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Effect of Fe–Mo–Cr pre-alloyed powder on the microstructure and mechanical properties of TiC–high-Mn-steel cermet
International Journal of Refractory Metals & Hard Materials 84 (2019) 105031
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Effect of Fe–Mo–Cr pre-alloyed powder on the microstructure and mechanical properties of TiC–high-Mn-steel cermet Guoping Lia, Hao Yanga, Yinghai Lyub, Haojun Zhoua, Fenghua Luoa, a b
T
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State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China Department of Bioengineering, College of Chemical and Environmental Engineering,Shandong University of Science and Technology, Qingdao 266590, China
A R T I C LE I N FO
A B S T R A C T
Keywords: TiC Fe-Mo-Cr pre-alloyed powder Microstructure Mechanical properties Toughening
In the study, a TiC–high-Mn-steel cermet is fabricated using FeeMo and Fe–Mo–Cr pre-alloyed powders as metallic binders by powder metallurgy techniques. The effect of Cr on the microstructure and mechanical properties of the cermet is studied and the cermet preparation process is optimized. The microstructure and fracture morphology of the cermets are observed with scanning electron microscopy, while phase identification and analysis are performed by X-ray diffractometry. The results show that the particles of Cr-free cermet are angular and polygonal, while those of cermet with added Cr are rounded and ellipsoidal. The grains of Cr-free cermet are larger than those of the Cr-added cermet, which is unlike similar conventional cermets. The grain size of the Cr-added cermet increases slightly with increased Cr content. In addition, the relative density of the cermet decreases slightly with increased Cr content. The hardness of cermet is maximized at HRC 64.8 with the Cr content of 1.0 wt%; with further increases in Cr, the hardness decreases gradually. The transverse rupture strength and impact toughness first increase and then decrease with increasing Cr content, reaching the maxima of 2355 MPa and 13.42 J/cm2, respectively, at the Cr content of 1.5 wt%. The strength and toughness of the cermet are improved greatly compared to those of conventional similar TiC–high-Mn-steel cermets.
1. Introduction Because TiC- and/or Ti(C/N)-based cermets show excellent properties such as high hardness, good thermal resistance, superior stability at elevated temperature, and excellent wear and oxidation resistance, they are considered promising materials for tools, wear-resistant parts, extrusion dies and punches, as well as components for surface finishing, high-speed milling, and machining of both carbon and stainless steels [1–6]. However, TiC and Ti(C,N)-based cermets are limited in applicability because they have relatively low toughness and strength [7]. To overcome this limit, a strong and ductile binder phase could be used to fabricate dense TiC- or Ti(C,N)-based cermets with high toughness and strength [2]. Metals (Fe, Ni, Co, Cr, Ti, Mo, and alloys thereof) can be used as binders in TiC- and Ti(C,N)-based cermets [8–10]; the binder chemical composition can be selected depending on specific requirements for practical applications [3]. Fe-based alloys are the most commonly used as binders because of their low cost, non-toxicity, and good fracture toughness [3,6,10]. In this research, TiC–high-Mn-steel cermets were manufactured by powder metallurgy techniques with different Cr contents. Mo and Cr were added to the cermet in the form of FeeMo or Fe–Mo–Cr pre-
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alloyed powders. Pre-alloyed powder-form metallic binders are advantageous in guaranteeing uniformity of the composition and microstructure of the prepared cermet compared to conventional cermets. However, most cermet producers do not produce their own pre-alloyed powders, preferring to add pure metal powders to cermets. Additionally, the specifications of pre-alloyed powders are significantly limited; frequent chemical composition variations in the cermet can be very inconvenient. Many researchers have confirmed that Mo is indispensable in TiCand Ti(C,N)-based cermets because it improves the wettability of metallic binders on the ceramic phase, refines the ceramic phase grain size, and enhances the mechanical properties of the cermet [11–15]. Some studies have demonstrated that transition-metal carbides can also improve the metallic binder wettability on the ceramic phase; among these carbides, Mo2C is considered the most effective additive and applied frequently to TiC- and Ti(C,N)-based cermets [16–21]. Cr is also important in high-Mn steel, but it is seldom used in TiC–high-Mn-steel cermets, because Cr deteriorates the sinterability and mechanical properties of the cermet by its strong affinity with oxygen [22]. Cr also decreases the eutectic temperatures of alloys, causing liquid phases to appear at lower temperatures and coexist with solid phases for long
Corresponding author. E-mail addresses: [email protected] (G. Li), [email protected] (F. Luo).
https://doi.org/10.1016/j.ijrmhm.2019.105031 Received 4 July 2019; Received in revised form 18 July 2019; Accepted 22 July 2019 Available online 23 July 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 84 (2019) 105031
G. Li, et al.
Table 1 Main characteristics of the starting powders. Powders
Purity
Oxygen
(μm)
(wt%)
(wt%)
≥99.5 ≥99.8 ≥99.6 ≥99.9 ≥99.5
≤0.28 < 0.10 < 0.12 < 0.01 < 0.2
a
TiC Ni Mn/Fe Graphite Fe–Mo–Cr a
Particle size
3.1 43 43 30 147
Manufacturer
Zhuzhou GuangYuan Cemented Material Co., Ltd., China Shanghai Xtnami Science & Technology Co., Ltd., China Jinzhou Honda New Material Co., Ltd., China Qingdao Baichuan Graphite Co., Ltd., China Own manufacturing
Fisher particle size.
was observed by SEM in secondary electron (SE) mode. The core–rim structure was observed by high-resolution transmission electron microscopy (HRTEM); the chemical composition of the core–rim structure and binder was determined with an energy-dispersive X-ray spectroscope (EDS). Phase identification of each sample was performed by Xray diffraction (XRD; TD-3500, China). The densities of the bulk specimens were measured by the Archimedes method. The hardness was tested by a Rockwell hardness tester (HR-150B, China). The transverse rupture strength (TRS) was determined by a universal material testing machine (WDW-100, China) using specimens of 5 mm × 5 mm × 35 mm with the span distance of 20 mm and the crosshead velocity of 0.5 mm/min. Impact toughness (IM) testing was performed by an impact testing machine (JBW, China) using specimens of 10 mm × 10 mm × 55 mm in size.
