High-temperature Laser Scanning Confocal Microscopy Research Articles

Characterization of the γ-loop in the Fe-P system by coupling DSC and HT-LSCM with complementary in-situ experimental techniques

Solid-state phase transformations in the γ-loop of the binary Fe-P system were studied using differential scanning calorimetry (DSC) and high-temperature laser scanning confocal microscopy (HT-LSCM). In total, eight alloys with varying P content from 0.026 to 0.48 mass pct. P were investigated in the temperature range of 800 °C to 1450 °C. The first part of the present work deals with the critical evaluation of the approach to couple DSC experiments and HT-LSCM observations in order to characterize bcc/fcc phase equilibria in Fe-based γ-loops. The phase transformation temperatures of a selected alloy with 0.394%P were analyzed by DSC and HT-LSCM and compared with results of the well-established techniques of dilatometry and high-temperature X-ray diffraction (HT-XRD). Then, the overall phase boundaries of the γ-loop were reconstructed by HT-LSCM and DSC data and the phase diagram was compared with thermodynamic assessments from literature. Finally, the quantitative phase fractions of fcc and bcc at 0.394%P were analyzed by Rietveld refinement at temperatures of 1050 °C, 1100 °C and 1150 °C using in-situ HT-XRD. Although the phase boundaries of the γ-loop and phase transformation temperatures have been reproduced accurately by recently published thermodynamic optimizations, larger deviations between HT-XRD measurements and the calculations were identified for the phase fraction prediction. The present work clearly demonstrates that coupling DSC and HT-LSCM is a powerful tool to characterize γ-loops in steel for future research work.

The Spontaneous Emulsification of Entrained Inclusions During Casting of High Aluminum Steels

Mold slag entrainment during the continuous casting process presents a late stage source of non-metallic inclusions (NMI) with a high likelihood of ending up in the final product. The reaction between the entrained slag phase and surrounding liquid steel in the continuous casting mold affects the inclusion morphology and properties. However, there is a lack of information on the kinetics of the NMI-steel reaction. A novel approach, utilizing controlled synthetic inclusion/metal samples, has been developed to study the reactions between free inclusion-slag droplets and steel. The technique combines High-Temperature Confocal Scanning Laser Microscopy (HT-CSLM), X-ray Computed Tomography (XCT) and advanced electron microscopy techniques offering rapid controlled heating performance and extensive characterization of the samples. This method offers the ability to observe the size, shape and composition of an unconstrained reacting inclusion and to investigate the interface between the materials with respect to reaction time. This study interrogates a low aluminum steel (0.04 wt pct) and a high aluminum steel (1 wt pct) in contact with an inclusion-slag phase with a starting composition aligned to a typical mold slag. It was found that the reaction between silica and aluminum across the interface of the two phases provided a driving force for spontaneous emulsification to occur. Products of such emulsification will have a significant effect on the inclusion size distribution and potentially the prevalence of inclusion retention in molten steels solidifying in the continuous caster (for example if emulsified buoyancy forces are reduced to near zero) and hence in the subsequent solid product.

STUDY ON FORMATION OF LAMELLAR MICROSTRUCTURE OF Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y ALLOY BY HIGH TEMPERATURE LASER SCANNING CONFOCAL MICROSCOPE

In order to study the formation mechanism of fine fully lamellar (FL) microstructure of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy, the heat treatments of holding at 1450 ℃ for 35s followed by cooling at the rates of 3-600 ℃/min were conducted by high temperature laser scanning confocal microscope (HTLSCM). Evolution of lamellae were observed in situ and the resultant microstructures were analyzed by SEM and TEM. The results show that the surface of the sample was contaminated by oxygen coming from the alumina crucible and the phenomena observed in situ are not as reliable as those occurred in the interior of the sample. The results observed in situ give some qualitative conclusions on α to lamellar colony transformation: γ lamellae nucleate at α grain boundaries then propagate mainly from one end of grain to the other; at the same cooling rate, the start temperatures of the transformation are different for different α grains; with the increase of the cooling rate, the start temperature of the transformation decreases. The interior microstructures give some quantitative results: the alloy is in α+β two phase region at 1450 ℃; with the increase of cooling rate, the amount of B2 phase increases, the lamellar colony size and lamellar spacing decrease. When the cooling rate is higher than 30 ℃/s, the nearly lamellar (NL) microstructure with volume fraction of B2 phase, lamellar colony size and lamellar spacing of about 2%, about 60 μm and about 100-500 nm, respectively, is obtained.

