KE-HAN WU FABRICATION OF TITANIUM CARBIDE HAI-PENG …

Chemical Industry & Chemical Engineering Quarterly

Available on line at Association of the Chemical Engineers of Serbia AChE www.ache.org.rs/CICEQ

Chem. Ind. Chem. Eng. Q. 25 (1) 21−29 (2019) CI&CEQ

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KE-HAN WU1

HAI-PENG GOU2

GUO-HUA ZHANG1

KUO-CHIH CHOU1,2

1State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing,

China 2Collaborative Innovation Center of

Steel Technology, University of Science and Technology Beijing,

Beijing, China

SCIENTIFIC PAPER

UDC 669.018.9:669.1:544:66

FABRICATION OF TITANIUM CARBIDE REINFORCED IRON MATRIX CERMET BY VACUUM CARBOTHERMAL REDUCTION OF ILMENITE

Article Highlights • Vacuum carbothermic reduction was used to remove impurity elements Mg, Ca and Mn • After reduction, most of Si and part of Al were dissolved in the iron matrix • High reduction temperature helped increase hardness and bending strength of Fe-TiC

cermet • Both excessive and insufficient carbon were detrimental to performances of the cermet Abstract

Iron matrix cermet reinforced with TiC has been produced by vacuum carbo-thermal reduction of ilmenite followed by sintering processes. The influences of reduction temperature and carbon mass ratio were discussed in detail. X-Ray diffraction (XRD), electron probe micro-analyzer (EPMA) and scanning elec-tron microscope (SEM) with energy dispersive spectrometer (EDS) were emp-loyed to characterize the phase composition and microstructures. After carbo-thermic reduction, most of Mg, Mn, Ca evaporated from the sample; Si and part of Al was dissolved in the iron matrix. The obtained powders were used as the raw materials to produce TiC-Fe cermet by vacuum sintering. Density, hardness and bending strength of the samples were examined. The optimal cermet products after heat treatment had a density of 5.38 g·cm-1, a hardness of 1125.5 HV and a bending strength of 667 MPa, which was obtained at the carbon/ilmenite mass ratio of 0.378:1 at 1773 K under the pressure of 10 Pa.

Keywords: titanium carbide reinforced iron, ilmenite, vacuum, carbo-thermal reduction.

Ceramic-metal composites (cermet) have become the focus of present studies due to their excellent wear resistance and hardness, and admirable specific modulus and strength. Steel bonded carbide first appeared in the early 1960s as a creative cermet integrating the characteristics of both steel and carbide [1]. In various carbides, titanium carbide is a promising reinforcing material in cermet due to its high melting point, high chemical and thermal stabil-ity, excellent wear resistance and hardness [2-5].

Correspondence: G.-H. Zhang, State Key Laboratory of Adv-anced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China. E-mail: [email protected] Paper received: 24 November, 2017 Paper revised: 16 May, 2018 Paper accepted: 29 May, 2018

https://doi.org/10.2298/CICEQ171114015W

Fe/TiC cermet is a prospective cermet owing to its excellent wettability among most of metal/TiC cermets [6-11].

For decades, various methods have been emp-loyed to synthesize titanium carbide-reinforced Fe-based composites, such as carbothermal reduction [12,13], powder metallurgy [14], in situ synthesis [15- –18], and electrochemical synthesis [19]. In addition, several densification routes have been developed, including vacuum sintering, hot-pressing, microwave sintering, and spark plasma sintering [20-23]. Among these methods, the carbothermal reduction of ilmenite is one of the most promising routes due to the low cost of raw materials. After carbothermal reduction, vacuum sintering is an effective densification tech-nique because of its simplicity [24]. Welham and Wills put forward the production of TiN/TiC-Fe composites directly from ilmenite, and the phase evolution during

K.-H. WU et al.: FABRICATION OF TITANIUM CARBIDE REINFORCED IRON… Chem. Ind. Chem. Eng. Q. 25 (1) 21−29 (2019)

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carbothermic reduction process of ilmenite in Ar atmosphere was studied [25]. Gupta et al. found that the addition of FeCl3·6H2O provided nuclei of iron and increased the reaction rate during the reduction pro-cess [26].

