High-temperature Confocal Laser Scanning Research Articles

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.

The investigation of precipitation behavior of titanium compounds for high titanium steel based on in situ observation.

The effects of cooling rate, Ti content, and casting temperature on titanium compounds for high titanium steel were investigated. In-situ observation of high titanium steel during remelting and solidification was carried out by using a High Temperature Confocal Scanning Laser Microscope (HTCSLM), and the observed results were in good agreement with the thermodynamic and kinetic calculations. The observation and calculation results both show that the inclusions in high titanium steel first precipitate in the form of TiN, followed by TiC precipitates as temperature decreases, eventually forming TiCxN1-x type inclusions at room temperature. The initial precipitation temperature of the inclusions increases with the increase of Ti content in molten steel, whereas casting temperature has little effect on the initial precipitation temperature of inclusions. In addition, the size of TiN inclusions increases with the increase of Ti content in steel but decreases with the increase in cooling rate.

In-Situ Observation of Steel/Slag/Inclusion Interaction by Means of High-Temperature Confocal Scanning Laser Microscopy

Non-metallic inclusions (NMIs) in steels have been the focus of various experimental studies due to their detrimental character. While isolated processes, such as agglomeration or dissolution, have been well investigated, holistic in-situ views with high-temperature confocal scanning laser microscopy (HT-CSLM) have been rare. In this work a novel and suitable method is presented to simultaneously observe agglomeration, movement, detachment, and dissolution of non-metallic inclusions by combined the melting of steel and slag with a dual-crucible approach. Therefore, different steel/slag combinations were tested. It is shown that, with transparent that does not become tarnished by the ions stemming from the steel, the steel’s surface and non-metallic inclusions are observable. Furthermore, technological limitations are discussed, including restrictions regarding the melting point of steels and certain steel/slag combinations.

In Situ Observation of Solidification and Crystallization of Low-Alloy Steels: A Review

Crystallization during the solidification process of steels is of vital importance for controlling the quality of final products. This paper summarizes the in situ characterization research activities of crystallization behaviors of low-alloy steels during the solidification process. The results obtained using high-temperature confocal laser scanning microscope (HT-CLSM) are critically reviewed, and other relevant methodologies, i.e., either classical method using differential scanning calorimetry (DSC) or large-scale facility (LSF), are also briefly mentioned. The evolution of the crystallization front from a planar to a cellular and further to a dendritic one, and subsequential microstructure evolutions, i.e., delta-ferrite (δ) formation from the liquid, austenite (γ) transformation and decomposition, are mainly discussed. The current review aims to highlight the state-of-the-art research outputs obtained by the novel in situ characterization techniques, and the obtained knowledge aims to shed light on the further development of the quality low-alloy steel products by controlling the processing and structure correlation.

Potential for carbide dispersive precipitation strengthening Ni-based alloy for high temperature

Increasing service temperature of wrought Ni-based superalloy by increasing alloying degree and fraction of γ′ phase has come to a bottleneck. In present work, an attempt is made to make maximum use of carbide to strengthen Ni-based alloy with a simpler chemical composition. Multiple morphology characterization methods and high temperature tensile test show that the newly designed Ni(bal.)-20Cr-0.80C–12Ta (wt%) alloy can be strengthened by TaC and Cr-rich M7C3 carbide and shows good ductility at 800 °C. Phase transformation characteristics detected by differential scanning calorimetry (DSC) and high-temperature confocal laser scanning microscopy (CLSM) images shows that there is a potential for dispersive precipitation of carbide in the alloy to get better mechanical property.

