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Journal of Alloys and Compounds 766 (2018) 759e768
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Rapid synthesis of ZrB2eB4C composite powders via induction heating
and its effect on the properties of Al2O3eSiCeC castables
Huan Xu, Xitang Wang*, Zhoufu Wang, Yan Ma, Hao Liu, Yulong Wang
The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 May 2018
Received in revised form
28 June 2018
Accepted 30 June 2018
Available online 3 July 2018
ZrB2eB4C composite powders (ZBCs) were synthesized by B4C reduction method via induction heating in
an argon atmosphere using ZrO2, B4C, and pitch powder as raw materials. The effect of ZBCs addition on
the performance of Al2O3eSiCeC (ASC) castables was evaluated by considering their mechanical properties, oxidation resistance, and slag resistance. The results showed that high-purity ZBCs with a median
particle diameter of 3.07 mm could be easily obtained by induction heating at 1400 C for 30 min. The
bulk density, cold modulus of rupture (CMOR), hot modulus of rupture (HMOR), and anti-oxidation
performance of ASC castables can be synchronously improved via the introduction of synthesized
ZBCs. The isothermal oxidation test at 1450 C revealed that the oxidation behavior of the samples with
ZBCs addition follows a parabolic law. The slag corrosion resistance of ASC castables was slightly
impaired, but the slag penetration resistance was obviously enhanced due to improved oxidation
resistance.
© 2018 Elsevier B.V. All rights reserved.
Keywords:
Al2O3eSiCeC castables
ZrB2eB4C composite powders
B4C reduction method
Oxidation resistance
Slag resistance
1. Introduction
Al2O3eSiCeC (ASC) castable refractories have been widely used
in blast furnace trough linings due to their excellent physical and
chemical characteristics including high strength, good anti-slag
performance, and good resistance to both abrasion and thermal
shock [1e4]. Despite developments in blast furnaces and smelting,
the properties of ASC castables must still be improved to meet the
new challenges. Approaches for improving the service life of ASC
castables mainly focus on enhancing their anti-oxidation performance, mechanical properties and slag resistance [5]. In the
Al2O3eSiCeC system, carbon and SiC offer high thermal conductivity and non-wettability with liquid slag. However, the high
susceptibility to oxidation always leads to a porous material with
diminished properties. Antioxidants have been extensively investigated to improve their oxidation resistance of these materials.
Examples include metal-powders (such as Al and Si) and their alloys, Al4SiC4 [6,7], as well as boron-based non-oxide ceramics
[8e10]. Of these, boron-based non-oxide ceramics (such as B4C and
ZrB2, etc) [11e13] display excellent anti-oxidization performance
when incorporated into the carbon-containing refractories due to
* Corresponding author.
E-mail addresses: wangxt_wust@126.com, wangxitang@wust.edu.cn (X. Wang).
https://doi.org/10.1016/j.jallcom.2018.06.375
0925-8388/© 2018 Elsevier B.V. All rights reserved.
their self-repairing ability.
It is well known that B4C can react with O2 or CO to form a B2O3
liquid phase. This can further form a compound silicate glass layer
containing multi-component oxides including SiO2, Al2O3, and CaO
at elevated temperatures [14]. The resulting liquid phases can fill up
the pores and cracks and hinder oxygen diffusion. Meanwhile, the
B4C action can result in the deposition of C and reduce the loss of C
[15,16]. Wu et al. [17] recently indicated that the addition of B4C
could greatly improve the anti-oxidation performance of ASC
castables and promote the nucleation of SiC whiskers. However,
excessive addition of B4C into the refractories is detrimental to the
hot modulus of rupture (HMOR) and slag corrosion resistance [18].
