High-temperature Confocal Laser Scanning Research Articles

Influence of Solute Drag Effect and Interphase Precipitation of Nb on Ferrite Transformation.

The significant impact of Nb on ferrite transformation, both in terms of solute drag effect (SDE) and interphase precipitation, was investigated quantitatively. Ferrite transformation kinetics were characterized using thermal expansion experiments and theoretical calculations. The microstructures were characterized using high-temperature confocal laser scanning microscopy (CLSM), a field-emission scanning electron microscope (FESEM), and a transmission electron microscope (TEM). Under a higher driving force, interphase precipitations were observed in the sample with a higher Nb content. A three-dimensional (3D) reconstruction method was used to convert the two-dimensional (2D) image of interphase precipitation into a three-dimensional model for a more typical view. The SDE and interphase precipitation had opposite effects on the kinetics of ferrite transformation. A lower Nb content showed a strong contribution to the SDE, which delayed ferrite transformation. A higher concentration of Nb was expected to enhance the SDE, but the inhibition effect was eliminated by the interphase precipitation of NbC during interfacial migration. Both the experimental results and theoretical calculations confirmed this phenomenon.

Mechanistic insight into phosphorus migration pathways from oolitic hematite using in-situ observations during roasting and reduction process

Oolitic hematite is a promising alternative resource, in facing the iron ore downgrading. As oolitic hematite is often rich in phosphorus, an efficient separation of iron and phosphorus is necessary, in terms of utilizing both elements. Therefore, a close insight into the transformation of phosphorus-bearing minerals in the unique oolitic structure during the reduction process is important. To capture the real-time transformation of the apatite ring in the oolitic structure, in-situ observations under roasting, and CO/carbon reducing conditions were adopted by high-temperature confocal laser scanning microscopy. The results indicated that the apatite ring was transformed by four stages during the roasting process from 25 °C to 1400 °C: stabilization, defluorination, partial melting, and complete melting, which was: Ca5(PO4)3F → Ca3(PO4)2 → Ca3(PO4)2 + P2O5 liquid → P2O5 liquid. Under the CO reduction, the partial reduction of the phosphate at 1400 °C generated a Fe-Plow alloy. While the carbon reduction promoted iron phase absorption of phosphorus, forming Fe-Phigh alloy at 1400 °C. In addition, interfacial phenomena between apatite and slag melt were observed, and the dissolution of apatite (Ca/P molar ratio) into the slag melt was affected by the Fe/(Al + Si) molar ratio of the slag. Finally, using inverse error function methods, the diffusion coefficients of P and Ca increased significantly from the roasting to CO reduction and then to carbon reduction. These findings can benefit to understand phosphorus migration mechanisms, improving effective dephosphorization strategies for high-phosphorus oolitic hematite.

In-situ observation of restoration of tungsten plate via high-temperature confocal laser scanning microscopy

The restoration of an yttria dispersion strengthened tungsten plate and the corresponding microstructural evolution are investigated through in situ annealing using high-temperature confocal laser scanning microscopy. The geometrically necessary dislocation density and subgrain/grain evolution at each annealing stage are confirmed using the electron backscatter diffraction technique. The microstructure's evolution is divided into distinct stages of recovery, recrystallization, and grain growth based on their characteristic features. During the recovery process, the fomation of subgrains occurs. The primary mechanism of recrystallization nucleation is dominated by the strain-induced boundary migration mechanism, and the subgrains preexisting at grain boundaries are the main nucleation sites.

Physicochemical transformation of gasification slags with different residue carbon contents during sintering/melting process