In-situ observations of solidification process of Fe–Al–Ta eutectic alloy

The Fe–Al–Ta eutectic alloy can be formed by eutectic reaction in order to improve the performance of Fe–Al intermetallic compounds, and thus it is important to further clarify the crystal growth mechanism of the Fe–Al–Ta eutectic alloy during solidification. Thus, the melting and solidification temperatures of the Fe–Al–Ta eutectic alloy were determined by differential scanning calorimetry, and the melting and solidification processes of the Fe–Al–Ta eutectic alloy were observed in situ by high-temperature confocal scanning laser microscopy. In addition, the solidified structure of the Fe–Al–Ta eutectic alloy was observed and characterized by scanning electron microscopy and energy dispersive spectroscopy. No other types of phase transitions were found in the Fe–Al–Ta eutectic alloy when the temperature was below 1600 °C. The reinforced phase exhibited a certain orientation after solidification, and the microstructure at the center of the eutectic cell was much more regular than that at the grain boundary. The effect of the cooling rate on the microstructure morphology and the formation mechanism of the irregular divorced eutectic were analyzed.

Agglomeration behavior of rare earth inclusions and alumina inclusions at the interface between argon and molten steel

Nozzle clogging frequently occurs in steel containing rare earth (RE) due to the aggregation of RE inclusions, which seriously interferes with the application of RE in steel. To provide insights into nozzle clogging, the agglomeration behavior of inclusions in RE-containing steel was investigated by laboratory experiments and theoretical calculations. High-temperature confocal laser scanning microscopy (HT-CLSM) was applied to observe the behavior of inclusions at molten steel surfaces and their interactive forces. Kralchevsky–Paunov (K–P) model was used to predict the capillary force acting on different inclusions. The results show that the interactive forces that acted on inclusions were investigated as a function of inclusion size, inter-distance, and inclusion composition. It is found that the capillary force acting on the inclusions decreases in the order of Ce2O3 > CeAlO3 > Al2O3. This study verified the applicability of the K–P model in RE-containing steel, which can provide theoretical support to help solve the nozzle clogging problem during rare earth steel production.

In-situ observation of peritectic solidification of Fe-Mn-Al-C steel with medium manganese

The design of the continuous casting process for the third generation of Fe-Mn-Al-C steel with medium manganese should be based on the understanding of the peritectic solidification mechanism. So, in the present study, Fe-5.56Mn-0.078Al-0.157C (wt%) steel was taken as the research object. By means of high-temperature confocal scanning laser microscopy (HT-CSLM), Thermo-Calc thermodynamic software and optical microscopy, the peritectic solidification process and microstructure characteristics of the medium manganese steel at different cooling rates were studied and revealed. And thus, the influence of the cooling rate on the peritectic solidification mechanism was clarified. Meanwhile, the kinetics of the γ-Fe grain wrapping the δ-Fe grain and the subsequent growth were determined. The results show that at low cooling rate of 10 °C/min, the initial δ-Fe phase is cellular. The γ-Fe phase firstly nucleates at the L/δ interface, and then grows along the δ/L interface until the L and δ-Fe phases are separated. Subsequently, the peritectic transformation occurs, that is the L and δ-Fe phases are directly transformed into the γ-Fe phase. Under the conditions with higher cooling rates of 20 and 30 °C/min, when the γ-Fe phase separates the L and δ-Fe phases completely, multiple γ-Fe grains form in a single δ-Fe grain at the same time, and the whole δ-Fe grain is covered within a very short time, which is the typical massive-like transformation. In the initial stage of the peritectic reaction, the linear velocity of the γ-Fe phase wrapping the δ-Fe phase is inversely proportional to the cooling rate, which is closely related to the size and morphology of δ-Fe grains. At cooling rates of 10, 20 and 30 °C/min, the initial migration rates of the δ/γ interface are 45.9, 8.9 and 12.5 µm/s, respectively. At higher cooling rates, the occurrence of the massive-like transformation greatly promotes the peritectic transformation, and the migration rate of the δ/γ interface increases to 16.7 and 17.8 µm/s. The initial migration rates of the γ/L interface are 6.0, 1.8 and 6.5 µm/s, respectively. It is found that the migration rate of the δ/γ interface is much larger than that of the γ/L interface. Dendrite etching experiment in the top, bottom and longitudinal sections of the medium manganese steel samples shows that the larger the cooling rate, the finer the solidification structure. • The δ/L interface of Fe-5.56Mn-0.078Al-0.157C (wt%) steel becomes more unstable with the increase of the cooling rate. • The mechanisms of the peritectic solidification of the medium Mn steel at different cooling rates are revealed. • Kinetics of the γ-Fe phase wrapping δ-Fe phase and then growing into δ-Fe and L phases are measured for the medium Mn steel. • The relationships between the dendrites arm spacings of the medium manganese steel and cooling rates are presented.

