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Corrosion mechanism of Mo-W-ZrO2 cermet in molten steel
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Corrosion mechanism of Mo-W-ZrO2 cermet in molten steel ⁎
Jiu Zhang , Guohui Mei, Zhi Xie, Shumao Zhao College of Information Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, China
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Sintering B. Microstructure-final C. Corrosion D. ZrO2, Mo, W
Mo-W-ZrO2 cermet has potential application in metallurgical industry. To optimize its corrosion resistance, the corrosion mechanism of Mo-35W-35ZrO2 cermet in molten steel has been investigated. Results showed that the corrosion resistance of the cermet mainly depended on the corrosion of its ceramic phase (ZrO2). The ceramic phase formed a ZrO2 layer (thickness: about 75 µm) on the cermet surface during the corrosion, while the metal phase (Mo-W) was corroded firstly. It was attributed to that the ceramic phase was corroded slowly by physical erosion of molten steel while the corrosion of the metal phase caused by its melting in molten steel was much faster. Meanwhile, the ZrO2 layer prevented molten steel from penetrating into cermet, lowering the melting corrosion rate of the metal phase to the physical erosion rate of the ceramic phase. As a result, the corrosion of the cermet reached dynamic equilibrium, which was corresponded to a small corrosion rate of about 0.05 mm/h. Therefore, the ZrO2 layer formation was the key to improve corrosion resistance, which might have important implication for solving the corrosion resistance problem of the similar cermet.
1. Introduction The materials for the metallurgical applications (such as nozzle, stopper and molten steel temperature sensor) have to solve the two key problems of thermal shock and corrosion. The severe thermal shock involves to a large temperature difference up to about 1500 °C in a short time. And the strong corrosion of molten steel involves to a long period of about 20 h. Generally, oxide-carbon refractories (such as Al2O3-C, MgO-C, ZrO2-C, etc.) are used for these applications [1–4]. However, all of them almost have low strength and high porosity, restraining the performances of these applications. For instance, the molten steel temperature sensor has to be made into a thick-walled structure with a thickness of about 20–30 mm, to satisfy the demand of structural strength and service life of corrosion resistance. As a result, the heat transfer of the sensor is slow and its temperature response time is long of about 5–6 min [4]. To make the molten steel temperature sensor with a thin-walled structure for a fast temperature response, the metal-oxide cermet that has much higher strength and lower porosity has been proposed. Addition of the ductility metal phase improves thermal shock resistance of the cermet significantly (e. g.: ZrO2-Nb [5]), which has a better toughening effect than the addition of the brittle phase (e. g.: BN-ZrO2SiC [6]). As the thermal shock fracture occurs, the metal phase can bridge crack face of the brittle ceramic phase, preventing the crack from propagating. Meanwhile, the plastic deformation of metal phase that can consume the fracture energy occurs, also preventing the crack from ⁎
propagating [7–10]. Thus, the cermet has the potential to satisfy the demand of thermal shock resistance for the metallurgical applications [11,12]. Therefore, the Mo-35W-35ZrO2 cermet that has satisfied the demand of thermal shock resistance has been developed. The Mo and W are used to improve the high temperature strength of the metal phase. The ZrO2 that has relatively low Young's modulus and coefficient of linear expansion is also benefit to improve thermal shock resistance. In addition to the thermal shock resistance, the service life of corrosion resistance is another important performance for the metallurgical applications. Corrosion of the cermet is governed by the special features of the metal and ceramic phases. The corrosion of the metal (Mo, W) in molten steel is fast, resulting from the chemical co-melting reaction between the metal and Fe. On the contrary, the corrosion of the ceramic (ZrO2) is much slower, due to the fact that the ZrO2 cannot be wetted and dissolved by molten steel [6,13,14]. Its corrosion is mainly caused by the physical erosion of the flow of molten steel. As the corrosion of the cermet is similar to that of the metal, the cermet will have a short service life. To obtain good corrosion resistance, the corrosion of the cermet should be closer to that of the ZrO2, which has been investigated in this work. The aim of the present work is to clarify the corrosion mechanism of the Mo-W-ZrO2 cermet, and then to obtain implications for optimizing the corrosion resistance. This investigation studies the microstructure evolution and composition variation of the cermet during the corrosion. They are checked by the scanning electron microscopic (SEM), energy
Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected] (G. Mei), [email protected] (Z. Xie), [email protected] (S. Zhao).
https://doi.org/10.1016/j.ceramint.2018.02.076 Received 1 January 2018; Received in revised form 7 February 2018; Accepted 8 February 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhang, J., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.02.076
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disperse spectroscopy (EDS), X-ray diffraction (XRD) and phase diagram analysis, respectively. In addition, some suggestions for improving the corrosion resistance are given. 2. Experimental procedure 2.1. Materials Mo-W-ZrO2 cermet was prepared by the powder metallurgy method. Its compositions were 30 wt% Mo, 35 wt% W, and 35 wt% ZrO2. Raw materials included the following powders: (1) calcium stabilized zirconia (purity: 98%, Astron Ltd, Shenyang, China) with an average particle size d50 = 45 µm. It was synthesized through the electrofusion method for cost reduction. (2) 3Y-ZrO2 (partially stabilized zirconia, 10 wt%, purity: 99.7%, specific surface area: 10 m2/g, L. Feng Ltd, Jiaozuo, China) with an average particle size d50 = 50 nm. It was synthesized through the chemical doping method for improving the thermal shock resistance based on the phase transformation toughening [15]. (3) Molybdenum (purity: 99.5%, Cemented Carbide Corp., Ltd, Chengdu, China) with an average particle size d50 = 3–4 µm; (4) tungsten (purity: > 99.9%, Cemented Carbide Corp., Ltd, Chengdu, China) with an average particle size d50 = 2.5–3 µm. The cermet sample was a tube with a hemisphere bottom (Diameter × Thickness × Length: Φ22 × 3 × 100 mm3). Its manufacture processes were as follows (see Fig. 1): (1) the initial powders of raw materials were ball-milled in air atmosphere for 48 h with WC grinders; (2) the powders were molded into a tube by the cold-isostatic pressing (CIP) at 180 MPa; (3) the tube was sintered in the hydrogen atmosphere (purity: 99.99%, flux: 50 ml/min, pressure: 0.2 MPa) at 1800 °C for 1 h. And then, the properties of the cermet that might be related to the corrosion were checked. Its flexural strength that was measured by the three-point loading bending test method (sample size: 5 × 5 × 25 mm3, crosshead speed: 0.5 mm/min, span length: 20 mm) was 245.3 MPa. And its relative density dr that was measured by the following equation was 93.2%.
