Under the background of the strategic goal of ‘‘double carbon,’’ the carbon reduction and consumption reduction of the iron and steel industry, especially in the ironmaking process, need to be further improved. The raceway of tuyere provides the chemical environment, fuel and power source for blast furnace smelting. The research on the characteristics of its action mode and mechanism is of great significance to clarify the way of reducing carbon and consumption of blast furnace. In general, the formation mechanism, energy distribution, research progress, extended resource injection and directional regulation are studied and expounded. The research results of various scholars on the characteristics of the raceway show that the raceway is a complex process including multiphase turbulent flow, heat, momentum, mass and homogeneous and heterogeneous chemical reactions. With the development of multi-source fuel injection technology, the complexity of problem research is more obvious. Therefore, the collection of multi-factor, multi-directional and multi-process characteristic information in the raceway can provide guarantee for the stability, smooth operation, high yield, carbon reduction and consumption reduction of blast furnace and provide new ideas for the green and low-carbon development of iron and steel industry.
The effect of Mo on dual-phase precipitation behavior and tensile properties of Fe26Mn8Al1.2C-(2-3.5 wt.%) Mo lightweight austenitic steels after annealing at 700 ℃ was investigated by electron backscatter diffraction, transmission electron microscopy, hardness and tensile tests. Alloying with Mo in the steels promotes the precipitation of Mo2C carbides while inhibits the precipitation of j-carbides. The addition of Mo exceeding 2.5 wt.% facilitates the precipitation of intragranular Mo2C carbides, whereas with up to 2.5 wt.% Mo, only intergranular Mo2C carbides precipitate. With containing more Mo in the steels, the strength increases due to enhancement of precipitation strengthening and solid solution strengthening, while ductility gradually decreases. 3Mo steel exhibits excellent overall mechanical properties, with the synergistic increase in strength, ductility, and work-hardening rate, which can be attributed to the precipitation of fine intragranular Mo2C distributed uniformly in the matrix and the suppression of the formation of coarsened j-carbides. However, in 3.5Mo steel, abundant coarsened Mo2C precipitation strongly interacts with dislocations to promote crack propagation along non-coherent interfaces, leading to a high initial work-hardening rate but severe ductility loss.
The gasification behaviors of coke were investigated under conditions simulating a hydrogen-rich blast furnace atmosphere, composed of N2, CO, CO2, H2, and H2O. Systematic experimental studies were conducted to examine the effects of gasification temperature and H2O content on the microstructural and macroscopic properties of coke. The results indicated that increasing temperature and H2O content enhanced the gasification and dissolution loss of coke, with temperature having a more significant impact. Pore structure analysis of the gasified coke revealed that small pores and micropores predominated at 900 and 1000 ℃. However, at gasification temperatures above 1100 ℃, oversized holes formed, some of which extended into the coke’s interior. The compressive strength of the coke was also assessed, showing that higher gasification temperatures or increased H2O content reduced this property. This reduction is primarily due to the increased coke porosity and the degradation of the pore wall structure. X-ray diffraction analysis results suggested that higher gasification temperatures and H2O content could improve the degree of order in the carbon microcrystals of the gasified coke.
Coke is the only solid charge component in the lower part of the blast furnace, and its strength is crucial to its production. Si and Al are the two most abundant elements in coke ash. The influences of these oxides on the tensile strength of the coke matrix were studied by splitting tests. According to the Weibull analysis, with increasing Si and Al oxide concentrations, the fracture stress range of the coke widened, the upper and lower limits decreased, the probability of fracture under the same stress conditions increased, and the randomness and dispersion of strength increased. These results can be attributed to the inhibitory effect of ash during coal pyrolysis. Ash impedes the growth and contact of mesophase, leading to a decrease in graphitic carbon structures and an increase in edge carbon and aliphatic carbon structures in the resulting coke. Consequently, the overall ordering of the carbon structure is reduced. Moreover, SiO2 and Al2O3 promoted the development of coke pores, thinned the coke pore wall, and significantly increased the proportion of large pores ([500 lm). Moreover, Al2O3 had more significant influences on the coke strength, carbon structure and stomatal ratio than SiO2. In addition, the position where the ash particles bonded to the carbon matrix easily produced cracks and holes, and the sharp edge of the matrix was likely to produce stress concentration points when subjected to an external force, leading to structural damage. Therefore, controlling the concentration of ash could effectively reduce the number of structural defects inside coke, which is conducive to improving the strength.