The ash residue after gasification in the coal gasifier contains a certain proportion of residual carbon, which exerts a significant effect on the slag properties such as sintering, melting, and flow within the furnace. The residues discharged outside the gasifier retain the distribution patterns of ash and residual carbon under gasification conditions. This study selected coarse and fine residues generated from an entrained-flow gasifier and investigated the transformation of slags with different carbon contents during heating process. The obtained ash residues underwent characterization to determine the modes of the compositions occurrence. Subsequently, ash residues with different carbon contents were observed for the sintering-melting processes with a high-temperature confocal scanning laser microscope (CSLM). With the aid of FactSage, the physicochemical transformation of slag particles was determined. The impact of residual carbon on particle characteristics manifests in both physical structure and chemical reactions. On one hand, residue residual carbon can act as a skeleton during the melting process, causing the molten phase to enter the pores of porous carbon, intensifying particle shrinkage. After the inorganic components melt, the liquid encapsulated by carbon retains the original particle shape. On the other hand, the carbon can react with the inorganic ash components. If the carbon content is less than 6%, a new FexSi with a high ash melting point can be formed. As the carbon content increases, the carbon reacts with the Si to form SiC. This implies a reduction in the liquid content. Therefore, the flow temperature is greatly delayed by an increase in carbon content above 14%.

Effect Mechanism of α-Ferrite Sustained Precipitation on High-Temperature Properties in Continuous Casting for Peritectic Steel

Exploring the mechanism of the α-ferrite precipitation process on high-temperature properties plays an important guiding role in avoiding slab cracks and effectively regulating quality. In this work, in situ observation of the α-ferrite sustained precipitation behavior for peritectic steel during the austenitic phase transition process has been investigated using high-temperature confocal scanning laser microscopy. Meanwhile, the high-temperature evolution of the phase fractions during the phase transition process was quantitatively analyzed based on the high-temperature expansion experiment using the peak separation method. Furthermore, the high-temperature properties variations of the casting slab during the α-ferrite sustained precipitation process were investigated with the Gleeble thermomechanical simulator. The results show that the film-like ferrite precipitated along the austenite grain boundaries at the initial stage of phase transition, then needle-like ferrite initiates rapid precipitation on film-like ferrite when the average thickness reaches 15~20 μm. Hot ductility reached a minimum at the ferrite phase fraction fα = 10~15%, while high-temperature properties returned to a higher level after fα > 40~45%. The appearance of a considerable amount of needle-like ferrite and grain refinement effectively improves the high-temperature properties with the α-ferrite precipitation process advances.

Detachment of a Soluble Particle at the Slag-Argon Interface: CFD Study and Experimental Observations

The behavior of non-metallic inclusions at interfaces of high-temperature melt and molten slag affects the removal of inclusions and the consequent melt cleanness. This study presents real-time in situ observations on the behavior of an oxide particle in the vicinity of the slag-argon interface by means of high-temperature confocal scanning laser microscopy (HT-CSLM). On top of that, CFD simulations are conducted to investigate the underlying mechanisms of particle-interface interactions. In addition to revealing the particle motion process from the argon phase toward the slag, a significant particle morphology alteration associated with its dissolution in the slag is experimentally observed. Particularly, upon detachment from the slag-argon interface, the particle exhibits more dissolution at the near-interface area. By combining with numerical simulations, this study indicates that particle separation at the interface can be characterized as two stages. First, a short-term capillary force-driven motion stage happens until the particle initially settles at the interface. The settling position estimated by simulation shows good consistency with experimental measurement. Second, the particle takes a relatively long time to eventually detach from the interface, and this period is accompanied by particle dissolution. Investigations suggest that the concentration variation near the interface arising from particle dissolution triggers a Marangoni flow. This flow, in turn, enhances the local dissolution rate, consequently causing a significant particle morphology change that influences the detachment. This study provides new insight into the mechanism of inclusion removal through slag absorption in metallurgical processes. Both particle dynamics and dissolution kinetics, especially the effect of solutal Marangoni convection, are highlighted in detaching a small-scale particle from the fluid-fluid interface.

