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Corrosion mechanism of Al–MgO–MgAl2O4 refractories in RH refining furnace during production of rail steel
Journal Pre-proof Corrosion mechanism of Al–MgO–MgAl2O4 refractories in RH refining furnace during production of rail steel Shanghao Tong, Jizeng Zhao, Yucui Zhang, Qingyang Cui, Rongrong Wang, Yong Li PII:
S0272-8842(19)33779-4
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.277
Reference:
CERI 23930
To appear in:
Ceramics International
Received Date: 7 October 2019 Revised Date:
28 December 2019
Accepted Date: 31 December 2019
Please cite this article as: S. Tong, J. Zhao, Y. Zhang, Q. Cui, R. Wang, Y. Li, Corrosion mechanism of Al–MgO–MgAl2O4 refractories in RH refining furnace during production of rail steel, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2019.12.277. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Corrosion mechanism of Al-MgO-MgAl2O4 refractories in RH refining furnace during production of rail steel Shanghao Tonga, Jizeng Zhaob, Yucui Zhangb, Qingyang Cuib, Rongrong Wangb and Yong Lia,∗ a
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China b
Beijing Lier High-Temperature Materials Co., Ltd., Beijing 102200, China
Abstract: The corrosion behavior of Al-MgO-MgAl2O4 refractories by liquid rail steel was studied by analyzing the worn bricks used in an RH refining furnace of a steel mill in China. The corrosion mechanism of Al-MgO-MgAl2O4 refractories is expressed as follows: on one hand, during vacuum treatment, with the increasing temperature and declining oxygen partial pressure, a minor amount of Fe in the steel was oxidized into FeO at steel/refractory interface. Then FeO dissolved with MgO particles and fine powder, forming (Mg, Fe)O solid solution. During the refining interval, as the temperature decreased and the partial oxygen pressure increased, Fe2+ was oxidized into Fe3+ and MgO·Fe2O3 solid solution separated from (Mg, Fe)O solid solution. In the cyclic process of solid solution and desolution, MgO particles and fine powder were structurally weakened. On the other hand, the CaO-SiO-based slag infiltrated along the open pores and the grain boundaries into the Al-MgO-MgAl2O4 refractories, forming CaO·MgO·SiO2 and 3CaO·MgO·2SiO2 phases, which led to ∗
Corresponding author: Yong Li: Male, Ph.D, Professor;
Email: [email protected] Tel./Fax: +86-10-6233 2666
poor binding ability of refractories. At the same time, due to the Al-O reaction, the oxygen partial pressure in the system was further reduced, which provided conditions for the formation of MgAlON and MgO dense layers, improving the resistance to slag corrosion and slag permeability of Al-MgO-MgAl2O4 refractories. When the dense layer structure was completely destroyed, the interaction zones of the refractories were washed away into the liquid steel. Keywords: Al-MgO-Al2O3 refractories; Degradation mechanisms; RH; MgAlON; 1. Introduction RH refining is widely used in steelmaking production, especially in clean steel production because of its short working cycle, good refining effect, high production efficiency and strong applicability of treatment capacity. The service environment of refractories for RH refining is harsh: high temperatures (≥1580 °C), low pressures (50-20000Pa), turbulence of the molten steel, aggressive fluid slags [1]. Until recently, fired magnesia-chrome bricks were the most extensively used materials in RH, showing excellent slag resistance and thermal stability [2,3]. However, the high cost of MgO-Cr2O3 bricks and the disposal problem of used bricks containing carcinogenic Cr6+ are providing an impetus for replacement of MgO-Cr2O3 by chrome-free bricks. Magnesium aluminate spinel (MgAl2O4) is the only stable compound in MgO-Al2O3 binary system. It has a high melting point (2135 °C), a low thermal expansion coefficient (7.6×10−6 K−1), high mechanical strength both at room (135-216 MPa) and elevated temperatures (120-205 MPa at 1300 °C) and excellent resistance to slag corrosion [4-6]. However, MgAl2O4 refractories possess poor resistance to
thermal shocks and slag penetration, which restrains their development and wide application. The most effective method to solve this problem is to form composites with oxides and non-oxides. MgAlON has a low linear thermal expansion coefficient (5.3×10−6 K−1-7.4×10−6 K−1) and a high thermal conductivity (10.89 W/(m·K), room temperature, 20 °C) which means better thermal shock resistance and excellent spalling resistance [7-9]. Meanwhile, there are cation vacancies in the crystal lattices which can accommodate Fe3+ and Fe2+ ions [10]. Thus, MgAlON as well as its composites has good slag resistance at high temperatures. Therefore environmentally friendly MgO-MgAlON composites are expected to replace MgO-chromite bricks. Some researches have been carried out on the corrosion mechanism of magnesia bricks. Liugang Chen et al [11] investigated the corrosion behaviour of magnesiachrome and magnesia-dolomite refractories through crucible tests in a vacuum induction furnace at elevated temperatures (1650 and 1750 °C) and low oxygen partial pressures (0.53 and 3.0×10-12 MPa). The results reveal that MgO-based refractories are corroded due to the dissolution of MgO. Kiyoshi Goto et al investigated the corrosion of MgO-MgAl2O4 spinel refractory bricks by calcium aluminosilicate slags through crucible tests at 1400-1450 °C [12]. The results show that the corrosion mechanism is believed to involve initial dissolution of fine matrix MgO in the slag leading to secondary spinel precipitation from the MgO-enriched slag. However, previous studies mostly focused on laboratory simulation of refining conditions, which could not take the hot molten steel scouring, slag corrosion,
temperature fluctuations, alternating stress and other factors into account, affecting understanding on degradation mechanism mechanism of magnesia refractories. In the present work, commercial Al-MgO-MgAl2O4 bricks were prepared, using fused magnesia, metal aluminum, fused spinel and sintered high purity magnesia. The prepared bricks were applied in an RH refining furnace of a steel mill in China. The worn Al-MgO-MgAl2O4 bricks were analyzed by XRD, SEM and EDS to investigate the corrosion mechanisms of Al-MgO-MgAl2O4 bricks during production of rail steel. 2. Experimental Procedure 2.1. Materials preparation In this work, fused magnesia, metal aluminum, fused spinel and sintered high purity magnesia were taken as raw materials, and magnesium aluminate sol as the binder. The raw materials were batched in an aggregate and matrix ratio of 72:28 according to the formulations shown in Table 1. The materials were mixed in a mixer for 30 min and then pressed into bricks with dimensions of 250 mm×150 mm×100 mm under 1000 t pressure. The green bricks were dried at 300 °C for 24 h. The chemical composition is illustrated in Table 2. 2.2. Experimental procedure and conditions Al-MgO-MgAl2O4 bricks were laid at the bottom of an RH refining furnace of a steel mill in China. A schematic drawing of the bottom masonry of the RH refining furnace is shown in Fig. 1. The refining conditions of rail steel and rail steel slag compositions are listed in Table 3 and Table 4, respectively. The basicity (C/S: CaO/SiO2 ratio) was 1.32. Working layer bricks of the refining furnace were