Study on microsegregation and homogenization process of a novel nickel-based wrought superalloy

In this paper, the as-cast microstructure, microsegregation, precipitates and suitable homogenization process of a novel Ni-based wrought superalloy prepared by vacuum induction melting (VIM) + vacuum arc remelting (VAR) were investigated by optical microscope (OM), scanning electron microscope (SEM), electron probe microanalysis (EPMA), differential scanning calorimetry (DSC), high temperature confocal laser scanning microscope (HT-CLSM) and thermal simulation compression. The results show that severe element segregation exists in the ingot. Nb, Ti and Al are mainly segregated in the interdendritic region, while W is mainly segregated in the dendrite arm. The precipitates in the alloy mainly include non-uniform γ′ strengthening phase, coarse (γ + γ′) eutectic phase and MC carbide, among which, the redissolution temperature range of γ′ phase is 1060–1220 °C; the (γ + γ′) eutectic phase can be redissolved obviously above 1180 °C but melted into liquid at 1250 °C; the redissolution temperature range of MC carbide is 1349–1358 °C, exceeding the solidus temperature of the alloy. After homogenization at 1220 °C/10 h, all the γ′ phase and most of the (γ + γ′) eutectic phase can be redissolved, and the element segregation can be significantly alleviated, moreover, the hot deformation resistance of the alloy is lower and the dynamic recrystallization (DRX) degree is higher, indicating that 1220 °C/10 h is an ideal single-stage homogenization process.

Investigation into the Temperature of Metallic High-Temperature Confocal Scanning Laser Microscope Samples

Investigation into the Temperature of Metallic High-Temperature Confocal Scanning Laser Microscope Samples

Effect of Al content on the δ→γ transformation and M23C6 precipitation behavior in 18Cr-xAl-Si ferritic heat-resistant stainless steel during solidification process

Aimed at clarifying δ→γ transformation and M23C6 precipitation behavior in18Cr-xAl-Si (x = 0, 0.43wt.%, 0.91wt.% and 1.82wt.%) ferritic heat-resistant stainless steel (FHSS), high temperature confocal scanning laser microscopy (CSLM) was used for in-situ observation during solidification. The results demonstrated that increasing of Al content reduced the nucleation temperature of δ-ferrite, and at the same time reduced the free energy difference between δ-ferrite and γ-austenite, which increased the difficulty of δ→γ transformation, in turn reduced the temperature of δ→γ transformation. The γ-austenite preferentially precipitated at the δ-ferrite phase boundaries and gradually along the sides of the δ-ferrite phase boundaries as the temperature decreased, and part of the martensite transformed inside the γ-austenite phases. M23C6 carbides mainly precipitated at the phase boundaries between δ-ferrite and γ-austenite/martensite. With the increase of Al content, the amount of γ-austenite/martensite and M23C6 carbides decreased. In addition, the solidification mechanism of the complex nucleus was also discussed.