The mineral composition of the sinter affects the quality of cold-bonded briquettes (CBB), which are prepared from returned sinter fines and serve as a cleaner blast furnace charge. Pulverization rate, compressive strength, reduction disintegration index (RDI) and compressive strength after reduction experiment were tested to analyze the experimental parameters of CBB under the influence of different basicities and sintering time. The results show that when the basicity of CBB is increased from 0.5 to 1.5, the pulverization rate increases, and performance indexes such as compressive strength, RDI, and compressive strength exhibit a decreasing trend. When the basicity is increased from 1.5 to 3.0, all the aforementioned performance indexes are improved. When the sintering time is extended from 0 to 8 h, the properties mentioned above are improved. The results of X-ray diffraction, microstructure, and thermodynamic calculations confirm that the hematite in the mineral composition of CBB tends to convert into calcium ferrite, which leads to the increased compressive strength of CBB. The reasonable basicity and sintering time during sinter preparation not only form the desired mineral composition but also improve the properties of the CBB.
To study the combustion behavior of municipal solid waste (MSW) in blast furnace, the combustion process within the raceway was simulated using computational fluid dynamics. Based on the parameters of an actual blast furnace, a threedimensional model including coal lance, blowpipe, tuyere, and raceway was established. The model was then used to compare the combustion characteristics of pulverized coal and MSW in the raceway and to investigate the effects of blast temperature and particle size on the combustion characteristics of MSW in the raceway. The results showed that MSW combusted more rapidly, achieving a maximum temperature of 3839 K in the raceway, comparing to 2974 K during pulverized coal injection. However, the average temperature during MSW injection was 1790 K, which was 73 K lower than that of pulverized coal injection. The maximum velocity during MSW injection was 120 m/s, lower than 188 m/s obtained during pulverized coal injection. MSW could be completely burned out in the middle of the raceway, while the burnout of pulverized coal at this position was only 50%. The combustion effect of MSW makes no difference when the blast temperature increased from 1273 to 1673 K, due to its excellent combustion characteristic. When the MSW particle size was increased from 0.074 to 2 mm, the burnout was 75%, which was still higher than that of pulverized coal injection with a particle size of 0.074 mm. However, injecting larger-sized fuel might increase the risk of tuyere wear. To ensure stable furnace conditions and great combustion, a blast temperature of 1473 K and a MSW particle size of about 1 mm will be better.
The dripping zone in a blast furnace plays a crucial role in connecting the cohesive zone with the hearth, and its stability significantly impacts low-carbon smelting processes. Based on a detailed anatomical study of a 2200-m3 blast furnace in China, it involves core sampling of the furnace dripping zone and uses scanning electron microscopy to investigate the micro-morphology of potassium (K) and sulfur (S) within this region. The formation process of kalsilite (KAlSiO4) and CaS inside the furnace is elucidated. The results show that when potassium vapor rises to the upper area of the dripping zone, some of it adsorbs onto the coke pore walls and reacts with the dripping slag and coke ash to form kalsilite. The formation pathways of CaS differ between upper and lower areas of the dripping zone. It forms mainly from the reaction of slag with SO2 in the gas flow and from the slag-coke interface reaction. The CaS generated from the slag-coke interface reaction is the major source of CaS in the dripping zone. Based on the formation mechanisms of kalsilite and CaS in the dripping zone, it is possible to regulate their formation by adjusting the temperature, slag phase composition, and the content of harmful elements in the raw materials. It provides theoretical insights into the behavior of harmful elements in the blast furnace, offering guidance for steel enterprises to ensure the stable operation of the dripping zone, reduce fuel consumption, and achieve greener production.
Analysis of the energy balance of various parts during the basic oxygen furnace (BOF) steelmaking is of vital importance for revealing the blowing characteristics of the swirl-type oxygen lance. The energy transfer behavior between the oxygen jet and the molten bath in the top-blowing steelmaking process was investigated using the volume of fluid method. The energy of the reflected jet and the slag was introduced, and the energy balance model of the BOF converter was modified. The influences of lance height and operation pressure on energy transfer were analyzed. Compared with the traditional oxygen lance, the energy of reflected jet, splashing, and cavity formation of the swirl-type oxygen lance was decreased. However, the energy of jet attenuation, slag, and molten steel increased. The energy proportion of the reflected jet was about 8%, while the energy of slag was 15% of molten steel. The maximum energy was transferred from the jet to the slag and molten steel at H = 40de (H is lance height and de is outlet diameter). When the operation pressure increased from 0.8P0 to 1.2P0 (P0 is the designed pressure), the energy of slag and molten steel was increased by 33% and 25.9%, respectively.