preheated at 900 °C for 72 h in air atmosphere. 2.3. Sample analysis techniques After reaching the service life of 82 heats, samples were taken from the worn bricks and examined. The phases were identified by X-ray diffraction (PANalytical, X'Pert Powder, working voltage: 40 kV; working currency: 40 mA; Cu Kα radiation; steps: 0.013°; counting time: 2 min, scanning range: 10°-90°). The morphology and composition of the phases were investigated using a field emission scanning electron microscope (Σ ΣIGMA HD, Germany) equipped with an energy dispersive X-ray spectroscope (BRUKER XFlash 6130, American). The samples after polishing for SEM investigation were gold coated to make electrically conductive. 3. Results and discussion 3.1 Macroscopical morphology of the worn brick Fig. 2 shows the macroscopical morphology and horizontal section of the worn brick. As shown in Fig. 2(b), according to the different colors, the worn brick can be divided into the following areas: the slag zone, the interaction zone, the infiltration zone and the original brick zone. The depth of the interaction zone and the infiltration zone is around 1.5 mm and 3.5 mm, respectively. Each zone of the worn brick was analyzed by XRD. 3.2 XRD analysis Fig. 3 shows the XRD patterns of the worn brick. Since the relative intensity of periclase (MgO) phase is high, the characteristic peaks at the range of 28-40° are not obvious. At higher magnification, It can be seen that the original brick zone consists
of MgO and spinel, while the interaction zone and infiltration zone is composed of MgO, (Mg, Fe)Oss and compound spinel Mg(AlxFe2-x)O4. According to the experimental conditions, the spinel in the XRD results may be MgAlON. However, magnesia alumina spinel and MgAlON are spinel structures and it is difficult to distinguish them in the diffraction results. This will be further confirmed in the EDS analysis below 3.3 Microstructural observations The microstructure images of the Al-MgO-MgAl2O4 worn brick are shown in Fig. 4. The EDS results of the particles and matrix are shown in Table 5. It can be seen from Fig. 4(a) that the worn brick can be divided into three areas, including the interaction zone, the infiltration zone and the original brick zone. Fig. 4(b) and Fig. 4(c) are the partial magnification of region 1 in Fig. 4(a). A reaction zone is formed around magnesia particles, accompanied by dissolution. The scanning results of Fig. 4(b) are shown in Fig. 5. The main elements of the reaction zone are Mg, Fe and O. The Fe solution in the MgO particles decreases gradually from the outside to the inside. Combined with the EDS results, the main phase composition of the reaction zone is MgFe2O4(containing small amounts of Al) and MgO (containing small amounts of Fe), which is consistent with the XRD results. Dense layers are formed in the matrix, and the pores are mostly closed. Fig. 4(d) presents a further enlargement of the infiltrated zone in Fig. 4(a). As seen from Fig. 4(d), the worn brick is infiltrated by liquid slag up to its centre along the open pore network and the grain boundaries. Compared with the matrix of the interaction zone, the matrix in the infiltration zone is loose. The
white binding phase in the matrix are CaO·MgO·SiO2 and a small amount of 3CaO·MgO·2SiO2. Fig. 4(e) and Fig. 4(f) shows the microstructures of the original brick zone. Some flake structures has a network distribution in the matrix. In combination of the XRD results as well as some references [13, 14], the flake structure is confirmed as MgAlON. 3.3 Corrosion mechanism of Al-MgO-MgAl2O4 bricks 3.3.1 Phase relations Fig. 6 shows phase diagram of the CaO-MgO-SiO2 system [15]. According to the equilibrium relationship of coexistence, ternary and polynary mineral phases coexisting with MgO vary on the basis of the ratio of CaO/SiO2 in the system. From this, composite compounds coexisting with MgO components can be drawn, as shown in Table 6. In this work, the steel slag has a CaO/SiO2 mass ratio of 1.32. In Fig. 6, the liquid line in equilibrium with periclase at high temperatures is marked in red. So, when the Al-MgO-MgAl2O4 bricks contact with steel slag, the phase zones in equilibrium with MgO at high temperatures are 3CaO·MgO·2SiO2 (abbreviated as C3MS2) and CaO·MgO·SiO2 (abbreviated as CMS), which is consistent with the EDS results. Fig. 7 shows the phase diagram of the MgO-FeO-Fe2O3 system [16]. From Fig. 7, we can see that the phase diagram contains three important areas including the (MgO·FeO)ss region, the (MgO·FeO)ss and spinelss region and the spinelss region. The spinel region refers to (MgO·Fe2O3 + FeO·Fe2O3)ss. Oxygen partial pressure is an important variable for MgO-FeO-Fe2O3 system as well as temperature. The lower the
oxygen partial pressure is, the larger the (MgO·FeO)ss region is. In contrast, the higher the oxygen partial pressure is, the larger the (MgO·FeO)ss and spinelss region is. So when the temperature rises and oxygen partial pressure reduces, MgO·Fe2O3 gradually decomposes and the content of (MgO·FeO)ss increases. 3.3.2 Thermodynamic calculation Iron oxides are variable valence oxides. At high temperatures, the thermodynamic reactions involving iron oxides are as follows [17]: 2Fe 3 O 4 (s)=6FeO(l)+O 2 (g) ∆G1 =978228-458.97T +RTln
lg
PO P
2
θ
458.97
=
-
PO P
2
(1)
θ
978228
2.303R 2.303RT
2Fe 3O 4 (s)=6FeO(s)+O 2 (g)
∆G2 =624682-250.62T +RTln lg
PO
2
P
θ
250.62
=
-
PO P
2
θ
(2)
624682
2.303R 2.303RT
2FeO(l)=2Fe(l)+O 2 (g)
∆G3 =459400-87.45T +RTln lg
PO P
87.45
=
2
θ
-
PO P
(3)
2
θ
459400
2.303R 2.303RT
2FeO(l)=2Fe(s)+O 2 (g)
∆G4 =441410-77.82T +RTln lg
PO P
2
θ
=
77.82
-
PO P
2
θ
(4)
441410
2.303R 2.303RT
2FeO(s)=2Fe(s)+O 2 (g)
∆G5 =519230-125.10T +RTln lg
PO P
2
θ
=
125.10
-
PO
2
P
θ
(5)
519230
2.303R 2.303RT
Based on the above thermodynamic equations of FeO reactions, the stable region
containing FeO are plotted, as shown in Fig. 8. As can be seen from Fig. 8, the valence state of iron is related to the temperature and the partial pressure of oxygen. During refining and interval of the RH furnace, the oxygen partial pressure and the temperature change periodically, leading to periodic change of valence state of iron. During Vacuum treatment with oxygen blowing, the vacuum and the temperature are up to 13 KPa and 1923 K, respectively. lg
PO P
2
θ
= lg
13 101.325
=-0.89
Since the molten steel is circulating in the RH refining process, when the reaction: 2Fe (l) +O2 (g) =2FeO (l) reaches the equilibrium, the oxygen pressure in the vacuum chamber is the lowest. It can be seen from Fig. 7 that the oxygen pressure at Fe-FeO equilibrium at 1923K is as follows. lg .