In China, more than 90 wt.% of ilmenite is loc-ated in Panzhihua, Sichuan Province [27]. However, besides FeO, Fe2O3 and TiO2, there are many other components in ilmenite, such as MgO, SiO2, Al2O3, MnO and CaO. Especially, the content of MgO is as high as 6 wt.% [28]. Gou et al. found that it was hard to separate the main impurity element Mg by the con-ventional process [29]. MgAl2O4 and Mg2SiO4 existed in the products after carbothermic reduction of ilme-nite in Ar atmosphere and were hard to remove [30].

Even if many investigations have been done on the carbothermic reduction of ilmenite, most of them have focused on preparation of composite powders without extending their investigation on densification of the powders and mechanical performance testing. In this paper, vacuum carbothermal reduction served to remove the impurity elements (such as Mg, Mn) from Panzhihua ilmenite, and vacuum sintering served to densify the products. The hardness, density, and the bending strength of the cermet were measured.

MATERIALS AND METHODS

Activated carbon and ilmenite were used in the experiment. The activated carbon (analytical reagent) was supplied by Sinopharm Chemical Reagent Bei-jing Co., Ltd. The ilmenite was supplied by Panzhihua Iron and Steel (Group) Co., Ltd. Its compositions were detected by inductively coupled plasma atomic emis-sion spectrometry (ICP-AES) method and are shown in Table 1 [31-35]. To determine the contents of Al2O3 and SiO2, 0.2 g sample mixed with 3 g anhydrous sodium hydroxide was melted at 923 K in a nickel crucible. Then the sample was leached by hot deion-ized water, and after that leached by concentrated hydrochloric acid. After the salt was completely dis-solved, the above solution was diluted with deionized water to 200 ml for ICP-AES measurement. To det-ermine the contents of iron oxide and titanium oxide, the sample was leached in polytetrafluoroethylene cru-cible by aqua regia, sulfuric acid and hydrofluoric acid, with heating. Then the solution was measured by ICP-AES. The titration method with potassium dichro-mate was used to measure Fe3+ and Fe2+ contents.

Ilmenite and activated carbon were blended in a blender for 5 min with a rotating speed of 11000 rpm. The mixed powders, with the addition of polyvinyl alcohol solution (PVA, 3 wt.%), were compressed into cylindrical pellets of ∅ 18 mm×18 mm by uniaxial pressing in a stainless-steel die under 230 MPa. Then the pellets, together with alumina crucible, were put into an electric resistance furnace with a vacuum pressure control system. The sample was heated to the desired temperature at a heating rate of 5 K∙min−1 and held for 4 h, and then was cooled down to ambient temperature at a cooling rate of 5 K∙min−1. The detailed experimental parameters are shown in Table 2 where the No. 6L-8L was the product No. 6-8 reduced at low temperature (1673 K). The atmo-sphere of the system was maintained at 10 Pa by a vacuum gauge (ZF-VPM-1, Huachang eternal Beijing Vacuum Technology Co., Ltd.) for all of the reduction process. In previous work [27], it was found that air atmosphere (or without special control) and argon atmosphere had no difference in carbothermal reduct-ion when the pressure was maintained at 10 Pa. Therefore, by considering the cost, the air atmo-sphere was selected for the reduction process. The obtained products were examined by X-ray diffraction (XRD, Rigaku Ultima IV), scanning electron micro-scope (SEM, MLA-250, voltage 200 V-30 kV), and electron probe micro-analyzer (EPMA, JXA-8230, voltage 0.2 V-30 KV). Contents of main impurity ele-ments are shown in Table 3, where content of ele-ment Si was detected by sulfuric acid dehydration gravimetric method and contents of elements Al, Ca, Mg and Mn were detected by ICP-AES.

Table 2. Experimental conditions of carbothermic reduction in vacuum; pressure: 10 Pa; atmosphere: air; holding time: 4 h

Group Mass ratio (carbon:ilmenite) Temperature, K

No. 1 0.358:1 1773

No. 2 0.365:1 1773

No. 3 0.371:1 1773

No. 4 0.378:1 1773

No. 5 0.385:1 1773

No. 6 0.391:1 1773

No. 7 0.424:1 1773

No. 8 0.456:1 1773

No. 6L 0.391:1 1673

No. 7L 0.424:1 1673

No. 8L 0.456:1 1673

Table 1. Chemical compositions of Panzhihua ilmenite (wt.%)