Thus, the composite additive of antioxidants is a promising method
to solve these problems [19e21]. Many studies have shown that the
addition of ZrB2 can significantly improve oxidation resistance by
modifying the formed glass protective layer without impairing the
hot mechanical properties [22e24]. This is because the ZrB2 belongs to the family of ultra-high temperature ceramics (UHTCs). It
has outstanding mechanical and thermo-chemical properties
[25,26]. During oxidation, the ZrO2 product can react with SiO2 to
form a thermally stable ZrSiO4 phase [27]. Corral et al. improved the
ablation of CeC composites at high temperature via ZrB2eB4C
composite powders (ZBCs). The ablation rates decreased by 30%
when the CeC composites were filled with a combination of
ZrB2eB4C particles over carbon black and B4C filled CeC composites
760
H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768
[28]. The ZBCs might be an appropriate candidate antioxidant for
ACS castables, but applications of ZBCs are limited by the high
synthetic cost of ZrB2 and B4C powders. Moreover, the resulting
powders are usually relatively coarse, and the subsequent ZBCs
machining process is time-consuming and costly. Furthermore, the
ZBCs obtained from directly mechanical mixing of ZrB2 and B4C
powders suffers from agglomeration. To address these problems, it
is critical to develop an alternative technique to one-step ZBCs
synthesis.
Induction heating is a new rapid heating method recently
developed for the synthesis of ceramics and composites, including
HfB2 [29], LaB6 [30], NbSi2eSiC [31], and HfSi2eSiC [32]. This process is fast and effectively inhibits grain growth. Another important
advantage of induction heating is the role of rapid heat transfer to
the product via electromagnetic waves [33]. Consequently, small
and well-dispersed B4C and ZrB2 particles can be produced in situ
via rapid induction heating. These materials enhance the antioxidation performance.
Here, ZrO2, B4C, and pitch powder were used to synthesize ZBCs
by a simple B4C reduction method via rapid induction heating for
the first time. The effect of different contents of the ZBCs on the
anti-oxidation properties of ASC castables was evaluated. Simultaneously, the analysis of physical properties and slag corrosion
resistance as well as thermodynamic calculations were also performed in order to better understand the effect of ZBCs introduction on the performance of ASC castables.
2. Experimental methods
2.1. Synthesis of ZBC
ZrO2 (98.5 wt%, 3 mm, Macklin Biochemical Co., Ltd.), B4C (98 wt
%, 5 mm, Macklin Biochemical Co., Ltd.) and pitch powder (C: 51.3 wt
%, Vd: 48.55 wt%, Ad: 0.15 wt%, 200 mesh, Handan, China) were
used as the starting materials. According to the following reaction
(1), weighed quantities of ZrO2, B4C and pitch powder with a molar
ratio of 2:2:3 were mixed thoroughly in a polythene bottle for 3 h in
order to obtain ZBCs. The powder mixture was then pressed into
cylinders with 20 mm in diameter under a pressure of 100 MPa. The
pellets were then loaded in a graphite crucible and heated in an
induction furnace.
At the start of the experiments, the furnace chamber was sealed,
evacuated and purged with argon (99 wt% purity). The samples
were then held at a fixed temperature (1400 C, 1500 C, 1600 C,
and 1700 C) for 30 min in an argon atmosphere at a rate of 2.5 L/
min. The temperature was measured using a pyrometer with an
accuracy of ±10 C. At the end of the process, the reacted pellets
were cooled in the furnace until it reached room temperature and
was removed. The black products were fluffy and easily ground into
powders with a corundum mortar and a pestle for 5 min.
2ZrO2 ðsÞ þ B4 CðsÞ þ 3CðsÞ/2ZrB2 ðsÞ þ 4COðgÞ
(1)
2.2. Preparation of Al2O3eSiCeC castable specimens
The raw materials used in this work were brown alumina with
different particle grades (95 wt%, D 8), silicon carbide (98 wt%,
3-1 mm, 1-0 mm and 0.075 mm), white fused alumina (99 wt%,
200 mesh), reactive alumina powder (99 wt%, CL370, Almatis), ball
pitch (60 wt% C), metallic Al (98 wt%, 120 mesh) and metallic Si
(97 wt%, 200 mesh) and calcium aluminate cement (70 wt% of
Al2O3, Secar 71, Kerneos) as a binder. Several groups of ASC castable
samples with different synthesized ZBCs contents (0 wt%, 0.5 wt%,
1 wt%, 1.5 wt%, and 2 wt%) were designed with these raw materials,
and the details of composition of ASC castables are listed in Table 1.