Formation and evolution of inclusions in low-aluminum Ti-containing 51CrV4 spring steel under BOF (basic oxygen furnace)-LF (ladle furnace)-CC (continuous casting) process were investigated by industrial trials and thermodynamic calculations. During LF refining, deoxidation products including Al2O3, Al2O3-Ti3O5-SiO2-MnO and Al2O3-SiO2-MnO are modified as MgO-Al2O3, CaO-Al2O3-SiO2, CaO-Al2O3-MgO and CaO-Al2O3-SiO2-MgO. When reoxidation during ladle casting is quite serious, inclusions such as Al2O3, Al2O3-Ti3O5-SiO2-MnO, and Al2O3-SiO2-MnO may regenerate. A handful of Ti carried by alloy into liquid steel has less influence on inclusions during LF refining; Ti-containing inclusions mainly transiently exist as an intermediate product of deoxidation process and then are gradually modified by [Al], [Ca] or [Mg]. Thermodynamic calculation and experimental results reveal that tundish flux is the main source of reoxidation in ladle casting process. Further calculations taking into account of the influence of inclusions before casting indicate that reoxidation within a certain of degree leads to the generation of a large amount of high melting point inclusions including CaO∙2MgO∙8Al2O3, CaO∙MgO∙7Al2O3, MgO∙Al2O3, CaO∙6Al2O3 and Al2O3 in molten steel, which is basically consistent with experimental results, and more high melting point inclusions will generate as reoxidation becomes severer. On this basis, severer reoxidation will deplete [Si], [Mn], and [Ti] in steel melt, resulting in the formation of liquid inclusions composed of Al2O3-Ti3O5-SiO2-MnO(-CaO). These results are of guiding significance for controlling inclusions in Al-killed Ti-containing spring steel.
Large-sized titanium alloy ingots produced by vacuum arc remelting (VAR) technology are susceptible to metallurgical imperfections such as compositional segregation, inconsistent solidification microstructures, black spots, and inclusions. These defects are intricately linked to the electromagnetic effects, temperature distribution, and fluid dynamics during the melting process. The self-induced magnetic field created by the electric current, along with the axial magnetic field applied to stabilize the arc, significantly influences the solidification of titanium alloy ingots. A mathematical model optimized for the integrated analysis of multiple fields—electromagnetic, fluid, and thermal—was developed for the VAR solidification process of titanium alloys. The influence mechanism of electromagnetic field on the macroscopic solidification process of titanium alloy was investigated. The findings indicate the presence of two competing forces within the VAR molten pool, namely, thermal buoyancy and the Lorentz force. Introducing a coupled self-induced magnetic field and elevating the current to 15 kA led to an increase in the molten pool depth by 42.9% and a reduction in the thickness of the mushy zone by 25.2%. The application of a constant axial magnetic field enhances a unidirectional momentum buildup within the molten pool, thereby enhancing the flow velocity and cooling efficiency of melt.
Slagging and calcium treatment are commonly used methods to control cleanliness and inclusions in steel. However, the inappropriate slagging and calcium treatment operations resulted in the generation of large-sized inclusions and deterioration of steel cleanliness; meanwhile, changed inclusions from Al2O3-SiO2-MnO type to Al2O3-SiO2-CaO type after the calcium treatment during the production of an H-beam steel. Combining the thermodynamic analysis and industrial trials, measurements including reducing the basicity of refining slag to be less than 2.0 and the Al2O3 content in slag to be less than 10 wt.% and the cancelation of calcium treatment under the total content less than 15 9 10-6 have been taken. After optimization, the content of total oxygen in tundish decreased by 24%; meanwhile, inclusions were changed from the Al2O3-SiO2-CaO system to the Al2O3-SiO2-MnO system with a low-melting point and a obvious decrease in the number density, area fraction, and maximum size of inclusions. It has achieved the improvement of steel cleanliness while reducing production costs.
Steel-flux reactions involving the high aluminum (0.75-3.85 wt.% Al) low manganese (2.2 wt.% Mn) steel and the 18 wt.% SiO2-18 wt.% Al2O3 mold flux were investigated. The results indicated that the reaction rate increased when the initial aluminum content increased from 0.76 to 3.85 wt.%. Utilizing the two-film theory, a steel-flux reaction kinetic model controlled by mass transfer was established, which considered the influence of the initial composition on the density of liquid steel and flux. The mass transfer of aluminum in the steel phase was the reaction rate-determining step. It was confirmed that the mass transfer coefficient of Al was 1.87 9 10-4. The predicted results of the kinetic model were consistent and reliable with the experimental results. Thermodynamic equilibrium calculation was performed using FactSage 8.2, which was compared with the steel and flux final composition after 30 min. The content of initial aluminum in the liquid steel played a critical role in the SiO2 equilibrium content of the mold flux. In addition, the steel-flux reaction between [Al] and (SiO2) occurred with the initial SiO2 content in the mold flux lower than 3 wt.%.
To address the increasing demand for corrosion-resistant shaft components, a bi-metallic composite shaft comprising carbon steel, which is known for its high thermal strength, and stainless cladding, which offers excellent corrosion resistance, was introduced. A novel method for manufacturing these composite shaft parts using cross-wedge rolling (CWR) was proposed and explored. Thermal simulation experiments, CWR forming trials and finite element analysis were conducted to examine the coordinated deformation during the CWR process. The results revealed a downhill diffusion pattern of elements from higher to lower-concentration areas, forming a smooth and uniform concentration gradient. When the cladding thickness (CT) ranged from 3 to 4 mm, the trajectories of the points on both the cladding material and the substrate coincided, indicating strong bonding at the transitional interface of the composite shaft. Conversely, with a CT of 5 mm, coordinated deformation between the substrate and cladding material was not achieved. Shear strength tests demonstrated a gradual decrease in strength with increasing CT. The microscopic morphology of the interface showed that the metal grains near both sides of the interface were refined, and the binding interface displayed a slightly curved shape. A viable method was provided for producing high-performance corrosion-resistant composite shaft components using CWR technology.