dr =
dm × 100% dt
Fig. 2. Schematic diagram of the corrosion test device: h = 200 mm, d = 250 mm, corrosion time: 20 h, molten steel temperature: 1520 °C.
have a thick-walled structure with a thickness of 20–30 mm while the new sensor (cermet tube) could be made into a much thinner structure with a thickness of about 3 mm. The corrosive environment was as follows: (1) chemical compositions of the molten steel for the test are given in Table 1; (2) the testing temperature of molten steel was about 1520 °C; (3) the testing time was 20 h that was consistent with the practical demand (a regular casting cycle); (4) the flow state of molten steel around the cermet tube was indirectly illustrated by the position (h = 200 mm, d = 250 mm) of the new temperature sensor. 2.3. Microstructure and composition characterization After the cermet was dipped into molten steel, complexly physical and chemical reactions occurred between the cermet and molten steel. They involved to the chemical co-melting reaction between the metal phase and molten steel and the physical erosion caused by the flow of molten steel. The microstructure evolution and composite variation that were caused by the above physical and chemical reactions were checked by the SEM, EDS (SSX-550) and XRD (PW3040/60), respectively, which directly reflected the corrosion process.
(1)
dm was the measured density that was measured by the Archimedes method in distilled water (sample size: 5 × 5 × 5 mm3); dt was the true density that was measured by the pycnometry method. 2.2. Corrosion test In order to reflect the actual corrosive conditions of the industrial application, the corrosion of the cermet was tested in the industrial field (Nanjing Steel Company, China). Fig. 2 shows the schematic diagram of the corrosion test device. The cermet tube that was used as the temperature measuring unit was integrated with the bottom of the molten steel temperature sensor. The support unit for the cermet tube was made of Al2O3-C refractory that was used for the traditional sensor. With the limitation of the sensor material, the traditional sensor had to
3. Results and discussion 3.1. Cermet characterization Composites of the Mo-35W-35ZrO2 cermet were illustrated by the XRD patterns (see Fig. 3). Remarkable reflection peaks of ZrO2, W and Mo were observed. The ZrO2 involves to the cubic-phase zirconia, tetragonal-phase zirconia, and monoclinic-phase zirconia. The reflection peaks of monoclinic-phase zirconia seem more obvious, which may be corresponded to that the electrofusion method is not good enough for stabilizing the monoclinic-phase zirconia. The reflection peaks of W and Mo are near, attributing to their same body-centered cubic (BCC) structure and similar lattice parameters. These near reflection peaks may also be associated with the solid solution formation of the metal Table 1 Chemical compositions of the molten steel (major only).
Fig. 1. Manufacture processes of the cermet.
2
Composition
C
Si
Mn
Cr
Ni
P
S
Fe
wt%
0.39
0.29
0.66
0.95
0.19
0.016
0.027
Bal.
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structure and uniformly distributed in the cermet, playing an important role in preventing crack from propagating. On the one hand, it establishes a semi-continuous network for bridging the crack. On the other hand, as the fracture occurs, the metal phase has a plastic deformation and consumes the fracture energy, which has been verified by Sbaizero et al. [10]. These can seal the propagating of the crack in the metal phase network as soon as possible and improve the thermal shock resistance significantly. The composition and microstructure are the basic reasons for the good thermal shock resistance of the cermet, also governing the corrosion resistance.
3.2. Microstructure evolution Fig. 5(a) shows the industrial corrosion test of the cermet tube that is located at the bottom of the new molten steel temperature sensor. After being tested in molten steel for 20 h, the thickness of the cermet was corroded by only about 1 mm (3 mm → 2 mm), which is illustrated in Fig. 5(b). It implies that the cermet tube has a low corrosion rate of about 0.05 mm/h, leading to that the new molten steel temperature sensor can be made into a thin-walled structure with a thickness of about 3 mm while the thickness of the traditional sensor is about 20–30 mm. Meanwhile, Fig. 5(a) also illustrates that the cermet tube had not thermal shock damage after it was pull out from the molten steel, verifying its good thermal shock resistance. Besides, it can also be found that the cermet tube was much darker than the Al2O3-C support unit after they were pulled out from the molten steel for a while, which is consistent with that the heat transfer of the cermet tube is much faster than that of the Al2O3-C support unit. The corrosion of the cermet tube may be in dynamic equilibrium, attributing to that it has been corroded for a long term. As a result, the microstructure of the corroded cermet may also have reached dynamic equilibrium. The microstructure evolution is used to investigate the corrosion process of the cermet intuitively. The cross-sectional microstructure of the corroded cermet tube is presented in Fig. 6(a). It can be observed that a ZrO2 skeleton layer (Z) with a thickness of about 75 µm formed on the corroded cermet surface after the melting of the metal phase, indicating that the corrosion rate of the ZrO2 layer is much slower than that of the metal phase (Mo-W). The black area in this ZrO2 layer was slag (S) and its elements (checked by EDS) included 34.4 wt% O, 24.5 wt% Al, 17.6 wt% Si, 16.4 wt% Ca, 4.7 wt% Mn and 2.1 wt% Mg. The slag area originally belongs to molten steel. It is formed through the following processes. As the cermet tube is pulled out from the slag layer (see Fig. 2), the molten steel flows out from the ZrO2 layer. Then, the molten slag adheres on the cermet tube surface and penetrates into the ZrO2 layer.