All the raw materials were first dry-mixed in a mixer, and then
4.8 wt% of extra water containing 0.1% of a polycarboxylate
dispersant (FS20, BASF, Germany) was added to the dry mixed
batches. After 5 min of mixing, the batches were cast into rectangle
shaped molds with dimensions of 40 mm 40 mm 160 mm on a
vibratory table. Castables were cured at about 20 C ambient temperature and 70% relative humidity within the mold for 48 h. They
were then dried at 110 C for 24 h. Finally, the samples were fired in
air at 600 C, 800 C, 1100 C and 1450 C respectively for 3 h at a
heating rate of 5 C/min below 1000 C and then 3 C/min to the
target temperature.
2.3. Test and characterization methods
The phase compositions of the synthesized ZBCs and the ASC
castables samples were identified by X-ray diffraction (XRD, X0 Pert
pro). The purity of the obtained ZBCs were examined by (XRF, ARL
9900 series, Thermo Scientific) and chemical analysis. The
morphology of the obtained powders and microstructure of the
ASC castables samples were examined in field emission scanning
electron microscopy (SEM, Nova NanoSem400) supported with
energy dispersive spectroscopy (EDS, Phoenix). The median particle
diameter and particle size distribution of ZBCs were measured with
laser particle size analysis.
The bulk density and apparent porosity of the ASC castable
samples were tested via the Archimedes Principles. The linear
change rate was tested according to ISO 2477:2005, MOD. CMOR
was determined using the three-point bending test at ambient
temperature. HMOR of the sintered samples was tested via threepoint bending tests at 1400 C for 30 min. The standard deviations quantified the amount of variation of data values. The
oxidation resistance properties of the samples were evaluated by
calculating the oxidation index (Id) using the Image-Pro Plus 6.0
software. The oxidation index Id was obtained via equation (2):
.
Abefore
Id ð%Þ ¼ 100 Abefore Aafter
(2)
Here, Abefore and Aafter are the cross-sectional area of the samples
before and after oxidation, respectively. The isothermal oxidation
kinetics of ASC castables was also evaluated by analyzing the mass
change vs time curve at a heating rate of 10 C/min, from room
temperature to 1450 C, followed by thermal insulation for 8 h.
The slag corrosion resistance experiments used crucible-shaped
specimens inserted into the lining of an induction furnace
(21WGJL0.025-100-2.5P). Subsequently, the steel and slag were
added to the lining. The samples fired at 1450 C for 3 h were used
for slag corrosion resistance experiments. The blast furnace slag
with a basicity (CaO/SiO2) of 1.28 was selected, and the details of
the chemical composition are provided in Table 2. After testing, the
corroded samples were cut, and the corrosion rates were calculated
to measure the corrosion damage using Image-Pro Plus 6.0
software.
Thermodynamic simulations of the matrix used FactSage 6.2
software with an excessive amount of O2 for oxidation. This software could predict the castables' phase compositions as a function
of the amount of ZBCs. The Fact53 and FToxid databases were used
along with Equilib modules.
3. Results and discussion
3.1. Synthesis of ZBCs
Fig. 1(a) shows well-defined peaks for ZrB2 and B4C. These can
be identified in the XRD patterns at all temperatures ranging from
H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768
761
Table 1
Batch composition of the prepared ASC castables.
Raw materials
Content, wt%
Brown fused alumina aggregate
Silicon carbide
White fused alumina powder
Ball pitch
Binder (fumed silica and calcium aluminate cement)
Reactive alumina powder
Metallic Si
Metallic Al
ZBCs
FS20
ZBC0
ZBC0.5
ZBC1
ZBC1.5
ZBC2
52
23
8
3
4
8
2
0.1
0
0.1
52
23
7.5
3
4
8
2
0.1
0.5
0.1
52
23
7
3
4
8
2
0.1
1
0.1
52
23
6.5
3
4
8
2
0.1
1.5
0.1
52
23
6
3
4
8
2
0.1
2
0.1
prepare ZBCs.
Table 2
Chemical composition of blast furnace slag (wt%).