Component FeO Fe2O3 TiO2 MgO SiO2 Al2O3 CaO MnO Total

Content 36.85 5.59 45.34 5.76 3.46 1.35 0.96 0.69 100


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Table 3. Contents of the main impurity elements in the products after vacuum reduction

Group Temperature,

K

Contents of the main impurity elements, wt.%

Si Al Ca Mg Mn

No. 0 – 2.19 0.97 0.93 4.70 0.73

No. 1 1773 2.58 1.70 0.33 0.023 <0.0005

No. 2 1773 2.55 1.78 030 0.037 <0.0005

No. 3 1773 2.60 1.55 0.31 0.044 <0.0005

No. 4 1773 2.54 1.23 0.25 0.037 <0.0005

No. 5 1773 2.55 1.32 0.23 0.038 <0.0005

No. 6 1773 2.58 1.30 0.37 0.0047 <0.0005

No. 7 1773 2.64 1.14 0.23 0.015 <0.0005

No. 8 1773 2.52 1.65 0.29 0.019 <0.0005

No. 6L 1673 2.62 1.12 0.58 0.27 <0.0005

No. 7L 1673 2.50 1.06 0.63 0.62 <0.0005

No. 8L 1673 2.50 1.01 0.65 0.78 <0.0005

In order to synthesize cermet, the powder pro-ducts after vacuum carbothermic reduction were milled with alcohol in air atmosphere at normal atmo-spheric temperature by a planetary ball mill (QM- -3SP2) with a rotating speed of 580 rpm for 8 h. The products after ball milling were measured by particle size analyzer (SA-CP3) and the results are shown in Table 4. It was found that both the particle sizes of iron and TiC reduced at 1773 K were larger than those obtained at 1673 K, owing to the faster rate of grain growth at higher temperatures. The particle size of iron was substantially decreased after ball milling. However, the particle size of TiC had no change after ball milling. The mixture of 5 g sample and 0.18 g PVA were compressed into a cylindrical pellet with the size of ∅ 18 mm×4 mm by uniaxial pressing in a stainless-steel die under 308 MPa. Then the pellet was placed into alumina crucibles which were put into an electric resistance furnace connected with a turbo molecular pump (TMP, JTFB 300). The samples were heated to 600 K at a heating rate of 2 K∙min−1 and held for 2 h to remove PVA. Then the samples were heated to 1673 K at a heating rate of 5 K∙min−1 and held for 6 h. The samples were ultimately cooled down to ambient temperature at a cooling rate of 5 K∙min−1. The vacuum pressure was maintained at 0.002 Pa during the whole sintering process. The obtained products were characterized by EPMA and SEM with energy dispersive spectrometer (EDS) after polishing. The Vickers hardness (HV) of the cermet was investigated by a micro-hardness tester (MH-6, 100X). The bending strength was examined by the three-point bending method (CDW-5, 5KN). The size of the bending test sample, distance between sup-

ports, and loading rate were 16 mm×2 mm×2 mm, 13.1 mm, 0.5 mm/min, respectively. Each group of samples was tested three times and then the average value was taken. The density was investigated by the Archimedes’ principle. The detailed experimental procedure flowchart is given in Figure 1.

Table 4. Effect of carbothermal reduction temperature and ball milling on the particle size of iron and TiC (μm)

Temperature, K Process Iron TiC

1673 Before ball milling 19.9 3.3

1673 After ball milling 7.8 3.3

1773 Before ball milling 55.6 7.5

1773 After ball milling 7.8 7.5

Figure 1. Experimental procedure flowchart.

RESULTS AND DISCUSSION

Vacuum carbothermal reduction

The theoretical contents of main impurity ele-ments in the products are shown as No. 0 in Table 3. It was calculated by assuming that final products were Fe, TiC, [Si], [Al], [Ca], [Mg] and [Mn], where [Si], [Al], [Ca], [Mg] and [Mn] were elements Si, Al, Ca, Mg and Mn dissolved in liquid iron, respectively. By compar-ing the actually measured contents of various ele-ments with the theoretical contents shown in Table 3, the products of different elements can be approx-imately deduced. If the measured content of one ele-ment was lower than the corresponding theoretical value, it was conjectured that the element was separ-ated by gas state. Therefore, it could be seen that element Mn was eliminated; contents of elements Mg, Ca substantially decreased and a high temperature