The phenomenological and physically based models, using the true stress-true strain curve data obtained under various hot working conditions of 850-1200 °C and 0.001-10 s-1, were developed and improved for AerMet 100 ultra-high strength steel. The predictability of the developed constitutive models was verified and compared. The determination coefficient and average absolute relative error were 0.9988 and 3.72% for the improved version of the modified Zerilli-Armstrong model, 0.9985 and 3.96% for the improved version of the modified Johnson-Cook model, 0.9947 and 4.59% for the straincompensated Arrhenius-type model and 0.9913 and 5.43% for the improved Khan-Huang-Liang model, respectively. The results showed that the improved versions of the modified Zerilli-Armstrong model have the best predictability among the studied constitutive models. Comparing the predictability before and after the improvement, the average absolute relative error was increased by 65.14% for the modified Zerilli-Armstrong model and 58.45% for the modified Johnson-Cook model. This indicates that the phenomenological improvement of physically based constitutive models allows us to develop effectively constitutive equations with high prediction accuracy.
As high-speed railway transportation advances toward increased velocities, it is imperative to enhance the mechanical performance of EA4T axle steel, especially through microstructures regulation by thermal-mechanical processing. However, little research has been conducted on the phase transformation and microstructure evolution mechanism of EA4T steel under thermal-mechanical load, resulting in a lack of theoretical guidance. The hot deformation behavior and phase transformation mechanism of EA4T steel were investigated under different conditions of strain rates (0.01-10 s-1) and temperatures (850-1200℃). A relation of deformation stresses with Zener-Hollomon parameter was established to characterize the mechanical response and dynamic softening effect of EA4T steel during hot compression. The evolution of grain boundaries with different misorientations has been analyzed to evaluate the influence of strain rates and temperatures on the dynamic recrystallization. It was found that the grain refinement mechanisms of EA4T steel by dynamic recrystallization including twin-assisted boundary bulging, sub-grain rotation, and sub-grain growth. Transmission electron microscopy observations confirmed that dynamic recrystallization nuclei and small recrystallized grains impeded martensite phase nucleation during hot deformation, while the ongoing dynamic recrystallization consumed deformation stored energy and reduced dislocation density, which mitigated the stress concentration in the parent phase of martensite, thereby facilitating the uniform growth of martensite lath with a mixing structure of nanotwins and dislocations during quenching.
The tensile properties and deformation mechanisms of the reduced activation ferritic/martensitic steel—China low activation martensitic (CLAM) steel are determined from tests carried out over a wider range of strain rate and temperature. During high-temperature deformation, the plastic deformation modes involve dynamic recrystallization (DRX) and dynamic recovery (DRV) processes, which govern the mechanical behaviors of CLAM steel under different loading conditions. This work systematically explored the effects of increasing strain rates and temperatures, finding that the microstructure evolution process is facilitated by nano-sized M23C6 precipitates and the grain boundaries of the initial microstructure. Under quasi-static loading conditions, DRX grains preferentially nucleate around M23C6 precipitates, and the dominant deformation mechanism is DRX. However, under dynamic loading conditions, the number of DRX grains decreases significantly, and the dominant deformation mechanism converts to DRV. It was concluded that the coupling effects of strain rates and temperatures strongly influence DRX and DRV processes, which ultimately determine the mechanical properties and microstructure evolution. Moreover, dynamic deformation at elevated temperatures achieves much finer grain sizes, offering a novel method for grain refinement through dynamic straining processes.
Isothermal compression tests were carried out to investigate the hot deformation behavior of a multi-alloyed high-Mn austenitic steel, 110Mn12Cr2NY, at temperatures ranging from 800 to 1200 ℃ and strain rates ranging from 0.01 to 1 s-1. The results revealed that the critical strain for dynamic recrystallization (DRX) lowered with increasing deformation temperature and decreasing strain rate. The analysis of microstructure pointed to discontinuous dynamic recrystallization (DDRX) as the dominant DRX mechanism, characterized by Σ3 twin boundaries and necklace-like structure during deformation at relatively low temperature and high strain rate. The decrease in strain rate facilitated continuous dynamic recrystallization (CDRX) as an auxiliary nucleation mechanism, leading to a significant decrease in the softening rate in the flow stress curves. When deformed at high temperatures and low strain rates, the preferential growth of h001i oriented grains resulted in the formation of a strong h001i//CD texture, and CDRX associated with h001i grains emerged as the predominant DRX mechanism. Significant DRX occurring at high temperatures and high strain rates yielded fine, defectfree equiaxed grains. Consequently, this region could be employed as the optimal hot working window for 110Mn12Cr2NY steel, with a temperature range of 950-1200 ℃ and a strain rate range of 0.4-1 s-1.