Image Processing Procedure to Evaluate Inclusion Dissolution in a Slag Observed by High-Temperature Confocal Scanning Laser Microscopy

In situ study of the inclusion dissolution behavior in a slag utilizing high-temperature confocal scanning laser microscopy helps to understand the phenomenon of the removal of non-metallic inclusions from liquid steel before the casting process. The current work introduces an image processing procedure to automate and measure the inclusion diameter during its dissolution. Silica and alumina particle dissolution were considered as they appear differently when suspended in the slag (bubble-like transparent and solid-like opaque, respectively). The procedure consists of two parts: (1) morphological isolation and construction of the inclusion, and (2) inclusion diameter calculation. The inclusion diameters could be successfully measured over a series of images and showed good agreement with the manually measured diameters. The image processing procedure has the advantages of significant time saving and standardization compared to manual measurements.

Influence of cooling rate on δ-ferrite/γ-austenite formation and precipitation behavior of 18Cr–Al–Si ferritic heat-resistant stainless steel

The δ-ferrite/γ-austenite formation and precipitation behavior of 18Cr–Al–Si FHSS (ferritic heat-resistant stainless steel) during solidification has been investigated by a high temperature confocal scanning laser microscopy (CSLM) at the cooling rates from 5 °C min−1 to 100 °C min−1. The results have demonstrated that the increase of cooling rate will decrease nucleation temperature of δ-ferrite phase, which will affect δ-ferrite/γ-austenite formation process, and the δ→γ transformation process all occurred at the texted cooling rates. The γ-austenite phase first precipitated at δ-ferrite phase boundaries and then appeared within δ-ferrite phase, and in the subsequent cooling process, part of the γ-austenite begins to transform into martensite. The content of γ-austenite/martensite phase and M23C6 carbides increased with the increment of cooling rate. In addition, the solidification mechanism of the complex nucleus at the cooling rates of 5 °C min−1 and 100 °C min−1 were also discussed.

Origin of fine equiaxed grains in industrial low-temperature grain-oriented silicon steel normalized sheet and their influence on magnetic properties

In the industrial low-temperature grain-oriented (LTGO) silicon steel normalized sheet, there are a certain number of fine equiaxed grains distributed along the rolling direction. However, the formation mode of the fine equiaxed grains and the influence of them on magnetic properties have not yet been well understood. In the present work, the above issues were investigated using field emission scanning electron microscopy (SEM), field emission transmission electron microscopy (TEM), X-ray diffraction (XRD), high temperature confocal scanning laser microscopy (HTCSLM), etc. It was found that (1) the fine equiaxed grains could be divided into many grain colonies according to grain orientation, and within each colony, the grain orientations were similar; (2) during normalizing, the fine equiaxed grains were mainly generated inside the elongated deformed grains which contained a large number of subgrains; (3) the amount of α → γ phase transformation during the normalizing was very small. Therefore, it can be inferred that the fine equiaxed grains were mainly produced by subgrain coalescence, rather than by phase transformation. Moreover, γ-grain (〈111〉 // ND) colonies with ideal grain size could be formed in the fine equiaxed grain regions after ideal normalizing process. Due to the heredity of microstructure and texture, γ-grain colonies, which can facilitate the formation of the precise Goss texture ({110} 〈001〉 ) during high-temperature annealing, were formed in the primary recrystallized matrix after cold rolling, decarburizing and nitriding, as a result, the magnetic properties of the final product were significantly improved. The present work can provide help to further improve the understandings of the microstructural evolution in the LTGO silicon steel normalized sheet and the influence of normalized microstructure on magnetic properties.

On the solid-state dendritic growth of [formula omitted] carbide at interfaces in an austenitic system

Solid-state growth of M7C3 carbide at different interfaces in an austenitic Ni–Cr–Fe alloy (Alloy 600) was studied in-situ by high temperature confocal laser scanning microscopy during continuous cooling from 1140 °C. The carbide develops a dendritic morphology and grows laterally across the free surface via a diffusion-assisted mechanism. Primary dendrite arms can extend beyond 100 μm, with some dendrites also developing secondary arms with spacing approximately 3.0 μm. By using focused ion beam tomography, some of the surface carbides are found to be directly connected to dendritic carbide at grain boundaries in the bulk confirming they originated from the same nucleus. Matrix deformation is induced in the vicinity of a carbide during growth. This work provides significant insight into the important topic of carbide development by a rare combination of both in-situ microscopy and a 3D tomographic technique; the new knowledge discovered is expected to apply to other austenitic systems.

