High-temperature Laser Scanning Confocal Microscopy Research Articles

Characterization of Roughness, Porosity and Thermal Resistances of Continuous Casting Mold Slag Layers Devitrified and Crystallized in Laboratory

This paper presents an experimental investigation about the variation, with temperature, of surface roughness, porosity, degree of crystallization and microstructure, occurring in solid layers of a commercial mold powder slag used for casting medium carbon steel thin slabs. Characterization is done with samples devitrified and crystallized at 698 K, 823 K, 898 K and 973 K (425 °C, 550 °C, 625 °C and 700 °C) for 10,800 seconds, in the furnace of a high-temperature confocal laser scanning microscope. As well as, with slag disks devitrified non-isothermally in a heat flow apparatus based on the application of Fourier's law in steady state conditions, where the high temperature surface is maintained at selected values, between 927 K and 1241 K (654 °C and 968 °C). Surface roughness and porosity determinations are performed by image analysis of confocal laser scanning and scanning electron micrographs, respectively. For the time considered and temperatures ≥ 823 K (550 °C) the slag crystallizes completely, and variations of roughness and porosity with temperature are associated with changes in size and packing of the primary crystalline-phase grains. Crystallization induced porosity is significant and causes slag layer expansion. Hence crystallization does not lead to gap formation with a neighboring surface (e.g., mold wall) by shrinkage. Instead, surface separation arises from development of roughness, and measurements with the heat flow apparatus confirm that interfacial thermal resistance between a slag layer and a metallic wall can be estimated from the sum of the means of maximum peak to valley height of their surfaces. Furthermore, the results establish that slag layers roughness and porosity increase with temperature and reveal effective thermal conductivities in the range of ~ 0.3 to ~ 0.5 W/m K. Low effective thermal conductivities must be a consequence of porous bands, whose porosity depends on temperature and reaches values as high as ~ 0.20 and ~ 0.50.

Investigations of the peritectic reaction and transformation in a hypo-peritectic steel: Using high-temperature confocal scanning laser microscopy and differential scanning calorimetry

The present work focuses on the peritectic reaction and transformation in a commercial hypo-peritectic steel. This multiple phase transformation was studied by means of the high temperature confocal scanning laser microscopy (HTCSLM) and the differential scanning calorimetry (DSC) analysis. The results indicate that the technique of combining HTCSLM and DSC can effectively distinguish and characterize the peritectic reaction and transformation. The results of these two methods verify and complement each other in interpreting the peritectic reaction and transformation. In situ observation of HTCSLM reveals that the austenite → δ-ferrite transformation during the continuous heating firstly occurred around the inclusion and at austenite grain boundary. The emerging δ/γ interfaces travel through previous γ grains by a finger-like morphology. With further heating, the unstable liquid phase formed by the peritectic reaction transforms into the δ-ferrite phases again. These stable δ-ferrite phases subsequently melt into liquid phase. During the peritectic solidification, in situ observation reveals that the peritectic reaction is firstly observed to occur at the junction interface of the liquid phase and δ-ferrite phase. The γ intermediate phase propagates and thickens toward the liquid phase side and the δ-ferrite phase side in the subsequent peritectic transformation. The δ-ferrite and γ phases were observed to coexist in solid phase when the peritectic transformation finishes. Based on the linear regression of DSC results, it is found that the activation energy of the peritectic reaction is larger than that of the other phase transformations during heating, but it is on the contrary during cooling.

In-Situ Observation of Martensitic Transformation in a Fe–C–Mn–Si Bainitic Steel During Austempering