periods; this induces complex reactions in sintering [22]. Finally, Cr can react with carbon or graphite to form carbides or compound carbides that can strengthen and improve the mechanical properties of the cermet. Based on these considerations, Mo and Cr are selected as alloying elements to produce FeeMo and Fe–Mo–Cr pre-alloyed powders in this study. The effect of Fe–Mo–Cr pre-alloyed powders on the microstructure and mechanical properties of TiC–high-Mn-steel cermets has not yet been reported. In this study, TiC–high-Mn-steel cermets were manufactured by conventional powder metallurgy techniques using FeeMo or Fe–Mo–Cr pre-alloyed powders as binders and the microstructure and mechanical properties of the cermets were investigated and discussed. 2. Experimental procedure
3. Results and discussion
In the present study, TiC–50 wt% high-Mn-steel cermets using FeeMo or Fe–Mo–Cr pre-alloyed powders as the binder with additives including Ni, Mn, and graphite, among others, were manufactured by powder metallurgy techniques. The main characteristics of the starting powders are listed in Table 1; all starting powders are industrial grade to reduce cost. The nominal chemical compositions of the cermets are shown in Table 2. The Mo content of the pre-alloyed powder is optimized according to a previous study [23]. The pre-alloyed powders of Fe–3.0Mo and Fe–3.0Mo–xCr (x = 1.25, 2.5, 3.75, and 5.0 wt%) were used as the binders for cermet A and cermets B, C, D, and E, respectively. The mixed powders were wet-milled using stainless steel balls in an ethyl alcohol bath in a planetary ball mill for 24 h. The rotation speed of the planetary ball mill was 220 rpm and the weight ratio of milling balls to powder was 7:1. The obtained slurry was dried in a vacuum oven at 85 °C for 12 h, and 2 wt% rubber was added to the powder mixture as a forming agent in the form of rubber-gasoline solution. The powder mixture was pressed into a columnar green compact under the uniaxial pressure of 180 MPa. The green compacts were dewaxed and then sintered in vacuum. Hydrogen was used in the dewaxing process to reduce the oxygen content of the green compacts to improve the wettability of the metallic binder on the ceramic phase during the subsequent sintering. The dewaxed green compacts were vacuum-sintered at 1430 °C for 60 min and then low-pressure treated with a pressure of 1 MPa for 40 min by Ar. The bulk specimens were then machined and polished for testing. The microstructure of the polished bulk specimens was observed by scanning electron microscopy (SEM; FEI Nova NanoSEM, America) in backscattered electron (BSE) mode and the fracture surface morphology
3.1. Investigation and optimization of preparation process In this research, the Fe–3.0Mo and Fe–3.0Mo–xCr (x = 1.25, 2.5, 3.75, and 5.0 wt%) pre-alloyed powders were produced according to the demands of industrial-scale production to reduce the cost of the starting powders. The particle size of the Fe–3.0Mo and Fe–3.0Mo–xCr pre-alloyed powders is larger than that of the Fe powder used in conventional TiC–high-Mn-steel cermet. The weight ratio of milling balls to powder must therefore be increased to strengthen the milling effect. The pre-alloyed powder has low activity compared to pure metal powder; the experimental cermet preparation process was adjusted correspondingly. The temperature–time dewaxing and sintering curves of the green compacts are given in Figs. 1 and 2, respectively. Although ball milling was performed in an ethyl alcohol bath, oxidizing of the powder mixture was inevitable during preparation. This decreased the wettability of metallic binder on ceramic phase, retarded densification, and deteriorated the mechanical properties of the cermet. The gas dewaxing of the green compacts in hydrogen decreased the oxygen content significantly to improve the wettability of metallic binder on ceramic phase and promote densification of the cermet. Fig. 2, the sintering curve, shows two important insulation stages. The first at 1100–1150 °C corresponds to the reduction of oxygen in Fe powder by carbon–oxygen reaction. Considering the oxidation of Cr and Mo, the higher temperature in this insulation stage of 1150 °C is selected. The other insulation stage occurs approximatelyat 1300 °C; the metallic binder is molten or semi-molten at this temperature, and the solid-liquid change is less abrupt and occurs within a certain temperature range. The so-called molten or quasi-molten liquid flows in the spaces between ceramic particles, although it differs somewhat from the ordinary liquid state. The flowing of the metallic binder separates ceramic particles from each other and promotes the diffusion, migration, and deposition of alloying elements on ceramic particles to form rim structures that primarily inhibit coalescencegrain growth of the cermet. The rim structure continues to develop through the dissolution–precipitation mechanism to improve the wettability between the ceramic phase and metallic binder. The grain growth of the cermet
Table 2 Nominal chemical compositions of the experimental cermets (wt%). Cermets
TiC
Ni
Mo
Cr
Mn
C
Fe
A B C D E
50 50 50 50 50
2.25 2.25 2.25 2.25 2.25
1.19 1.19 1.19 1.19 1.19
0 0.5 1.0 1.5 2.0
7.0 7.0 7.0 7.0 7.0
0.7 0.7 0.7 0.7 0.7
Bal. Bal. Bal. Bal. Bal.