Fig. 3. X-ray diffraction patterns of cermet (c = cubic-phase zirconia, t = tetragonalphase zirconia, and m = monoclinic-phase zirconia).
Fig. 4. Microstructure of the cermet (SEM): ceramic phase (C), metal phase (M) and pore (P).
phases (Mo and W). In addition, no other obvious reflection peck is observed, implying that there is a good chemical stability between the ZrO2 and metal phase. Microstructure of the cermet is illustrated in Fig. 4, including ceramic phase (C), metal phase (M) and pore (P). The ceramic phase (ZrO2) was the matrix, which is benefit to improve the corrosion resistance. The size of the metal phase (Mo and W) becomes much larger during the sintering while the original average sizes of the metal particles are small. The metal phase has formed a semi-continuous
Fig. 5. (a) The industrial corrosion test of the cermet tube; (b) the thickness variation of the cermet tube after being corroded for 20 h in molten steel.
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Fig. 6. (a) Cross-sectional microstructure of the corroded cermet (SEM): ZrO2 layer (Z), ceramic phase (C), metal phase (M), slag (S) and pore (P); (b) Fe distribution on AB: deteriorated layer (D).
Fig. 7. Microstructure (SEM) and Zr distribution (EDS) of the corroded cermet: slag (S), metal phase (M) and ceramic phase (C).
These reactions result in that the metal phase transforms into the solid solution before its melting. To investigate the deteriorating of the metal phase, the Fe content variation from the surface to substrate was examined by the EDS (see Fig. 6(b)). It can be found that the Fe content variation has the following characteristics: (1) in the distance range from 0 to 80 µm, there was a low content of the Fe, corresponding to the ZrO2 layer (Z) area. It is attributed to that the molten steel has flowed out from the ZrO2 layer after the cermet is pulled out form molten steel. (2) In the distance range from 80 to 320 µm, a high content of the Fe existed in this area, corresponding to the deteriorated region with a thickness of about 240 µm. It is associated with that the deteriorating reactions (5)–(7) have occurred. (3) Above 320 µm, the Fe content almost disappeared, implying that the diffusion of the Fe has decreased significantly.
The corrosion of the ceramic phase (ZrO2) is mainly resulted from the physical erosion of the flow of molten steel, associating with that the ZrO2 cannot be wetted and dissolved by molten steel [6,13,14]. As a result, the ZrO2 has a small corrosion rate. However, the corrosion of the metal phase (Mo-W) is much faster, attributing to that the metal phase is mainly corroded by the following one or more chemical comelting reactions. Mo (s) + Fe (l) = Mo-Fe (l)
(2)
W (s) + Fe (l) = W-Fe (l)
(3)
Mo-W (s) + Fe (l) = Mo-W-Fe (l)
(4)
Thus, the ZrO2 layer forms and increases to a certain thickness of about 75 µm gradually. Meanwhile, the ZrO2 layer prevents molten steel from further penetrating into the cermet and slows down the comelting reaction of the metal phase. As the co-melting corrosion rate of the metal phase decreases to the physical erosion rate of the ZrO2 layer, the corrosion of the cermet reaches dynamic equilibrium. Beneath the ZrO2 layer, the deteriorated layer forms, attributing to the interaction between the metal phase (Mo and W) and Fe in molten steel through the following one or more reactions. Mo (s) + Fe (l) = Mo-Fe (s)
(5)
W (s) + Fe (l) = W-Fe (s)
(6)
Mo-W (s) + Fe (l) = Mo-W-Fe (s)
(7)
3.3. Composition variation To further clarify the corrosion mechanism, the composition variation also has been investigated. The cross-sectional component distributions of the ceramic and metal phases in the corroded cermet have been investigated respectively. Microstructure and Zr distribution are presented in Fig. 7, representing the ceramic phase distribution. It indicated that the ZrO2 distribution was continuous and uniform, meaning a good microstructure of the matrix. In addition, the thickness of the ZrO2 layer (Z) was about 78 µm on the surface of the corroded cermet, which is agreement with the result as illustrated in Fig. 6(b). The small difference of the thicknesses mainly comes from the material 4
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Fig. 8. Microstructure (SEM) and Fe, Mo and W distributions (EDS) of the corroded cermet: metal phase (M), slag (S), ceramic phase (C) and deteriorated layer (D).
Besides, the distributions of Mo and W were coincident as shown in Fig. 8(c)–(d), implying that the metal powders disperse uniformly and have a good sintering performance. The solid-liquid transforming process of the metal phase are analyed through the phase diagrams. According to the phase diagram of Fe-Mo [16–18], at the testing temperature (1520 °C) of molten steel, the comelting reaction (2) of Mo starts to occur as its content is less than about 59.2 wt%. As the Mo content deceases to below about 43.2 wt%, it converts into liquid completely. As the Mo content is more than 59.2 wt%, the Mo and Fe form solid solution that causes the deteriorated layer formation through the reaction (5). Besides, with the increase of the molten steel temperature, the Fe content that is needed for the solid-liquid transforming decreases. The lowest temperature of the solid-liquid transformation of Mo is 1449 °C that is significantly lower than the molten steel temperature in tundish (about 1470–1560 °C). That is to say, the Mo is easily corroded by molten steel. According to the phase diagram of Fe-W [19,20], the co-melting reaction between the W and any content of the Fe cannot occur at 1520 °C. Until the molten steel temperature increases to 1529 °C and the W content is 13.2 wt%, there is a solid-liquid transformation. However, the W is corroded at about 1520 °C in practice. This temperature is lower than the lowest temperature (1529 °C) of the solid-liquid transformation. There may be two reasons for this phenomenon: (1) Mo and W have formed the solid solution, lowering the temperature of the solid-liquid transformation; (2) other components of the molten steel may lower the solid-liquid transforming temperature. These are the basic reasons for the metal phase corrosion.