Analysis on phase nucleation in Sn-Ni peritectic alloy combining confocal laser scanning in-situ observation and differential scanning calorimetry

The analysis of the phase nucleation requires not only the in-situ images but also the accurate nucleation information. Thus, the nucleation of both the primary Ni3Sn2 and peritectic Ni3Sn4 phases were studied during peritectic solidification of Sn-Ni alloy using the high temperature confocal laser scanning microscope (HT-CLSM) and differential scanning calorimetry (DSC). The results by both methods confirm that the nucleation undercooling ∆T of the solid phases decrease with increasing cooling rates, which is demonstrated to be caused by higher concentration difference at larger cooling rates. Besides, the freezing range of the alloy is also found to decrease with the increase in cooling rate. However, the nucleation undercooling of the peritectic Ni3Sn4 phase determined by HT-CLSM is larger than that by DSC at each cooling rate. This is attributed to the slower capture of peritectic reaction when using HT-CLSM. Furthermore, the nucleation of peritectic Ni3Sn4 phase is confirmed to be controlled by the heterogeneous nucleation on primary Ni3Sn2 phase by comparison between the HT-CLSM observation and DSC result. In addition, the wetting angles for heterogeneous nucleation of both phases which are important parameters in analyzing the nucleation of solid phases were determined through the entropy of solidification data from the DSC results. The smaller wetting angle of peritectic Ni3Sn4 phase as compared with that of the primary Ni3Sn2 phase further proving the smaller nucleation undercooling of Ni3Sn4 phase during solidification.

Artificial Inclusion Environments—Replicating Industry in the Laboratory

The authors present a series of complementary test methods which were developed and used to investigate reactions between high aluminium steel and silica rich inclusions. Non-metallic inclusions (NMIs) cause many defects in the final steel product, therefore the ability to track their size, morphology and composition and correlate this with fundamental reaction kinetics provides important knowledge to support the production of clean quality steel products. Novel steel grades such as TRIP, TWIP and low-density steels have high aluminium contents; aluminium is a readily oxidisable species presenting the potential for instability and excessive reaction with commonly used mould powders that contain silica. A novel combination of techniques including HT-CLSM (High-Temperature Confocal Laser Scanning Microscope), XCT (X-ray computed tomography) and SEM/EDS (scanning electron microscopy/electron dispersive spectroscopy) have been used to study the interaction of entrained mould powder inclusions with steel at high temperatures simulating industrial conditions. This report presents a discussion on the development of techniques and samples to achieve representative and repeatable results that can provide information on the complex chemical and physical interaction phenomena with confidence. Each experimental technique had its own learning points and consequent results. Outcomes presented include possible confirmation of the chemical reaction rate controlling step being aluminium mass transfer; heterogeneous local environmental conditions including fluidity and chemical composition; and occurrence of spontaneous emulsification where the mould powder inclusion breaks apart into a cloud of smaller fragments.

Systematic study on orientation relationships between acicular ferrite and Ti-Mg oxide at different cooling rates in low-carbon steel