2
International Journal of Refractory Metals & Hard Materials 84 (2019) 105031
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Fig. 1. Gas dewaxing temperature–time curve of green compacts. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
insulation stage is not fixed and related closely to the binder chemical composition, which varies within a certain temperature range. Finally, the sintering temperature of the experimental cermet is higher than that of a similar conventional cermet because of the low activity and large particle size of the pre-alloyed powder.
by liquid sintering is hindered with complete formation of a sufficiently thick rim structure. Two grain growth mechanisms occur in the cermets, namely coalescence grain growth and dissolution–precipitation grain growth, the latter is also called Ostwald ripening [24]. Generally speaking,coalescence grain growth of the cermet occurs during solid sintering because of poor wettability of the metallic binder on the ceramic phase. Therefore, inhibitingcoalescence grain growth of the cermet is important. The application of the pre-alloyed powder significantly facilitates inhibiting coalescence grain growth of the cermet. In this study,holding at an insulation stage while the metallic binder is molten or quasi-molten is found to be crucial. Of course, the temperature of the
3.2. Microstructure and phase structure identification The microstructures of cermets with different Cr contents are shown in Fig. 3. It can be seen that the average grain size of the ceramic phase of the Cr-free cermet A is greater than those of Cr-added cermets B, C, D, and E. Previous studies have confirmed that Cr reduces the eutectic
Fig. 2. Sintering temperature–time curve after gas dewaxing of the cermets. 3
International Journal of Refractory Metals & Hard Materials 84 (2019) 105031
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Fig. 3. SEM (BSE) micrographs of the cermets with varied Cr content: (a) 0, (b) 0.5 wt%, (c) 1.0 wt%, (d) 1.5 wt%, and (e) 2.0 wt%.
size, and the metallic binder content and composition. Obtaining comprehensive and correct results considering only a single influencing factor is infeasible. The particle shape of cermets B, C, D, and E is round orellipsoidal, consistent with previous observations [16,22]. The microstructures of the Cr-added cermets are homogeneous without abnormal grain growth, which may greatly improve the mechanical properties of the cermets. Fig. 3 shows the microstructural characteristics of ceramic-phase particles embedded in metallic binder. The black regions in the micrograph are ceramic particles, while the gray regions are the metallic binder. Core–rim structures are observed in the five cermets, typical of TiC- and Ti(C,N)-based cermets. The formation mechanism of rim structures in TiC- and Ti(C,N)based cermets is complex and subject to some argument [11,25]. The black core structures in the cermet are remnant undissolved large TiC particles; the gray rim structures form during sintering. Some reports state that the gray rim structure comprises inner and outer rims formed during solid- and liquid-state sintering, respectively [12,13,26]. The amounts of heavy elements such as Mo and W are higher in the inner rim than in the outer rim [26]. However, the rim structure in the five cermets fabricated here is less distinct than those in conventional TiCbased cermets with an unclear border separating the inner and outer rim. The low magnification and resolution of the test instrument may
temperature of alloys and initiates earliergrain growth of the cermet; generally, the grain size of Cr-added cermet is larger than those of Crfree cermets [16,22]. However, the results observed here differ from this trend. The uniform distribution of Mo in the binder may contribute to this difference. With liquid formation, Mo precipitates on the surface of ceramic-phase grains to form rim structures that inhibit the dissolution of the ceramic phase and suppress grain growth. The liquid phase appears earlier because of the Cr additive, but the ceramic grains of the cermet show less growth than those of similar conventional cermets. The grain size of the cermets increases only slightly with increasing Cr content. Cermet A shows angular and polygonal particles, which are unlike those of similar conventional cermets prepared using pure metal powder as additives. The FeeMo pre-alloyed powder is inferred to induce this phenomenon. The cermet with larger-sized and angular particles has both advantages and disadvantages. The cermet is probably tougher than other similar cermets because larger particles can withstand larger impact loads and cracks tend to propagate through the binder because of the high stiffness of the ceramic phase. However, more defects may be present in the particle–binder interface, which deteriorate the mechanical properties of the cermet. The properties of the cermet all result from the competition of influencing factors such as chemical composition, content, ceramic-phase distribution and grain 4
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strengthen the cermet. In order to further investigate the phases, XRD analysis of the cermets with varied Cr content was performed, with resulting patterns shown in Fig. 5. All peaks in the XRD patterns are associated with TiC and Fe in the binder; other elements, such as Ni, Mn, and carbon (graphite), are dissolved in the binder. Mo and Cr are dissolved completely in Fe during preparation of the pre-alloyed powder by atomization. Mo and Cr react with TiC to form (Ti,Mo)C and/or (Ti,Mo,Cr)C solid solutions; these have the same crystal structure as TiC and similar lattice parameters, so the XRD peak position of TiC changes negligibly. The Fe-based binder contains other metals, but accurate identification of the type of carbide and solid solution is difficult, so overall, it presents the peaks of ɑ-Fe. 3.3. Density and mechanical properties The densities, hardness, TRS, and IM of the experimental cermets were measured at room temperature, as shown in Figs. 6 and 7. Fig. 6 shows that thedensity of cermet decreases slightly with increases in Cr content; Cr negatively affects the sinterability. The hardness of the cermets first increases and then decreases with increases in Cr content, reaching the maximum value of HRC 64.8 at the Cr content of 1.0 wt%. The hardness of the cermet is not correlated to density because of the strengthening of the binder by Cr. In practical situations, TiC–high-Mnsteel cermets are typically applied in scenarios subject to violent impact and vibration; therefore, IM is a more valuable characteristic than fracture toughness. Fig. 7 shows that the TRS and IM of the cermets all increase first and then decrease with increases in Cr content, reaching maxima of 2355 MPa and 13.42 J/cm2, respectively, at theCr content of 1.5 wt%. With further increase in Cr, the TRS and IM decrease remarkably, because the cermet becomes brittle with excessive