PO2 Pθ
= − 7.8
During the refining interval, the temperature drops to around 1173 K. The air enters the furnace, the partial pressure of oxygen is the partial pressure of oxygen in the air. lg
PO
2
P
θ
=lg
0.21 × 101.325 101.325
= − 0.06
Combined with the above thermodynamic analysis, the transition region of iron oxides with the change of the temperature and the oxygen partial pressure during the refining process and refining interval is marked in red in Fig. 8. 3.3.3 Corrosion mechanisms
In the Al-MgO-MgAl2O4 system, the possible reactions of metal Al are as follows. 2Al(l)+1.5O2(g)=Al2O3(s)
(6)
2Al(l)+0.5O2(g)=Al2O(g)
(7)
Al2O(g)+O2(g)=Al2O3(s)
(8)
Al(l)+0.5N2(g)=AlN(s)
(9)
Al(g)+0.5N2(g)=AlN(s)
(10)
During the RH refining process, the Al-MgO-MgAl2O4 bricks contact with the molten steel. Under high temperature and vacuum conditions, Al is oxidized as reaction (6) and (7), which reduces the oxygen partial pressure in the brick. Then the molten steel and the CaO-SiO2-based slag infiltrate into the brick along the pores, cracks, or grain boundaries. On one hand, a minor amount of Fe in the steel is oxidized into FeO on the surface of bricks. Then FeO dissolves with MgO particles and fine powder and MgAl2O4, forming (Mg, Fe)O and (Mg, Fe)Al2O4 solid solution, respectively [18]. On the other hand, the slag reacts with MgO, forming CMS and a small amount of C3MS2 which are liquid at the refining temperature, distributing between particles and matrix and between matrix and matrix, resulting in the poor structure of the interaction zone and the infiltration zone of Al-MgO-MgAl2O4 brick. In the infiltration zone, as the temperature increases and the partial oxygen pressure decreases owing to the oxygen consumed according to the Al-O reaction, MgO decomposes into Mg(g): MgO( s) → Mg(g)+0.5O2 (g) . Continuous vacuuming will accelerate the reaction to the right. Mg(g) migrates outward to form a MgO and spinel dense layer in the interaction zone, preventing further penetration of molten steel and slag [19]. At the same time, some of the metal aluminum is nitrided into AlN which dissolves with MgAl2O4 forming MgAlON [20]and some of the metal aluminum is oxidized into Al2O(g) which diffuses along pores and gaps to the matrix and reacts
with N2, Mg(g) and O2 in the system generating flake MgAlON, as expressed: Al2O(g)+ O2(g)+ N2(g)+Mg(g) → MgAlON(s) [21]. Due to the poor wettability, MgAlON can effectively prevent the infiltration of CaO-SiO2-based slag. During the refining interval, as the temperature decreases and the partial oxygen pressure increases, the valence state of Fe changes. (MgO·Fe2O3)ss containing Al precipitates from (MgO·FeO)ss. The reaction equation is as follows. +2 → 3(MgO0.33 Fe0.67 MgO • Fe 2O3 ← )O − O2 + O2
（11）
In the circulating process of the above reaction (11), there will be a change in volume in the Al-MgO-MgAl2O4 brick [22], as shown in Fig. 9, resulting in structural damage to the interaction zone of the brick. The MgO granules and fine powders gradually dissolved into the infiltrating slag. Finally, under the combined action of valence changes of iron oxide and CaO-SiO2-based slag, the interaction zone of the brick is washed away into the liquid steel and a new interaction zone forms. The corrosion mechanism model is shown in Fig. 10. 4. Conclusions The corrosion behavior of Al-MgO-MgAl2O4 refractories by liquid rail steel was studied by analyzing the worn bricks from bottom zones in an RH refining furnace. The worn bricks were characterized and analyzed by XRD, SEM and EDS. The following conclusions can be drawn. (1) During vacuum treatment, with the increasing temperature and declining oxygen partial pressure, a minor amount of Fe in the steel was oxidized into FeO at
steel/refractory interface. (2) FeO dissolved with MgO particles and fine powder, forming (Mg, Fe)O solid solution. As the temperature decreases and the partial oxygen pressure increases, the Fe2+ was oxidized into Fe3+ and MgO·Fe2O3 solid solution separated from (Mg, Fe)O solid solution. In the process of circulating between solid solution and desolution, the refractories were structurally weakened. (3) The CaO-SiO-based slag infiltrated along the open-pores and the grain boundaries into the Al-MgO-MgAl2O4 refractories, forming CMS and C3MS2 phases which were liquid at refining temperature, leading to poor binding ability of refractories. (4) Under the combined action of valence changes of iron oxide and CaO-SiO2-based slag, When the MgO dense layer structure is completely destroyed, the interaction zone of the refractories were washed away into the liquid steel. (5) In the Al-MgO-MgAl2O4 system, the formation of dense layer and MgAlON can effectively prevent the infiltration of CaO-SiO2-based slag and molten steel due to the poor wettability. Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Conflict of Interest: The authors declare that they have no conflict of interest. References [1] B. Liu, G. Zhu, H. Li, B. Li, Y. Cui, A. Cui, Decarburization rate of RH refining
for ultra low carbon steel, Int. J. Min. Met. Mater. 17(2010) 22-27. [2] V. Petkov, P.T. Jones, E. Boydens, B. Blanpain, P. Wollants, Chemical corrosion mechanisms of magnesia–chromite and chrome-free refractory bricks by copper metal and anode slag, J. Eur. Ceram. Soc. 27(2007) 2433-2444. [3] K. Gotod, W.E. Lee, The “Direct bond” in magnesia chromite and magnesia spinel refractories, J. Am. Ceram. Soc. 78(1995) 1753-1760. [4] I. Ganesh, R. Johnson, G.V.N. Rao, Y.R. Mahajan, S.S. Madavendra, B.M. Reddy, Microwave-assisted combustion synthesis of nanocrystalline MgAl2O4 spinel powder, Ceram. Int. 31(2005) 67-74. [5] C. Păcurariu, I. Lazău, Z. Ecsedi, P. Lazău. Barvinschi, G. Mărgineanc, New synthesis methods of MgAl2O4 spinel, J. Eur. Ceram. Soc. 27(2007) 707-710. [6] R.K. Pati, P. Pramanik, Low-Temperature Chemical Synthesis of Nanocrystalline MgAl2O4 Spinel Powder, J. Am. Ceram. Soc. 83(2000) 1822-1824. [7] D. Yang, H. Zhang, X. Zhong, Study on preparation and mechanical property of MgO-MgAl2O4-MgAlON composite, J. Naihuo Cailiao/Refract., 40(2006) 12-15. [8] S. Pichlbauer, H. Harmuth, Z. Lenčéš, P. Šajgalík, Preliminary investigations of the production of MgAlON bonded refractories, J. Ceram. Soc. 32(2012) 2013-2018. [9] S. Bandyopadhyay, G. Rixecker, F. Aldinger, H. S. Maiti, Effect of controlling parameters on the reaction sintering sequences of formation of nitrogencontaining magnesium aluminate spinel from MgO, Al2O3 and AlN, J. Am. Ceram. Soc. 87(2004) 480–482.