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was beneficial for the removals of them; elements Al, Si almost remained in the products, revealing that gas products Al2O and SiO were not generated during the reduction process. The volume fraction of TiC in this composite was about 65 vol.%, which was approx-imately calculated by Eqs. (1) and (2):

TiC TiC

TiC TiCTiC

TiC Fe TiC TiC

TiC Fe TiC Fe

100 100vol. %

1

W W

W W W Wρ ρ

ρ ρ ρ ρ

= = −+ + (1)

TiC TiCTiTiC Ti Si

Ti Si Ti

M MWW W WM W M

= = (2)

where WTiC is the mass fraction of TiC in the com-posite; WFe is the mass fraction of Fe in the com-posite; WTi is the mass fraction of element Ti in the composite; WSi is the mass fraction of Si in the com-posite (about 2.56% in Table 3); ρTiC is the density of TiC (4.93 g·cm-3) [36]; ρFe is the density of Fe (7.8 g·cm-3) [36]; MTiC is the relative molecular mass of TiC (60); MTi is the relative molecular mass of Ti (48); wTi/wSi is the mass ratio of element Ti to element Si in Panzhihua ilmenite (about 21:1 in Table 1).

XRD patterns of the products after vacuum red-uction are given in Figure 2. It was found that the pro-ducts were composed of a titanium carbide phase and α-Fe phase after reduction. However, there were graphite phases formed in products as shown in Figure 2a. The graphite phase would reduce the purity of Fe/TiC composites and the strength of the composites materials. Therefore, the mass ratio of carbon to ilmenite should be less than 0.424:1. It was also found that there was a Ti2O3 phase formed in products No. 1 and No. 2 which indicated the initial carbon content was not enough to reduce ilmenite completely, while the mass ratio of carbon to ilmenite was lower than 0.365:1.

The oxygen contents in Panzhihua ilmenite and products were measured by oxygen, nitrogen and hydrogen analyzer (EMGA-830 OK, HORIBA) and are shown in Table 5. The extent of reduction was cal-culated by Eq. (3):

O

O

100(1 )WW

α = −°

(3)

where α is the extent of reduction; WO is the mass fraction of oxygen in products; WO

o is the mass fraction of oxygen in ilmenite. It was found that when the mass ratio of carbon to ilmenite was larger than 0.378:1, the initial carbon content was enough to make the degree of reduction be over 99%. Whereas when the ratio was lower than this value, reduction was incomplete.

Figure 2. XRD patterns of product obtained after reduction:

a) No. 1-5; b) No. 6L-8L and No. 6-8.

Table 5. Oxygen content in the products and extent of reduct-ion after vacuum reduction

Group Oxygen content, wt.% Extent of reduction

Ilmenite 33.2 0%

No. 1 2.3 93.1%

No. 2 1.4 95.8%

No. 3 0.7 97.9%

No. 4 0.3 99.1%

No. 5 0.3 99.1%

No. 6 0.3 99.1%

No. 7 0.3 99.1%

No. 8 0.3 99.1%

No. 6L 1.0 97.0%

No. 7L 1.0 97.0%

No. 8L 0.9 97.3%


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SEM images of the products after vacuum red-uction with different molar ratios of ilmenite to carbon were almost the same and Figure 3 shows the typical pictures of products No. 6L and No. 6, a light white area and grey area were iron and titanium carbide by EDS analysis, respectively. It was found that particle sizes of TiC obtained at 1773 K were about 7 μm, larger than those obtained at 1673K (3 μm), owing to fast crystal growth rate at high temperatures. The contents of the main elements in the iron area were investigated by EPMA and are given in Table 6. It was found that, in the iron area, contents of elements Fe, Si, Al and C were about 91.9, 6.0, 0.9 and 0.5 wt.%, respectively. The contents of these elements in the iron area were not affected by temperature and car-bon ratio essentially. It was worth noting that 6.0 wt.% of Si content in liquid iron was not enough to form the SiC phase. The critical silicon content for SiC and C saturated Fe-Si-C melts was about 22.4 wt.% by Chipman et al. at 1763 K [37]. Only when the content of Si in liquid iron was higher than this value, SiC could be formed. Based on the above analyses, SiC cannot be formed during the vacuum reduction pro-cess. Therefore, nearly all of the element Si should be dissolved in the iron matrix. Besides, the element compositions were detected by EPMA, and it was found that only Ti and C existed in the titanium car-bide phase.