In-situ observation of Ni-11.5 wt%Si eutectic microstructure evolution and exploration of strengthening mode of Ni-Ni3Si eutectic composites

Microstructure evolution of the Ni-11.5 wt%Si eutectic alloy during the melting and cooling process was elaborately examined using a high-temperature confocal laser scanning microscope (HT-CLSM). The thermal stability of the phase composition within the Ni-11.5 wt%Si eutectic alloy was thoroughly analyzed. The intricate mechanism governing the solidification of the Ni-11.5 wt%Si eutectic alloy, along with the formation of the metastable Ni31Si12 phase under varying cooling rates, was systematically investigated. Through in-depth in-situ observations, a well-defined heat treatment procedure was developed for Ni-Ni3Si eutectic composites. Remarkably, the metastable Ni31Si12 phase can completely transform into Ni3Si upon undergoing the prescribed heat treatment, yielding a refined lamellar structure. This structural refinement notably results in substantial enhancements in the compressive properties and fracture toughness of the Ni-Ni3Si eutectic composite at ambient temperatures.

Phase-field simulation and high-temperature confocal laser scanning microscope experimental study of Martensite transformation during the disk laser quenching process

Laser quenching has significant advantages over conventional heat treatment. The main microstructure of C30E4 steel after quenching at a specific temperature is Martensite. Accurately obtaining the microscopic characteristics of the Martensite transformation process is the key to obtaining the best material properties, which is significant to material design. In this paper, the C30E4 steel was disk-laser-quenched, the metallographic and high-temperature confocal laser scanning microscope experiment was carried out. The microstructure of the fully phase-change hardened zone, heat-affected transition zone and matrix zone after quenching was obtained. The C30E4 martensitic transformation process in-situ was obtained. The mathematical physics model of Martensite transformation from metastable to steady state was established by phase field method. The finite-element method was used to solve the model, and the quenching Martensite transformation law was obtained. The Martensite formation process in austenite grain boundaries and microstructure defects was discussed. The experimental and the numerical simulation were compared to get consistent results, which verify the validity of the calculation method. This study lays a significant foundation for effectively revealing the Martensite formation and transformation during the disk laser quenching process, and provides a theoretical basis for quantifying the influence of Martensite transformation process on material properties.

Formation and modulation mechanisms of Fe-As phases in melting-processed Ba(Fe1-xCox)2As2 superconductor

The Fe-As phases play a crucial role in determining the superconducting properties of iron arsenic-based superconductors. In this study, we employed high-temperature confocal laser scanning microscopy (HT-CLSM) to investigate the formation mechanism of Fe-As phases in the melting processed Ba(Fe0.92Co0.08)2As2 superconductor through in situ observations. By combining these observations with microstructural and magnetic characterizations, we have confirmed that the high-temperature Fe-As phases primarily originate from the partial incongruent melting of the Ba122 superconducting phase. Moreover, we have elucidated the transition process of the Fe-As liquid and identified two dominant eutectic reactions in normal cooling sample: "L → Fe2As + FeAs" and "L → Fe2As + α-Fe". In case of extremely slow cooling rate, a further eutectic reaction between the Fe-As and superconducting phases ("L → Ba122 + Fe2As + FeAs") occurs, leading to the elimination of α-Fe and an increased volume of non-superconducting phases. We have also analyzed the effects of the Fe-As phases on the magnetic behavior, critical current density, and vortex pinning behavior. This study provides valuable insights for designing the microstructure of new type iron arsenic-based superconductors through the melting process.

Effect of P and Ti on the agglomeration behavior of Al2O3 inclusions in Fe–P–Ti alloys

Abstract Nozzle clogging occurs in the interstitial free (IF) steel with high phosphorus (P) more frequently than in IF steel with lower P. To explore the effect of P and Ti on the inclusion behavior in liquid steel, the in situ experiment and theoretical calculations were conducted. High-temperature confocal laser scanning microscopy was used in situ to observe the inclusion behavior at the liquid Fe–P–Ti alloy surfaces, and the attractive and capillary forces were also calculated to quantitatively estimate the effect of P and Ti on the inclusion behavior. The results show that the agglomeration of Al2O3 inclusions involves four steps: dispersed Al2O3 particles in liquid alloy; formation of Al2O3 chain structure; bending of the Al2O3 chain, and sintering and densification of the chain structure. The addition of Ti and P in the steel can increase the agglomeration time of inclusions, indicating the impeding effect of P and Ti on the inclusion aggregation. Furthermore, the orientation factor is proposed to estimate the direction of movement of the small inclusion crossing between large inclusions, and the experimental results confirm its validity.