[10] H.X. Willems, G.D. With, R. Metselaar, Thermodynamics of Alon III: stabilization of Alon with MgO, J. Eur. Ceram. Soc. 12(1993) 43-49. [11] L. Chen, A. Malfliet, P.T. Jones, B. Blanpain, M. Guo, Comparison of the chemical corrosion resistance of magnesia-based refractories by stainless steelmaking slags under vacuum conditions, Ceram. Int. 42(2016) 743-751. [12] K. Goto, B.B. Argent, W.E. Lee, Corrosion of MgO-MgAl2O4 spinel refractory bricks by calcium aluminosilicate slag, J. Am. Ceram. Soc. 80(1997) 461-471. [13] A. Granon, P. Goeuriot, F. Thevenot, J. Guyader, P. L'Haridon, Y. Laurent, Reactivity in the Al2O3-AlN-MgO system. The MgAlON spinel phase, J. Eur. Ceram. Soc. 13(1994) 365-370. [14] Z. Zhang, L. Teng, W. Li, Mechanical properties and microstructures of hot-pressed MgAlON–BN composites, J. Eur. Ceram. Soc. 27(2007) 319-326. [15] W.M. Huang, M. Hillert, X.Z. Wang, Thermodynamic assessment of the CaO-MgO-SiO2 system, Metall. Mater. Trans. A. 26A (1995) 2293-2310. [16] J.C. Willshee, J. White, An investigation of equilibrium relationships in system MgO-FeO-Fe2O3 up to 1750 degrees in air, Trans. Br. Ceram. Soc. 66(1967) 541-555 [17] Z.Y. Chen, Chemical Thermodynamics of Refractories, First ed., Metallurgical industry press, Beijing, 2005. [18] G.W. Groves, M. E. Fine, Solid solution and precipitation hardening in Mg-Fe-O alloys, J. Appl. Phy. 35(1964) 3587-3593. [19] M. Yan, Y. Li, L. Li, Y. Sun, S. Tong, J. Sun, In-situ synthesis and reaction
mechanism of MgAlON in Al2O3-MgO composites produced in flowing nitrogen, Ceram. Int. 43(2017) 14791-14797. [20] S. Tong, Y. Li, M. Yan, P. Jiang, J. Ma, D. Yue, In situ reaction mechanism of MgAlON in Al-Al2O3-MgO composites at 1700°C under flowing N2, Int. J. Min. Metall. Mater. 24(2017) 1061-1066. [21] M. Yan, Y. Li, H. Li, Y. Sun, H. Chen, C. Ma, J. Sun, Evolution mechanism of MgAlON in the Al-Al2O3–MgO composite at 1800 °C in flowing nitrogen, Ceram. Int. 44(2018) 3856-3861. [22] Y.F. Sun, X.M. Wang, C.X. Wang, P.Q. Sun, Magnesia and Magnesia Based Composite Refractories, First ed., Metallurgical industry press, Beijing, 2010.
Caption of tables
Table 1. Formulations of Al-MgO-MgAl2O4 bricks (wt)/% Table 2. Chemical composition of raw materials Table 3 Refining conditions of rail steel Table 4 Composition of the rail steel during the refining process ( in wt% ) Table 5. EDS results of areas marked in Fig. 4 Table 6 Composite compounds coexisted with MgO
Table 1. Formulations of Al-MgO-MgAl2O4 bricks (wt)/% Raw material
Fused magnesia particles Fused magnesia powder Fused spinel Metal aluminum powder Sintered high purity magnesia powder
Particle size
Specimen ratio
5-3mm 3-1mm 1-0mm 200mesh 325mesh 200mesh
15 35 22 12 5 6
200mesh
5
Table 2. Chemical composition of raw materials Chemical composition /wt% Raw materials MgO Fused magnesia
Al2O3
97.5
Metal aluminum
magnesia Fused spinel
SiO2
CaO
0.63
0.76
0.5
1.34
Fe2O3
K2O
Na2O
0.13
0.01
0.14
99.3
powder Sintered high purity
Al
97.8 23.57
75.16
0.10
Table 3 Refining conditions of rail steel Smelting parameter
Steel grade Rail steel
Temperature (°C)
Time (min)
Vacuum degree(Pa)
1580-1650
30
50
Table 4 Composition of the rail steel during the refining process (in wt%) SiO2
Al2O3
CaO
MgO
Cr2O3
MnO
Total Fe
Basicity (C/S)
35.85
3.2
47.32
7.2
1.9
1.1
0.9
1.32
Table 5. EDS results of areas marked in Fig. 4 Atomic percentage /%
Marked areas
Mg
1
12
2
50
3
48
2
50
4
46
8
46
5
44
11
45
6
43
1
16
40
7
14
2
35
49
8
13
2
27
58
9
13
14
14
59
10
11
35
17
37
11
16
12
54
13
15
32
52
1
14
6
42
27
25
15
9
39
27
25
Al
Ca
Si
14
13
Fe
O
N
61 50
31
53 46
Table 6 Composite compounds coexisted with MgO CaO/SiO2,
ratio value
Mole ratio,%
0
0-1.0
1.0
1.0-1.5
1.5
1.5-2.0
2.0
Mass ratio,%
0
0-0.93
0.93
0.93-1.4
1.4
1.4-1.87
1.87
Mineral phase
M2S
M2S-CMS
CMS
CMS-C3MS2
C3MS2
C3MS2-C2S
C 2S
Caption of figures
Fig.1 Schematic drawing of the bottom masonry of RH refining furnace Fig.2 Images of the worn brick after used (a) macroscopical morphology and (b) horizontal section Fig.3 XRD patterns of the Al-MgO-MgAl2O4 worn brick Fig.4 SEM images of Al-MgO-MgAl2O4 worn brick Fig.5 Scanning results of Fig. 4(b) Fig. 6 Phase diagram of CaO-MgO-SiO2 system Fig. 7 Phase diagram of MgO-FeO-Fe2O3 system Fig. 8 lg
PO2 Pθ
~T curves of Fe-O system
Fig. 9 Volume change caused by change in iron's valence state Fig. 10 Corrosion mechanism model of Al-MgO-MgAl2O4 refractories
Fig.1 Schematic drawing of the bottom masonry of RH refining furnace
Fig.2 Images of the worn brick after used (a) macroscopical morphology and (b) horizontal section
Fig.3 XRD patterns of the Al-MgO-MgAl2O4 worn brick
Fig.4 SEM images of Al-MgO-MgAl2O4 worn brick
Fig.5 Scanning results of Fig. 4(b)
Fig. 6 Phase diagram of CaO-MgO-SiO2 system [15] (Lime = CaO; Per = periclase; Hat = hatrurite; Ra = rankinite; Psw = pseudowollastonite; Wo = wollastonite; Ak = akermanite (MgCa2Si2O7); Mer = merwinite; Mtc = monticelite; Dsp = diaspore; Pgt = pigeonite; Opx = orthopyroxene; Ppx = protopyroxene; Fo = forsterite; Trd = tridymite; Crs = cristobalite.)