Figure 3. SEM images of product obtained after reduction: a) No. 6L; b) No. 6.

Data of Al and Si shown in Tables 3 and 6 rep-resent the contents of Al and Si in the products and the iron matrix after the reduction process, respect-ively. If all the Al and Si existed in the iron matrix, the mass ratio of Al:Si in Tables 3 and 6 should be the same. However, it was found that the mass ratio of Al:Si shown in Table 6 was about 1:5~8, while that shown in Table 3 was about 1:2, which indicated that not all of the element Al existed in the iron matrix. A certain amount of element Al may still exist in the form of oxides.

Table 6. Contents of the main elements in the iron area after reduction and sintering

Group T / K Process Contents of the main elements,

wt.%

Fe Si Al Ti C

No. 1 1773 After reduction 92.4 5.35 1.10 0.75 0.38

After sintering 92.5 5.44 0.22 1.47 0.36


After sintering 91.2 6.84 0.23 1.36 0.41


After sintering 91.2 6.82 0.21 1.41 0.40


After sintering 91.2 6.51 0.30 1.65 0.37


After sintering 91.8 5.55 0.24 2.01 0.43


After sintering 86.6 10.94 0.18 1.97 0.35


After sintering 91.5 5.53 0.05 1.99 0.95


After sintering 90.7 7.00 0.21 1.48 0.60

No. 6L 1673 After reduction 92.3 5.64 1.03 0.60 0.40

After sintering 89.2 8.00 0.13 2.15 0.47


After sintering 92.1 5.57 0.01 1.79 0.50


After sintering 91.7 6.24 0.20 1.44 0.38

According to the above analyses, it was specul-ated that Eqs. (4)-(11) may happen during the red-uction process. The changes in Gibbs energy of dif-ferent reactions under 10 Pa are given in Figure 4. It was found that at 1673 or 1773 K, the thermodynamic reduction sequence of the different oxides was Fe2O3 > FeO > MnO > TiO2 > MgO > SiO2 > CaO > Al2O3. Most of Mg, Mn, Ca evaporated from the sample, while Si and part of Al were dissolved in the iron matrix:

(solid) (solid) (gas) (liquid)FeO C CO Fe+ = + (4)

( (CO) 10 Pa) 232.18 159580pG T=Δ = − +

2 3(solid) (solid) (gas) (liguid)1 1

Fe O C CO Fe3 3

+ = + (5)

( (CO) 10 Pa) 248.25 162012pG T=Δ = − +

2(solid) (solid) (gas) (solid)1 3 1

TiO C CO TiC2 2 2

+ = + (6)

( (CO) 10 Pa) 242.32 260711pG T=Δ = − +

(solid) (solid) (gas) (gas)MgO C CO Mg+ = + (7)


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( (CO) (Mg) 10 Pa) 445.09 617233p pG T= =Δ = − +

(solid) (solid) (gas) (gas)MnO C CO Mn+ = + (8)

( (CO) (Mn) 10 Pa) 430.37 531589p pG T= =Δ = − +

2 3(solid) (solid) (gas)1 2

Al O C CO [Al]3 3

+ = + (9)

( (CO) 10 Pa) 270.45 445930pG T=Δ = − +

2(solid) (solid) (gas)1 1

SiO C CO [Si]2 2

+ = + (10)

( (CO) 10 Pa) 261.01 357924pG T=Δ = − +

(solid) (solid) (gas) (gas)CaO C CO Ca+ = + (11)

( (CO) (Ca) 10 Pa) 435.38 678070p pG T= =Δ = − +

where ΔG is the change of Gibbs energy of reaction [36]; T is the reaction temperature; p(CO), p(Mg), p(Ca) and p(Mn) are the partial pressures of CO, Mg, Ca and Mn, respectively.

Figure 4. Changes of Gibbs energy of different reactions.