Cr. The conventional TiC–high-Mn-steel cermet TM52 shows the TRS and IM of 1800–2000 MPa and 8–10 J/cm2, respectively. Compared to TM52, the TRS and IM of the experimental cermets are improved by > 20% and 40%, respectively. In addition, the TRS and IM values of the Cr-added cermets are higher than those of the Cr-free cermet because of the strengthening provided by Cr. Fig. 8 shows the impact fracture surface morphologies of the experimental cermets. The impact fracture surfaces of the cermets show mixed transgranular and intergranular fracture. A previous study indicated that intergranular fracture was the dominant failure mode for cermets because the interface and binder phase are weak regions [28]; therefore, strengthening the interface and binder is a primary approach to improve cermet toughness. In this study, transgranular fracture is also apparent because many of the ceramic particles are split under impact. In situations of instantaneous and violent impact, cracks propagate rapidly and incompletely in the binder, so more ceramic particles are split; this positively affects the IM. With Cr additive and increases in its concentration, ductile tearing occurs within the binder because of the strengthening effect of Cr. The toughening mechanisms include crack deflection, branching, bridging, and so on. A previous study showed that the contribution of the binder to fracture toughness was much greater than that of the ceramic phase [29]. Therefore, strengthening the cermet binder is necessary and effective during crack initiation and propagation. During fracture, the fracture resistance of the ceramic phase is much greater than that of the binder. Because the grains of cermet are fine in size and the binder is strengthened by added Cr, more ceramic particles are split via the transgranular fracture mode; the IM is therefore improved significantly, especially for cermets applied in situations with violent impact and vibration. The strengthening effect of Cr benefits the binder and modifies the deformation and fracture behavior of the ceramic phase. In addition to strengthening the binder, the toughening effect is also attributed to the strengthening ceramic phase of the cermet, especially in violent impact and vibration situations. For example, for TiC- and Ti (C,N)-based cermets with WC additives, WC improves the wettability of
Fig. 4. High-resolution TEM image of cermet D. Letters C, R, and B indicate core, rim, and binder, respectively. Table 3 TEM/EDS results of the core–rim structure and the binder of cermet D. Elements (at.%)
Core
Rim
Binder
C Mo Ti Cr Mn Fe Ni
46.2 0.4 52.6 0.3 0 0.4 0.1
37.6 1.6 59.5 0.7 0.1 0.5 0.1
16.1 0.5 1.9 3.3 7.4 62.6 8.2
contribute to this lack of clarity. Another contributing factor is the low concentrations of Mo and Cr in the cermets because of the large volume fraction of the pre-alloyed powder. To further investigate the core–rim structures and analyze the chemical composition of the cermet, a highmagnification TEM image of cermet D is shown in Fig. 4. The chemical compositions of the core, rim structure, and binder of the cermet are tested and shown in Table 3. The rim structure of cermet D is very distinct in Fig. 4. Table 3 shows that the core mainly comprises Ti and C, indicating that the cores are undissolved TiC particles remaining after sintering. The contents of other elements such as Cr, Mo, Ni, and Fe are very low, and no Mn is found in the core. Some trace elements may appear because of the test instrument, leading to erroneous results. The chemical composition of the rim structure indicates that it has the highest Mo content, forming a (Ti,M)C (where M = Mo, Cr, Mn, Fe, and other metals) solid solution that improves the wettability of the metallic binder on the ceramic phase and promotes densification of the cermet, which is consistent with previous studies [13,14,21]. Previous research has also demonstrated high dislocation densities in the rim structures; these improve the mechanical properties of the cermet [27]. However, the thickness of the rim structure must be controlled to avoid large interior stresses arising from incoherence with the ceramic-phase core; such stresses could deteriorate the mechanical properties of the cermet. The chemical composition of the binder shows the highest Cr content; a small quantity of Cr is present in the rim structure of the ceramic phase to form a (Ti,Mo,Cr)C solid solution. Generally, Cr, Mo, Ti, Mn, graphitic carbon, and other elements dissolve in the binder to form a solid solution that modifies the mechanical properties of the cermet. The dissolved atoms may also form carbides and/or compound carbides to 5
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Fig. 5. XRD patterns of the cermets with varied Cr content: (a) 0; (b) 0.5 wt%; (c) 1.0 wt%; (d) 1.5 wt%; (e) 2.0 wt%.
Fig. 6. Relative density and hardness of the cermets as a function of Cr content.
because WC is more rigid than TiC; therefore, the toughness of the TiCbased cermet is increased. However, WC is seldom used because of its high cost.
the metallic binder on the ceramic phase, refines the ceramic phase, and strengthens the metallic binder effectively [17,21,30]. The toughening role of WC in TiC-based cermets is also attributed to the formation of a (Ti,W)C rim structure that increases the rigidity of the ceramic phase, 6
International Journal of Refractory Metals & Hard Materials 84 (2019) 105031
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Fig. 7. TRS and IM of the cermets as a function of Cr content.
4. Conclusions
(2) The particle size of the cermet using FeeMo pre-alloyed powder is much larger than that of the cermet with Fe–Mo–Cr pre-alloyed powder. However, the particle sizes of the Cr-containing cermets are increased slightly with increases in Cr. The particles in the Crfree cermet are angular and polygonal, while those of the Cr-containing cermets are rounded and ellipsoidal. With compositional
(1) The preparation of experimental cermets is optimized with a sintering temperature exceeding that used for conventional cermets with similar chemical compositions because the pre-alloyed powder is less active than pure metallic powders.
Fig. 8. Fracture surface morphologies of the experimental cermets. 7
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variations in Mo and Cr in the pre-alloyed powder, the grain sizes of the Cr-containing cermets are increased slightly, unlike the trends observed in conventional cermets with Cr or Cr carbide additives. (3) Core–rim structures appear in the experimental cermets, which is consistent with data from similar conventional cermets. However, the formation mechanism or process of the rim structure is not clarified by the presented observations. (4) The TRS and IM reach the maximum values of 2355 MPa and 13.42 J/cm2, respectively, at the Cr content of 1.5 wt%. These values exceed those measured in similar conventional cermets because of the strengthening effect of Cr and Mo.