Dendrite growth in nickel-based superalloy with in-situ observation by high temperature confocal laser scanning microscopy and numerical simulation

Dendrite growth of nickel-based superalloy was investigated by using high temperature confocal laser scanning microscopy (HT-CLSM) and numerical simulation method. The influence of cooling rate on the evolution of dendrite was discussed through in-situ observation experiments and simulation cases. The results indicate that secondaryand tertiary dendrites grow luxuriantly at the higher cooling rate and primary dendrite present well growth tendency at the lower cooling rate. The variation of secondary dendrite arm spacing (SDAS) with cooling rate was also analyzed, and the results show that SDAS decrease exponentially with the increasing of cooling rate, which is helpful for designing process parameters to obtain desirable dendrites.

New insights of the peritectic phase transition in steel through in-situ measurement of thermal response in a high-temperature confocal microscope

The topic of peritectic phase transition in steel has been widely studied using various in-situ experimental techniques including visual observation on a surface of a sample in a high temperature microscope and measurement of the thermal response from the bulk of the sample using differential thermal analysis (DTA) and differential scanning calorimetry (DSC). These two different techniques have inherent limitations leading to difficulties in interpretation of the results in the case of complex phase transformations such as the peritectic reaction, and there have been discrepancies among the interpretations from many previous studies of the peritectic transition in steel. A new technique was developed through a spatial combination of a DTA apparatus and a high-temperature laser-scanning confocal microscope to overcome these limitations. Solidification experiments with carbon steels of 0.061, 0.182 and 0.451 wt%C were conducted under conditions typical of conventional thermal analysis experiment, i.e. 90 mg of sample mass and 10 °C/min cooling rate, and the findings of earlier studies were revisited. We concluded that whether or not a peritectic reaction in a steel specimen of peritectic composition occurs, largely depends on the size of the sample used in the experiment. Deep undercooling for austenite nucleation can be experienced, in the absence of the occurrence of a peritectic reaction as a result of constrained nucleation of austenite, due to a limited number of nucleation sites in the small samples used.

In-situ investigation of β/α transformation in β-solidifying γ-TiAl alloys at different cooling rates

We report an in-situ study on β/α transformation of a β-solidifying γ-TiAl alloy using high-temperature laser scanning confocal microscopy. Combining the current theories of Ti alloys and a method to distinguish the crystal orientation, the special effect of cooling rate on the nucleation, growth of α phase from β matrix as well as the surface relief in Ti-40Al were researched. The results indicate that Ti-40Al exhibits both sympathetic and interface instability nucleation and grows in the form of Widmannstatten plate and equiaxed grain cooling at 2 K/min; just interface instability nucleation and Widmannstatten plate growth occur in the alloy at 108 K/min. Besides, high cooling rate can promote the growth rate of Widmannstatten α plates, whose growth direction is parallel to γ lamellar orientation nearly. These findings will provide insight into controlling the orientation and size of lamellar colonies of TiAl alloys to improve mechanical properties.

In-situ observation of ancient co-fusion steelmaking process based on HT-CLSM

Observing and analyzing the microstructure of zone samples had been an essential concept in the study of co-fusion steelmaking. High-temperature confocal laser scanning microscope (HT-CLSM) provided a new method in the in-situ observation and real-time analysis of the co-fusion steelmaking process. In this research, a series of experiments had been designed based on Shen Kuo’s The Dream Pool Essays (a famous ancient Chinese literature) and carried out by HT-CLSM. The results showed that a new interface was formed with the phase transition during the heating process, which had an essential influence on promoting the carbon diffusion rate in co-fusion steelmaking. The cast iron zone occurred in mushy solidification because of decarbonization. There were collision, aggregation, and other behaviors of inclusions in the cast iron zone and they moved towards the boundary, which led to the purity of the matrix and the aggregation of a large number of inclusions at the boundary.