Evolution of primary carbides during accelerated solidification process of high-carbon martensitic stainless steels

In order to evaluate the potential application advantage for development of sub-rapid cooling strip casting of high-carbon martensitic stainless steels, the effects of cooling rate on the solidification microstructure, solute element segregation, and primary carbide precipitation in high-carbon martensitic stainless steels were investigated using an association of high-temperature confocal laser scanning microscopy (HT-CLSM), optical microscopy (OM), scanning electron microscopy (SEM) and electro-probe microanalysis (EPMA). As a result, the microstructure becomes more refined and the secondary dendrite arm spacing (SDAS) decreases greatly by 51 μm from 65.9 μm as the increase of cooling rate from 30 to 6000 °C·min−1. The equation of SDAS versus cooling rate was formulated as λ2 = 191.62·RC−0.317. Solute elements are enriched in the inter-dendritic region, leading to the precipitation of primary carbides. The elemental segregation ratio reduces by more than 20 % for all elements, meanwhile, the size of primary carbides decreases significantly and the mean area fraction of the primary carbides decreases from 4.77 % to 0.82 % with increase of cooling rate from 30 to 6000 °C·min−1. Moreover, the reticulated carbides gradually become less continuous. Despite the type of carbide is not sensitive to the cooling rate, the content of Cr and Fe in the primary carbide is sensitive to cooling rate. Compared to the reported methods, sub-rapid cooling of strip casting shows great advantage in control of primary carbides of high-carbon martensitic stainless steels.

In Situ Observation the Effect of Y on the Solidification Process of 7Mo-SASS under a Low Cooling Rate.

The effects of Y on the solidification process of 7Mo super austenitic stainless steel (7MoSASS) under low cooling rate conditions (10 °C/min) were investigated using high-temperature confocal laser scanning microscopy (HT-CLSM). The in situ observation results indicate that Y samples promote an increase in austenite nucleation density. After 10 s of nucleation, the nucleation density increased by 149.53/mm2 for the Y sample. Furthermore, variance analysis indicated that Y addition improved the uniformity of the 7MoSASS solidification microstructure under low cooling rate conditions. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory results showed that when the solid phase ratio was 0.5, the nucleation mode of the Y sample transitioned from saturation site nucleation to saturation site nucleation + Avrami nucleation. YAlO3 has a low lattice disregistry value with austenite, making it a suitable heterogeneous nucleation core for promoting the early nucleation of austenite. During the late stages of solidification, Y accumulates in the residual liquid phase, providing a greater degree of compositional undercooling. SEM-EDS analysis showed that Y contributed to the refinement of the 7MoSASS solidification microstructure, with the proportion of precipitated phases decreasing by approximately 7.5%. Cr and Mo were the main elements exhibiting positive segregation in 7MoSASS, and the Cr segregation ratio increased in the Y sample, while the Mo segregation ratio decreased.

Research on hot isostatic pressing sintering behavior of 90W–Ni–Fe–Cu alloy

In order to understand the underlying material evolution during liquid phase sintering (LPS) process in hot isostatic pressing process (HIP), microstructure and mechanical properties of 90 W–Ni–Fe–Cu alloys fabricated at 1400 °C/150 MPa are examined systematically. For 90W-4.2Ni-1.8Fe–4Cu alloy, the typical LPS microstructure, which are spherical W grains uniformly surrounded by continuously distributed γ- (Ni, Fe, Cu) matrix phase with the lowest W–W contiguity of 0.31, is achieved successfully after HIP at 1400 °C/150 MPa. The excellent tensile strength reaches as high as 873 MPa with elongation of 17.3% due to the soft γ- (Ni, Fe, Cu) matrix phase. Moreover, the relationship between the microstructure parameters VM(1−CW−W) and elongation ε are established and analyze the association with dislocation motion in depth through electron backscattered diffraction (EBSD) characterization. To our best knowledge, the LPS behavior of tungsten alloy is in-situ revealed by high-temperature confocal scanning laser microscopy (HT-CSLM) for the first time. The current research provides a novel strategy for designing and fabricating powder metallurgy materials by LPS with pressure.