Constituent
SiO2
CaO
Al2O3
MgO
Fe2O3
K2O
Na2O
S
Others
Percentage
32.75
39.16
16.62
8.75
0.36
0.18
0.06
1.37
0.75
1400 C to 1700 C; no other phases were found. We used the XRF
analysis combined with chemical analysis to identify the purity of
ZBCs. The total content of ZrB2 and B4C is more than 98%. The impurity content is less than 1.6% (residual C: 0.45 wt%, Ce:0.08 wt%,
Mo: 0.41 wt%, Nd: 0.12 wt%, Ca: 0.23 wt%, Fe: 0.06 wt%, Al: 0.06 wt%,
Si: 0.05 wt%; Ti: 0.02 wt%; O:<0.1 wt%). This indicates that highpurity ZBCs can be easily synthesized at temperatures as low as
1400 C using this induction heating method. Fig. 1(b) shows the
backscattered electron image of the ZBCs synthesized at 1400 C for
30 min. There are clearly two different color phases in the productdthe EDS analysis proved that the white phases are ZrB2, and
the black ones are B4C originated from the raw materials.
The ZrB2 and B4C particles are well distributed with no obvious
agglomeration. The laser particle size analysis results at the topright corner of Fig. 1(b) show the particle size distribution of the
samples synthesized at various temperatures. The ZBCs synthesized at 1400 C has a narrow particle size distribution with a
median particle diameter of 3.07 mm. As the temperature increases
from 1400 C to 1700 C, the median particle diameter slightly
increases from 3.07 mm to 4.60 mm. The preparation temperature
used to synthesize ZBCs here is much lower than that used for
preparing single powders via conventional heating methods
(about 1500e2000 C) [34,35]. These results indicate that this
induction heating method is a fast, low energy, and capable tool to
3.2. Physical properties of ASC castables upon the introduction of
ZBCs
The linear change rate of the prepared ASC castables containing
different amounts of ZBCs are depicted in Fig. 2(a). The linear
change rate gradually increases with temperature. At 800 C and
1100 C, the linear change rate slightly varies with increasing ZBCs
content. However, all samples have a volumetric expansion after
heat treatment at 1450 C. The linear change rate tends to increase
with increasing ZBCs content.
The ZrB2 and B4C can easily react with oxygen to form B2O3 via
reactions (3) and (4). These are accompanied by a volume expansion. At the same time, many new phases such as mullite and ZrSiO4
are formed in the matrix at this temperature. This can lead to a
volumetric expansion.
2B4 C þ 7O2 ðgÞ/4B2 O3 ðlÞ þ 2COðgÞ
(3)
2ZrB2 ðsÞ þ 5O2 ðgÞ/2B2 O3 ðlÞ þ 2ZrO2 ðsÞ
(4)
The variation of bulk density and apparent porosity of the
samples heat treated at various temperatures as a function of ZBCs
content are illustrated in Fig. 2(b) and (c), respectively. Fig. 2(b)
shows that the bulk density strongly depends on the heat treatment temperature and the ZBCs content. The bulk density first
decreases and then increases with the increase of the temperature.
In all cases, the highest bulk density ranges from 2.94 to
2.99 g cm3. This can be obtained after drying at 110 C for 24 h. The
sample then sharply decreases when the temperature reaches
Fig. 1. (a) XRD patterns and (b) backscattered electron image, EDS, and laser particle size analysis of the synthesized ZBCs.
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H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768
fine powder (3.95 g cm3). Furthermore, the B2O3 liquid phase
produced by the oxidation of ZrB2 and B4C improves the bulk
density. However, the sample containing 1.5% ZBCs presents the
highest bulk density when firing at 1450 C. It is generally accepted
that an appropriate extent of volume expansion was proved to be
advantageous as the pores were filled. However, excessive volume
expansion is harmful to the density of the castables with a
tremendous permanent linear change rate of 0.57% (shown in
Fig. 2(a)). The apparent porosity indicates an opposite tendency
versus bulk density (Fig. 2(c)).
The CMOR of the ASC castable samples after heat treatment at
different temperatures are shown in Fig. 3. The samples dried at
110 C for 24 h introduce ZBC but do not significantly influence
CMOR. All the samples have a considerable strength that mainly
originates from the hydration of the calcium aluminate cement.
Meanwhile, the softening of some ball pitch at this temperature
also improves CMOR. However, the samples heat treated above
800 C show that CMOR is greatly enhanced via ZBCs. The CMOR
increases by 5 times from 4.2 MPa to 20.6 MPa and 2.5 times from
7.4 MPa to 18.7 MPa for the samples fired at 800 C and 1100 C
respectively, versus the unmodified samples. This significant
enhancement is likely due to B2O3 glass phase [36]. When heating
over 1450 C, the sample containing 1.5% ZBCs has the largest
CMOR value of 23.1 MPa. This slightly decreases to 21.2 MPa for the
sample ZBC2. This phenomenon might be associated with the
excessive volume expansion resulting in destruction of the
microstructure.
