High-temperature Laser Scanning Confocal Microscopy 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.

Improvement in Grain Size Distribution Uniformity for Nuclear-Grade Austenitic Stainless Steel through Thermomechanical Treatment.

In this work, thermomechanical treatment (single-pass rolling at 800 °C and solution treatment) was applied to nuclear-grade hot-rolled austenitic stainless steel to eliminate the mixed grain induced by the uneven hot-rolled microstructure. By employing high-temperature laser scanning confocal microscopy, microstructure evolution during solution treatment was observed in situ, and the effect of single-pass rolling reduction on it was investigated. In uneven hot-rolled microstructure, the millimeter-grade elongated grains (MEGs) possessed an extremely large size and a high Schmid factor for slip compared to the fine grains, which led to greater plastic deformation and increased dislocation density and deformation energy storage during single-pass rolling. During subsequent solution treatment, there were fewer nucleation sites for the new grain, and the grain boundary (GB) was the main nucleation site in MEGs at a lower rolling reduction. In contrast, at a higher reduction, increased uniformly distributed rolling deformation and more nucleation sites were developed in MEGs. As the reduction increased, the number of in-grain nucleation sites gradually exceeded that of GB nucleation sites, and in-grain nucleation preferentially occurred. This was beneficial for promoting the refinement of new recrystallized grains and a reduction in the size difference of new grains during recrystallization. The single-pass rolling reduction of 15-20% can effectively increase the nucleation sites and improve the uniformity of rolling deformation distribution in the MEGs, promote in-grain nucleation, and finally refine the abnormally coarse elongated grain, and eliminate the mixed-grain structure after solution treatment.

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.

In situ study and assessment of the phosphorus-induced solute drag effect on the grain boundary mobility of austenite

The solute drag effect of phosphorus (P) in the single-phase austenite (γ-Fe) region was studied under isothermal annealing conditions at temperatures of 1050 °C, 1150 °C, 1250 °C and 1350 °C. High-temperature laser scanning confocal microscopy (HT-LSCM) was used to observe and quantify in situ grain growth on the surface of three different samples containing 0.026 wt.-% P, 0.044 wt.-% P and 0.102 wt.-% P. The dependence of the arithmetic mean grain sizes on time and temperature were modeled mathematically using a simple ordinary differential equation (ODE) according to normal grain growth (NGG) theory. As no other major effects, i.e., Zener-pinning by precipitates, occurred under the selected experimental conditions, grain growth interference was only considered by grain boundary (GB) segregation of P. Thus, the total grain boundary mobility (M) was directly determined depending on the P content and isothermal annealing temperature. The fitted GB mobility values enabled the determination of an average binding energy value between impurity P atoms and grain boundaries (E0 = -1.1 - -0.6 eV) in the system. Finally, the GB segregation of P in γ-Fe was derived from the observed grain growth kinetics. The results showed reasonable agreement with calculations using segregation enthalpies from the literature.

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.

Inclusions and microstructures in coarse-grained heat-affected zone of Al–Ti–Ca deoxidized shipbuilding steels with different Al contents after high-heat input welding

The inclusions, microstructure evolution and impact toughness of the coarse-grained heat-affected zone (CGHAZ) of Al–Ti–Ca deoxidized shipbuilding steel plates with different Al contents after high-heat input welding of 400 kJ cm−1 are investigated. The characteristics of inclusions and their influence on the refinement of microstructure and impact toughness in CGHAZ was elucidated. Increasing the Al content from 0.016 wt% to 0.022 wt% and 0.043 wt% results in an increase in the average size of prior austenite grain and the brittle phase fractions, while decreasing the content of intragranular acicular ferrites (IAFs). The results of the thermodynamic calculations, high-temperature laser scanning confocal microscopy and differential scanning calorimetry all show that increasing the Al content can diminish the austenitic stable region, elevate the transition temperature from austenite to ferrite and suppress the nucleation of IAF. In the present steels, the IAF formation is successfully induced by the specific inclusions, namely Al2O3–CaO–Ti2O3–MnS(-CaS), in which the calcium aluminate inclusions are embedded with Ti-oxides and CaS–MnS precipitates. The effective inclusions have the number density of 93 mm−2 in 16Al steel and 27 mm−2 in 22Al steel, but they are rarely found in 43Al steel. On average, an effective inclusion can induce 3.4 IAFs in 16Al steel and 2.8 IAFs in 22Al steel. Thus, as the Al content increases from 0.016 wt% to 0.22 wt% and 0.043 wt%, the impact toughness of CGHAZ at −40 °C decreases from 134 J to 78 J and 24 J with the welding heat input of 400 kJ cm−1.