The martensitic transformation in a Fe–C–Mn–Si bainitic steel was examined by in situ high-temperature laser scanning confocal microscopy (LSCM) and dilatometry. The phenomenon of continuous martensitic transformation during austempering was firstly dynamically observed by LSCM. Differing from the commonly accepted viewpoint on martensite formation in bainitic steels, the martensitic transformation in the conventional medium-carbon bainitic steel was not instantaneous and proceeded gradually when the sample was austempered below martensite starting temperature (MS). It can be attributed to the generation of internal stresses, thermal activation, stimulating nucleation, and the segregation of Mn. In addition, apart from the continuous martensitic transformation, the bainitic transformation was also directly observed by LSCM during austempering below MS. Moreover, it was clear from the results of dilatation during austempering that the inflection point in the dilatation curve against time was not the demarcation point between martensitic and bainitic transformation, and in situ observations confirmed that martensite was still formed after the inflection point. Therefore, the obtained results could be an excellent reference to further understand the mechanism of bainitic and martensitic transformations in Fe–C–Mn–Si bainitic steel during austempering below MS. Graphical abstract presents a series of frames that reveals the rapid growth of some black units during austempering at 240 °C, and these rapidly growing units (marked by arrows) were not present in the previous frame of the video. These black growing units (martensite laths) shown in Graphical abstract appeared over a time interval of ~ 420 s from 837.39 to 1257.37 s during austempering. This phenomenon differs from the previously held viewpoint where martensitic transformation is expected to be finished in a very short period of time after a sample is cooled to a certain temperature below MS because of the diffusionless nature of martensite transformation in conventional medium-carbon bainitic steels. The growth of martensite laths during the austempering process at 240 °C.

In situ characterisation of MnS precipitation in high carbon steel

Manganese sulphide (MnS) is one of the major non-metallic inclusions in steel with huge impact on steel property. In the case of high carbon steel, due to higher sulphur content and its brittleness, controlling MnS formation is one of the main issues. MnS has a complicated precipitation mechanism during solidification in liquid and solid steel and at the interface with oxide inclusions. Higher sulphur content, lower melting point and different oxide inclusions in high carbon steel will cause MnS precipitation at different stages. In this study, different stages of MnS precipitation from liquid and/or solid in high carbon steel and at the interface with oxide inclusion were investigated comprehensively via two different types of High Temperature Confocal Scanning Laser Microscope (HTCSLM). Samples were analysed further using SEM-EDS for better understanding the pertaining mechanisms. MnS precipitation on the surface of liquid steel was observed in situ in a HTCSLM by the use of a concentric solidification technique. Additionally, formation of MnS following solidification and at the interfaces of oxide inclusions, was investigated in situ in a HTCSLM, which has a uniform temperature profile across the specimen. These comprehensive descriptions about different stages of MnS precipitation in high carbon steel have been conducted for the first time and provide crucial information for controlling MnS morphology in high carbon steel.

Nucleation and Growth of Iron Whiskers during Gaseous Reduction of Hematite Iron Ore Fines

A high-temperature confocal scanning laser microscope and an online reduction–water quenching experiment system were used to systematically study the generation of iron whiskers during the reduction of hematite ore particles with CO/CO2 gas. The "blooming" phenomenon of the surface during the reduction of iron ore particles was found in this experiment. The orientation of the grain on the longitudinal section of an iron whisker was measured to be uniform by applying the electron back-scattered diffraction technique, which proved that the iron whiskers are most likely to exist in single crystal form. According to the in-situ online experimental video, the average diffusion flux of iron atoms when the layered iron completely covers the surface of the ore particle is about 0.0072 mol/(m2·s). While the iron atom diffusion flux at the root of the iron whisker during the pre-growth process is much larger than the flux when the layered iron is produced, which are defined to be 0.081 mol/(m2·s), 0.045 mol/(m2·s), 0.013 mol/(m2· s), and 0.0046 mol/(m2·s), respectively during the four stages of the growth of an iron whisker. The quantitative relationship between the chemical driving force and the whisker growth is established as Δ G θ + R T ln p CO 2 p CO + 2 n 0.056 r ρ E s T = 0 .