The mechanical properties, microstructure and second phase precipitation behavior of flange forgings for high-pressure hydrogen storage vessels at different tempering temperatures (620-700 ℃) were studied. The results showed that when tempered at 620-680 ℃, the main microstructure of the test steel was tempered sorbite, and the main microstructure of tempered steel changed to martensite at 700 ℃. At 700 ℃, the dislocation density increased and some retained austenite existed. With the tempering temperature increasing, the yield strength showed a decreasing trend, the formation of fresh martensite made the tensile strength first decrease and then increase slightly, the impact energy at -40 ℃ increased first and then decreased, and the impact energy at 660 ℃ had the maximum value. The precipitates of MC type were mainly (Mo, V, Ti)C. The test steel had excellent strength and toughness matching at 660 ℃ tempering, the tensile strength at different cross section locations was above 750 MPa, the impact energy was above 200 J at -40 ℃, and the relative percentage reduction of area (ZH2/ZN2) was above 75% at hydrogen environment of 6.3 MPa.
Hot deformation tests were performed under various temperature and strain rate conditions to determine the optimal hot working conditions for the Co-Cr-Fe alloy, extensively used in the aerospace industry for its excellent hardness and high wear resistance. The mechanical properties and microstructure observations showed that the flow stress of the sample, composed of M7C3-M2C carbides and face-centered cubic matrix, increased with decreasing temperature and increasing strain rate. Furthermore, as the deformation temperature increased, the volume fraction of recrystallized grains increased at equivalent strain levels, and the dynamic recrystallization mechanism transitioned from continuous dynamic recrystallization to discontinuous dynamic recrystallization. Based on the activation energy (Qc = 419.4 kJ/mol) and power exponent (n = 5.2) achieved from the true strain-stress curves, it was concluded that dislocation climb creep was the dominant deformation mechanism during hot working of the Co-Cr-Fe alloy.
The melting and solidification process of DZ411 superalloy at different cooling rates (50, 200, 500 ℃/min) was observed in situ by high-temperature confocal laser scanning microscopy. The solidification behaviour of this alloy was also studied through other methods such as differential scanning calorimetry and scanning electron microscopy. The results show that the precipitation sequence of the main phases during the solidification of DZ411 alloy is c matrix phase, carbide phase and Laves phase. Besides, during the solidification process of DZ411 alloy, both the dendrite thickness and dendrite spacing decreased with the increasing of cooling rate. In addition, a large amount of Ta is enriched in the dendrite stems at the end of solidification, which is the main reason for the formation of Laves phase. As the cooling rate increases, the size of the Laves phase becomes smaller and the distribution becomes more dispersed, which effectively inhibits the segregation of the alloy.
With the evolution of nickel-based single crystal superalloys, there is an increase in heavy elements such as Re and Ru. This has made solutal convection more pronounced during the directional solidification process, leading to solute redistribution and increasing the risk of casting defects such as low-angle grain boundaries. To avoid casting defects, downward directional solidification (DWS) method is adopted to eliminate solutal convection and change solute redistribution. However, there is currently no in-situ characterization or quantitative simulation studying the solute redistribution during DWS and upward directional solidification (UWS) processes. A multicomponent phase field simulation coupled with lattice Boltzmann method was employed to quantitatively investigate changes in dendrite morphology, solutal convection and deviation of dendrite tips from the perspective of solute redistribution during UWS and DWS processes. The simulation of microstructure agrees well with the experimental results. The mechanism that explains how solutal convection affects side branching behavior is depicted. A novel approach is introduced to characterize dendrite deviation, elucidating the reasons why defects are prone to occur under the influence of natural convection and solute redistribution.
In recent years, the effect of pulsed magnetic fields on improving the solidification structure of alloys has attracted significant attention. A GH4738 nickel-based alloy smelted using a self-designed 20-kg electromagnetic casting system was taken as the research object. Finite element software was used to numerically simulate the magnetic field intensity, distribution, and temperature field of the casting device. The effect of the pulsed magnetic field on the solidification process of the GH4738 alloy was studied by means of low-magnification microstructural analysis. The measured magnetic field shows that when the duty cycle is 20%, the pulse frequency is 50 Hz, the output current is in the range of 150-250 A, and the peak magnetic field intensity of the crucible center is 68-116 mT. The crucible temperature is heated to 600 ℃ and the melt center solidification time is 12.844 s. The microstructural analysis of the ingot shows that its shrinkage hole is reduced from 130 to 100 mm, and the equiaxed crystal area is increased from 2275 to 3150 mm2. The solidification angle of the dendrite is changed under the action of the pulsed magnetic field, and the tilt angle is 45°. The results show that the pulsed magnetic field promotes the primary crystal core of the GH4738 alloy, improves the nucleation rate of the melt, reduces the size difference of the solidification structure between the center and the edge of the ingot, and improves the uniformity of the solidification structure.