Dissolution of Magnesia in Silicate Melts and Diffusivity Determination from CLSM Studies

Magnesia is one of the vital and extensively used refractory components. In this study, the dissolution of magnesia is investigated at 1450, 1500, and 1550 °C in three silicate slags in the CaO–Al2O3–SiO2–MgO system using high-temperature confocal laser scanning microscopy to determine its effective binary diffusivity. The pore-free fragments of single-crystal fused magnesia particles were used, and the effects of experimental parameters and slag properties on the dissolution of magnesia were assessed. The ranking of dissolution times in the three slags at the three temperatures did not agree with the trend expected from the CaO/SiO2 ratio of each slag. Instead, several quotients serving as reference numbers were tested. The effective binary diffusivities were calculated considering all the impacting phenomena and parameters. The diffusivities of magnesia at 1500 °C in the slags with CaO/SiO2 weight ratios of 0.65, 0.93, and 1.17 are 2.67 × 10−10, 1.81 × 10−10, and 3.20 × 10−10 m2/s, respectively. The diffusivity of magnesia in one of the three slags was compared with rotating finger test results, which showed good agreement. The plausibility of diffusivity was checked using an Arrhenius plot.

Effect of Cooling Rate on Crystallization Behavior during Solidification of Hyper Duplex Stainless Steel S33207: An In Situ Confocal Microscopy Study

Hyper duplex stainless steel (HDSS) is a new alloy group of duplex stainless steels with the excellent corrosion resistance and mechanical properties among the existing modern stainless steels. Due to the incorporation of the high content of alloying elements, e.g., Cr, Ni, Mo, etc., the crystallization behavior of δ-ferrite from liquid is of vital importance to be controlled. In this work, the effect of the cooling rate (i.e., 4 °C/min and 150 °C/min) on the nucleation and growth behavior of δ-ferrite in S33207 during the solidification was investigated using a high-temperature confocal scanning laser microscope (HT-CLSM) in combination with electron microscopies and thermodynamic calculations. The obtained results showed that the solidification mode of S33207 steel was a ferrite–austenite type (FA mode). L→δ-ferrite transformation occurred at a certain degree of undercooling, and merging occurred during the growth of the δ-ferrite phase dendrites. Similar microstructure characteristics were observed after solidification under two different cooling rates. The variation in the area fraction of δ-ferrite with different temperatures and time intervals during the solidification of S33207 steels was calculated at different cooling rates. The post-microstructure as well as its composition evolution were also briefly investigated. This work shed light on the real-time insights for the crystallization behavior of hyper duplex stainless steels during their solidification process.

Precipitation mechanism of σ phase in S32654 super austenitic stainless steel

By utilizing High-Temperature Confocal Laser Scanning Microscopy (HT-CLSM), the microstructure evolution of S32654 super austenitic stainless steel was observed in situ during both solidification and solid-state phase transition. Moreover, the predominant focus was on investigating the precipitation mechanism of the σ phase. The findings revealed that the initial stages of the solidification process involved the nucleation and growth of austenite, followed by a gradual reduction in the residual liquid phase between dendrites until complete solidification was achieved. Furthermore, as the temperature decreased, the composition of the austenite formed from the residual liquid phase differed from the initially formed austenite, ultimately leading to the formation of granular austenite through extrusion. Below the critical temperature, the excessive amounts of Cr and Mo could not be dissolved in the supersaturated austenite between dendrites, thereby resulting in the instantaneous eutectoid decomposition of austenite, namely γ → γ2 + σ.