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Acknowledgements Support for this work by the Shandong Science and Technology Development Plan Project No. 2016GGX102044, is gratefully acknowledged. We would also like to thank Editage (www.editage.cn) for English language editing. References [1] J. Jung, S. Kang, Effect of ultra-fine powders on the microstructure of Ti(CN)-WC-Ni cermets, Acta Mater. 52 (2004) 1379–1386. [2] B. Liu, S. Huang, J.V. Humbeeck, J. Vleugels, Influence of Mo addition on the microstructure and mechanical properties of TiC-NiTi cermets, J. Alloys Compd. 712 (2017) 579–587. [3] A. Rajabi, M.J. Ghazali, J. Syarif, A.R. Daud, Development and application of tool wear: a review of the characterization of TiC-based cermets with different binders, Chem. Eng. J. 255 (2014) 445–452. [4] Z.Z. Fu, J.H. Kong, S.R. Gajjala, R. Koc, Sintering, mechanical, and oxidation properties of TiC-Ni-Mo cermets obtained from ultra-fine TiC powders, J. Alloys Compd. 751 (2018) 316–323. [5] S.Y. Ahn, S. Kang, Formation of core/rim structures in Ti(C,N)-WC-Ni cermets via a dissolution and precipitation process, J. Am. Ceram. Soc. 1489–94 (2000) (2000) 83. [6] A. Jam, L. Nikzad, M. Razavi, TiC-based cermet prepared by high-energy ballmilling and reactive spark plasma sintering, Ceram. Int. 43 (2017) 2448–2455. [7] B. Huang, W. Xiong, M. Zhang, Y. Jing, B. Li, H. Luo, S. Wang, Effect of W content in solid solution on properties and microstructure of (Ti, W)C-Ni3Al cermets, J. Alloys Compd. 676 (2016) 142–149. [8] G. Upadhyaya, Materials science of cemented carbides—an overview, Mater. Des. 22 (2001) 483–489. [9] W. Zhang, X. Zhang, J. Wang, C. Hong, Effect of Fe on the phases and microstructure of TiC-Fe cermets by combustion synthesis/quasi-isostatic pressing, Mater. Sci. Eng. A 381 (2004) 92–97. [10] J. Pirso, M. Viljus, K. Juhani, M. Kuningas, Three-body abrasive wear of TiC-NiMo cermets, Tribol. Int. 43 (2010) 340–346. [11] N. Liu, Y. Xu, Z. Li, M. Chen, G. Li, L. Zhang, Influence of molybdenum addition on the microstructure and mechanical properties of TiC-based cermets with nano-TiN
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International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effect of Fe–Mo–Cr pre-alloyed powder on the microstructure and mechanical properties of TiC–high-Mn-steel cermet Guoping Lia, Hao Yanga, Yinghai Lyub, Haojun Zhoua, Fenghua Luoa, a b
T
⁎
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China Department of Bioengineering, College of Chemical and Environmental Engineering,Shandong University of Science and Technology, Qingdao 266590, China
A R T I C LE I N FO
A B S T R A C T
Keywords: TiC Fe-Mo-Cr pre-alloyed powder Microstructure Mechanical properties Toughening
In the study, a TiC–high-Mn-steel cermet is fabricated using FeeMo and Fe–Mo–Cr pre-alloyed powders as metallic binders by powder metallurgy techniques. The effect of Cr on the microstructure and mechanical properties of the cermet is studied and the cermet preparation process is optimized. The microstructure and fracture morphology of the cermets are observed with scanning electron microscopy, while phase identification and analysis are performed by X-ray diffractometry. The results show that the particles of Cr-free cermet are angular and polygonal, while those of cermet with added Cr are rounded and ellipsoidal. The grains of Cr-free cermet are larger than those of the Cr-added cermet, which is unlike similar conventional cermets. The grain size of the Cr-added cermet increases slightly with increased Cr content. In addition, the relative density of the cermet decreases slightly with increased Cr content. The hardness of cermet is maximized at HRC 64.8 with the Cr content of 1.0 wt%; with further increases in Cr, the hardness decreases gradually. The transverse rupture strength and impact toughness first increase and then decrease with increasing Cr content, reaching the maxima of 2355 MPa and 13.42 J/cm2, respectively, at the Cr content of 1.5 wt%. The strength and toughness of the cermet are improved greatly compared to those of conventional similar TiC–high-Mn-steel cermets.
1. Introduction Because TiC- and/or Ti(C/N)-based cermets show excellent properties such as high hardness, good thermal resistance, superior stability at elevated temperature, and excellent wear and oxidation resistance, they are considered promising materials for tools, wear-resistant parts, extrusion dies and punches, as well as components for surface finishing, high-speed milling, and machining of both carbon and stainless steels [1–6]. However, TiC and Ti(C,N)-based cermets are limited in applicability because they have relatively low toughness and strength [7]. To overcome this limit, a strong and ductile binder phase could be used to fabricate dense TiC- or Ti(C,N)-based cermets with high toughness and strength [2]. Metals (Fe, Ni, Co, Cr, Ti, Mo, and alloys thereof) can be used as binders in TiC- and Ti(C,N)-based cermets [8–10]; the binder chemical composition can be selected depending on specific requirements for practical applications [3]. Fe-based alloys are the most commonly used as binders because of their low cost, non-toxicity, and good fracture toughness [3,6,10]. In this research, TiC–high-Mn-steel cermets were manufactured by powder metallurgy techniques with different Cr contents. Mo and Cr were added to the cermet in the form of FeeMo or Fe–Mo–Cr pre-
⁎
alloyed powders. Pre-alloyed powder-form metallic binders are advantageous in guaranteeing uniformity of the composition and microstructure of the prepared cermet compared to conventional cermets. However, most cermet producers do not produce their own pre-alloyed powders, preferring to add pure metal powders to cermets. Additionally, the specifications of pre-alloyed powders are significantly limited; frequent chemical composition variations in the cermet can be very inconvenient. Many researchers have confirmed that Mo is indispensable in TiCand Ti(C,N)-based cermets because it improves the wettability of metallic binders on the ceramic phase, refines the ceramic phase grain size, and enhances the mechanical properties of the cermet [11–15]. Some studies have demonstrated that transition-metal carbides can also improve the metallic binder wettability on the ceramic phase; among these carbides, Mo2C is considered the most effective additive and applied frequently to TiC- and Ti(C,N)-based cermets [16–21]. Cr is also important in high-Mn steel, but it is seldom used in TiC–high-Mn-steel cermets, because Cr deteriorates the sinterability and mechanical properties of the cermet by its strong affinity with oxygen [22]. Cr also decreases the eutectic temperatures of alloys, causing liquid phases to appear at lower temperatures and coexist with solid phases for long
Corresponding author. E-mail addresses: [email protected] (G. Li), [email protected] (F. Luo).
