Al2O3 Spinel Research Articles

Evolution of oxide inclusions in stainless steel containing yttrium during thermo-mechanical treatment

To get more fundamental information on the control of rare earth inclusions in solid steel, the evolution behavior of oxide inclusions during thermo-mechanical treatment of stainless steel containing yttrium at 1423K (1150°C) was investigated in detail. Homogeneous spherical SiO2–MnO–Al2O3–Y2O3 inclusions were observed in the as-cast steel. After thermo-mechanical treatment, the homogeneous oxide inclusions changed to heterogeneous ones with gray-colored (SiO2–Y2O3)-rich phase and dark-colored MnO–Cr2O3–Al2O3 spinel phase owing to the mutual effects of (i) crystallization of the glassy oxide and (ii) reaction between inclusions and steel matrix. In addition, the oxide inclusions were elongated along the deforming direction of steel during hot deformation process. With increasing reduction of the steel, the MnO–Cr2O3–Al2O3 spinel phase (dark phase) were crushed into small pieces. And the two phases (gray and dark phases) in inclusions gradually became separated from each other due to different deformability. The deformation index of inclusions gradually decreased with an increase in the deformation degree of the steel, indicating that the deformability of inclusions got worse.

Characterization of Multiphase Mixtures of Calcium Aluminates and Magnesium Aluminate Spinel Using Time‐Gated Raman Spectroscopy

In aluminum‐killed calcium‐treated steels, the characterization of multiphase oxides found in the CaO–MgO–Al2O3 system is essential for the study of inclusion formation, modification, and the control process. Herein, Raman spectroscopy is used as a characterization technique for the multiphase mixtures of MgO·Al2O3 spinel and calcium aluminate phases of C12A7, C3A, and CA. Multiphase samples of C12A7–CA–MA and C12A7–C3A–MA are synthesized from MgO·Al2O3 spinel and calcium aluminates with varying phase content. The relative intensity of the Raman peaks is qualitatively used for the most primary phases present in the sample. Regression models are also identified for predicting the phase composition. Prediction model identification is conducted using the successive projection algorithm (SPA) for feature selection and the partial least squares regression (PLS) model as a regressor. The identified prediction models show a reasonably good prediction performance. Herein, the time‐gated Raman spectroscopy technique to characterize the multiphase mixture of magnesium aluminate spinel and calcium aluminate phases of CA, C12A7, C3A is used, and the potential application for inclusion characterization is demonstrated.

Revolution and Control of Fe-Al-(Mg, Ti)-O Oxide Inclusions in IF Steel during 260t BOF-RH-CC Process

The evolution of inclusions that contain Al, Mg, and Ti was studied through industrial-grade experiments. Field emission scanning electron microscopy, energy dispersive spectrometry, inductively coupled plasma atomic emission spectrometry, and FactSage software were used to analyze the evolution mechanisms of inclusions in Al-killed titanium alloyed interstitial free (IF) steel. The research found that the evolution of inclusions during the smelting process of IF steel is results in ‘large sphere-like SiO2-CaO-FeO-MgO-MnO’ and ‘small cluster spherical FeO-MnO’ change to cluster-like Al2O3 and irregular MgO·Al2O3, then change to Al2O3·TiOx and Al2O3, and finally change to Al2O3. It is difficult for Al2O3·TiOx to stably exist in the IF molten steel. It is the key to extend the holding time properly after Ruhrstahl Heraeus (RH) to ensure the removal of Al2O3 inclusion. With the increase of Mg content, the change path of MgAl2O4 inclusion in IF steel is that Al2O3 changes to MgO·Al2O3, and finally changes to MgO. It is difficult to suppress MgO·Al2O3 spinel formation by controlling the oxygen in the steel, but Ca can modify part of the MgO·Al2O3 spinel inclusions during RH refining. In order to ensure the removal of 6–10 μm inclusions, the holding time is suitable for 19–42 min.

Analysis of Spinel Based Inclusions During the Last Stage of The Steelmaking Process of SAE 52100