Fig. 7 Phase diagram of MgO-FeO-Fe2O3 system [16]
Fig. 8 lg
PO2 Pθ
~T curves of Fe-O system
Fig. 9 Volume change caused by change in iron's valence state [22]
Fig. 10 Corrosion mechanism model of Al-MgO-MgAl2O4 refractories
Declaration of Interest Statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Corrosion mechanisms of Al-MgO-MgAl2O4 refractories in RH refining furnace during production of rail steel”
S0272-8842(19)33779-4
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.277
Reference:
CERI 23930
To appear in:
Ceramics International
Received Date: 7 October 2019 Revised Date:
28 December 2019
Accepted Date: 31 December 2019
Please cite this article as: S. Tong, J. Zhao, Y. Zhang, Q. Cui, R. Wang, Y. Li, Corrosion mechanism of Al–MgO–MgAl2O4 refractories in RH refining furnace during production of rail steel, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2019.12.277. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Corrosion mechanism of Al-MgO-MgAl2O4 refractories in RH refining furnace during production of rail steel Shanghao Tonga, Jizeng Zhaob, Yucui Zhangb, Qingyang Cuib, Rongrong Wangb and Yong Lia,∗ a
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China b
Beijing Lier High-Temperature Materials Co., Ltd., Beijing 102200, China
Abstract: The corrosion behavior of Al-MgO-MgAl2O4 refractories by liquid rail steel was studied by analyzing the worn bricks used in an RH refining furnace of a steel mill in China. The corrosion mechanism of Al-MgO-MgAl2O4 refractories is expressed as follows: on one hand, during vacuum treatment, with the increasing temperature and declining oxygen partial pressure, a minor amount of Fe in the steel was oxidized into FeO at steel/refractory interface. Then FeO dissolved with MgO particles and fine powder, forming (Mg, Fe)O solid solution. During the refining interval, as the temperature decreased and the partial oxygen pressure increased, Fe2+ was oxidized into Fe3+ and MgO·Fe2O3 solid solution separated from (Mg, Fe)O solid solution. In the cyclic process of solid solution and desolution, MgO particles and fine powder were structurally weakened. On the other hand, the CaO-SiO-based slag infiltrated along the open pores and the grain boundaries into the Al-MgO-MgAl2O4 refractories, forming CaO·MgO·SiO2 and 3CaO·MgO·2SiO2 phases, which led to ∗
Corresponding author: Yong Li: Male, Ph.D, Professor;
Email: [email protected] Tel./Fax: +86-10-6233 2666
poor binding ability of refractories. At the same time, due to the Al-O reaction, the oxygen partial pressure in the system was further reduced, which provided conditions for the formation of MgAlON and MgO dense layers, improving the resistance to slag corrosion and slag permeability of Al-MgO-MgAl2O4 refractories. When the dense layer structure was completely destroyed, the interaction zones of the refractories were washed away into the liquid steel. Keywords: Al-MgO-Al2O3 refractories; Degradation mechanisms; RH; MgAlON; 1. Introduction RH refining is widely used in steelmaking production, especially in clean steel production because of its short working cycle, good refining effect, high production efficiency and strong applicability of treatment capacity. The service environment of refractories for RH refining is harsh: high temperatures (≥1580 °C), low pressures (50-20000Pa), turbulence of the molten steel, aggressive fluid slags [1]. Until recently, fired magnesia-chrome bricks were the most extensively used materials in RH, showing excellent slag resistance and thermal stability [2,3]. However, the high cost of MgO-Cr2O3 bricks and the disposal problem of used bricks containing carcinogenic Cr6+ are providing an impetus for replacement of MgO-Cr2O3 by chrome-free bricks. Magnesium aluminate spinel (MgAl2O4) is the only stable compound in MgO-Al2O3 binary system. It has a high melting point (2135 °C), a low thermal expansion coefficient (7.6×10−6 K−1), high mechanical strength both at room (135-216 MPa) and elevated temperatures (120-205 MPa at 1300 °C) and excellent resistance to slag corrosion [4-6]. However, MgAl2O4 refractories possess poor resistance to
thermal shocks and slag penetration, which restrains their development and wide application. The most effective method to solve this problem is to form composites with oxides and non-oxides. MgAlON has a low linear thermal expansion coefficient (5.3×10−6 K−1-7.4×10−6 K−1) and a high thermal conductivity (10.89 W/(m·K), room temperature, 20 °C) which means better thermal shock resistance and excellent spalling resistance [7-9]. Meanwhile, there are cation vacancies in the crystal lattices which can accommodate Fe3+ and Fe2+ ions [10]. Thus, MgAlON as well as its composites has good slag resistance at high temperatures. Therefore environmentally friendly MgO-MgAlON composites are expected to replace MgO-chromite bricks. Some researches have been carried out on the corrosion mechanism of magnesia bricks. Liugang Chen et al [11] investigated the corrosion behaviour of magnesiachrome and magnesia-dolomite refractories through crucible tests in a vacuum induction furnace at elevated temperatures (1650 and 1750 °C) and low oxygen partial pressures (0.53 and 3.0×10-12 MPa). The results reveal that MgO-based refractories are corroded due to the dissolution of MgO. Kiyoshi Goto et al investigated the corrosion of MgO-MgAl2O4 spinel refractory bricks by calcium aluminosilicate slags through crucible tests at 1400-1450 °C [12]. The results show that the corrosion mechanism is believed to involve initial dissolution of fine matrix MgO in the slag leading to secondary spinel precipitation from the MgO-enriched slag. However, previous studies mostly focused on laboratory simulation of refining conditions, which could not take the hot molten steel scouring, slag corrosion,
temperature fluctuations, alternating stress and other factors into account, affecting understanding on degradation mechanism mechanism of magnesia refractories. In the present work, commercial Al-MgO-MgAl2O4 bricks were prepared, using fused magnesia, metal aluminum, fused spinel and sintered high purity magnesia. The prepared bricks were applied in an RH refining furnace of a steel mill in China. The worn Al-MgO-MgAl2O4 bricks were analyzed by XRD, SEM and EDS to investigate the corrosion mechanisms of Al-MgO-MgAl2O4 bricks during production of rail steel. 2. Experimental Procedure 2.1. Materials preparation In this work, fused magnesia, metal aluminum, fused spinel and sintered high purity magnesia were taken as raw materials, and magnesium aluminate sol as the binder. The raw materials were batched in an aggregate and matrix ratio of 72:28 according to the formulations shown in Table 1. The materials were mixed in a mixer for 30 min and then pressed into bricks with dimensions of 250 mm×150 mm×100 mm under 1000 t pressure. The green bricks were dried at 300 °C for 24 h. The chemical composition is illustrated in Table 2. 2.2. Experimental procedure and conditions Al-MgO-MgAl2O4 bricks were laid at the bottom of an RH refining furnace of a steel mill in China. A schematic drawing of the bottom masonry of the RH refining furnace is shown in Fig. 1. The refining conditions of rail steel and rail steel slag compositions are listed in Table 3 and Table 4, respectively. The basicity (C/S: CaO/SiO2 ratio) was 1.32. Working layer bricks of the refining furnace were