The hot deformation behavior of the premium GH4738 alloy was investigated in the temperature range of 1313 to 1353 K at strain rates of 0.01 to 1 s-1 using the hot compression test. To accurately predict flow stress, three novel strain compensation constitutive equations were developed and rigorously assessed. The results indicate that the power function model (correlation coefficients r = 0.98544) demonstrates greater prediction accuracy compared to other functions, with a calculated average activation energy of 507.968 kJ mol-1. Additionally, electron backscattered diffraction technology and transmission electron microscopy were used to analyze the evolution of the alloy microstructure during dynamic recrystallization under different deformation conditions. The results show that under high-temperature and large deformation conditions, the dislocation density and the degree of grain rotation increase, which promotes the formation and growth of new recrystallized grains, so that recrystallization is completed when the deformation amount reaches 30%. Besides, the increase in the temperature not only enhances the thermal activation mechanism, but also improves the grain size uniformity and texture consistency. Meanwhile, the carbide inhibits grain overgrowth by pinning grain boundaries, maintaining a fine and uniform grain structure of the alloy, and thereby improving the plasticity of the material.
The effects of Ti/N ratio on the number densities of nano particles, the size of the prior austenite grain (PAG) and the toughness of the heat-affected zone (HAZ) of Mg-deoxidized steels were studied after high heat input welding of 400 kJ/ cm. With increasing the Ti/N ratio from 2.7 to 5.7, the cuboid nano-sized particles are formed, and their number density increases. The area fractions of ductile intragranular acicular ferrites (IAFs) have the highest value and the area fractions of brittle microstructures of ferrite side plates and upper bainites have the lowest value in TN30 steel. With the Ti/N ratio of about 3.0, the HAZ of steel plate has the best low-temperature toughness. With increasing the Ti/N ratio from 2.7 to 5.7, the PAG sizes after the high-temperature laser scanning confocal microscopy observation decrease linearly with increasing the number densities of nano-sized particles. The PAG size of TN30 steel is between 100 and 150 lm, which is conducive to the nucleation of IAFs.
Fe-based slag-free self-shielded flux-cored welding wires with different Mn and Al additions were fabricated, and their processing properties and microstructures were studied. The results show that the slag coverage of high-Al and high-Mn wires is 9.03% and 15.32%, respectively. At the current of 260 A and voltage of 27 V, a higher deposition rate of 25.06% and lower spatter loss rate of 24.42% are achieved with the high-Al welding wire, as compared with the high-Mn welding wire. The microstructure of the high-Al wire hardfacing alloy is flake ferrite and d-ferrite, while the microstructure of the high-Mn wire hardfacing alloy is acicular ferrite. Since Al is a ferrite stabilizing element and Mn is an austenite stabilizing element, the addition of Al and Mn is capable of manipulating the type of precipitates by altering the degree of austenitization. Besides, inclusions in the hardfacing alloy may also be potential nucleation sites for acicular ferrite. Due to the better mechanical properties of acicular ferrite, the microhardness of high-Mn hardfacing alloys is higher than that of high-Al alloys.
Cu-Ni and Cu-Co-Ni superhydrophobic films were constructed on the surface of B10 copper-nickel alloy welded joints using a two-step process of electrodeposition and stearic acid modification. The chemical composition of the film surface was determined using surface characterization techniques. The corrosion resistance of the films was characterized using electrochemical impedance spectroscopy, potentiodynamic polarization, and scanning Kelvin probe microscopy at multiple scales. The thermal stability, mechanical stability, and self-cleaning properties of the films were also characterized. It was determined that the Cu-Co-Ni superhydrophobic film exhibited the best performance, with a static water contact angle of 159.3°, a roll-off angle of 2.3°, a charge transfer resistance 3300 times higher than the substrate, a self-corrosion current density nearly three orders of magnitude lower, and a surface Kelvin potential increase of 420 mV. The film demonstrated good thermal stability, excellent mechanical stability, and outstanding self-cleaning properties. Combining with previous studies, it was found that Co elements in the film contribute to the formation of a uniform and dense film, Ni elements enhance the adhesion and corrosion resistance between the films, and the combination of Co and Ni elements promotes uniform surface potential and further improves the corrosion resistance and interfilm adhesion of the films.