Fig. 4 shows the HMOR of the ASC castables with different ZBCs
contents. It is widely accepted that the HMOR is markedly affected
by their ceramic bonding phases in the matrix [37]. Fig. 4 shows
that the HMOR value tends to increase with increasing ZBCs content. This significant enhancement is because the samples containing a higher amount of ZBCs have a higher bulk density and a
lower apparent porosity. Second, the introduction of ZBCs can
greatly improve their anti-oxidation properties, and this can
minimize the structural damage induced by the oxidation of C and
SiC. Furthermore, the addition of ZBCs can lead to the formation of
ceramic bonding phases such as columnar mullite and ZrSiO4. This
can improve the strength at high temperature [38]. These observations were confirmed via XRD and SEM analysis (as shown in
Figs. 5 and 6).
Next, the phase composition and microstructure evolution of
the samples with various ZBCs contents were analyzed to better
Fig. 2. (a) Linear change rate, (b) bulk density, and (c) apparent porosity of the ASC
castable samples containing different contents of ZBCs after heat-treating at different
temperatures.
800 C due to the pyrolyzation of ball pitch and the oxidation of C
and SiC. This leaves numerous cavities inside the castables.
Next, the bulk density again begins to increase when the temperature increases to 1100 Cdboth B4C and ZrB2 start reacting
with oxygen at temperatures below 800 C. This leads to pore
blocking due to the formation of B2O3 liquid phases. At 1450 C, the
bulk density further increases due to the sintering process of
castable composites and the formation of new phases in the matrix.
This makes the castables denser. In addition, the bulk density of the
samples all increase with increasing ZBCs content at the temperatures ranging from 110 C to 1100 C. This is because the microsized ZBCs have an excellent capability to fill the voids between
aggregates. This leads to a higher packing density. The ZBCs also has
a much higher density (z4.74 g cm3) than the white fused Al2O3
Fig. 3. CMOR of the ASC castable samples after heat-treating at different temperatures.
H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768
763
Fig. 4. HMOR of the ASC castables with different ZBCs contents.
understand the effect of ZBCs addition on the physical properties of
the ASC castables. Fig. 5(a) depicts the XRD patterns of the oxidized
Fig. 6. Typical microstructures of the non-oxidized zone of samples with different
ZBCs contents after firing at 1450 C: (a), (b): 0%; (c), (d): 0.5%; (e), (f): 1%; (g), (h): 1.5%;
and (i), (j): 2%.
Fig. 5. XRD patterns of the oxidation layer of (a) the samples with different ZBCs
contents after heat-treating at 1450 C (b) the sample ZBC2 after heat-treating at
different temperatures.
region of samples with different amounts of ZBCs after firing at
1450 C. The Al2O3, SiC, and mullite are the major crystal phases in
all the samples. ZrO2 is generated when ZBCs is added. This is
attributed to the oxidation of ZrB2. The relative diffraction peak
intensity of mullite/Al2O3 is gradually enhanced with increasing
ZBCs content. This suggests that the incorporation of ZBCs into ASC
castables can promote the formation of mullite. This improves the
high temperature strength.
Significant diffraction peaks from ZrSiO4 are observed in the
samples with 1.5% and 2% ZBCs, but these are not seen detected in
the samples with a ZBCs content below 1.5% (the concentration is
under the XRD detection limit). Previous studies reported that ZrO2
and ZrSiO4 can act as an “embedding structure” on the surface of
the silicate glass layer. This increases the viscosity of the glass layer,
and reduces its porosity, improves the anti-oxidation ability, and
promotes the mechanical strength [39].
764
H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768
Fig. 5(b) shows the XRD patterns of the oxidized region in the
ZBC2 sample coked at various temperatures (600 C, 800 C,
1100 C, and 1450 C). At 600 C, both ZrB2 and m-ZrO2 are identified in the XRD spectrum suggesting that ZrB2 was partially
oxidized at this temperature. When the temperature increases to
800 C, the diffraction peaks of ZrB2 fully convert into ZrO2. The
accompanying oxidation scale of B2O3 is fluid. It fills the pores and
cracks, and this accounts for the high CMOR above 800 C. When
the temperature is elevated to 1100 C, the Si intensity obviously
decreases, and the t-ZrO2 transforms into m-ZrO2. Up to 1450 C,
many mullite phases can be observed along with ZrSiO4.