preheated at 900 °C for 72 h in air atmosphere. 2.3. Sample analysis techniques After reaching the service life of 82 heats, samples were taken from the worn bricks and examined. The phases were identified by X-ray diffraction (PANalytical, X'Pert Powder, working voltage: 40 kV; working currency: 40 mA; Cu Kα radiation; steps: 0.013°; counting time: 2 min, scanning range: 10°-90°). The morphology and composition of the phases were investigated using a field emission scanning electron microscope (Σ ΣIGMA HD, Germany) equipped with an energy dispersive X-ray spectroscope (BRUKER XFlash 6130, American). The samples after polishing for SEM investigation were gold coated to make electrically conductive. 3. Results and discussion 3.1 Macroscopical morphology of the worn brick Fig. 2 shows the macroscopical morphology and horizontal section of the worn brick. As shown in Fig. 2(b), according to the different colors, the worn brick can be divided into the following areas: the slag zone, the interaction zone, the infiltration zone and the original brick zone. The depth of the interaction zone and the infiltration zone is around 1.5 mm and 3.5 mm, respectively. Each zone of the worn brick was analyzed by XRD. 3.2 XRD analysis Fig. 3 shows the XRD patterns of the worn brick. Since the relative intensity of periclase (MgO) phase is high, the characteristic peaks at the range of 28-40° are not obvious. At higher magnification, It can be seen that the original brick zone consists
of MgO and spinel, while the interaction zone and infiltration zone is composed of MgO, (Mg, Fe)Oss and compound spinel Mg(AlxFe2-x)O4. According to the experimental conditions, the spinel in the XRD results may be MgAlON. However, magnesia alumina spinel and MgAlON are spinel structures and it is difficult to distinguish them in the diffraction results. This will be further confirmed in the EDS analysis below 3.3 Microstructural observations The microstructure images of the Al-MgO-MgAl2O4 worn brick are shown in Fig. 4. The EDS results of the particles and matrix are shown in Table 5. It can be seen from Fig. 4(a) that the worn brick can be divided into three areas, including the interaction zone, the infiltration zone and the original brick zone. Fig. 4(b) and Fig. 4(c) are the partial magnification of region 1 in Fig. 4(a). A reaction zone is formed around magnesia particles, accompanied by dissolution. The scanning results of Fig. 4(b) are shown in Fig. 5. The main elements of the reaction zone are Mg, Fe and O. The Fe solution in the MgO particles decreases gradually from the outside to the inside. Combined with the EDS results, the main phase composition of the reaction zone is MgFe2O4(containing small amounts of Al) and MgO (containing small amounts of Fe), which is consistent with the XRD results. Dense layers are formed in the matrix, and the pores are mostly closed. Fig. 4(d) presents a further enlargement of the infiltrated zone in Fig. 4(a). As seen from Fig. 4(d), the worn brick is infiltrated by liquid slag up to its centre along the open pore network and the grain boundaries. Compared with the matrix of the interaction zone, the matrix in the infiltration zone is loose. The
white binding phase in the matrix are CaO·MgO·SiO2 and a small amount of 3CaO·MgO·2SiO2. Fig. 4(e) and Fig. 4(f) shows the microstructures of the original brick zone. Some flake structures has a network distribution in the matrix. In combination of the XRD results as well as some references [13, 14], the flake structure is confirmed as MgAlON. 3.3 Corrosion mechanism of Al-MgO-MgAl2O4 bricks 3.3.1 Phase relations Fig. 6 shows phase diagram of the CaO-MgO-SiO2 system [15]. According to the equilibrium relationship of coexistence, ternary and polynary mineral phases coexisting with MgO vary on the basis of the ratio of CaO/SiO2 in the system. From this, composite compounds coexisting with MgO components can be drawn, as shown in Table 6. In this work, the steel slag has a CaO/SiO2 mass ratio of 1.32. In Fig. 6, the liquid line in equilibrium with periclase at high temperatures is marked in red. So, when the Al-MgO-MgAl2O4 bricks contact with steel slag, the phase zones in equilibrium with MgO at high temperatures are 3CaO·MgO·2SiO2 (abbreviated as C3MS2) and CaO·MgO·SiO2 (abbreviated as CMS), which is consistent with the EDS results. Fig. 7 shows the phase diagram of the MgO-FeO-Fe2O3 system [16]. From Fig. 7, we can see that the phase diagram contains three important areas including the (MgO·FeO)ss region, the (MgO·FeO)ss and spinelss region and the spinelss region. The spinel region refers to (MgO·Fe2O3 + FeO·Fe2O3)ss. Oxygen partial pressure is an important variable for MgO-FeO-Fe2O3 system as well as temperature. The lower the
oxygen partial pressure is, the larger the (MgO·FeO)ss region is. In contrast, the higher the oxygen partial pressure is, the larger the (MgO·FeO)ss and spinelss region is. So when the temperature rises and oxygen partial pressure reduces, MgO·Fe2O3 gradually decomposes and the content of (MgO·FeO)ss increases. 3.3.2 Thermodynamic calculation Iron oxides are variable valence oxides. At high temperatures, the thermodynamic reactions involving iron oxides are as follows [17]: 2Fe 3 O 4 (s)=6FeO(l)+O 2 (g) ∆G1 =978228-458.97T +RTln
lg
PO P
2
θ
458.97
=
-
PO P
2
(1)
θ
978228
2.303R 2.303RT
2Fe 3O 4 (s)=6FeO(s)+O 2 (g)
∆G2 =624682-250.62T +RTln lg
PO
2
P
θ
250.62
=
-
PO P
2
θ
(2)
624682
2.303R 2.303RT
2FeO(l)=2Fe(l)+O 2 (g)
∆G3 =459400-87.45T +RTln lg
PO P
87.45
=
2
θ
-
PO P
(3)
2
θ
459400
2.303R 2.303RT
2FeO(l)=2Fe(s)+O 2 (g)
∆G4 =441410-77.82T +RTln lg
PO P
2
θ
=
77.82
-
PO P
2
θ
(4)
441410
2.303R 2.303RT
2FeO(s)=2Fe(s)+O 2 (g)
∆G5 =519230-125.10T +RTln lg
PO P
2
θ
=
125.10
-
PO
2
P
θ
(5)
519230
2.303R 2.303RT
Based on the above thermodynamic equations of FeO reactions, the stable region
containing FeO are plotted, as shown in Fig. 8. As can be seen from Fig. 8, the valence state of iron is related to the temperature and the partial pressure of oxygen. During refining and interval of the RH furnace, the oxygen partial pressure and the temperature change periodically, leading to periodic change of valence state of iron. During Vacuum treatment with oxygen blowing, the vacuum and the temperature are up to 13 KPa and 1923 K, respectively. lg
PO P
2
θ
= lg
13 101.325
=-0.89
Since the molten steel is circulating in the RH refining process, when the reaction: 2Fe (l) +O2 (g) =2FeO (l) reaches the equilibrium, the oxygen pressure in the vacuum chamber is the lowest. It can be seen from Fig. 7 that the oxygen pressure at Fe-FeO equilibrium at 1923K is as follows. lg .