In this paper, the influence of cooling rate on acicular ferrite (AF) nucleation and growth on Ti-Mg oxide was studied using high-temperature confocal laser scanning microscopy. The compositions and elemental distributions of the inclusions were studied via electron probe microanalysis, and the crystallographic orientation relationships between AF and inclusions were investigated systematically via electron back scattered diffraction. The results indicated that the initial transformation temperature of AF decreased from ~656 to ~609 °C and the growth rate of AF raised from 6.74 to 55.52 μm/s as the cooling rate increased from 1 to 12 °C/s. The area fraction and the nucleation probability of AF reached a maximum at 2 °C/s, which was the most suitable cooling rate for AF formation. The Ti-Mg-Mn-O, Ti-Mg-Mn-O·TiN and Ti-Mg-Mn-O·MnS inclusions in Ti-Mg deoxidized steel were all effective for AF nucleation. For the four samples at different cooling rates, all AF grains had four kinds of orientation relationships with Ti-Mg-Mn-O oxides: Baker–Nutting (B-N), Nishiyama–Wasserman (N-W), {100} Ferrite ║{110} Oxide , and irrational orientation relationships. The {100} Ferrite ║{110} Oxide orientation was also a common parallel relationship between the B-N and N-W orientation relationships. The lattice mismatch between the AF and oxide was low, and the interface between the AF and oxide was close to a coherent interface when the first three kinds of orientation relationships were adopted. The formation of a Mn-depleted zone (MDZ) around Ti-Mg-Mn-O oxide could increase the driving force of ferrite formation, and it contributed more to the nucleation of those AF grains that had no orientation relationships with oxides. • The nucleation and growth behaviors of acicular ferrite on Ti-Mg oxides at different cooling rates were observed. • Orientation relationships between acicular ferrite and Ti-Mg oxides were examined via electron back scattered diffraction. • The nucleation mechanisms of acicular ferrite induced by Ti-Mg oxides were discussed in detail.

Gasification and structural behaviour of different carbon sources and resultant chars from rapid devolatilization for HIsarna alternative ironmaking process

To evaluate the potential of using renewable biomass in the novel HIsarna technology, reactivity of thermal coal (TC), charcoal (CC), Bana grass char (BGC) before and after rapid devolatilization at 1500 °C in a drop tube furnace (DTF) was investigated. Thermogravimetric (TG) was used for CO2 gasification study, and high temperature confocal scanning laser microscope (HT-CSLM), Brunauer-Emmett-Teller (BET) and scanning electron microscopy (SEM) were used to characterise the morphology of all three raw carbonaceous materials and their chars produced by rapid devolatilization. CC has fastest gasification reaction before and after rapid heat treatment, and BGC raw is more reactive than TC raw but BGC 1500 °C and TC 1500 °C have very similar gasification behaviour. The reactivity index of the rapidly devolatilized char is reduced to 84.21% (BGC), 92.11% (TC) and 94.23% (CC) compared to their raw materials. This shows that BGC is more severely affected by the rapid devolatilization, and this behaviour is likely to be governed by the high ash content which will melt and cause pore blockage during heat treatment. According to HT-CSLM results the average particle sizes decreased by 28%, 24% and 20 % for TC, CC and BGC respectively. While the SEM images shown that TC has gone through significant structural changes during the rapid devolatilization, but CC and BGC maintained their parent structural shapes. The BET results indicate that TC is non-porous, but both CC and BGC contain a large number of constricted micropores with significantly larger surface area.

Effect of Cooling Rates on the Local—Overall Morphology Characteristics of Solidification Structure at Different Stages for High Carbon Steel

The solidification characteristics of 70 steel at the stage of the superheat elimination and the liquid–solid phase transformation were analyzed at cooling rates from 10 to 150 °C/min based on a high-temperature confocal scanning laser microscope (HT-CSLM). Secondary dendrite arm spacing (SDAS) and fractal dimension (D) were used to quantitatively describe the local compactness and overall self-similar complexity of the solidification morphology. It was found that the cooling rate had a very important influence on the local and overall morphology characteristics of solidification structures. At the superheat elimination stage, the cooling rate affected the morphology of the microstructure through the dynamic structural fluctuation between the generation and disappearance of atomic clusters in the molten steel. At the liquid–solid phase transformation stage, the cooling rate affected the local morphology of the microstructure by affecting the solute diffusion rate between dendrite arms, while it affected the overall morphology by changing the concentration undercooling at the front of all solidified interfaces. The presented results show that adjusting the cooling system at the superheat elimination stage can also be an important way to control the solidified morphology of different alloys.