Mechanism of MgO dissolution in MgF2–CaF2–MF (M=Li or Na) melts: Kinetic analysis via in-situ high temperature confocal scanning laser microscopy (HT-CSLM)

Abstract The solid oxide membrane (SOM) process is a direct electrolysis method for refining magnesium and has become a popular and promising technology. In the electrolysis process of SOM, the metal oxide is dissociated into the metal cation and oxygen anion. Thus, it is important to investigate the dissolution reaction of metal oxides in molten fluoride flux, which contributes to the overall reaction mechanism and reaction rate. However, there are few fundamental studies on the reaction between oxide particles and fluoride flux. Notably, the dissolution behavior of magnesium oxide (MgO), which is a major source of magnesium production, into fluoride flux has not been reported. In addition, the dissolution behavior is mediated by the chemical and physical properties of the flux. Therefore, we investigated the dissolution reaction of MgO in fluoride flux using high temperature confocal scanning laser microscopy (HT-CSLM) measurements to demonstrate the reaction mechanism governing the dissolution rate of MgO particles. Consequently, the rate-limiting mechanism is a diffusion of O2− ion, dissociated from MgO, through the boundary layer.

A phase‐field model for the study of isothermal dissolution behavior of alumina particles into molten silicates

AbstractParticle dissolution process refers to the dissolving of one liquid or solid phase into another solvent phase. This process is of vital importance in the engineering material science, and chemical engineering, etc. Here, we develop a phase‐field model with diffusion‐controlled processes at interfaces for isothermal dissolution of alumina inclusion particles in molten silicates, referred as “slag,” with comprehensive compositions applied in process metallurgy. The interfacial energy between solid inclusion particle and molten silicate can vary with different temperatures and chemical compositions. This model is validated using experimental data of high‐temperature confocal laser scanning microscopy (HT‐CLSM) technique. Moreover, the developed model is applicable to predict the dissolution behavior of solid particle in a realistic size scale. The possibility of the model application is illustrated with studies of different physical parameters that affect particle dissolution behavior. The phase‐field simulations show that inclusion morphology influences both the dissolution profile and time, and the influence decreases with the increased temperature. Increasing same amount of Al2O3 component in slag, the increased degrees of the alumina dissolution time in silicates with different V‐ratio values, a ratio between the mass percent of CaO and that of SiO2, are almost the same. If the V‐ratio of slag is relatively small, an increased MgO component in the silicate will significantly decrease the particle dissolution time. On the contrary, the change in MgO component in the silicate will have a minor effect if the value of V‐ratio is quite large. In order to rapidly dissolve the inclusion, basicity index (BI) of molten silicate should be larger than 1.4. A balance between BI and MgO component in slag is suggested to be considered to design or optimize molten silicate (slag) for its application in the refining process of steel manufacturing.

Study on Bainitic Transformation by Dilatometer and In Situ LSCM.

This study investigates the bainitic transformation kinetics of carbide-free bainitic steel with Si + Al and carbide-bearing bainitic steel without Si + Al, as well as the phase transformation and microstructure through in situ high-temperature laser scanning confocal microscopy. Results show that bainitic ferrite plates preferentially nucleate at the grain boundary. New plates nucleate on previously formed ones, including two dimensions which appear on a plane where a three-dimensional space of bainitic ferrite forms. Nucleation on the formed bainitic ferrite is faster than that at the grain boundary in some grains. The bainitic ferrite growth at the austenite grain boundary is longer and has a faster transformation rate. The bainitic ferrite growth on the formed bainitic ferrite plate is shorter and has a slower transformation rate. The location and number of nucleation sites influence the thickness of the bainitic ferrite. The higher the number of plates preferentially nucleating at the original austenite grain boundary, the greater the thickness of the bainitic ferrite.

The Influence of Mg-Based Inclusions on the Grain Boundary Mobility of Austenite in SS400 Steel

In this study, the effects of the addition of Mg to the grain growth of austenite and the magnesium-based inclusions to mobility were investigated in SS400 steel at high temperatures. A high-temperature confocal scanning laser microscope (HT-CSLM) was employed to directly observe, in situ, the grain structure of austenite under 25 torr Ar at high temperatures. The grain size distribution of austenite showed the log-normal distribution. The results of the grain growth curves using 3D surface fitting showed that the n and Q values of the growth equation parameters ranged from 0.2 to 0.26 and from 405 kJ/mole to 752 kJ/mole, respectively, when adding 5.6–22 ppm of Mg. Increasing the temperature from 1150 to 1250 °C for 20 min and increasing the addition of Mg by 5.6, 11, and 22 ppm resulted in increases in the grain boundary velocity. The effects of solute drag and Zener pinning on grain boundary mobility were also calculated in this study.