https://doi.org/10.1016/j.ijrmhm.2019.105031 Received 4 July 2019; Received in revised form 18 July 2019; Accepted 22 July 2019 Available online 23 July 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 84 (2019) 105031
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Table 1 Main characteristics of the starting powders. Powders
Purity
Oxygen
(μm)
(wt%)
(wt%)
≥99.5 ≥99.8 ≥99.6 ≥99.9 ≥99.5
≤0.28 < 0.10 < 0.12 < 0.01 < 0.2
a
TiC Ni Mn/Fe Graphite Fe–Mo–Cr a
Particle size
3.1 43 43 30 147
Manufacturer
Zhuzhou GuangYuan Cemented Material Co., Ltd., China Shanghai Xtnami Science & Technology Co., Ltd., China Jinzhou Honda New Material Co., Ltd., China Qingdao Baichuan Graphite Co., Ltd., China Own manufacturing
Fisher particle size.
was observed by SEM in secondary electron (SE) mode. The core–rim structure was observed by high-resolution transmission electron microscopy (HRTEM); the chemical composition of the core–rim structure and binder was determined with an energy-dispersive X-ray spectroscope (EDS). Phase identification of each sample was performed by Xray diffraction (XRD; TD-3500, China). The densities of the bulk specimens were measured by the Archimedes method. The hardness was tested by a Rockwell hardness tester (HR-150B, China). The transverse rupture strength (TRS) was determined by a universal material testing machine (WDW-100, China) using specimens of 5 mm × 5 mm × 35 mm with the span distance of 20 mm and the crosshead velocity of 0.5 mm/min. Impact toughness (IM) testing was performed by an impact testing machine (JBW, China) using specimens of 10 mm × 10 mm × 55 mm in size.
periods; this induces complex reactions in sintering [22]. Finally, Cr can react with carbon or graphite to form carbides or compound carbides that can strengthen and improve the mechanical properties of the cermet. Based on these considerations, Mo and Cr are selected as alloying elements to produce FeeMo and Fe–Mo–Cr pre-alloyed powders in this study. The effect of Fe–Mo–Cr pre-alloyed powders on the microstructure and mechanical properties of TiC–high-Mn-steel cermets has not yet been reported. In this study, TiC–high-Mn-steel cermets were manufactured by conventional powder metallurgy techniques using FeeMo or Fe–Mo–Cr pre-alloyed powders as binders and the microstructure and mechanical properties of the cermets were investigated and discussed. 2. Experimental procedure
3. Results and discussion
In the present study, TiC–50 wt% high-Mn-steel cermets using FeeMo or Fe–Mo–Cr pre-alloyed powders as the binder with additives including Ni, Mn, and graphite, among others, were manufactured by powder metallurgy techniques. The main characteristics of the starting powders are listed in Table 1; all starting powders are industrial grade to reduce cost. The nominal chemical compositions of the cermets are shown in Table 2. The Mo content of the pre-alloyed powder is optimized according to a previous study [23]. The pre-alloyed powders of Fe–3.0Mo and Fe–3.0Mo–xCr (x = 1.25, 2.5, 3.75, and 5.0 wt%) were used as the binders for cermet A and cermets B, C, D, and E, respectively. The mixed powders were wet-milled using stainless steel balls in an ethyl alcohol bath in a planetary ball mill for 24 h. The rotation speed of the planetary ball mill was 220 rpm and the weight ratio of milling balls to powder was 7:1. The obtained slurry was dried in a vacuum oven at 85 °C for 12 h, and 2 wt% rubber was added to the powder mixture as a forming agent in the form of rubber-gasoline solution. The powder mixture was pressed into a columnar green compact under the uniaxial pressure of 180 MPa. The green compacts were dewaxed and then sintered in vacuum. Hydrogen was used in the dewaxing process to reduce the oxygen content of the green compacts to improve the wettability of the metallic binder on the ceramic phase during the subsequent sintering. The dewaxed green compacts were vacuum-sintered at 1430 °C for 60 min and then low-pressure treated with a pressure of 1 MPa for 40 min by Ar. The bulk specimens were then machined and polished for testing. The microstructure of the polished bulk specimens was observed by scanning electron microscopy (SEM; FEI Nova NanoSEM, America) in backscattered electron (BSE) mode and the fracture surface morphology
3.1. Investigation and optimization of preparation process In this research, the Fe–3.0Mo and Fe–3.0Mo–xCr (x = 1.25, 2.5, 3.75, and 5.0 wt%) pre-alloyed powders were produced according to the demands of industrial-scale production to reduce the cost of the starting powders. The particle size of the Fe–3.0Mo and Fe–3.0Mo–xCr pre-alloyed powders is larger than that of the Fe powder used in conventional TiC–high-Mn-steel cermet. The weight ratio of milling balls to powder must therefore be increased to strengthen the milling effect. The pre-alloyed powder has low activity compared to pure metal powder; the experimental cermet preparation process was adjusted correspondingly. The temperature–time dewaxing and sintering curves of the green compacts are given in Figs. 1 and 2, respectively. Although ball milling was performed in an ethyl alcohol bath, oxidizing of the powder mixture was inevitable during preparation. This decreased the wettability of metallic binder on ceramic phase, retarded densification, and deteriorated the mechanical properties of the cermet. The gas dewaxing of the green compacts in hydrogen decreased the oxygen content significantly to improve the wettability of metallic binder on ceramic phase and promote densification of the cermet. Fig. 2, the sintering curve, shows two important insulation stages. The first at 1100–1150 °C corresponds to the reduction of oxygen in Fe powder by carbon–oxygen reaction. Considering the oxidation of Cr and Mo, the higher temperature in this insulation stage of 1150 °C is selected. The other insulation stage occurs approximatelyat 1300 °C; the metallic binder is molten or semi-molten at this temperature, and the solid-liquid change is less abrupt and occurs within a certain temperature range. The so-called molten or quasi-molten liquid flows in the spaces between ceramic particles, although it differs somewhat from the ordinary liquid state. The flowing of the metallic binder separates ceramic particles from each other and promotes the diffusion, migration, and deposition of alloying elements on ceramic particles to form rim structures that primarily inhibit coalescencegrain growth of the cermet. The rim structure continues to develop through the dissolution–precipitation mechanism to improve the wettability between the ceramic phase and metallic binder. The grain growth of the cermet
Table 2 Nominal chemical compositions of the experimental cermets (wt%). Cermets
TiC
Ni
Mo
Cr
Mn
C
Fe
A B C D E
50 50 50 50 50
2.25 2.25 2.25 2.25 2.25
1.19 1.19 1.19 1.19 1.19
0 0.5 1.0 1.5 2.0
7.0 7.0 7.0 7.0 7.0
0.7 0.7 0.7 0.7 0.7
Bal. Bal. Bal. Bal. Bal.