Catalytic graphitization of Glassy Carbon by Molten Fe-Csat

The campaign life of blast furnaces can be greatly limited by the interactions of carbon walls with liquid Fe-Csat. Numerous carbon forms have been investigated but not glassy carbon. In this work, liquid Fe-Csat catalyzed the graphitization of glassy carbon crucibles heated in a high-temperature confocal scanning laser microscope. Graphitization created readily wetted pores into which the hot metal penetrated, similar to the embrittlement layer seen in blast furnace hearth refractory.

In-situ real time observation of martensite transformation in duplex fcc+hcp cobalt based entropic alloys

Athermal martensite transformation in duplex fcc+hcp Co-based entropic alloys during continuous cooling was investigated in-situ. The real time observation was carried out using high temperature confocal laser scanning microscopy (HT-CLSM). This technique enables the detection of the athermal fcc to hcp transformation in entropic alloys, which is not sensitively detected by conventional thermomechanical methods e.g. dilatometer. The martensite fraction increases with increasing martensite starting temperature, and vice versa. Meanwhile, the martensite starting temperature decreases with the increasing grain size. In addition, the morphology and nucleation sites for martensite formation is discussed. This is the first time the that HT-CLSM technique is utilized in the field of entropic alloys. This in-situ observation technique coupled with thermodynamic calculations may help in the design of entropic alloys through the tailoring of the desired microstructure.

Investigation of Fe–C–Cr and Fe–C–Cr–Ni-based systems with the use of DTA and HT-LSCM methods

Three Fe–C–Cr and three Fe–C–Cr–Ni-based alloys containing carbon between 0.318 and 0.410 mass%, chromium and nickel between of 0.001 and 4.990 mass% were studied. Original temperatures of solidus, liquidus and peritectic transformation were determined and clarified for all six specific alloys. Phase transition temperatures were experimentally obtained using differential thermal analysis (DTA), and the start of melting of alloys was investigated using the high-temperature laser scanning confocal microscopy. Additionally, experimental data were compared and discussed with calculation results using SW IDS and Thermo-Calc operating with commercially available thermodynamic database TCFE8. The combination of both experimental methods contributed substantially to investigate more clearly the start of melting. Determination of solidus temperatures/incipient melting point temperatures on DTA curves is often challenging, and there are significant differences when compared with the determination of liquidus and peritectic transformation temperatures. HT-LSCM method brings an improvement in that regard because it can reveal thermal events which are hardly identified on DTA curves. The relation between homogeneity/inhomogeneity of investigated alloys and the initiation of their melting was observed depending mainly on the state of alloys.

In situ observation of Li2O-Al2O3-SiO2 glass crystallization process and kinetics analysis

In this work, we have observed the crystallization process of Li2O-Al2O3-SiO2 glass using a high-temperature confocal laser scanning microscope and studied its crystallization kinetics with differential thermal analysis. The value of activation energy has been calculated out using Kissinger and Flynn–Wall–Ozawa methods. The results show that LiAlSi3O8 crystal particles precipitate at 850 °C, and then gradually grow to 75 um with the irregular hexagonal structure until 980 °C. The activation energy of two exothermal peaks through Kissinger method is 121.78 ± 13.42 and 189.74 ± 7.76 kJ/mol. Glass transition and crystallization both have remarkable kinetics effects, and kinetic crystallization parameter, Avrami index (n), indicates that crystallization mechanism of two exothermal peaks corresponds to one-dimensional and surface crystallization, respectively.