In-situ observation of interaction between precipitates and austenite during δ→γ phase transformations

ABSTRACTThe δ-ferrite to γ-austenite phase transformation in duplex stainless steels was observed using ultra high-temperature confocal laser scanning microscope, and the orientation relationship between the γ phase, and precipitate is discussed. Owing to mutual promotion action, the γ phase was observed at δ/δ grain boundary at the beginning of δ-ferrite→γ-austenite transformation, followed by two-dimensional γ-phase growth at the same speed (0.625 µm s−1) along the grain boundary and into the δ grain matrix. The γ-phase growth rate decreases to 0.244 µm s−1 when precipitate stops growing. In the process of grain growth at high temperature, the precipitated pinning grain boundary will slow the movement speed of the grain boundary.The mutual promotion action leads to preferential nucleation of the γ phase, and the nucleation and growth of the austenite also promoting the growth of MnS the growth.

Microstructure Evolution and Orientation Relationship of Reverted Austenite in 13Cr Supermartensitic Stainless Steel During the Tempering Process.

The transformation mechanism of reverted austenite and the amount of reverted austenite during the tempering process in supermartensitic stainless steel have been investigated by X-ray diffraction (XRD), electron backscattered diffraction (EBSD), and a high-temperature laser scanning confocal microscope (HTLSCM). The results indicate that the microstructure mainly consists of tempered martensite and reverted austenite. The reverted austenite nucleates uniformly at the sub-block boundary and prior grain austenite boundary. The amount of reverted austenite strongly relies on the tempering time, showing a positive correlation in the supermartensitic stainless steel. The crystallographic orientation relationship between reverted austenite and martensite meets the Kurdjumov-Sachs(K-S) relationship and the deviation angle is mainly concentrated at about 2 degrees. The mechanism of reverted austenite transformed from martensite is a diffusion mechanism. The growth kinetics of the reverted austenite are dominated by diffusion of the Ni element and there is no shear deformation of the martensite matrix in the in situ observation. It can be deduced that the reverted austenite is formed by nickel diffusion during tempering at 620 °C for different tempering times.

A Glassy Carbon Electrode Modified with Molybdenite and Ag Nanoparticle Composite for Selectively Sensing of Ascorbic Acid.

Molybdenite (MLN) was physically co-adsorbed with Ag nanoparticles (Ag) on a glassy carbon electrode (GCE) for selectively sensing of ascorbic acid (AA). The composite was characterized with a scan electron microscope, a high-temperature confocal laser scanning microscope, an X-ray diffractometer, an X-ray fluorescence analyzer and electrochemical methods. The prepared MLN/Ag-GCE sensor exhibited good properties including a linear range from 3.0 × 10-5 to 1.0 × 10-3 M toward AA, a low detection limit of 1.5 × 10-5 M, good selectivity, excellent reproducibility, and good stability. The synergistic effect between MLN and Ag nanoparticles results in an enhancement of the electrocatalytic activity for molybdenite.