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.

An Investigation into Microstructure Evolution and High-temperature Deformation Behavior during the Instability of In-service Welding

It is of great significance to reveal deformation behavior coupling with both the high-temperature effect and microstructure for the in-service welding instability zone. In the present paper, microstructure evolution and high-temperature deformation behavior at the representative temperatures 900 °C and 1100 °C inside the in-service welding instability zone were investigated based on the in-situ high-temperature laser scanning confocal microscope (LSCM). The results indicated that a sufficient condition of carbide precipitation is to hold it at 900 °C for a certain time. However, the phenomenon of carbide precipitation failed to be presented during continuous heating to 1150 °C. Although the slip and twining deformation occurred inside austenite grain, the fracture mechanism gave priority to intergranular cracking, which could be ascribed to the carbides with higher hardness and the grain boundary weakening caused by the temperature effect.

Some Considerations on Austenite Grain Growth Kinetics from High‐Temperature Laser Scanning Confocal Microscopy Observations

For more than 20 years, high‐temperature laser scanning confocal microscopy (HT‐LSCM) has become worldwide a proven technique for observing various metallurgical phenomena in situ. HT‐LSCM turns out to be a reliable and effective method for the observation of quantification austenite grain growth kinetics in steel, being highly relevant in casting and steel processing. In the present article, the measurement technique is briefly introduced followed by some basics on austenite grain growth. Finally, selected examples of recent research activities are discussed, including investigations into pure iron and the systems Fe–P, Fe–C, Fe–Al–N, and Fe–C–Nb–N. The influence of P on the grain growth kinetics is remarkable. At 1350 °C, the final grain size decreases from 255 μm in pure iron to 145 μm by adding 0.044 wt% P. In contrast, C enhances the grain growth in specific alloying ranges (0–0.30 wt%C), particularly at elevated temperatures. For a Al content of 0.025 wt% and only 50 wt ppm N, AlN loses the pinning effect above 1150 °C. In case of Nb(C,N), an elevated Nb content (0.085 wt%) provides stabilization of Nb(C,N) at 1150 °C, but at 1250 °C, no pinning of Nb(C,N) is visible anymore.

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.

Combination of In Situ Confocal Microscopy and Calorimetry to Investigate Solidification of Super‐ and Hyper‐Duplex Stainless Steels

The solidification processes of super‐ and hyper‐duplex stainless steels (i.e., UNS S32750 and S 33 207) are investigated in situ by a high‐temperature confocal laser scanning microscope (HT‐CLSM) and differential scanning calorimetry (DSC). The variations of δ‐ferrite phase fraction during solidification are measured quantitatively. The results show that liquid L→δ‐ferrite transformation first occurs at a certain degree of supercooling during the solidification process of steel. UNS S3DSS 3207 with a higher Cr content can result in a higher nucleation temperature and faster growth of δ‐ferrite compared to those of UNS S332750 steel. Moreover, both the liquidus (TL) and solidus temperatures (TS) are increased with the increasing Cr content, while TL increases greater than TS. Electron microscopies are used to quantify the fraction and composition of each phase. Scheil equation is employed to predict the distribution behavior of the main alloying elements in the solidification process, and the predicted results are consistent with the experimental findings. This study aims to provide real‐time experimental insights into the solidification kinetics of state‐of‐the‐art high‐alloy‐grade duplex steels and benefits for controlling the casting process in the real production of stainless steels.

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.