defects, such as pores and composition segregation. The ZrO2 layer that governs the corrosion resistance of the cermet is the key to improve the corrosion resistance. The factors that govern the ZrO2 layer formation mainly involve ZrO2 content, original raw material powder size, sintering temperature and so on. The ZrO2 content is the critical factor, more ZrO2 content leads to easier formation of the ZrO2 layer. However, in order to satisfy the demand of the thermal shock resistance, the ZrO2 content has to be limited. The least ZrO2 content of the cermet should assure that a ZrO2 layer can form during the corrosion. As the ZrO2 layer forms, the corrosion of the cermet is mainly controlled by the physical erosion of the ZrO2 layer, meaning a low corrosion rate. Otherwise, the corrosion rate mainly depends on the chemical co-melting reaction of the metal phase, meaning a fast corrosion rate and a short service life. Fig. 8 illustrates the microstructure and component distributions of Fe, Mo and W, reflecting the corrosion process of the metal phases (Mo and W). The co-melting reaction between the metal phase and Fe in molten steel is one of the root corrosion types. Actually, the oxygen, carbon and other components of molten steel may also affect the corrosion. Due to their low contents, this work does not discuss their influences. Beneath the ZrO2 layer, the interaction between the metal phases (Mo and W) and Fe forms solid solution that results in the deteriorated layer (D) formation through these reactions (5)–(7). The solid solution formation leads to that the metal phase area has enlarged (see Fig. 8(a)). As the Fe content increases to a certain value, the metal phase transforms from the solid into liquid through these reactions (2)–(4), resulting in that the ZrO2 layer forms. Meanwhile, it can be found that the Fe content was higher near to the ZrO2 layer (see Fig. 8(b)), which is agreement with the result as illustrated in Fig. 6(b). 5
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Fig. 10. Variation of the metal phase's co-melting corrosion rate (Vm) and the ZrO2 layer's physical erosion rate (Ve) during the corrosion process.
erosion rate of ZrO2 layer (Ve) are represented in Fig. 10. In brief, the physical erosion of the ZrO2 layer governs the corrosion of the cermet. To improve the corrosion resistance, the network of the ZrO2 phase in the cermet is needed, mainly depending on its volume content. The least ZrO2 content should assure that the ZrO2 network can form in the cermet. Only under this situation, the ZrO2 layer can form during the corrosion, which is the key factor for good corrosion resistance. The ZrO2 network formation is similar to a percolation phenomenon [21,22]. The least ZrO2 content can be seen as the percolation threshold. (1) At the ZrO2 content below the percolation threshold, there is not continuous contact between ZrO2 particles, meaning that the percolation path cannot form. The ZrO2 phase is discrete and cannot form a ZrO2 layer. Without its barrier protection, the melting corrosion of the metal phase will keep a constant fast rate during the whole corrosion process. What's worse, the discrete ZrO2 phase may accelerate the corrosion. It is easily shed from the metal matrix and increases the interaction interface between the metal phase and molten steel. (2) At the ZrO2 content above the percolation threshold, continuous contact of ZrO2 particles forms percolation paths that establish the network of the ZrO2 phase. As a result, the ZrO2 layer can form. Under this situation, the cermet exhibits good corrosion resistance which mainly comes from the barrier protection of the ZrO2 layer.
Fig. 9. Schematic diagram of the corrosion process: metal phase (M), ceramic matrix (C), deteriorated layer (D), ZrO2 layer (Z) and liquid steel (L).
3.4. Corrosion mechanism
4. Conclusions
On the basis of the above investigations of the microstructure evolution and composition variation, the microstructure characteristics of different corrosion stages are qualitatively described in Fig. 9. They are as follows: (1) t = t0, initial period. As the cermet tube is dipped into molten steel, a solid solution reaction between the metal phase (Mo-W) and Fe occurs on the cermet surface, leading to the deteriorating of the metal phase and the formation of the deteriorated layer (D). (2) t = t1, intermediate period. As the Fe content in the solid solutions increases to a certain value, the deteriorated metal phase melts. And then, a ZrO2 layer (Z) forms on the corroded cermet surface. It is mainly corroded by the physical erosion that is much slower than the melting corrosion of the metal phase. Meanwhile, the ZrO2 layer prevents molten steel from further penetrating into the cermet and slows down the metal phase melting. The initial period and intermediate period belong to an unstable period that may be short. (3) t = t2, stable period. The melting corrosion rate of the metal phase decreases to the physical erosion rate of the ZrO2 layer, the corrosion of the cermet reaches dynamic equilibrium, the thicknesses of the ZrO2 layer and deteriorated layer increase to almost constant values. During the above processes, the variations of the co-melting corrosion rate of metal phase (Vm) and the physical
The corrosion mechanism of the Mo-35W-35ZrO2 cermet in molten steel has been investigated. Its corrosion processes were as follows: (1) Unstable period. At beginning, a solid solution reaction between the metal phase (Mo-W) and Fe occurred on the cermet surface, leading to the deteriorating of the metal phase. As the Fe content increased up to a certain value, the deteriorated metal phase melted, leading to that a ZrO2 layer formed on the cermet surface. The melting corrosion of the metal phase was much faster than the corrosion of the ZrO2 layer that was mainly caused by the physical erosion of the flow of molten steel. Meanwhile, the ZrO2 layer prevented molten steel from further penetrating into the cermet, slowing down the melting corrosion of the metal phase. (2) Stable period. As the thickness of the ZrO2 layer increased to about 75 µm, the melting corrosion rate of the metal phase was lowered to the physical erosion rate of the ZrO2 layer. As a result, the corrosion of the cermet reached dynamic equilibrium, corresponding to a small corrosion rate of about 0.05 mm/h and a good corrosion resistance. 6
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In brief, the ZrO2 layer formation was the key to improve corrosion resistance. The least ZrO2 content of the cermet should assure that a ZrO2 layer could form during the corrosion, which might have important implication for solving the corrosion resistance problem of the similar cermet.