Abstract Al2O3 (alumina) and MgO·Al2O3 (spinel) inclusions cause valve obstruction (clogging) in continuous casting and can deteriorate the quality of the final product. In this context, industrial heats of the bearing steel SAE 52100 was examined. Samples were collected in the final steps of the steelmaking process, both after vacuum treatment and during continuous casting. A scanning electron microscope (SEM) equipped with a energy-dispersive spectrometer (EDS) and automated particle characterization analysis was used to characterize the inclusions present in the steel samples. Thermodynamic calculations were performed with the commercial software FactSageTM 7.2. Based on thermodynamic predictions, parameters such as solid fraction, liquid fraction, MgO saturation point of the slags, content of dissolved elements in steel (Al, Mg, Ca,...) and the construction of a phase stability diagram were determined. The results in this study showed a tendency for increase in MgO content in the inclusions with the decrease of %FeO and SiO2 contents in the slag, an increase of binary basicity (%CaO/%SiO2). It is verified that the MgO contents in the slag were close to the saturation, increasing the probability for the formation of inclusions rich in MgO and/or spinel. On the other hand, stability diagrams confirm the formation of spinel inclusions for each of the heats analyzed. During the final step of the steelmaking process, there is a tendency for re-oxidation, which is verified by an increase in the density of inclusions (or total oxygen TO values).

Effect of Ce-La on inclusion evolution in Al-killed high strength steel

The mechanism of inclusion evolution after rare earth addition based on oxide metallurgy was investigated experimentally and using thermodynamic calculations, where Ce-La was added to Al-killed high strength steel during Ruhrstahl-Heraeus refining to modify the oxide inclusions within the steel. The typical inclusions observed before Ce-La addition were spherical magnesium aluminate spinel inclusions. And fewer individual Al2O3 inclusions and Al2O3–TiOx inclusions were also observed. The addition of Ce-La led to transformation of MgO · Al2O3 spinel inclusions to (Ce,La)2O3, (Ce,La)2O2S and (Ce,La)2O2S + MgO · Al2O3 inclusions. Thermodynamic calculations indicated that Ce-La combined with dissolved oxygen and sulfur in molten steel to form rare earth inclusions, while the remainder of the Ce and La modified MgO · Al2O3 to (Ce,La)2O3 and (Ce,La)2O2S.

Formation and Evolution of Non-metallic Inclusions in Ti-Bearing Al-Killed Steel During Secondary Refining Process

Industrial and laboratory experiments were carried out to investigate the formation and evolution of the inclusions in Ti-bearing Al-killed steel, and thermodynamic calculations were also conducted to check the stability of inclusions in steel. It is found that the addition of Ti influences the inclusions evidently. Before Ti addition, the inclusions transform along the route of “Al2O3 → MgO·Al2O3 spinel → CaO-Al2O3(-MgO) system” with the generation of dissolved Mg and Ca in liquid steel. When there is no sufficient Ca in liquid steel, the addition of Ti during refining process would lead to the transformation of MgO·Al2O3 spinel inclusions into MgO-Al2O3-TiOx inclusions. When a trace of dissolved Ca is formed, CaO-Al2O3-TiOx system inclusions can be generated from MgO-Al2O3-TiOx inclusions or CaO-Al2O3 inclusions. Higher calcium content originated from calcium treatment would result in the formation of CaO·TiO2 inclusions, and then weaken the castability of Ti-bearing Al-killed steel grades. Calcium treatment could also increase the frequency of macro-inclusions in steel, and evidently decline the quality of the steel. Cancellation of calcium treatment is suggested to improve the cleanliness of Ti-bearing Al-killed steel grades.

Effect of Refractory on Nonmetallic Inclusions in Si–Mn‐Killed Steel

Some laboratory experiments are carried out to study the effect of different refractories on the original and exogenous inclusions in Si–Mn‐killed steel. As expected, it is found that Al2O3 refractory causes high melting points of both SiO2–Al2O3‐MnO system and SiO2–Al2O3‐CaO system inclusions, while MgO crucible reduces the melting points of both systems of inclusions to a certain extent. The purity of MgO refractory is crucial for tire cord steel grades, and even 0.3% of Al2O3 impurity in refractory results in an evident increase (by 10%‐20%) of Al2O3 content in the inclusions. Similar to Al‐killed steel grades, MgO·Al2O3 spinel can also be stable in Si–Mn‐killed steel due to the effect of MgO refractory, and MgO and Al2O3 inclusions can transform into MgO·Al2O3 spinel. These inclusions should be strictly controlled during refining process. High‐purity and high‐quality MgO refractory as well as high‐quality alloys are recommended for Si–Mn‐killed steel grades, and Al2O3‐based refractory should be avoided.

Formation pathways for MgO·Al2O3 inclusions in iron melt

The crystallization of inclusions plays a decisive role in determining the structure, properties and size distribution of inclusions in steel. MgO·Al2O3 (MA) spinel is one of the most common inclusions having great influence on steel performance. Understanding the crystallization pathways of MA from molten steel is important to control their behaviors. Based on the thermodynamic properties of metastable phases of MA calculated by first principle prior to critical nucleus formation, the present work suggests that there are four possible pathways for MA inclusions nucleation from molten iron. According to the four nucleation pathways, the reaction among [Mg], [Al] and [O] could be the most probable pathway leading to MA inclusion formation, while Al2O3 reacting with MgO is almost impossible pathway for MA inclusion formation. The size of MA inclusions is dominated by the formation pathways, which have a close relationship with the adding ways of deoxidizer.