Experimental Study of High Temperature Phase Equilibria in the Iron-Rich Part of the Fe-P and Fe-C-P Systems

During the solidification of steel, phosphorus strongly segregates in the interdendritic liquid phase. In the continuous casting process, even low levels of P may have a detrimental effect on the final product quality. However, phosphorus is partly added up to 0.10 wt pct to improve the mechanical properties of advanced steel grades nowadays, e.g., High-Strength Interstitial-Free (HSIF). To provide new experimental data for the development of thermodynamic databases and solidification models for P alloyed steel grades, phase equilibria in the Fe-P and Fe-C-P key systems were studied up to 1550 °C using differential scanning calorimetry (DSC) and high temperature laser scanning confocal microscopy (HT-LSCM). Special focus was placed on solid/liquid equilibrium temperatures in the Fe-rich part of the binary Fe-P system between 0.025 and 9 wt pct P. In the ternary system, three isoplethal sections with 0.10 mass pct. P, 0.20 mass pct. C and constant mass percent ratio P/C of 2 were investigated. In the latter section, HT-LSCM observations were linked with DSC signals to optically identify present phase stabilities. Particularly at [pct P] < 1, significant differences between performed measurements and calculated phase equilibrium temperatures using thermodynamic assessments from the literature were identified. In all ternary sections, the experiments indicate less influence of P on the hypo-peritectic range compared to the thermodynamic calculations.

Hidden Phenomena During Transient Reaction Trajectories in Liquid Metals Processing

The transient trajectory taken for a system striving toward equilibration has consequences on the rate of processes and on the chemical and physical state of products in metallurgical processes. A case study approach to recent advancements in liquid steel processing is given. A combination of techniques and knowledge developed is given as a targeted showcase of the authors’ contributions to the understanding of liquid metal droplet reactions and their contribution to the large-scale production processes within the steel industry. Examples relevant to novel ironmaking technologies, oxygen steelmaking, ladle metallurgy, and continuous casting are discussed, showing the range of processes that benefit from greater understanding in this area. This article considers specifically the reaction of liquid ferrous droplets, immersed in molten oxides, involving key alloying components, including phosphorus, aluminum, and carbon. The studies use high-temperature–confocal scanning laser microscopy (HT-CSLM), X-ray computed tomography (XCT), phase-field modeling, and in situ limited angle X-ray imaging. These techniques have seen significant development over recent years, and the combination of these powerful tools reveals the occurrence of spontaneous emulsification driven by chemical reaction (in the case of oxygen/phosphorus/aluminum reactions) and gas-phase formation (in the case of decarburization) both internally and externally to a steel droplet. A key finding is that the interfacial area pertinent for the heterogenous reactions to occur changes considerably (by up to an order of magnitude) depending on the chemical driving force. Additional key findings include the shift between preferential internal and external gas nucleation during decarburization, an inflection point of behavior as to whether or not spontaneous emulsification will occur (within the study discussed, this is between 3 and 4 wt pct Al) and the pathway of perturbation growth through which spontaneous emulsification occurs, including the physical maxima a perturbation will grow to before breaking away from the parent droplet.

A New Cooling Strategy in Curved Continuous Casting Process of Vanadium Micro-alloyed YQ450NQR1 Steel Bloom Combining Experimental and Modeling Approach

In the continuous casting process of steel, the bloom surfaces would experience intensive cooling from the water-cooled copper mold to secondary cooling water spray. If the cooling process is not controlled properly, hot ductility of the bloom surface microstructures would deteriorate, and bloom surface cracks would form easily under straightening deformation in a curved caster. Considering the above facts, the cooling scheme for the continuous casting of YQ450NQR1 steel bloom, a kind of vanadium-containing micro-alloyed steel, is studied with both experimental investigation and mathematical modeling in this work. The authors first investigate the hot ductility of bloom surface microstructures at various cooling rates using a Gleeble thermal simulator. Then, the precipitation of V(C, N) particles and its influence on ferrite formation during continuous cooling are studied and characterized using High-Temperature Laser Scanning Confocal Microscopy. Based on these, the preferred cooling rate for surface microstructures at the straightening position in the caster is obtained. To further reduce the solute macro-segregation through enlargement of the equiaxed crystal zone, a cellular automaton-finite element model is used to calculate heat transfer and solidification structure evolution during the continuous casting process. After calibration with industrial trials, the model is utilized to determine the critical position for columnar to equiaxed transition and to adjust the pouring temperature for the melt. Combining the above research, a new cooling strategy for YQ450NQR1 steel bloom is obtained, which can improve crack resistance of bloom surface microstructures and reduce solute macro-segregation at the same time.

