number of broken bubbles, and push bubbles to further area, which is beneficial to strengthen the bubble distribution and improve the degassing effect. The appropriate amplitude, length, and position of ultrasonic end-face are 23 μm, 50 mm, and 185 mm.
A novel flash ironmaking technology (FIT), based on the direct reduction of iron ore concentrate with a reductant gas such as hydrogen, natural gas, coal gas, or a combination thereof in a flash furnace, has been developed at the University of Utah. This technology aims to overcome the shortcomings of the conventional blast (BF) and alternate technologies by eliminating the problematic sintering and pelletization steps, thereby reducing the overall energy requirements and CO2 emissions.
In the development of the FIT and its proposed scale-up in the near future, the choice and design of refractories is expected to play a pivotal role. In this work, interactions of iron, FeO, and slags with selected refractory materials have been studied in the temperature range 1100°-1500°C under H2-CO-CO2-H2O environments relevant to the FIT.
Thermodynamic bases for the interaction phenomena between iron, wustite and slags with pure Al2O3 and MgO-C refractories have been developed by considering the relevant systems. Then, kinetic models based on solid-state diffusion have been proposed to describe the growth of the phases formed as a result of the interactions. Reacted samples from the refractory-iron, wustite or slag experiments were analyzed using XRD, SEM-EDX and EPMA techniques. The results showed that the proposed models satisfactorily describe the growth of the product phases and using the proposed models, the kinetic parameters governing the solid-sate diffusion processes were calculated. Good agreement between the values of these kinetic parameters calculated independently from the experiments with iron, FeO or slags further corroborated the proposed reaction schemes and showed that the interaction processes in the three cases follow the same overall mechanism. Also, the mechanical properties of the interaction product layers and the unaffected refractory materials have been measured and compared with the as-received refractory samples and the suitability of these refractory materials in the FIT has been analyzed. Finally, representative testing with three other industrial refractories has been carried out and their refractory performances under flash ironmaking conditions have been evaluated.
Phase equilibria in the ZnS-Ag2GeS3-Ge-GeS2 part of the Ag-Zn-Ge-S system were investigated using differential thermal analysis, X-ray diffraction, and EMF methods. The data was used to model Ag2GeS3-ZnS polythermal section. Further, the mechanism of formation and thermal stability of the Ag2ZnGeS4 compound were established. The results suggest presence of another quaternary phase Ag4ZnGe2S7 in the temperature range of 695 to 853 K. The determined phase relations were used to express the chemical reactions. Based on the electromotive force vs. temperature measurements, experimental thermodynamic data of the Ag2ZnGeS4 quaternary phase were derived for the first time. The calculated Gibbs energy, enthalpy and entropy values of the Ag2ZnGeS4 compound in both phase regions are consistent, which indicates that Ag2ZnGeS4 has stoichiometric composition.