The FESEM micrographs of the non-oxidized zone of samples at
1450 C with different ZBCs contents are shown in Fig. 6. Many SiC
whiskers are formed in the pores of the samples with 0% and 0.5%
ZBCs. The formation mechanism has been depicted previously [40].
However, a few of SiC whiskers were found in the ZBC1 sample and
the SiC whiskers almost disappear when the ZBCs content increases
to 2%. Instead, a large amount of the columnar structure mullite
crystals were observed in the pores, and the crystals are oriented
and interlocked with each other to form an intertexture that might
improve the mechanical strength of the ASC castable.
The decrease in SiC whiskers might be associated with a change
in CO partial pressure in the castables. The formation of SiC fibers is
usually dependent on the amount of CO and SiO gases with a high
amount of these gases considered favorable for this transformation.
However, we found that the addition of ZBCs led to a dense protective layer on the surface of castables. This layer could inhibit
oxygen infiltration. Moreover, the CO gases in the matrix could be
reduced to C by ZrB2 and B4C through reaction (5) and (6). Both of
these can decrease the CO partial pressure in the inner parts leading
to the disappearance of SiC whiskers.
The mullite is generated through two paths: First, the active
alumina particles directly react with SiO2 derived from the additive
microsilia, the oxidation of Si, and SiC, and the CaAl2Si2O8. This
product is mullite based on reactions (7) and (8). Second, mullites
are easily formed by the Al18B4O33 at high temperatures due to the
similar crystal structure (9). Besides, the higher B2O3 liquid content
is also favorable for the formation of Al18B4O33 and mullites.
B4 CðsÞ þ 6COðgÞ/2B2 O3 ðlÞ þ 7CðsÞ
(5)
ZrB2 ðsÞ þ 5COðgÞ/ZrO2 ðsÞ þ B2 O3 ðlÞ þ 5CðsÞ
(6)
2SiCðsÞ þ 3O2 ðgÞ/2SiO2 ðsÞ þ 2COðgÞ
(7)
2SiO2 ðsÞ þ 3Al2 O3 ðsÞ/Al6 Si2 O13 ðsÞ
(8)
Al18 B4 O33 ðsÞ þ 6SiO2 ðl; sÞ/3Al6 Si2 O3 ðsÞ þ 2B2 O3 ðlÞ
(9)
Thermodynamic calculations were also made using FactSage
software to predict the phase evolution. Fig. 10 shows that alpha
stands for the ratio of ZBCs to the matrix of the ASC castable. The
mass of the matrix was 100 g according the experimental formula.
The N2 and O2 were set as 395 g and 105 g, respectively, according
to their ratios in the air. With increasing ZBCs contents, the partial
pressure of oxidizing gases O2 and CO2 decreases, while the partial
pressure of B2O3 and CO increase. The content of liquid phase increases with increasing ZBCs; mullite decreases. The trend is
different from our present observations. Because the prediction
presumed that the ZrB2 and B4C are completely oxidized in an
adequate O2 atmosphere, but that is impossible in reality.
3.3. Oxidation tests
Fig. 7 shows the oxidation cross area of the specimens heattreated at different temperatures. All the samples without ZBCs