2
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Fig. 1. Gas dewaxing temperature–time curve of green compacts. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
insulation stage is not fixed and related closely to the binder chemical composition, which varies within a certain temperature range. Finally, the sintering temperature of the experimental cermet is higher than that of a similar conventional cermet because of the low activity and large particle size of the pre-alloyed powder.
by liquid sintering is hindered with complete formation of a sufficiently thick rim structure. Two grain growth mechanisms occur in the cermets, namely coalescence grain growth and dissolution–precipitation grain growth, the latter is also called Ostwald ripening [24]. Generally speaking,coalescence grain growth of the cermet occurs during solid sintering because of poor wettability of the metallic binder on the ceramic phase. Therefore, inhibitingcoalescence grain growth of the cermet is important. The application of the pre-alloyed powder significantly facilitates inhibiting coalescence grain growth of the cermet. In this study,holding at an insulation stage while the metallic binder is molten or quasi-molten is found to be crucial. Of course, the temperature of the
3.2. Microstructure and phase structure identification The microstructures of cermets with different Cr contents are shown in Fig. 3. It can be seen that the average grain size of the ceramic phase of the Cr-free cermet A is greater than those of Cr-added cermets B, C, D, and E. Previous studies have confirmed that Cr reduces the eutectic
Fig. 2. Sintering temperature–time curve after gas dewaxing of the cermets. 3
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Fig. 3. SEM (BSE) micrographs of the cermets with varied Cr content: (a) 0, (b) 0.5 wt%, (c) 1.0 wt%, (d) 1.5 wt%, and (e) 2.0 wt%.
size, and the metallic binder content and composition. Obtaining comprehensive and correct results considering only a single influencing factor is infeasible. The particle shape of cermets B, C, D, and E is round orellipsoidal, consistent with previous observations [16,22]. The microstructures of the Cr-added cermets are homogeneous without abnormal grain growth, which may greatly improve the mechanical properties of the cermets. Fig. 3 shows the microstructural characteristics of ceramic-phase particles embedded in metallic binder. The black regions in the micrograph are ceramic particles, while the gray regions are the metallic binder. Core–rim structures are observed in the five cermets, typical of TiC- and Ti(C,N)-based cermets. The formation mechanism of rim structures in TiC- and Ti(C,N)based cermets is complex and subject to some argument [11,25]. The black core structures in the cermet are remnant undissolved large TiC particles; the gray rim structures form during sintering. Some reports state that the gray rim structure comprises inner and outer rims formed during solid- and liquid-state sintering, respectively [12,13,26]. The amounts of heavy elements such as Mo and W are higher in the inner rim than in the outer rim [26]. However, the rim structure in the five cermets fabricated here is less distinct than those in conventional TiCbased cermets with an unclear border separating the inner and outer rim. The low magnification and resolution of the test instrument may
temperature of alloys and initiates earliergrain growth of the cermet; generally, the grain size of Cr-added cermet is larger than those of Crfree cermets [16,22]. However, the results observed here differ from this trend. The uniform distribution of Mo in the binder may contribute to this difference. With liquid formation, Mo precipitates on the surface of ceramic-phase grains to form rim structures that inhibit the dissolution of the ceramic phase and suppress grain growth. The liquid phase appears earlier because of the Cr additive, but the ceramic grains of the cermet show less growth than those of similar conventional cermets. The grain size of the cermets increases only slightly with increasing Cr content. Cermet A shows angular and polygonal particles, which are unlike those of similar conventional cermets prepared using pure metal powder as additives. The FeeMo pre-alloyed powder is inferred to induce this phenomenon. The cermet with larger-sized and angular particles has both advantages and disadvantages. The cermet is probably tougher than other similar cermets because larger particles can withstand larger impact loads and cracks tend to propagate through the binder because of the high stiffness of the ceramic phase. However, more defects may be present in the particle–binder interface, which deteriorate the mechanical properties of the cermet. The properties of the cermet all result from the competition of influencing factors such as chemical composition, content, ceramic-phase distribution and grain 4
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strengthen the cermet. In order to further investigate the phases, XRD analysis of the cermets with varied Cr content was performed, with resulting patterns shown in Fig. 5. All peaks in the XRD patterns are associated with TiC and Fe in the binder; other elements, such as Ni, Mn, and carbon (graphite), are dissolved in the binder. Mo and Cr are dissolved completely in Fe during preparation of the pre-alloyed powder by atomization. Mo and Cr react with TiC to form (Ti,Mo)C and/or (Ti,Mo,Cr)C solid solutions; these have the same crystal structure as TiC and similar lattice parameters, so the XRD peak position of TiC changes negligibly. The Fe-based binder contains other metals, but accurate identification of the type of carbide and solid solution is difficult, so overall, it presents the peaks of ɑ-Fe. 3.3. Density and mechanical properties The densities, hardness, TRS, and IM of the experimental cermets were measured at room temperature, as shown in Figs. 6 and 7. Fig. 6 shows that thedensity of cermet decreases slightly with increases in Cr content; Cr negatively affects the sinterability. The