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Corrosion mechanism of Mo-W-ZrO2 cermet in molten steel ⁎
Jiu Zhang , Guohui Mei, Zhi Xie, Shumao Zhao College of Information Science and Engineering, Northeastern University, Shenyang, Liaoning 110819, China
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Sintering B. Microstructure-final C. Corrosion D. ZrO2, Mo, W
Mo-W-ZrO2 cermet has potential application in metallurgical industry. To optimize its corrosion resistance, the corrosion mechanism of Mo-35W-35ZrO2 cermet in molten steel has been investigated. Results showed that the corrosion resistance of the cermet mainly depended on the corrosion of its ceramic phase (ZrO2). The ceramic phase formed a ZrO2 layer (thickness: about 75 µm) on the cermet surface during the corrosion, while the metal phase (Mo-W) was corroded firstly. It was attributed to that the ceramic phase was corroded slowly by physical erosion of molten steel while the corrosion of the metal phase caused by its melting in molten steel was much faster. Meanwhile, the ZrO2 layer prevented molten steel from penetrating into cermet, lowering the melting corrosion rate of the metal phase to the physical erosion rate of the ceramic phase. As a result, the corrosion of the cermet reached dynamic equilibrium, which was corresponded to a small corrosion rate of about 0.05 mm/h. Therefore, the ZrO2 layer formation was the key to improve corrosion resistance, which might have important implication for solving the corrosion resistance problem of the similar cermet.
1. Introduction The materials for the metallurgical applications (such as nozzle, stopper and molten steel temperature sensor) have to solve the two key problems of thermal shock and corrosion. The severe thermal shock involves to a large temperature difference up to about 1500 °C in a short time. And the strong corrosion of molten steel involves to a long period of about 20 h. Generally, oxide-carbon refractories (such as Al2O3-C, MgO-C, ZrO2-C, etc.) are used for these applications [1–4]. However, all of them almost have low strength and high porosity, restraining the performances of these applications. For instance, the molten steel temperature sensor has to be made into a thick-walled structure with a thickness of about 20–30 mm, to satisfy the demand of structural strength and service life of corrosion resistance. As a result, the heat transfer of the sensor is slow and its temperature response time is long of about 5–6 min [4]. To make the molten steel temperature sensor with a thin-walled structure for a fast temperature response, the metal-oxide cermet that has much higher strength and lower porosity has been proposed. Addition of the ductility metal phase improves thermal shock resistance of the cermet significantly (e. g.: ZrO2-Nb [5]), which has a better toughening effect than the addition of the brittle phase (e. g.: BN-ZrO2SiC [6]). As the thermal shock fracture occurs, the metal phase can bridge crack face of the brittle ceramic phase, preventing the crack from propagating. Meanwhile, the plastic deformation of metal phase that can consume the fracture energy occurs, also preventing the crack from ⁎
propagating [7–10]. Thus, the cermet has the potential to satisfy the demand of thermal shock resistance for the metallurgical applications [11,12]. Therefore, the Mo-35W-35ZrO2 cermet that has satisfied the demand of thermal shock resistance has been developed. The Mo and W are used to improve the high temperature strength of the metal phase. The ZrO2 that has relatively low Young's modulus and coefficient of linear expansion is also benefit to improve thermal shock resistance. In addition to the thermal shock resistance, the service life of corrosion resistance is another important performance for the metallurgical applications. Corrosion of the cermet is governed by the special features of the metal and ceramic phases. The corrosion of the metal (Mo, W) in molten steel is fast, resulting from the chemical co-melting reaction between the metal and Fe. On the contrary, the corrosion of the ceramic (ZrO2) is much slower, due to the fact that the ZrO2 cannot be wetted and dissolved by molten steel [6,13,14]. Its corrosion is mainly caused by the physical erosion of the flow of molten steel. As the corrosion of the cermet is similar to that of the metal, the cermet will have a short service life. To obtain good corrosion resistance, the corrosion of the cermet should be closer to that of the ZrO2, which has been investigated in this work. The aim of the present work is to clarify the corrosion mechanism of the Mo-W-ZrO2 cermet, and then to obtain implications for optimizing the corrosion resistance. This investigation studies the microstructure evolution and composition variation of the cermet during the corrosion. They are checked by the scanning electron microscopic (SEM), energy
Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected] (G. Mei), [email protected] (Z. Xie), [email protected] (S. Zhao).
https://doi.org/10.1016/j.ceramint.2018.02.076 Received 1 January 2018; Received in revised form 7 February 2018; Accepted 8 February 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhang, J., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.02.076
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disperse spectroscopy (EDS), X-ray diffraction (XRD) and phase diagram analysis, respectively. In addition, some suggestions for improving the corrosion resistance are given. 2. Experimental procedure 2.1. Materials Mo-W-ZrO2 cermet was prepared by the powder metallurgy method. Its compositions were 30 wt% Mo, 35 wt% W, and 35 wt% ZrO2. Raw materials included the following powders: (1) calcium stabilized zirconia (purity: 98%, Astron Ltd, Shenyang, China) with an average particle size d50 = 45 µm. It was synthesized through the electrofusion method for cost reduction. (2) 3Y-ZrO2 (partially stabilized zirconia, 10 wt%, purity: 99.7%, specific surface area: 10 m2/g, L. Feng Ltd, Jiaozuo, China) with an average particle size d50 = 50 nm. It was synthesized through the chemical doping method for improving the thermal shock resistance based on the phase transformation toughening [15]. (3) Molybdenum (purity: 99.5%, Cemented Carbide Corp., Ltd, Chengdu, China) with an average particle size d50 = 3–4 µm; (4) tungsten (purity: > 99.9%, Cemented Carbide Corp., Ltd, Chengdu, China) with an average particle size d50 = 2.5–3 µm. The cermet sample was a tube with a hemisphere bottom (Diameter × Thickness × Length: Φ22 × 3 × 100 mm3). Its manufacture processes were as follows (see Fig. 1): (1) the initial powders of raw materials were ball-milled in air atmosphere for 48 h with WC grinders; (2) the powders were molded into a tube by the cold-isostatic pressing (CIP) at 180 MPa; (3) the tube was sintered in the hydrogen atmosphere (purity: 99.99%, flux: 50 ml/min, pressure: 0.2 MPa) at 1800 °C for 1 h. And then, the properties of the cermet that might be related to the corrosion were checked. Its flexural strength that was measured by the three-point loading bending test method (sample size: 5 × 5 × 25 mm3, crosshead speed: 0.5 mm/min, span length: 20 mm) was 245.3 MPa. And its relative density dr that was measured by the following equation was 93.2%.