Early Time Evolution of Selective Oxidation in a CMnSi AHSS

The influence of dew point temperature (DPT) and oxidation time on the selective oxidation of a Fe-0.09C-2.02Mn-0.91Si (wt pct) advanced high-strength steel during the early stages of oxidation was investigated in this study. Samples were annealed at 1123 K (850 °C) in a high-temperature confocal scanning laser microscope for 0, 30, 60, 90, and 120 seconds in an N2 + 5 pct H2 atmosphere with 243 K (− 30 °C) or 273 K (0 °C) DPT. Oxide morphology and chemistry were examined with respect to time and atmosphere DPT using scanning electron microscopy, scanning transmission electron microscopy, focused ion beam (FIB) milling, FIB serial sectioning, and energy dispersive spectroscopy techniques. Significant internal and external oxidation for all samples, including those with a 0 second isothermal hold, was observed. This indicated significant oxidation occurred upon heating. Samples oxidized in the 273 K (0 °C) DPT atmosphere exhibited intra- and intergranular internal oxidation and surfaces covered in discrete iron-rich nodules. These were attributed to stress generation during oxidation. Samples oxidized in the 243 K (− 30 °C) DPT atmosphere had shallow internal oxidation, sparse intragranular oxides, and surfaces rich in silicon and manganese containing oxide. Internal oxides for both atmospheres and all isothermal hold times were comprised of a silicon-rich core and manganese-rich outer shell was frequently observed. This core/shell structure was related to evolving oxide stability during the high-temperature exposure.

Analysis of the 355 Aluminium Alloy Microstructure for Application in Thixoforming

The 355 aluminium alloy is known to have excellent thermodynamic characteristics that render it suitable as a raw material for rheocasting and thixoforming. However, besides the controllable transition from solid to liquid phase, the refined microstructure required in the semisolid range is one of the key factors with a strong influence on the rheology of the material. This paper intends to analyse the in situ behaviour of the microstructure, in terms of morphological change, using high-temperature laser scanning confocal microscopy. The 355 alloy was prepared via conventional casting, and refined with a 30-s exposition via ultrasonic melt treatment (UST - 20 Hz, 2 kW). The material was reheated up to the thixoforming target temperature of 595 °C at which it was maintained for 0, 30, 60, 90, and 120 s, after which all the samples were cooled in water. The samples subjected to UST prior to the heat treatment were more refined in terms of microstructural evolution; they exhibited reduction in grain size (~107 ± 16 μm), smallest primary phase particle size (~81 ± 7 μm), and high circularity shape factor (~0.59 ± 0.19 μm). In situ observation methods were employed to analyse evolution mechanisms such as Ostwald ripening and coalescence.

Dissolution behavior of SiO2 in the molten blast furnace slags

AbstractThe kinetics of SiO2 dissolving in the molten blast furnace slags at 1743‐1803 K was studied using a High Temperature Confocal Scanning Laser Microscopy (HT‐CSLM), and then the effects of temperature, MgO and Al2O3 on the SiO2 dissolution were clarified in this study. The experiment results showed that the dissolution process is controlled by the solute diffusion in the slag. The SiO2 dissolution rate could be accelerated by increasing the temperature. Moreover, the SiO2 dissolution rate is the fastest in the slag with 11.0 pct MgO when the MgO varies between 7.0 and 13.0 pct. The SiO2 dissolution rate is the fastest in the slag with 18.0 pct Al2O3 when the Al2O3 varies between 14.0 and 20.0 pct. The effects of temperature, MgO and Al2O3 on the SiO2 dissolution rate could be indicated by a simple expression (csat−cb)/η that a larger of (csat−cb)/η is associated with the faster SiO2 dissolution rate, where (csat−cb) is the difference between the concentration of SiO2 in the saturated slag and in the bulk of slag, and η is the slag viscosity.

Columnar to equiaxed transition mechanism of pure copper induced by La

ABSTRACTThe solidification process of copper with and without La was observed by a high-temperature confocal laser scanning microscopy. The solidification microstructure was simulated by cellular-automaton and finite-element (CAFE) method and the source of equiaxed grains, the constitutional supercooling and nucleation rate were investigated. The results showed that some floats with regular shape appeared in copper melt with La addition, which is the source of equiaxed grains. The simulation results are in a good agreement with experimental results. The mechanism of columnar to equiaxed transition was that the spontaneous nucleation probability of copper melt was increased with La addition. The nucleation rate was increased due to the increase in critical nucleation temperature and constitutional supercooling, and the reduction of nucleation undercooling.