addition undergo significant oxidation. The sample is completely
oxidized by 1100 C. After the introduction of ZBCs, the oxidation
resistance properties are greatly enhanced at all temperatures. The
decarbonized areas calculated via Image-Pro Plus software are
shown in Fig. 8. The decarbonized areas all obviously decrease as
the ZBCs content increases. The carbonized area decreases by 64%
from 100% to 36% at 1100 C, and by 17% from 85% to 68% at 800 C,
respectively. The decarbonized area of ASC castables with ZBCs at
1100 C and 1400 C was much lower than that at 800 C. Because
ZBCs can provide oxidation protection through the formation of
continuous B2O3 layer at 800 C. When the temperature is above
1100 C, metal Si and SiC begin to sharply oxide to form SiO2. This
can further react with B2O3 to form borosilicate glass layer with
good oxidation resistance. The synergistic effect of ZBCs and Si
significantly improves the oxidation resistance at 1100 C and
1400 C.
Fig. 9 shows the mass change of the tested samples as a function
of ZBCs content. The mass loss for the sample without ZBCs undergoes a sequential increment before 1450 C for 200 min as a
result of the oxidation of C and the removal of crystal water and
organic compositions. However, for the samples with ZBCs addition, there was a significant mass increment phenomenon at
890 C. The mass change rate became sharper with increasing ZBCs
content. The ZBCs oxidation leads to weight gain, and somewhat
counteracts the weight loss. The generated liquid protective layer
also restricts the diffusion of oxygen into the inner castables. This
protects the C and SiC against oxidation. These results suggest that
the introduction of ZBCs powders can significantly improve the
oxidation resistance of ASC castables at middle temperature
(800e1100 C) and elevated temperatures.
Isothermal oxidation was also performed at 1450 C for 8 h to
better understand the effect of ZBCs on the oxidation mechanism.
The plot of (Dm/m)2 as a function of time (Fig. 9(b)) shows that for
the unmodified sample, the mass change rate continues to decrease
until 1400 C for 250 min. It then linearly increases with increasing
oxidation time. At the beginning of oxidation, C directly touches O2,
and their chemical reaction is the main control condition. After
oxidation, SiC begins to oxidize leading to a SiO2 film and a mass
increment.
Samples with ZBCs addition, however, have a linear (Dm/m)2
with oxidation time indicating that the oxidation mechanism is
parabolic. Previous work [36,41] has shown that ZrB2 and B4C
Fig. 7. Oxidation profiles of the evaluated ASC castables after heating in air.
H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768
765
Fig. 10. Phase changes of the ASC castables containing different amounts of ZBCs at
1450 C.
Fig. 8. Calculated decarbonized area of the castables as a function of ZBCs content.
Fig. 9. Isothermal oxidation weight gain vs. time curve: (a) from room temperature to 1450 C and then thermal insulation for 8 h, and (b) isothermal process (the mass at the
beginning of thermal insulation was zero).
initially react with O2 or CO to form C and B2O3 at temperatures
above 450 C and 540 C, respectively. Due to the expansion reactions, the open porosity of castables is markedly reduced (shown
in Fig. 2). The reacts not only provides carbon protection, but also
generates extra C. The B2O3 glass can further react with other
components (such as SiO2, Al2O3, and CaO) to form a protective
layer.
Oxidation layer is formed on the surface, and O2 needs to
overcome a large diffusion resistance in order to react with C and
SiC. As the diffusion path becomes longer, the rate of diffusion
766
H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768
Table 3
Values of rate constants kd.
Content of ZBCs (wt%)
Kd (%▪min1)
0
0.5
1
1.5
2
e
1.11 105
1.26 104
8.35 104
2.05 103
becomes the limiting factor. Thus, the rate of oxidation is equal to
the rate of oxygen diffusion as shown in Equation (10).
2
Dm
m
¼ kd $t
(10)
The values of kd were obtained by curve fitting and are summarized in Table 3. The oxidation rate constant obviously increases
with increasing amounts of ZBCs. This implies that introducing
ZBCs into ASC castables obviously retards the oxidation rate
compared to the unmodified sample.
3.4. Slag corrosion resistance
Slag corrosion tests were conducted to evaluate the effect of
ZBCs on the corrosion resistance of ASC castables. Fig. 11 presents
the samples' cross-sections after corrosion experiments in an induction furnace for 1 h. All the samples have an excellent slag
corrosion resistance. With increasing amounts of ZBCs, the erosion
layer become thinner, but the erosion region becomes wider. The
results suggest that the introducing of ZBCs has a significant influence on the slag penetration and slag corrosion.
The corrosion resistance of the castables can be characterized by
corrosion area per minute as shown in Fig. 11(b). The corrosion rate
initially increases and then decreases with increasing ZBCs. The
calculated corrosion rate for the sample without ZBCs is
1.8 mm2 min1. This increases to 3.5 mm2 min1 and 5.9 mm2 min1
for ZBC0.5 and ZBC1, respectively. It then slightly decreases to
5.72 mm2 min1 for ZBC2.