Post-annealing effect on transparent Mg-Zn aluminate solid solutions fabricated by spark plasma sintering

In MgAl2O4 (Mg aluminate) fabricated by pressure sintering, carbon contamination, oxygen vacancy, and other defects exist and cannot be completely removed even after post-annealing. ZnAl2O4 (Zn aluminate) has the same crystallographic structure as Mg aluminate and excellent light transmission by post-annealing. In this study, the transmission of (Mg1-XZnX)Al2O4 spinel, which is a solid solution between Mg aluminate and Zn aluminate, was investigated. After post-annealing, the transmission of (Mg1-XZnX)Al2O4 spinel was higher than Zn aluminate and Mg aluminate, which means that the absorption center disappeared. Besides, the real in-line transmission is improved by up to 30% by post-annealing, which is related to the driving force to increase the configurational entropy and form a more stable solid solution phase. In solid solution spinel, as the Zn content increases, the optical properties can be improved by post-annealing, but the mechanical properties are slightly decreased.

Generation Mechanism of MgO and Al2O3 Inclusions in 51CrV4 Spring Steel Based on the Ion–Molecule Coexistence Theory

The presence of MgO·Al2O3 inclusions in 51CrV4 spring steel is detrimental to the alloy’s castability and fatigue properties. To effectively suppress these inclusions during production, accretions were collected from the immersion nozzle, and the MgO·Al2O3 inclusions in the steel billet were investigated. The generation mechanism of the inclusions was evaluated based on the ion–molecule coexistence theory, and the mass action–concentration model of CaO–SiO2–Al2O3–MgO–FeO–MnO slag was developed. Industrial experiments showed that nozzle clogging was primarily caused by MgAl2O4 spinel inclusions, and the MgO·Al2O3 spinel inclusions in the steel billet were investigated by non-aqueous electrolysis. The model calculation results indicate that the Mg content increased with an increasing basicity, CaO/Al2O3 ratio, and Al content during the ladle furnace (LF) process. In contrast, the Mg content decreased with increasing CO pressure under Ruhstahl-Hausen vacuum degassing process (RH) conditions.

Self-lubricating bidirectional carbon fiber reinforced smart aluminum composites by squeeze infiltration process

Self-lubrication is one of the smart material properties required for producing components with enhanced wear resistance and low coefficient of friction. Bidirectional (BD) satin weave polyacrylonitrile (PAN) based carbon fiber (Cf) fabric preform was successfully infiltrated with Al 6061 alloy by squeeze infiltration process. The infiltrated composite shows uniform distribution of carbon fibers in the matrix with the elimination of porosities, fiber damage and close control on the formation of deleterious aluminum carbide (Al4C3) phase. Cf/Al composite exhibits remarkable wear resistance compared to unreinforced alloy due to the formation of self-lubricating tribolayer on the pin surface, which intercepts the contact of matrix metal to counter surface. The BD carbon fiber enhanced the hardness and compressive strength of the composite by restraining the plastic flow behavior of matrix. High resolution transmission electron microscopy shows the presence of Al2O3 and MgAl2O4 spinel, confirmed by EDS and SAD pattern, at the composite interface. The composite shows a lower density of 2.16 g/cm3 which is a major advantage for weight reduction compared to the monolithic alloy (2.7 g/cm3).

Gradient thermal cycling behavior of a thermal barrier coating system constituted by NiCoCrAlY bond coat and pure metastable tetragonal nano-4YSZ top coat

Abstract Pure metastable tetragonal (t’) phase 4YSZ top coats with thickness of 100 and 200 μm were deposited on NiCoCrAlY-coated second generation single crystal superalloy by air plasma spray (APS). The two thermal barrier coatings were evaluated under gradient thermal cycling test using gas mixture of propane and oxygen. After flame shock test, the values of Young's modulus, hardness and degree of densification all exhibited a gradient distribution across YSZ thickness. In contrast to intensive sintering at surface of 200 μm 4YSZ coating, the TBC sample with 4YSZ layer of 100 μm underwent poor oxidation at interface of YSZ and bond coat, forming a duplex oxide scale: (Ni,Co)(Cr,Al)2O4 spinel over Al2O3, which promoted the delamination at the top-coat/bond-coat interface. The resistance against gradient thermal cycling, the phase stability of 4YSZ and the failure mechanism of the TBCs, were discussed correlating to the effects of YSZ thickness.