A new type of corrosion-resistant alloyed-steel rebar, Cr10MoV, was researched using techniques such as corrosion electrochemistry, X-ray computed tomography, and zero resistance ammeter to systematically study the macro-cell corrosion behavior and corrosion resistance of alloyed-steel rebar in mortar and concrete samples induced by chloride ion concentration in the marine environment. The macro-cell corrosion characteristics and development patterns induced by chloride ion concentration in alloyed-steel rebar were preliminarily revealed. In the macro-cell corrosion system of rebar mortar samples induced by 29 times chloride ion concentration, the corrosion current density of the alloyed-steel rebar combination stabilizes at 1.6-2.4 lA/cm2, which is only one-third of that of the carbon-steel rebar combination, while the dissimilar steel rebar combination stabilizes at 0-0.4 lA/cm2. Alloyed-steel rebar and carbon-steel rebar are configured in high concentration and low concentration chlorine salt areas, respectively. With the help of high corrosion resistance, the long-term stable corrosion resistance of alloyed-steel rebar is ensured. The potential difference between carbon-steel rebar and alloyed-steel rebar is reduced to weaken the driving force of macro-cell corrosion. It is a useful way to inhibit the macro-cell corrosion of dissimilar steel rebar and ensure the high corrosion resistance and durability of marine reinforced concrete structures.
The applications of Al alloy foam require consideration of potential damage risks, which are closely related to the evolution of its internal pore structures. However, conventional ex situ experimental observation cannot provide information on the structure evolution during deformation. In order to investigate the failure mechanism of Al alloy foam under quasi-static compression, by utilizing X-ray imaging technology, in situ CT image data were obtained during the loading process. A geometric model characterizing the real structure of Al alloy foam was reconstructed from the initial CT images and used for finite element simulation. Besides, based on the digital volume correlation (DVC) method, the displacement and strain fields of Al alloy foam were calculated. The results show that the in situ experimental observation based on X-ray imaging can effectively obtain the failure information of Al alloy foam. The simulation results for deformation and failure behavior of Al alloy foam are consistent with experimental results. During the quasi-static compression, a shear band can be observed diagonally across the profile of Al alloy foam, with weak regions occurring in the cells with larger volume and higher aspect ratios. Using these weak regions as boundaries, the relative displacement of cell structures on one side compared to another side was identified as the intrinsic cause of shear band formation. The high-strain regions identified by DVC closely match the crack locations on the cell walls, validating the accuracy of DVC on localizing cracks on cell walls and predicting their propagation trends.
The precipitation behavior, corrosion, and passivation performance of solutionized and severely sensitized SAF 2507 super-duplex stainless steel subjected to a temperature of 900 ℃ for 10 h are investigated in a twofold concentrated seawater at 60 ℃. The sensitized alloy exhibits 66.1% c phases and 33.9% r phases, and the original a phases have completely decomposed through eutectoid transformation, resulting in a microstructure characterized by coarse blocky r/c2 aggregates. High defect densities and an increased amount of oxyhydroxides and hydroxides are present in the passive film on the sensitized alloy, thereby enhancing n-type semiconducting character. The inferior performance of the passive film on the sensitized alloy is ascribed to the increased potential drop across the film/solution interface, the high defect densities, and the pronounced n-type character of the passive film resulting from the variations in its constituents. The precipitation of r phase during sensitization significantly increases intergranular corrosion susceptibility and decreases critical pitting temperature, breakdown potential, and polarization resistance in hot concentrated seawater.
The effect of Cl- and SO42- on corrosion behavior of pure copper in simulated groundwater was investigated by electrochemical testing techniques, scanning electron microscope/energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction in 0.1 mol/L NaHCO3 solutions. The results indicate that increasing Cl- and SO42- reduces the corrosion resistance of Cu. Cl- and SO42- can promote anodic dissolution of Cu and deteriorate the passivation property. The breakdown potential (Eb) of Cu decreases with the increase in Cl- and SO42-. With the increase in immersion time, the polarization resistance in different solutions tends to be stable. After 55 days, polarization resistance (Rp) was almost equal in 0 and 0.01 mol/L Cl- and SO42- solutions. In 0.05 mol/L Cl- and SO42- solution, Rp was lower. HCO3- has a certain corrosion effect on Cu and the pits size increased with the increase in Cl- and SO42-. The corrosion products (Cu2(OH)2CO3) and CuO were detected in solutions without or at low Cl- and SO42- contents. The corrosion product after immersion in the solution containing 0.05 mol/L Cl- and SO42- was Cu2O.
TiAl alloys were melted in BaZrO3 crucibles without and with Y addition at 1550 ℃ for 5, 10, and 15 min, respectively. The effect of the melting time and Y addition on the interaction between the crucibles and the alloys was investigated. Results revealed that the interaction extent was intensified with the increasing melting time. However, it could be effectively suppressed by adding Y into the alloy due to the in situ generated Y2O3 protection layer. The thickness of the interaction layer could be decreased from 95.6 to 25.1 lm with Y addition at 1550 ℃ for 15 min. In addition, a significant deoxidation effect was achieved by adding Y, and the O concentration of the alloy was decreased from 0.2 to 0.0561 wt.%. In comparison with Al2O3, MgO, CaO, and Y2O3 crucibles, BaZrO3 crucible combined with Y addition exhibited the lowest O contamination to the alloy melt.