In-situ microstructural observation of Ti-Cu alloys for semi-solid processing

The semi-solid processing of metallic alloys strongly depends on the microstructural features exhibited by the raw material. Consequently, many characterization techniques have been used to elucidate semi-solid microstructures. In the present work, high-temperature laser-scanning confocal microscopy (HT-LSCM) was used to characterize Ti-Cu alloys intended for thixoforming. During the reheating stage, this in-situ technique enabled us to observe the Ti-β grains formed through the reverse eutectoid transformation. Furthermore, the results showed that thermomechanical treatment was primarily responsible for breaking up the dendritic microstructure. The liquid formation and peritectic temperature were easily determined via the HT-LSCM investigation. Prior to the reverse peritectic reaction, which corresponds to the onset of equilibrium melting, we obtained experimental evidence of the melting of non-equilibrium phases that originated via segregation during solidification. Finally, it was possible to qualitatively study the coarsening mechanisms that occurred in the semi-solid state. Irrespective of the alloy composition (liquid fraction), both Ostwald ripening and coalescence occurred during the isothermal heat treatments. Based on the obtained results, HT-LCSM can be considered a valuable technique for the characterization of thixotropic alloys.

In-situ investigation of phase transformation behaviors of 300M steel in continuous cooling process

Comprehensive understanding of continuous cooling transformation behaviors is important in heat treatment of steels, but there is still a lack of research in the phase transition behaviors of 300M steel, which would greatly hinder the microstructure controlling and finally affect the application of this material. In this work, in-situ observations by high temperature confocal laser scanning microscope (HTCLSM) was introduced by considering its superiorities in clarity, accuracy, and directness, to systemically investigate the microstructure evolutions of 300M steel under various cooling rates (0.01–100 °C/s). Pearlite, bainite, and martensite were observed to form at the cooling rate range of 0.01–0.15, 0.03–1, and 0.3–100 °C/s, respectively. A continuous cooling transformation diagram was constructed based on the in-situ observations, verified by metallography, and compared with dilatometry. Results showed that the transition temperatures by in-situ observations agreed well with the results of dilatometry, while the transition starting temperatures by in situ observation were lower and the phase transition termination temperatures were higher. Finally, models accurately describing the relationships between Vickers hardness, retained austenite content, and cooling rates were established, and the phase transformation mechanisms and kinetics were analyzed. Our finding not only understands fundamentally the transformation mechanisms of different microstructures, but also provides a useful reference for practical microstructure control in heat treatment of 300M steel.

Analysis of precipitation behavior of MnS in sulfur-bearing steel system with finite-difference segregation model

To further reveal the influence of micro-segregation on the precipitation behavior of MnS in sulfur-bearing steel system, a coupled model of micro-segregation and MnS precipitation was established by the finite-difference method based on various calculation domains and the solid diffusion degrees, and a new controlled diffusion equation with more stable convergence was also used. 49MnVS3 and 1215 steels were used to analyze the influence of calculation domain, segregation model and S content on the precipitation behavior of MnS. The calculation results were verified by a high-temperature confocal laser scanning microscope (HT-CLSM). The results show that the domain has little effect on the precipitation temperature, precipitation solid fraction and precipitation amount of MnS, but affects the precipitation location and segregation of the solutes. For low- and medium-sulfur steels, the temperatures calculated by the diffusion control growth (DCG) model and the Lever model were nearly identical, whereas the temperature calculated by the Scheil model was lower. However, for high-sulfur steels, the precipitation temperatures calculated by three segregation models were nearly same. The precipitation solid fraction is more reasonable to describe the precipitation behavior of MnS. The precipitation behavior of MnS, observed by the HT-CLSM, matches well with that in the DCG model.

In Situ Observation of the Nucleation and Growth of Ferrite Laths in the Heat-Affected Zone of EH36-Mg Shipbuilding Steel Subjected to Different Heat Inputs

The nucleation and growth behaviors of ferrite laths in the heat-affected zone (HAZ) of EH36-Mg shipbuilding steel with different heat inputs were observed in situ by high-temperature confocal scanning laser microscope (CSLM). It was found that ferrite laths prefer to nucleate on the surface of inclusions instead of grain boundaries under the heat input of 120 kJ/cm, while FSPs are easier to form in 210 kJ/cm due to a significantly reduced cooling rate.