Vacuum sintering

SEM images of products No. 6L-8L and No. 6-8 after vacuum sintering are given in Figure 5 and the volume fraction of TiC in Figure 5 calculated by Image-Pro Plus 6.0 Software was given in Table 7. This value was roughly in consistency with the value calculated by chemical compositions in Eqs. (1) and (2). As shown in Figure 5, the size of TiC particles at 1773 K was larger than that at 1673 K. Furthermore, the pores of products reduced at 1773 K in Figure 5 was also less than that at 1673 K, which indicated that high reduction temperature was beneficial for the densification of products during sintering. It was worth noting that several phases of different shapes and

brightness were observed in Figure 5, including a continuous white area, grey angularity, small black sphere, thin longish black strip and the large black pore. Map scanning analyses by SEM and EDS were used to determine these phases as shown in Figure 6. It was found that element Si was dissolved in the iron matrix; element Ti was combined with carbon as TiC; element Al was combined with oxygen as Al2O3. Accordingly, the continuous light white area, grey angularity, small black sphere and thin longish black strip in Figure 5 were iron, TiC, Al2O3 and graphite, respectively. The graphite should be precipitated from the iron phase during cooling. The existence of Al2O3 could decrease the purity of Fe/TiC cermet. However, the particle size of Al2O3 was ultrafine (<1 μm) which contributed to the pinning effect on the grain bound-ary [38]. Accordingly, the existence of Al2O3 could reinforce the strength of Fe/TiC cermet [39].

Figure 5. SEM images of as-prepared cermets: a) No. 6L, 1673 K, AC/Ilmenite 0.391:1; b) No. 6, 1773 K, AC/Ilmenite 0.391:1;

c) No. 7L, 1673 K, AC/Ilmenite 0.424:1; d) No. L, 1773 K, AC/Ilmenite 0.424:1; e) No. 8L, 1673 K, AC/Ilmenite 0.456:1;

f) No. 8, 1773 K, AC/Ilmenite 0.456:1.


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Table 7. Volume fraction of TiC in SEM patterns of products No.6L-8L and No. 6-8

Group No. 6L No. 6 No. 7L No. 7 No. 8L No. 8

vol.(TiC) % 67.2 65.4 64.7 63.5 68.3 61.9

Figure 6. SEM images with map scanning analyses.

Contents of the main elements after sintering in the iron area investigated by EPMA are given in Table 6. By contrasting the contents of various elements before and after sintering shown in Table 6, it was found that, in the iron area, the contents of elements Fe and Al decreased during the sintering process, while the contents of elements Si and Ti increased. Content of C was almost unchanged. Iron existed in liquid state at high temperature. Under a high vacuum degree during the sintering process, Fe could be vol-atilized in the gas phase. Therefore, the loss of Fe could be attributed to the evaporation under high vacuum conditions [40]. This evaporation of iron also resulted in the slight increase of Si and Ti.

Properties

Hardness, density, and bending strength of the obtained cermet are given in Table 8. The bending strength of No. 8L was too low to test. Considering the data shown in Table 8, it was found that both the reduction temperatures and initial carbon ratio had an effect on the properties of the final products. By con-trasting the density data in Table 8, it was found that a high reduction temperature was beneficial for dec-reasing the porosity of the product, which was also revealed by comparing Figure 5a, c and e with Figure 5b, d and f. The dense structure improved the adhe-sion strength of the interface between titanium car-bide and iron matrix and decreased dislocation move-ments within the iron matrix. Therefore, high tempe-ratures could enhance the hardness and bending strength by the aforementioned effects.

Table 8. Properties of the obtained cermet

Group T / K Density g·cm-3

Micro Vickers hardness

Bending strengthMPa

No. 1 1773 4.77 648 291

No. 2 1773 5.10 733 326

No. 3 1773 5.40 1001 603

No. 4 1773 5.38 1126 667

No. 5 1773 5.33 953 597

No. 6 1773 5.30 950 509

No. 7 1773 5.30 950 509

No. 8 1773 5.12 705 412

No. 6L 1673 5.00 648 138

No. 7L 1673 4.50 644 149

No. 8L 1673 3.84 641 N/A

Figure 7 shows the effect of carbon ratio on the properties of the products. Each point was tested three times and then the average value was taken.

Figure 7. The relationship between the carbon ratio and properties of the products No. 1-8.