dr =
dm × 100% dt
Fig. 2. Schematic diagram of the corrosion test device: h = 200 mm, d = 250 mm, corrosion time: 20 h, molten steel temperature: 1520 °C.
have a thick-walled structure with a thickness of 20–30 mm while the new sensor (cermet tube) could be made into a much thinner structure with a thickness of about 3 mm. The corrosive environment was as follows: (1) chemical compositions of the molten steel for the test are given in Table 1; (2) the testing temperature of molten steel was about 1520 °C; (3) the testing time was 20 h that was consistent with the practical demand (a regular casting cycle); (4) the flow state of molten steel around the cermet tube was indirectly illustrated by the position (h = 200 mm, d = 250 mm) of the new temperature sensor. 2.3. Microstructure and composition characterization After the cermet was dipped into molten steel, complexly physical and chemical reactions occurred between the cermet and molten steel. They involved to the chemical co-melting reaction between the metal phase and molten steel and the physical erosion caused by the flow of molten steel. The microstructure evolution and composite variation that were caused by the above physical and chemical reactions were checked by the SEM, EDS (SSX-550) and XRD (PW3040/60), respectively, which directly reflected the corrosion process.
(1)
dm was the measured density that was measured by the Archimedes method in distilled water (sample size: 5 × 5 × 5 mm3); dt was the true density that was measured by the pycnometry method. 2.2. Corrosion test In order to reflect the actual corrosive conditions of the industrial application, the corrosion of the cermet was tested in the industrial field (Nanjing Steel Company, China). Fig. 2 shows the schematic diagram of the corrosion test device. The cermet tube that was used as the temperature measuring unit was integrated with the bottom of the molten steel temperature sensor. The support unit for the cermet tube was made of Al2O3-C refractory that was used for the traditional sensor. With the limitation of the sensor material, the traditional sensor had to
3. Results and discussion 3.1. Cermet characterization Composites of the Mo-35W-35ZrO2 cermet were illustrated by the XRD patterns (see Fig. 3). Remarkable reflection peaks of ZrO2, W and Mo were observed. The ZrO2 involves to the cubic-phase zirconia, tetragonal-phase zirconia, and monoclinic-phase zirconia. The reflection peaks of monoclinic-phase zirconia seem more obvious, which may be corresponded to that the electrofusion method is not good enough for stabilizing the monoclinic-phase zirconia. The reflection peaks of W and Mo are near, attributing to their same body-centered cubic (BCC) structure and similar lattice parameters. These near reflection peaks may also be associated with the solid solution formation of the metal Table 1 Chemical compositions of the molten steel (major only).
Fig. 1. Manufacture processes of the cermet.
2
Composition
C
Si
Mn
Cr
Ni
P
S
Fe
wt%
0.39
0.29
0.66
0.95
0.19
0.016
0.027
Bal.
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structure and uniformly distributed in the cermet, playing an important role in preventing crack from propagating. On the one hand, it establishes a semi-continuous network for bridging the crack. On the other hand, as the fracture occurs, the metal phase has a plastic deformation and consumes the fracture energy, which has been verified by Sbaizero et al. [10]. These can seal the propagating of the crack in the metal phase network as soon as possible and improve the thermal shock resistance significantly. The composition and microstructure are the basic reasons for the good thermal shock resistance of the cermet, also governing the corrosion resistance.
3.2. Microstructure evolution Fig. 5(a) shows the industrial corrosion test of the cermet tube that is located at the bottom of the new molten steel temperature sensor. After being tested in molten steel for 20 h, the thickness of the cermet was corroded by only about 1 mm (3 mm → 2 mm), which is illustrated in Fig. 5(b). It implies that the cermet tube has a low corrosion rate of about 0.05 mm/h, leading to that the new molten steel temperature sensor can be made into a thin-walled structure with a thickness of about 3 mm while the thickness of the traditional sensor is about 20–30 mm. Meanwhile, Fig. 5(a) also illustrates that the cermet tube had not thermal shock damage after it was pull out from the molten steel, verifying its good thermal shock resistance. Besides, it can also be found that the cermet tube was much darker than the Al2O3-C support unit after they were pulled out from the molten steel for a while, which is consistent with that the heat transfer of the cermet tube is much faster than that of the Al2O3-C support unit. The corrosion of the cermet tube may be in dynamic equilibrium, attributing to that it has been corroded for a long term. As a result, the microstructure of the corroded cermet may also have reached dynamic equilibrium. The microstructure evolution is used to investigate the corrosion process of the cermet intuitively. The cross-sectional microstructure of the corroded cermet tube is presented in Fig. 6(a). It can be observed that a ZrO2 skeleton layer (Z) with a thickness of about 75 µm formed on the corroded cermet surface after the melting of the metal phase, indicating that the corrosion rate of the ZrO2 layer is much slower than that of the metal phase (Mo-W). The black area in this ZrO2 layer was slag (S) and its elements (checked by EDS) included 34.4 wt% O, 24.5 wt% Al, 17.6 wt% Si, 16.4 wt% Ca, 4.7 wt% Mn and 2.1 wt% Mg. The slag area originally belongs to molten steel. It is formed through the following processes. As the cermet tube is pulled out from the slag layer (see Fig. 2), the molten steel flows out from the ZrO2 layer. Then, the molten slag adheres on the cermet tube surface and penetrates into the ZrO2 layer.