In general, the slag corrosion process of ASC castables is mainly
described via the following three processes: (1) the surface of ASC
castables is directly dissolved into the melt or oxidized at high
temperatures to form a decarburized layer under the action of
molten steel; (2) The liquid slag penetrates into decarburized layer
through open pores, and the Al2O3, SiC, and ZrO2 aggregates are
isolated and subsequently eroded into the slag along the crystal
boundary; (3) The ASC castables are gradually eroded under the
alternation of slag and molten steel. Any process above that controls erosion will somewhat increase the erosion of the materials,
Fig. 11. (a) Cut-section view photographs and (b) slag corrosion rates of slag-corroded samples.
Fig. 12. Typical microstructures of the corroded samples with different ZBCs contents.
and the erosion rate is controlled by the slowest one of these steps.
Fig. 12 shows the microstructures of the corroded samples with
different ZBCs contents. Fig. 12(a) shows a significant decarburized
layer with a thickness of 1.6 mm. There are many coarse pores in
ZBC0, which is responsible for the bad slag penetration resistance.
An obviously thick penetration layer can be observed in ZBC0. The
slag penetration dominates the corrosion mechanism. Samples
with ZBCs addition have a markedly reduced decarburized layer.
This is only 0.40 mm and 0.25 mm for ZBC1 and ZBC2, respectively,
due to improvements in the anti-oxidation property. The penetration layer also becomes thinner versus ZBC0. This is because the
generated B2O3 liquid phase fills the open pores and blocks the
channels for molten slag. Hence, the samples exhibit excellent slag
penetration resistance.
However, the higher ZBCs content always generates more liquid
phase containing B at elevated temperature. The formed liquid
phase can easily dissolve into the alkaline molten slag that in turn
deteriorates the slag corrosion resistance. Here, the chemical
corrosion is the main control factor affecting the samples
H. Xu et al. / Journal of Alloys and Compounds 766 (2018) 759e768
containing ZBCs. This analysis shows that the proper ZBCs content
has a positive effect on corrosion penetration resistance, but excess
addition would impair slag corrosion resistance.
Thus, we concluded that ZBCs addition can effectively improve
the oxidation resistance performance and the high temperature
mechanical properties of the ASC castables. The incorporation of
ZBC increases slag penetration resistance but decreases slag
corrosion resistance against alkaline molten slag. If the synthesized
ZBCs are used in ASC castables, then the total ZBCs content should
be comprehensively considered.
4. Conclusion
The ZrB2eB4C composite powders were rapidly synthesized by a
simple B4C reduction method via induction heating. The effect of
ZBCs additives on the behavior of ASC castables was systematically
investigated. Based on the obtained results, the following conclusions can be drawn:
(1) High-purity ZrB2eB4C composite powders with a median
particle diameter of 3.07 mm were obtained via induction
heating at 1400 C for 30 min in an argon atmosphere. This
method is more efficient than conventional heating
methods.
(2) ZBCs addition effectively improved the anti-oxidation property and the thermal mechanical strength. As the addition
amount of ZBCs increased from 0 wt% to 2 wt%, the oxidation
index reduced by 19%, 60%, and 38% at 800 C, 1100 C, and
1450 C respectively; the HORM increased by 41%. The
isothermal oxidation kinetics at 1450 C indicated the
oxidation behavior of the samples with ZBCs addition follows
a parabolic law. The substantial enhancement in the thermomechanical properties was mainly attributed to the formation of more ceramic bonding, such as ZrSiO4 and columnar
mullite crystals.
(3) The slag corrosion results show that the increase in ZBCs
content has an obvious positive effect on the slag penetration
resistance of the castables due to the improved antioxidation properties. However, the slag corrosion resistance
property is reduced due to the formation of the liquid phase
containing B2O3 that can easily dissolve into the molten slag.
Acknowledgments
The present work was supported by National Natural Science
Foundation of China (No. 51474166, 51672195), Program for Innovative Teams of Outstanding Young and Middle-aged Researchers
in the Higher Education Institutions of Hubei Province, China (No.
T201602). We would like to thank LetPub (www.letpub.com) for
providing linguistic assistance during the preparation of this
manuscript.
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