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While the mass ratio of carbon to ilmenite was beyond 0.378:1, excess carbon was essentially a weakened phase with a low density and low hardness in the cer-met. The existence of residual carbon could decrease both the density and the mechanical properties. While the mass ratio was lower than 0.371:1, carbon was insufficient to reduce ilmenite completely. The lack of carbon resulted in the existence of Ti2O3, which was another brittle phase with low density, low hardness, and poor wettability with the iron matrix [41]. There-fore, deficient carbon could also decrease both the density and the mechanical properties, too. Conse-quently, a high reduction temperature could increase density, hardness, and bending strength, while both excessive and insufficient carbon decreased them. The highest hardness was 1125.5 HV while the bend-ing strength and density were 667 MPa and 5.38 g·cm-3, respectively. This hardness data was close to the others’ works, as shown in Table 9 [14,20,42,43].

Table 9. Hardness of TiC/Fe cermet from reference [14,20,42,43]

Sintering temperature, K

TiC (vol.%)

Substrate material Hardness

HV

1703 61 35CrMo steel 1249

1693 62 High manganese steel

1004

1623 61 465 stainless steel 920

1623 70 465 stainless steel 1140

Self-propagating synthesis

52 High manganese steel

600

Figure 8 shows the cross-section perpendicular to the fractured surfaces in product No. 4 after bend-ing testing. Transgranular cracking through TiC was found which revealed that TiC was the brittle phase and TiC had excellent bonding with the iron matrix. Each TiC particle failed essentially by cleavage frac-ture with well-defined facets [2].

Figure 8. SEM images showing the cross-sections perpen-

dicular to the fractured surfaces after the bending test.

CONCLUSION

In this paper, iron matrix cermet reinforced with TiC (65 vol.%) has been produced by carbothermal vacuum reduction of ilmenite followed by the sintering process. The main conclusions were drawn as follows: 1. During the reduction process, the element Mn

was eliminated; contents of elements Mg, Ca substantially decreased and the high temperature was beneficial for their removal; element Si and part of element Al were dissolved in the iron mat-rix.

2. The high reduction temperature helped increase the density, hardness and bending strength, while both excessive and insufficient carbon decreased them.

3. The optimal cermet with a density of 5.38 g·cm-1, a hardness of 1125.5 HV, and bending strength of 667 MPa was obtained at the carbon/ilmenite mass ratio of 0.378:1 at 1773 K under the pres-sure of 10 Pa.

Acknowledgements

This work was supported by the Natural Science Foundation of China (No. 51734002 and U1460201).

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KE-HAN WU1

HAI-PENG GOU2

GUO-HUA ZHANG1

KUO-CHIH CHOU1,2

1State Key Laboratory of Advanced Metallurgy, University of Science and

Technology Beijing, Beijing, China 2Collaborative Innovation Center of

Steel Technology, University of Science and Technology Beijing,

Beijing, China

NAUČNI RAD

IZRADA KERMETA TITAN-KARBIDOM OJAČANE MATRICE GVOŽĐA VAKUUM KARBOTERMIČKOM REDUKCIJOM ILMENITA

Kermet titan-karbidom ojačane matrice gvožđa je proizveden vakuum karbotermičkom redukcijom ilmenita i procesima sinterovanja. Detaljno su razmatrani uticaji temperature redukcije i masenog udela ugljenika. Za karakterizaciju sastava faze i mikrostruktura korišćeni su rentgenska difrakcija (XRD), elektronska sonda mikro-analizatora (EPMA) i skenirajući elektronski mikroskop (SEM) sa energetskim disperzijskim spektrometrom (EDS). Nakon karbotermičke redukcije, veći deo Mg, Mn i Ca je ispario iz uzorka; Si i deo Al je rastvoren u gvožđevoj matrici. Dobijeni praškovi su korišćeni kao sirovina za proizvodnju TiC-Fe kermeta vakuumskim sinterovanjem. Ispitane su gustine, tvrdoća i čvrstoća savijanja uzoraka. Optimalni proizvodi kermeta nakon toplotne obrade imaju gustinu od 5,38 g/cm, tvrdoću 1125,5 HV i čvrstoću savijanja od 667 MPa, koja je dobi-jena pri masenom odnosu ugljenik/ilmenit 0,378:1 na 1773 K i pritisku 10 Pa.

Ključne reči: gvožđe ojačano titan-karbidom; lmenit; vacuum; karbotermička redukcija.

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