PO2 Pθ
= − 7.8
During the refining interval, the temperature drops to around 1173 K. The air enters the furnace, the partial pressure of oxygen is the partial pressure of oxygen in the air. lg
PO
2
P
θ
=lg
0.21 × 101.325 101.325
= − 0.06
Combined with the above thermodynamic analysis, the transition region of iron oxides with the change of the temperature and the oxygen partial pressure during the refining process and refining interval is marked in red in Fig. 8. 3.3.3 Corrosion mechanisms
In the Al-MgO-MgAl2O4 system, the possible reactions of metal Al are as follows. 2Al(l)+1.5O2(g)=Al2O3(s)
(6)
2Al(l)+0.5O2(g)=Al2O(g)
(7)
Al2O(g)+O2(g)=Al2O3(s)
(8)
Al(l)+0.5N2(g)=AlN(s)
(9)
Al(g)+0.5N2(g)=AlN(s)
(10)
During the RH refining process, the Al-MgO-MgAl2O4 bricks contact with the molten steel. Under high temperature and vacuum conditions, Al is oxidized as reaction (6) and (7), which reduces the oxygen partial pressure in the brick. Then the molten steel and the CaO-SiO2-based slag infiltrate into the brick along the pores, cracks, or grain boundaries. On one hand, a minor amount of Fe in the steel is oxidized into FeO on the surface of bricks. Then FeO dissolves with MgO particles and fine powder and MgAl2O4, forming (Mg, Fe)O and (Mg, Fe)Al2O4 solid solution, respectively [18]. On the other hand, the slag reacts with MgO, forming CMS and a small amount of C3MS2 which are liquid at the refining temperature, distributing between particles and matrix and between matrix and matrix, resulting in the poor structure of the interaction zone and the infiltration zone of Al-MgO-MgAl2O4 brick. In the infiltration zone, as the temperature increases and the partial oxygen pressure decreases owing to the oxygen consumed according to the Al-O reaction, MgO decomposes into Mg(g): MgO( s) → Mg(g)+0.5O2 (g) . Continuous vacuuming will accelerate the reaction to the right. Mg(g) migrates outward to form a MgO and spinel dense layer in the interaction zone, preventing further penetration of molten steel and slag [19]. At the same time, some of the metal aluminum is nitrided into AlN which dissolves with MgAl2O4 forming MgAlON [20]and some of the metal aluminum is oxidized into Al2O(g) which diffuses along pores and gaps to the matrix and reacts
with N2, Mg(g) and O2 in the system generating flake MgAlON, as expressed: Al2O(g)+ O2(g)+ N2(g)+Mg(g) → MgAlON(s) [21]. Due to the poor wettability, MgAlON can effectively prevent the infiltration of CaO-SiO2-based slag. During the refining interval, as the temperature decreases and the partial oxygen pressure increases, the valence state of Fe changes. (MgO·Fe2O3)ss containing Al precipitates from (MgO·FeO)ss. The reaction equation is as follows. +2 → 3(MgO0.33 Fe0.67 MgO • Fe 2O3 ← )O − O2 + O2
（11）
In the circulating process of the above reaction (11), there will be a change in volume in the Al-MgO-MgAl2O4 brick [22], as shown in Fig. 9, resulting in structural damage to the interaction zone of the brick. The MgO granules and fine powders gradually dissolved into the infiltrating slag. Finally, under the combined action of valence changes of iron oxide and CaO-SiO2-based slag, the interaction zone of the brick is washed away into the liquid steel and a new interaction zone forms. The corrosion mechanism model is shown in Fig. 10. 4. Conclusions The corrosion behavior of Al-MgO-MgAl2O4 refractories by liquid rail steel was studied by analyzing the worn bricks from bottom zones in an RH refining furnace. The worn bricks were characterized and analyzed by XRD, SEM and EDS. The following conclusions can be drawn. (1) During vacuum treatment, with the increasing temperature and declining oxygen partial pressure, a minor amount of Fe in the steel was oxidized into FeO at
steel/refractory interface. (2) FeO dissolved with MgO particles and fine powder, forming (Mg, Fe)O solid solution. As the temperature decreases and the partial oxygen pressure increases, the Fe2+ was oxidized into Fe3+ and MgO·Fe2O3 solid solution separated from (Mg, Fe)O solid solution. In the process of circulating between solid solution and desolution, the refractories were structurally weakened. (3) The CaO-SiO-based slag infiltrated along the open-pores and the grain boundaries into the Al-MgO-MgAl2O4 refractories, forming CMS and C3MS2 phases which were liquid at refining temperature, leading to poor binding ability of refractories. (4) Under the combined action of valence changes of iron oxide and CaO-SiO2-based slag, When the MgO dense layer structure is completely destroyed, the interaction zone of the refractories were washed away into the liquid steel. (5) In the Al-MgO-MgAl2O4 system, the formation of dense layer and MgAlON can effectively prevent the infiltration of CaO-SiO2-based slag and molten steel due to the poor wettability. Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Conflict of Interest: The authors declare that they have no conflict of interest. References [1] B. Liu, G. Zhu, H. Li, B. Li, Y. Cui, A. Cui, Decarburization rate of RH refining
for ultra low carbon steel, Int. J. Min. Met. Mater. 17(2010) 22-27. [2] V. Petkov, P.T. Jones, E. Boydens, B. Blanpain, P. Wollants, Chemical corrosion mechanisms of magnesia–chromite and chrome-free refractory bricks by copper metal and anode slag, J. Eur. Ceram. Soc. 27(2007) 2433-2444. [3] K. Gotod, W.E. Lee, The “Direct bond” in magnesia chromite and magnesia spinel refractories, J. Am. Ceram. Soc. 78(1995) 1753-1760. [4] I. Ganesh, R. Johnson, G.V.N. Rao, Y.R. Mahajan, S.S. Madavendra, B.M. Reddy, Microwave-assisted combustion synthesis of nanocrystalline MgAl2O4 spinel powder, Ceram. Int. 31(2005) 67-74. [5] C. Păcurariu, I. Lazău, Z. Ecsedi, P. Lazău. Barvinschi, G. Mărgineanc, New synthesis methods of MgAl2O4 spinel, J. Eur. Ceram. Soc. 27(2007) 707-710. [6] R.K. Pati, P. Pramanik, Low-Temperature Chemical Synthesis of Nanocrystalline MgAl2O4 Spinel Powder, J. Am. Ceram. Soc. 83(2000) 1822-1824. [7] D. Yang, H. Zhang, X. Zhong, Study on preparation and mechanical property of MgO-MgAl2O4-MgAlON composite, J. Naihuo Cailiao/Refract., 40(2006) 12-15. [8] S. Pichlbauer, H. Harmuth, Z. Lenčéš, P. Šajgalík, Preliminary investigations of the production of MgAlON bonded refractories, J. Ceram. Soc. 32(2012) 2013-2018. [9] S. Bandyopadhyay, G. Rixecker, F. Aldinger, H. S. Maiti, Effect of controlling parameters on the reaction sintering sequences of formation of nitrogencontaining magnesium aluminate spinel from MgO, Al2O3 and AlN, J. Am. Ceram. Soc. 87(2004) 480–482.