High-temperature rheological behavior and non-isothermal pyrolysis mechanism of macerals separated from different coals

It is a challenge to accurately characterize the caking behavior of coal after thermal softening and its reaction mechanism. In this work, the formation behavior of metaplast during coal macerals coking was investigated using high-temperature confocal laser scanning microscopy and high-temperature small-amplitude oscillatory shear rheometer, the source of diversities in thermal fluidity of coal was revealed by thermogravimetric-mass spectrometry analysis and kinetic-thermodynamic theory calculations. The results showed that although vitrinite was the generator of metaplast in the thermoplastic stage, the expansion rate of vitrinite from coking coal is more than 70% greater than that from gas coal for two consecutive times. Moreover, the total weight loss of vitrinite from gas coal was about 1.262 times higher than that from coking coal, but the latter exhibited more generous aliphatic groups in pyrolysis products. In the thermal flow interval, the reaction activation energy of vitrinite and inertinite from coking coal and gas coal was 271.0 ± 0.7, 302.0 ± 1.5, 241.0 ± 0.1 and 300.25 ± 1.55 kJ/mol, respectively. Eventually, the kinetic and thermodynamic mechanism parameters of raw coal and macerals had favorable correspondence regularity, and macerals with the most prominent parameters would be dominant in the overall thermal properties of raw coal.

In-situ observation of the incipient melting of borides and its effect on the hot-workability of Ni-based superalloys

The excellent creep properties of advanced wrought Ni-based superalloys are achieved by adding grain boundary strengthening elements such as B and C. Their low solubility in the γ matrix may promote precipitation of micrometre-sized borides and carbides which potentially deteriorate hot-workability, e.g. through incipient melting. However, their incipient melting has not yet been directly linked to a decrease in hot-workability in Ni-based superalloys. Here, we study the susceptibility of two Ni-based superalloy variants with main differences in B and C contents (high B & C variant with>triple B and C contents) to incipient melting and its effect on hot-workability. We show that incipient melting causes severe intergranular cracking at low strains during compression at 1200 °C in the high B & C variant only. Compression to higher strains separates grain boundaries fully, partially pulverising the macroscopic specimen. Metallographic specimens already show incipient melting at heating rates they experience when put into a pre-heated furnace. During subsequent cooling, the melt re-solidifies and forms lamellar structures consisting of various phases rich in Mo-Cr-B, Zr, and Ti, the constituents of M3B2 and MC. In-situ observations using a customised high temperature confocal laser scanning microscope set-up unambiguously confirmed the origin of these lamellar structures. We demonstrate for the first time directly the constituent liquation of M3B2. This liquation is assumed to be aided by the partial dissolution of adjacent MC carbides into the melt. These results will help to improve thermo-mechanical simulations and microstructural control during industrial processing of advanced wrought superalloy parts.

High temperature confocal scanning laser microscopy analysis of dead‐burned magnesia aggregates

AbstractDead‐burned magnesia is a commonly used material in the manufacturing of refractories for the steelmaking industry. Aggregates of dead‐burned magnesia contain secondary phases due to the impurities within the magnesite rock used in its production. While these phases can aid in sintering magnesia, they may have some impact on the high‐temperature performance of the refractory product. High‐temperature confocal scanning laser microscopy was utilized to observe the behavior of dead‐burned magnesia aggregates at elevated temperatures (up to 1550°C). Liquid formation was detected even at temperatures below 1350°C. In some cases, this liquid quickly exuded from the aggregate surface. This liquid phase was characterized through microscopy and chemical analysis to determine the impact of impurity content on the formation and behavior of this liquid phase, and conclusions are drawn on its detrimental impact on the material's refractoriness in a steelmaking environment.

Cyclic Solid-Liquid Phase Transformations in the text {CaO}–text {SiO}_2 System—Experiments and Modelling

The cyclic growth and shrinkage of solid oxides (i.e. wollastonite) in text {CaO}–text {SiO}_2-based slags is investigated in-situ by means of High-Temperature Confocal Scanning Laser Microscopy (HT-CSLM). The compositions of the slags are carefully selected to induce different phase transformation conditions, i.e. congruent and incongruent melting/solidification. To complement the experimental results, the kinetics of growth and shrinkage of oxide crystals is investigated by means of a sharp interface model where the interfacial reaction and diffusion in the liquid bulk are considered as possible rate-controlling processes. The modelling approach combined with data analysis from key experiments reveals the underlying dissipative processes of solidification and melting phenomena, here, diffusion in the liquid bulk material and/or the interfacial reactions. The modelling approach is likely to be applicable for future materials design and processing problems from this perspective.