Effect of interactions between Fe–Al alloy and MgO-based refractory on the generation of MgO·Al2O3 spinel

ABSTRACTIn the present study, interactions between Fe-2.1 wt-%Al and Fe-5.1 wt-%Al alloys and different tundish refractories, and a spinel generation in the Fe–Al alloys were investigated in laboratory experiments at 1550°C in an alumina crucible using a high-frequency induction furnace. The results indicated that the formation of a spinel layer at the interface was owing to both the oxidation–reduction reactions and the phase transformation at the interface according to initial phases in refractories. The thickness of the interface layer increased with increasing the content of the [Al] in Fe–Al alloys, the SiO2 content in the refractory and the immersion time, however, the [Al] content was a dominant factor. Both experimental results and thermodynamic calculations concluded that MgO·Al2O3 spinel inclusions along with a small amount of CaO–MgO–Al2O3 were the main inclusions in Fe–Al alloys. MgO·Al2O3 inclusions were mainly formed at the interface by alloy/refractory interactions and then mechanically transferred into molten alloys.

Oxide-Inclusion Evolution in the Steelmaking Process of 304L Stainless Steel for Nuclear Power

The inclusions formed in 304L stainless steel for nuclear power produced by the electric arc furnace (EAF)-argon oxygen decarburization furnace (AOD)-ladle furnace (LF)-continuous casting (CC) process were investigated by thermodynamics calculations and experimental results. The results showed that the inclusions after AOD and LF refining were almost the same as the slag composition. The types of inclusions (sizes larger than 5 µm) were mainly CaSiO3 with high SiO2 content at the end of AOD, and Ca2SiO4 with high CaO content at the end of LF. The Al2O3 and MgO content of inclusions increased from AOD to LF. There were two types of inclusions in the tundish: CaO-SiO2-Al2O3-MgO and CaO-SiO2-Al2O3-MgO-MnO inclusions with MgO·Al2O3 spinel precipitation. The content of Al2O3 in the inclusions increased rapidly with the decrease in temperature from the end of LF refining to continuous casting, as calculated using FactSage6.3 software. The rapid increase of Al2O3 in the CaO-SiO2-Al2O3-MgO-(MnO) inclusions promoted the precipitation of MgO·Al2O3 spinel in continuous casting tundish, suggesting mechanisms for the formation of inclusions in the 304L stainless steel.

Mechanism of Interface Reactions Between Fe-2%Al Alloy and High-Silica Tundish Refractory

The reactions between a Fe-2%Al alloy and a high-silica tundish refractory were investigated at 1550 °C in an alumina crucible using a high-frequency induction furnace. The reaction between the dissolved aluminum in the molten alloy and the oxidizing components in the refractory took place at the interface, generating a MgO·Al2O3 spinel as the main product, which was confirmed by both experimental and FactSage™ 7.0 calculated results. Several layers were detected at the interface depending on an aluminum diffusion in the refractory material and the reaction time. However, the formation of MgO·Al2O3 spinel layer at the interface delayed the reduction of silica because aluminum diffusion became very slow through the interfacial layer. The mass transfer coefficient of silicon in the Fe-2%Al alloy was 7.91 × 10−4 m/s. FactSage™ 7.0 calculated results indicated that the content of silicon in the alloy was higher than that of magnesium, as the result of transformation of forsterite phase to MgO·Al2O3 spinel at the interface.

Study on the behaviour of DS-Class inclusions in advanced bearing steel

The source and generated mechanisms of DS-size inclusions in bearing steel were studied by sampling systematically and using ASPEX, SEM and EDS to analyse the morphology, composition and amount of non-metallic inclusions larger than 13 µm. Two kinds of typical DS-size inclusions were found in the refining process: type I is CaO-MgO-Al2O3-SiO2 distributed evenly and wrapped by CaS; type II is composite inclusion with MgO-Al2O3 as the core, CaO-Al2O3 and CaO-SiO2 as the inner layer, covered by CaS. Based on the FactSage and thermodynamic calculations, the DS inclusions formation mechanisms were drawn. There are two formation mechanisms of the type II inclusions: one is the solid phases will precipitate from the matrix of type I inclusions as the temperature drops; another is that when the reaction time is not sufficient, the MgO∙Al2O3 spinel core will not be transformed completely, and the evolution can be summarized as: Al2O3 → MgO-Al2O3 → CaO-MgO-Al2O3 → surrounded by CaO-Al2O3(SiO2) → covered by CaS.