[10] H.X. Willems, G.D. With, R. Metselaar, Thermodynamics of Alon III: stabilization of Alon with MgO, J. Eur. Ceram. Soc. 12(1993) 43-49. [11] L. Chen, A. Malfliet, P.T. Jones, B. Blanpain, M. Guo, Comparison of the chemical corrosion resistance of magnesia-based refractories by stainless steelmaking slags under vacuum conditions, Ceram. Int. 42(2016) 743-751. [12] K. Goto, B.B. Argent, W.E. Lee, Corrosion of MgO-MgAl2O4 spinel refractory bricks by calcium aluminosilicate slag, J. Am. Ceram. Soc. 80(1997) 461-471. [13] A. Granon, P. Goeuriot, F. Thevenot, J. Guyader, P. L'Haridon, Y. Laurent, Reactivity in the Al2O3-AlN-MgO system. The MgAlON spinel phase, J. Eur. Ceram. Soc. 13(1994) 365-370. [14] Z. Zhang, L. Teng, W. Li, Mechanical properties and microstructures of hot-pressed MgAlON–BN composites, J. Eur. Ceram. Soc. 27(2007) 319-326. [15] W.M. Huang, M. Hillert, X.Z. Wang, Thermodynamic assessment of the CaO-MgO-SiO2 system, Metall. Mater. Trans. A. 26A (1995) 2293-2310. [16] J.C. Willshee, J. White, An investigation of equilibrium relationships in system MgO-FeO-Fe2O3 up to 1750 degrees in air, Trans. Br. Ceram. Soc. 66(1967) 541-555 [17] Z.Y. Chen, Chemical Thermodynamics of Refractories, First ed., Metallurgical industry press, Beijing, 2005. [18] G.W. Groves, M. E. Fine, Solid solution and precipitation hardening in Mg-Fe-O alloys, J. Appl. Phy. 35(1964) 3587-3593. [19] M. Yan, Y. Li, L. Li, Y. Sun, S. Tong, J. Sun, In-situ synthesis and reaction
mechanism of MgAlON in Al2O3-MgO composites produced in flowing nitrogen, Ceram. Int. 43(2017) 14791-14797. [20] S. Tong, Y. Li, M. Yan, P. Jiang, J. Ma, D. Yue, In situ reaction mechanism of MgAlON in Al-Al2O3-MgO composites at 1700°C under flowing N2, Int. J. Min. Metall. Mater. 24(2017) 1061-1066. [21] M. Yan, Y. Li, H. Li, Y. Sun, H. Chen, C. Ma, J. Sun, Evolution mechanism of MgAlON in the Al-Al2O3–MgO composite at 1800 °C in flowing nitrogen, Ceram. Int. 44(2018) 3856-3861. [22] Y.F. Sun, X.M. Wang, C.X. Wang, P.Q. Sun, Magnesia and Magnesia Based Composite Refractories, First ed., Metallurgical industry press, Beijing, 2010.
Caption of tables
Table 1. Formulations of Al-MgO-MgAl2O4 bricks (wt)/% Table 2. Chemical composition of raw materials Table 3 Refining conditions of rail steel Table 4 Composition of the rail steel during the refining process ( in wt% ) Table 5. EDS results of areas marked in Fig. 4 Table 6 Composite compounds coexisted with MgO
Table 1. Formulations of Al-MgO-MgAl2O4 bricks (wt)/% Raw material
Fused magnesia particles Fused magnesia powder Fused spinel Metal aluminum powder Sintered high purity magnesia powder
Particle size
Specimen ratio
5-3mm 3-1mm 1-0mm 200mesh 325mesh 200mesh
15 35 22 12 5 6
200mesh
5
Table 2. Chemical composition of raw materials Chemical composition /wt% Raw materials MgO Fused magnesia
Al2O3
97.5
Metal aluminum
magnesia Fused spinel
SiO2
CaO
0.63
0.76
0.5
1.34
Fe2O3
K2O
Na2O
0.13
0.01
0.14
99.3
powder Sintered high purity
Al
97.8 23.57
75.16
0.10
Table 3 Refining conditions of rail steel Smelting parameter
Steel grade Rail steel
Temperature (°C)
Time (min)
Vacuum degree(Pa)
1580-1650
30
50
Table 4 Composition of the rail steel during the refining process (in wt%) SiO2
Al2O3
CaO
MgO
Cr2O3
MnO
Total Fe
Basicity (C/S)
35.85
3.2
47.32
7.2
1.9
1.1
0.9
1.32
Table 5. EDS results of areas marked in Fig. 4 Atomic percentage /%
Marked areas
Mg
1
12
2
50
3
48
2
50
4
46
8
46
5
44
11
45
6
43
1
16
40
7
14
2
35
49
8
13
2
27
58
9
13
14
14
59
10
11
35
17
37
11
16
12
54
13
15
32
52
1
14
6
42
27
25
15
9
39
27
25
Al
Ca
Si
14
13
Fe
O
N
61 50
31
53 46
Table 6 Composite compounds coexisted with MgO CaO/SiO2,
ratio value
Mole ratio,%
0
0-1.0
1.0
1.0-1.5
1.5
1.5-2.0
2.0
Mass ratio,%
0
0-0.93
0.93
0.93-1.4
1.4
1.4-1.87
1.87
Mineral phase
M2S
M2S-CMS
CMS
CMS-C3MS2
C3MS2
C3MS2-C2S
C 2S
Caption of figures
Fig.1 Schematic drawing of the bottom masonry of RH refining furnace Fig.2 Images of the worn brick after used (a) macroscopical morphology and (b) horizontal section Fig.3 XRD patterns of the Al-MgO-MgAl2O4 worn brick Fig.4 SEM images of Al-MgO-MgAl2O4 worn brick Fig.5 Scanning results of Fig. 4(b) Fig. 6 Phase diagram of CaO-MgO-SiO2 system Fig. 7 Phase diagram of MgO-FeO-Fe2O3 system Fig. 8 lg
PO2 Pθ
~T curves of Fe-O system
Fig. 9 Volume change caused by change in iron's valence state Fig. 10 Corrosion mechanism model of Al-MgO-MgAl2O4 refractories
Fig.1 Schematic drawing of the bottom masonry of RH refining furnace
Fig.2 Images of the worn brick after used (a) macroscopical morphology and (b) horizontal section
Fig.3 XRD patterns of the Al-MgO-MgAl2O4 worn brick
Fig.4 SEM images of Al-MgO-MgAl2O4 worn brick
Fig.5 Scanning results of Fig. 4(b)
Fig. 6 Phase diagram of CaO-MgO-SiO2 system [15] (Lime = CaO; Per = periclase; Hat = hatrurite; Ra = rankinite; Psw = pseudowollastonite; Wo = wollastonite; Ak = akermanite (MgCa2Si2O7); Mer = merwinite; Mtc = monticelite; Dsp = diaspore; Pgt = pigeonite; Opx = orthopyroxene; Ppx = protopyroxene; Fo = forsterite; Trd = tridymite; Crs = cristobalite.)
Fig. 7 Phase diagram of MgO-FeO-Fe2O3 system [16]
Fig. 8 lg
PO2 Pθ
~T curves of Fe-O system
Fig. 9 Volume change caused by change in iron's valence state [22]
Fig. 10 Corrosion mechanism model of Al-MgO-MgAl2O4 refractories
Declaration of Interest Statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Corrosion mechanisms of Al-MgO-MgAl2O4 refractories in RH refining furnace during production of rail steel”