Dissolution kinetics of arbitrarily-shaped alumina in oxide melt: An integration of phase-field modelling and real-time observation study

Abstract A feasibility study of utilizing a phase-field model considering the convection effect is performed in this work to investigate the dissolution kinetics of arbitrarily-shaped alumina particle in the oxide melt. The established model has been benchmarked by high temperature confocal laser scanning microscope (HT-CLSM) to observe the polycrystalline sapphire dissolution in multicomponent oxide melt. The effects of alumina morphology and temperature on the dissolution kinetics were studied quantitatively. It is for the first time found that the dissolution of irregular alumina particle in oxide melt shows a double-fold smooth anisotropy behavior. The dissolution rate of alumina particle is governed by the interface interactions rather than the self-diffusion of dissolved component in oxide melt. Increasing the temperature, the interfacial mobility coefficient increases, and the effect of alumina particle shape on the dissolution rate decreases. The temperature variation has a minor influence on the shape evolution of alumina during dissolution process. The convection intensity affects both the dissolution rate and shape evolution of alumina particle. An increased convection intensity increases the dissolution rate. As the convection partially affects the contact area of alumina particle with oxide melt, a decreased alumina particle curvature increases the dissolution rate. When the convection affects the whole contact area of alumina particle with oxide melt, the contact area plays an important role on the dissolution rate. The obtained methodology can be applied to investigate a general dissolution phenomenon in the high temperature oxide melt in materials science.

Microstructure evolution of hot work tool steel 5CrNiMoV throughout heating, deformation and quenching

The microstructure evolutions of hot work tool steel 5CrNiMoV throughout the heating, deformation and quenching procedures are investigated. Microstructures of the heated, compressed, and cooled specimens are observed by high-temperature confocal laser scanning microscope (HT-CLSM), optical microscope, and electron backscatter diffraction (EBSD). The measurement shows that the austenite transformation starts at 730 °C and ends at 765 °C. After the temperature exceeds 850 °C during the heating, the in-situ observed grains are first refined by static recrystallization (SRX) and then grow with the holding time, accompanied by the grain boundary gradually become straight. The microstructures of hot compressed specimens indicate that the dynamic recrystallization (DRX) fraction increases with growing temperature and strain, in which discontinuous dynamic recrystallization (DDRX) is the main mechanism of DRX. The grain boundaries formed by DDRX are serrated and have a larger curvature, which are different from those formed by SRX and subsequent grain growth. The lath martensite generated from the water quenching following the compression tends to align parallel to each other in the prior austenite grain (PAG). It comprises the packets containing the elongated blocks with the same habit planes.

In-situ observation of microstructure evolution and phase transformation under continuous cooling in Ru-containing TiAl alloys

Ruthenium is a promising element to heighten the mechanical properties of TiAl alloys. The microstructure evolution and phase transformation of two Ru-containing TiAl alloys have been in-situ observed by the high-temperature laser-scanning confocal microscopy from melting to 1000 °C under the cooling rates of 10 K/min and 100 K/min. For Ti-45Al-2.0Ru-0.2B alloy, some net-like and spherical microstructures precipitated at α grain boundaries and inside α grains by the eutectoid transformation of α → γ + τ1 at 10 K/min. While the rod-like τ1 phase replaced net-like morphology and dispersed in γ matrix at 100 K/min condition. For Ti-45Al-1.0Ru-0.2B alloy, the mixture of blocky γ phase and dispersive fine τ1 particles emanated from α grain boundaries via a divorced eutectoid transformation at 10 K/min. Upon 100 K/min, a complex microstructure appeared which consists of lamellar colony, blocky γ, γ/τ1 mixture and B2 phase. The near-equilibrium phase transformation of the present two Ru-containing TiAl alloys follows the sequence: L → L + β → α + β → α → α + γ → α + γ + τ1, which clarifies that the τ1-containing mixtures precipitate after the formation of α grain, rather than during solidification process. The related crystallographic orientation and microstructure evolution mechanism were investigated. In addition, it's found that the Ru element promotes the formation of duplex microstructure. The duplex microstructure containing dispersive τ1 particles of Ti-45Al-1.0Ru-0.2B alloy at 10 K/min obtained the maximum microhardness and can be considered as a potential optimal microstructure.