Fabrication and Property of Fine-grained MgO·1.44Al2O3 Spinel Transparent Ceramic

Fabrication and Property of Fine-grained MgO·1.44Al2O3 Spinel Transparent Ceramic

Simple identification of Al2O3 and MgO·Al2O3 spinel inclusions in steel using X‐ray‐excited optical luminescence

Nonmetallic inclusions in steel cause problems in steel products and steel production. In particular, an analysis of Al2O3 and MgO·Al2O3 spinel inclusions is important since they are one of the most harmful inclusions. A rapid and simple analysis of nonmetallic inclusions is required as the conventional analytical methods for nonmetallic inclusions are time‐consuming. In this study, we propose a simple method to identify Al2O3 and MgO·Al2O3 spinel inclusions in steels. X‐ray‐excited optical luminescence (XEOL) analysis was selected as a promising method because it can rapidly identify sizes, shapes, and compositions of nonmetallic inclusions and can be performed in air. A model steel sample prepared by heating a mixture of Fe, Al, and MgO powders at 1550°C in argon atmosphere was used. XEOL images of the model steel sample showed that Al2O3 inclusions emitted blue and red luminescences. Using a filter attached to the camera, blocking light in the wavelength region above 650 nm, only the blue luminescence of the Al2O3 inclusions was observed. This was implemented as the luminescences of the Al2O3 inclusions appeared in both blue and red regions at wavelengths of 350, 485, 695, and 750 nm; consequently, the luminescences at 695 nm and 750 nm were blocked by the filter. In contrast, MgO·Al2O3 spinel inclusions emitted green luminescence (peak at 520 nm), unaffected by the filter. This indicates that we can simply identify Al2O3 and MgO·Al2O3 spinel inclusions by an XEOL image in the wavelength range of 420–650 nm.

Formation and Evolution of Oxide Inclusions in Titanium-Stabilized 18Cr Stainless Steel

During Ti-stabilized stainless steelmaking process, oxide inclusions in steel generally cause the clogging of submerged entry nozzle and surface defects of cold-rolled products. Therefore, the evolution mechanism of oxide inclusions in Ti-stabilized 18Cr stainless steel was investigated by industrial experiments. The characteristics of inclusions in specimens were analyzed by scanning electron microscopy and energy dispersive spectroscopy. After Al deoxidation, the main inclusions were irregular MgO–Al2O3 spinel. After calcium treatment, MgO–Al2O3 inclusions were modified to be spherical multilayer CaO–MgO–Al2O3 inclusions consisting of spinel crystal embedded in CaO–Al2O3 liquid matrix. Thermodynamic calculation indicated that several ppm Ca could significantly expand the liquid oxides phase in Mg–Al–O phase diagram. After Ti addition, multilayer CaO–MgO–Al2O3–TiOx inclusions were formed. The compositions of steel were located close to Al2O3–TiOx liquid oxide phase, which would help to reduce oxide inclusions and increase titanium yield. Titanium addition has modified spinel inclusions to multilayer MgO–Al2O3–Ti3O5 inclusions containing solid spinel inner layer and MgO–Al2O3–Ti3O5 liquid oxide outer layer. As for improving the cleanliness of molten steel, the contents of magnesium, aluminum, and titanium could be considered simultaneously to liquefy oxide inclusions during Ti-stabilized stainless steelmaking process.

Modification of NiCoCrAlY with Pt: Part II. Application in TBC with pure metastable tetragonal (t′) phase YSZ and thermal cycling behavior

A thermal barrier coating system comprising Pt-modified NiCoCrAlY bond coating and nanostructured 4 mol.% yttria stabilized zirconia (4YSZ, hereafter) top coat was fabricated on a second generation Ni-base superalloy. Thermal cycling behavior of NiCoCrAlY-4YSZ thermal barrier coatings (TBCs) with and without Pt modification was evaluated in ambient air at 1100 °C up to 1000 cycles, aiming to investigate the effect of Pt on formation of thermally grown oxide (TGO) and oxidation resistance. Results indicated that a dual layered TGO, which consisted of top (Ni,Co)(Cr,Al)2O4 spinel and underlying α-Al2O3, was formed at the NiCoCrAlY/4YSZ interface with thickness of 8.4 μm, accompanying with visible cracks at the interface. In contrast, a single-layer and adherent α-Al2O3 scale with thickness of 5.6 μm was formed at the interface of Pt-modified NiCoCrAlY and 4YSZ top coating. The modification of Pt on NiCoCrAlY favored the exclusive formation of α-Al2O3 and the reduction of TGO growth rate, and thus could effectively improve overall oxidation performance and extend service life of TBCs. Oxidation and degradation mechanisms of the TBCs with/without Pt-modification were discussed.