Fig. 3. X-ray diffraction patterns of cermet (c = cubic-phase zirconia, t = tetragonalphase zirconia, and m = monoclinic-phase zirconia).
Fig. 4. Microstructure of the cermet (SEM): ceramic phase (C), metal phase (M) and pore (P).
phases (Mo and W). In addition, no other obvious reflection peck is observed, implying that there is a good chemical stability between the ZrO2 and metal phase. Microstructure of the cermet is illustrated in Fig. 4, including ceramic phase (C), metal phase (M) and pore (P). The ceramic phase (ZrO2) was the matrix, which is benefit to improve the corrosion resistance. The size of the metal phase (Mo and W) becomes much larger during the sintering while the original average sizes of the metal particles are small. The metal phase has formed a semi-continuous
Fig. 5. (a) The industrial corrosion test of the cermet tube; (b) the thickness variation of the cermet tube after being corroded for 20 h in molten steel.
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Fig. 6. (a) Cross-sectional microstructure of the corroded cermet (SEM): ZrO2 layer (Z), ceramic phase (C), metal phase (M), slag (S) and pore (P); (b) Fe distribution on AB: deteriorated layer (D).
Fig. 7. Microstructure (SEM) and Zr distribution (EDS) of the corroded cermet: slag (S), metal phase (M) and ceramic phase (C).
These reactions result in that the metal phase transforms into the solid solution before its melting. To investigate the deteriorating of the metal phase, the Fe content variation from the surface to substrate was examined by the EDS (see Fig. 6(b)). It can be found that the Fe content variation has the following characteristics: (1) in the distance range from 0 to 80 µm, there was a low content of the Fe, corresponding to the ZrO2 layer (Z) area. It is attributed to that the molten steel has flowed out from the ZrO2 layer after the cermet is pulled out form molten steel. (2) In the distance range from 80 to 320 µm, a high content of the Fe existed in this area, corresponding to the deteriorated region with a thickness of about 240 µm. It is associated with that the deteriorating reactions (5)–(7) have occurred. (3) Above 320 µm, the Fe content almost disappeared, implying that the diffusion of the Fe has decreased significantly.
The corrosion of the ceramic phase (ZrO2) is mainly resulted from the physical erosion of the flow of molten steel, associating with that the ZrO2 cannot be wetted and dissolved by molten steel [6,13,14]. As a result, the ZrO2 has a small corrosion rate. However, the corrosion of the metal phase (Mo-W) is much faster, attributing to that the metal phase is mainly corroded by the following one or more chemical comelting reactions. Mo (s) + Fe (l) = Mo-Fe (l)
(2)
W (s) + Fe (l) = W-Fe (l)
(3)
Mo-W (s) + Fe (l) = Mo-W-Fe (l)
(4)
Thus, the ZrO2 layer forms and increases to a certain thickness of about 75 µm gradually. Meanwhile, the ZrO2 layer prevents molten steel from further penetrating into the cermet and slows down the comelting reaction of the metal phase. As the co-melting corrosion rate of the metal phase decreases to the physical erosion rate of the ZrO2 layer, the corrosion of the cermet reaches dynamic equilibrium. Beneath the ZrO2 layer, the deteriorated layer forms, attributing to the interaction between the metal phase (Mo and W) and Fe in molten steel through the following one or more reactions. Mo (s) + Fe (l) = Mo-Fe (s)
(5)
W (s) + Fe (l) = W-Fe (s)
(6)
Mo-W (s) + Fe (l) = Mo-W-Fe (s)
(7)
3.3. Composition variation To further clarify the corrosion mechanism, the composition variation also has been investigated. The cross-sectional component distributions of the ceramic and metal phases in the corroded cermet have been investigated respectively. Microstructure and Zr distribution are presented in Fig. 7, representing the ceramic phase distribution. It indicated that the ZrO2 distribution was continuous and uniform, meaning a good microstructure of the matrix. In addition, the thickness of the ZrO2 layer (Z) was about 78 µm on the surface of the corroded cermet, which is agreement with the result as illustrated in Fig. 6(b). The small difference of the thicknesses mainly comes from the material 4
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Fig. 8. Microstructure (SEM) and Fe, Mo and W distributions (EDS) of the corroded cermet: metal phase (M), slag (S), ceramic phase (C) and deteriorated layer (D).
Besides, the distributions of Mo and W were coincident as shown in Fig. 8(c)–(d), implying that the metal powders disperse uniformly and have a good sintering performance. The solid-liquid transforming process of the metal phase are analyed through the phase diagrams. According to the phase diagram of Fe-Mo [16–18], at the testing temperature (1520 °C) of molten steel, the comelting reaction (2) of Mo starts to occur as its content is less than about 59.2 wt%. As the Mo content deceases to below about 43.2 wt%, it converts into liquid completely. As the Mo content is more than 59.2 wt%, the Mo and Fe form solid solution that causes the deteriorated layer formation through the reaction (5). Besides, with the increase of the molten steel temperature, the Fe content that is needed for the solid-liquid transforming decreases. The lowest temperature of the solid-liquid transformation of Mo is 1449 °C that is significantly lower than the molten steel temperature in tundish (about 1470–1560 °C). That is to say, the Mo is easily corroded by molten steel. According to the phase diagram of Fe-W [19,20], the co-melting reaction between the W and any content of the Fe cannot occur at 1520 °C. Until the molten steel temperature increases to 1529 °C and the W content is 13.2 wt%, there is a solid-liquid transformation. However, the W is corroded at about 1520 °C in practice. This temperature is lower than the lowest temperature (1529 °C) of the solid-liquid transformation. There may be two reasons for this phenomenon: (1) Mo and W have formed the solid solution, lowering the temperature of the solid-liquid transformation; (2) other components of the molten steel may lower the solid-liquid transforming temperature. These are the basic reasons for the metal phase corrosion.