An experiment was conducted to assess the impact of fused calcia-stabilized zirconia micro-powder on the thermal shock behavior of magnesia-spinel refractories. The effects of calcia-stabilized zirconia on the microstructure evolution and properties of magnesia-spinel refractories were characterized by the high-temperature elastic modulus, thermal shock damage resistance parameters, retainment of elastic modulus after thermal shock, and scanning electron microscopy. The results indicated that the incorporation of calcia-stabilized zirconia improved the thermomechanical properties and thermal shock behavior of magnesia-spinel specimens. The hot modulus of rupture of magnesia-spinel specimens increased by 2.5-fold due to the incorporation of calcia-stabilized zirconia micro-powder. The presence of a martensitic phase transformation in partially unstable ZrO2 and thermal mismatches among various phases contributed to a controlled formation of microcracks. And the pinning effect caused by the calcia-stabilized zirconia particles surrounding the grain boundaries played a crucial role in preventing the propagation of microcracks. This phenomenon significantly bolstered the thermal shock stability of magnesia-spinel refractories, consequently prolonging their service life.
Microporous MgO-MgAl2O4 refractory aggregates were prepared using calcined MgO powder and a-Al2O3 micro-powder as raw materials. The influence of a-Al2O3 micro-powder addition on the microstructures and properties of the aggregates was investigated. The results indicated that the addition of a-Al2O3 micro-powder to MgO powder not only promoted more pores in the MgO powder to being enclosed, but also caused the pores among the MgO powder to become micronano scale by the formation of continuous microporous MgAl2O4 bonding layers, which reduced the thermal conductivity of the aggregates. Furthermore, the microporous MgAl2O4 can induce crack deflection and generate crack branching when subjected to thermal shock, thus improving the thermal shock resistance of the microporous aggregates. The sample with 12.1 wt.% a-Al2O3 micro-powder addition exhibited the best comprehensive properties, with a bulk density of 3.44 g/cm3, a median pore size of 120.7 nm, a high flexural strength of 82.7 MPa, a high retention rate of flexural strength of 87.7%, and a thermal conductivity of 8.4 W/(m K) at 800 ℃. Compared to commercial fused magnesia and sintered magnesia, the thermal conductivity decreased by 47.2% and 18.4% at 800 ℃, respectively.
The dissolution behaviors of lime, limestone, and core-shell structured lime, as well as their effects on dephosphorization behavior were studied. The results show that the slow dissolution of lime in converter slag is mainly attributed to the calcium silicate layer at the lime/slag interface. CO2 generated by CaCO3 decomposition can destroy the calcium silicate layer, and thus accelerates the dissolution of limestone and core-shell structured lime. However, in the initial stage, a large amount of CO2 emission generated by limestone decomposition results in the poor contact between molten slag and limestone, and the dissolution rate is slower in the test of limestone than that of lime. For core-shell structured lime, the initial dissolution rate is not affected due to the lime surface, and is accelerated by the appropriate CO2 emission. Rapid CaO pickup in molten slag by fast dissolution of the lime sample can remarkably accelerate the dephosphorization reaction. Because of the fastest dissolution rate, the core-shell structured lime slagging mode shows the most promising prospects for the efficient dephosphorization.
A slurry-phase carbonation technique was utilized, employing argon oxygen decarburization slag (AOD slag) as a source of calcium and MgCl2 as a regulator for the crystal morphology of acicular aragonite. Subsequently, the carbonated AOD slag, enriched with acicular aragonite, was employed in fabricating composite cementitious materials, followed by an analysis of their evolution in hydration heat, hydration products, and microscopic morphology. Additionally, it delved into the mechanism through which acicular aragonite enhances the stength of composite cementitious materials. Moreover, advanced simulation software for engineering and sciences (ABAQUS) was utilized to simulate the compressive performance of composite mortar with varying dosages of acicular aragonite. The findings demonstrate that the carbonated AOD slag, containing 83.4% acicular aragonite (with an average aspect ratio of 21.31), exhibited commendable compatibility with cement. Moderate integration of carbonated AOD slag facilitated the formation of calcium sulfoaluminate hydrate (AFt) phases. The acicular aragonite within the cementitious matrix showcased remarkable filling effects. As the dosage of carbonated AOD slag increased, flexural and compressive strengths of cement mortar initially rose before declining. Upon reaching a 6% cement inclusion of carbonated AOD slag, the various constituents of the cementitious material displayed optimal synergy. The numerical simulation results confirmed the experimental findings, demonstrating a favorable increase in compressive strength and flexural strength with the addition of acicular aragonite. The acicular aragonite strengthened the matrix by serving bridging and pull-out functions.