In-situ observation of liquid zinc-induced erosion behavior diffusion mechanism in zinc-coated 22MnB5 steel

In this study, the erosion of liquid zinc penetrating 22MnB5 steel during austenitization was investigated by an in situ method. High-temperature confocal laser scanning microscopy (HT-CLSM) results show that erosion occurred in several areas after melting of the galvanized coating. Meanwhile, the morphology of the erosion tip at the substrate side was transformed from a cutting edge into a flat shape with increasing temperature. In addition, an inhibiting layer was presented on the substrate after the galvanizing coating melted; the inhibiting layer disappeared with further temperature increases. The erosion sample of liquid zinc penetrating the 22MnB5 substrate at different dwell times was captured by an austenitization experiment. The results show that the erosion occurred after the rupture of the Fe2Al5 inhibiting layer; in addition, the inhibiting layer found in the HT-CLSM experiment was shown to be Fe2Al5. Finally, the diffusion coefficient of zinc in α-Fe and γ-Fe at different temperatures was obtained by dynamic calculation based on zinc concentration; the results show that the relationship of zinc in α-Fe and γ-Fe is nonlinear and linear, respectively.

Coupling Effect of Prior Austenite Grain Size and Inclusion Characteristics on Acicular Ferrite Formation in Ti-Zr Deoxidized Low Carbon Steel

The coupling effect of prior austenite grain size and inclusion characteristics on acicular ferrite (AF) formation was investigated in Ti-Zr deoxidized low carbon steel by utilizing the high temperature confocal laser scanning microscope (HT-CLSM), optical microscope (OM), and scanning electron microscope (SEM) equipped with energy-dispersive spectrometer (EDS). The results indicated that with the target heating temperature increased from 1100 °C to 1350 °C, the average size of prior austenite grain varied from 58.22 to 237.40 μm, and the average grain size increased rapidly when the temperature was above 1200 °C. For inclusion characteristics, different target heating temperatures had no obvious effect on inclusion types, but had a great influence on the average size and number density of each, especially for the intragranular effective inclusions. In addition, as the increase of target heating temperature, the types of microstructure were identical, but both AF volume fraction and AF relative nucleation ability increased first and then decreased. When the target heating temperature of sample was 1250 °C, the AF volume fraction reached the maximum of 49.48 pct. However, the AF relative nucleation ability reached the maximum of 474.5 at the target heating temperature 1200 °C, at this time, the AF volume fraction was 47.92 pct, only 1.56 pct smaller than that at 1250 °C. Therefore, considering the AF volume fraction and AF relative nucleation ability, the optimal target heating temperature for AF formation in this study was 1200 °C, and the corresponding prior austenite grain size was 69.58 μm.

Carbide dissolution and grain growth behavior of a nickel-based alloy without γ′ phase during solid solution

In this paper, a new type of non-γ′ phase nickel-based heat-resistant alloy C-HRA-2 was studied by solution heat treatment. It was determined that after removing the γ′ phase, the carbide dissolution temperature was reduced and the grain growth process of nickel-based alloy was accelerated. It was also determined that the dissolution of M23C6 carbides is attributable to a gradual decomposition and diffusion process. High-temperature confocal scanning laser microscopy (HT-CLSM) was utilized to simulate the grain growth of the alloy during the solid solution treatment. The results revealed that only a portion of the grain boundaries migrated simultaneously, while most of the migration directions and distances followed the classical migration theory. Some grain boundaries expanded into new grain boundaries via sub-crystal rotation during grain growth. Following solution heat treatment, the yield strength of the alloy was mainly attributed to solution strengthening and grain boundary strengthening. The solution strengthening was found to be approximately 230 MPa and the grain boundary strengthening was found to be approximately 37–118 MPa. Carbides increase the precipitation strengthening of the alloy during dissolution.