defects, such as pores and composition segregation. The ZrO2 layer that governs the corrosion resistance of the cermet is the key to improve the corrosion resistance. The factors that govern the ZrO2 layer formation mainly involve ZrO2 content, original raw material powder size, sintering temperature and so on. The ZrO2 content is the critical factor, more ZrO2 content leads to easier formation of the ZrO2 layer. However, in order to satisfy the demand of the thermal shock resistance, the ZrO2 content has to be limited. The least ZrO2 content of the cermet should assure that a ZrO2 layer can form during the corrosion. As the ZrO2 layer forms, the corrosion of the cermet is mainly controlled by the physical erosion of the ZrO2 layer, meaning a low corrosion rate. Otherwise, the corrosion rate mainly depends on the chemical co-melting reaction of the metal phase, meaning a fast corrosion rate and a short service life. Fig. 8 illustrates the microstructure and component distributions of Fe, Mo and W, reflecting the corrosion process of the metal phases (Mo and W). The co-melting reaction between the metal phase and Fe in molten steel is one of the root corrosion types. Actually, the oxygen, carbon and other components of molten steel may also affect the corrosion. Due to their low contents, this work does not discuss their influences. Beneath the ZrO2 layer, the interaction between the metal phases (Mo and W) and Fe forms solid solution that results in the deteriorated layer (D) formation through these reactions (5)–(7). The solid solution formation leads to that the metal phase area has enlarged (see Fig. 8(a)). As the Fe content increases to a certain value, the metal phase transforms from the solid into liquid through these reactions (2)–(4), resulting in that the ZrO2 layer forms. Meanwhile, it can be found that the Fe content was higher near to the ZrO2 layer (see Fig. 8(b)), which is agreement with the result as illustrated in Fig. 6(b). 5
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Fig. 10. Variation of the metal phase's co-melting corrosion rate (Vm) and the ZrO2 layer's physical erosion rate (Ve) during the corrosion process.
erosion rate of ZrO2 layer (Ve) are represented in Fig. 10. In brief, the physical erosion of the ZrO2 layer governs the corrosion of the cermet. To improve the corrosion resistance, the network of the ZrO2 phase in the cermet is needed, mainly depending on its volume content. The least ZrO2 content should assure that the ZrO2 network can form in the cermet. Only under this situation, the ZrO2 layer can form during the corrosion, which is the key factor for good corrosion resistance. The ZrO2 network formation is similar to a percolation phenomenon [21,22]. The least ZrO2 content can be seen as the percolation threshold. (1) At the ZrO2 content below the percolation threshold, there is not continuous contact between ZrO2 particles, meaning that the percolation path cannot form. The ZrO2 phase is discrete and cannot form a ZrO2 layer. Without its barrier protection, the melting corrosion of the metal phase will keep a constant fast rate during the whole corrosion process. What's worse, the discrete ZrO2 phase may accelerate the corrosion. It is easily shed from the metal matrix and increases the interaction interface between the metal phase and molten steel. (2) At the ZrO2 content above the percolation threshold, continuous contact of ZrO2 particles forms percolation paths that establish the network of the ZrO2 phase. As a result, the ZrO2 layer can form. Under this situation, the cermet exhibits good corrosion resistance which mainly comes from the barrier protection of the ZrO2 layer.
Fig. 9. Schematic diagram of the corrosion process: metal phase (M), ceramic matrix (C), deteriorated layer (D), ZrO2 layer (Z) and liquid steel (L).
3.4. Corrosion mechanism
4. Conclusions
On the basis of the above investigations of the microstructure evolution and composition variation, the microstructure characteristics of different corrosion stages are qualitatively described in Fig. 9. They are as follows: (1) t = t0, initial period. As the cermet tube is dipped into molten steel, a solid solution reaction between the metal phase (Mo-W) and Fe occurs on the cermet surface, leading to the deteriorating of the metal phase and the formation of the deteriorated layer (D). (2) t = t1, intermediate period. As the Fe content in the solid solutions increases to a certain value, the deteriorated metal phase melts. And then, a ZrO2 layer (Z) forms on the corroded cermet surface. It is mainly corroded by the physical erosion that is much slower than the melting corrosion of the metal phase. Meanwhile, the ZrO2 layer prevents molten steel from further penetrating into the cermet and slows down the metal phase melting. The initial period and intermediate period belong to an unstable period that may be short. (3) t = t2, stable period. The melting corrosion rate of the metal phase decreases to the physical erosion rate of the ZrO2 layer, the corrosion of the cermet reaches dynamic equilibrium, the thicknesses of the ZrO2 layer and deteriorated layer increase to almost constant values. During the above processes, the variations of the co-melting corrosion rate of metal phase (Vm) and the physical
The corrosion mechanism of the Mo-35W-35ZrO2 cermet in molten steel has been investigated. Its corrosion processes were as follows: (1) Unstable period. At beginning, a solid solution reaction between the metal phase (Mo-W) and Fe occurred on the cermet surface, leading to the deteriorating of the metal phase. As the Fe content increased up to a certain value, the deteriorated metal phase melted, leading to that a ZrO2 layer formed on the cermet surface. The melting corrosion of the metal phase was much faster than the corrosion of the ZrO2 layer that was mainly caused by the physical erosion of the flow of molten steel. Meanwhile, the ZrO2 layer prevented molten steel from further penetrating into the cermet, slowing down the melting corrosion of the metal phase. (2) Stable period. As the thickness of the ZrO2 layer increased to about 75 µm, the melting corrosion rate of the metal phase was lowered to the physical erosion rate of the ZrO2 layer. As a result, the corrosion of the cermet reached dynamic equilibrium, corresponding to a small corrosion rate of about 0.05 mm/h and a good corrosion resistance. 6
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In brief, the ZrO2 layer formation was the key to improve corrosion resistance. The least ZrO2 content of the cermet should assure that a ZrO2 layer could form during the corrosion, which might have important implication for solving the corrosion resistance problem of the similar cermet.
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