hardness of the cermets first increases and then decreases with increases in Cr content, reaching the maximum value of HRC 64.8 at the Cr content of 1.0 wt%. The hardness of the cermet is not correlated to density because of the strengthening of the binder by Cr. In practical situations, TiC–high-Mnsteel cermets are typically applied in scenarios subject to violent impact and vibration; therefore, IM is a more valuable characteristic than fracture toughness. Fig. 7 shows that the TRS and IM of the cermets all increase first and then decrease with increases in Cr content, reaching maxima of 2355 MPa and 13.42 J/cm2, respectively, at theCr content of 1.5 wt%. With further increase in Cr, the TRS and IM decrease remarkably, because the cermet becomes brittle with excessive Cr. The conventional TiC–high-Mn-steel cermet TM52 shows the TRS and IM of 1800–2000 MPa and 8–10 J/cm2, respectively. Compared to TM52, the TRS and IM of the experimental cermets are improved by > 20% and 40%, respectively. In addition, the TRS and IM values of the Cr-added cermets are higher than those of the Cr-free cermet because of the strengthening provided by Cr. Fig. 8 shows the impact fracture surface morphologies of the experimental cermets. The impact fracture surfaces of the cermets show mixed transgranular and intergranular fracture. A previous study indicated that intergranular fracture was the dominant failure mode for cermets because the interface and binder phase are weak regions [28]; therefore, strengthening the interface and binder is a primary approach to improve cermet toughness. In this study, transgranular fracture is also apparent because many of the ceramic particles are split under impact. In situations of instantaneous and violent impact, cracks propagate rapidly and incompletely in the binder, so more ceramic particles are split; this positively affects the IM. With Cr additive and increases in its concentration, ductile tearing occurs within the binder because of the strengthening effect of Cr. The toughening mechanisms include crack deflection, branching, bridging, and so on. A previous study showed that the contribution of the binder to fracture toughness was much greater than that of the ceramic phase [29]. Therefore, strengthening the cermet binder is necessary and effective during crack initiation and propagation. During fracture, the fracture resistance of the ceramic phase is much greater than that of the binder. Because the grains of cermet are fine in size and the binder is strengthened by added Cr, more ceramic particles are split via the transgranular fracture mode; the IM is therefore improved significantly, especially for cermets applied in situations with violent impact and vibration. The strengthening effect of Cr benefits the binder and modifies the deformation and fracture behavior of the ceramic phase. In addition to strengthening the binder, the toughening effect is also attributed to the strengthening ceramic phase of the cermet, especially in violent impact and vibration situations. For example, for TiC- and Ti (C,N)-based cermets with WC additives, WC improves the wettability of
Fig. 4. High-resolution TEM image of cermet D. Letters C, R, and B indicate core, rim, and binder, respectively. Table 3 TEM/EDS results of the core–rim structure and the binder of cermet D. Elements (at.%)
Core
Rim
Binder
C Mo Ti Cr Mn Fe Ni
46.2 0.4 52.6 0.3 0 0.4 0.1
37.6 1.6 59.5 0.7 0.1 0.5 0.1
16.1 0.5 1.9 3.3 7.4 62.6 8.2
contribute to this lack of clarity. Another contributing factor is the low concentrations of Mo and Cr in the cermets because of the large volume fraction of the pre-alloyed powder. To further investigate the core–rim structures and analyze the chemical composition of the cermet, a highmagnification TEM image of cermet D is shown in Fig. 4. The chemical compositions of the core, rim structure, and binder of the cermet are tested and shown in Table 3. The rim structure of cermet D is very distinct in Fig. 4. Table 3 shows that the core mainly comprises Ti and C, indicating that the cores are undissolved TiC particles remaining after sintering. The contents of other elements such as Cr, Mo, Ni, and Fe are very low, and no Mn is found in the core. Some trace elements may appear because of the test instrument, leading to erroneous results. The chemical composition of the rim structure indicates that it has the highest Mo content, forming a (Ti,M)C (where M = Mo, Cr, Mn, Fe, and other metals) solid solution that improves the wettability of the metallic binder on the ceramic phase and promotes densification of the cermet, which is consistent with previous studies [13,14,21]. Previous research has also demonstrated high dislocation densities in the rim structures; these improve the mechanical properties of the cermet [27]. However, the thickness of the rim structure must be controlled to avoid large interior stresses arising from incoherence with the ceramic-phase core; such stresses could deteriorate the mechanical properties of the cermet. The chemical composition of the binder shows the highest Cr content; a small quantity of Cr is present in the rim structure of the ceramic phase to form a (Ti,Mo,Cr)C solid solution. Generally, Cr, Mo, Ti, Mn, graphitic carbon, and other elements dissolve in the binder to form a solid solution that modifies the mechanical properties of the cermet. The dissolved atoms may also form carbides and/or compound carbides to 5
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Fig. 5. XRD patterns of the cermets with varied Cr content: (a) 0; (b) 0.5 wt%; (c) 1.0 wt%; (d) 1.5 wt%; (e) 2.0 wt%.
Fig. 6. Relative density and hardness of the cermets as a function of Cr content.
because WC is more rigid than TiC; therefore, the toughness of the TiCbased cermet is increased. However, WC is seldom used because of its high cost.
the metallic binder on the ceramic phase, refines the ceramic phase, and strengthens the metallic binder effectively [17,21,30]. The toughening role of WC in TiC-based cermets is also attributed to the formation of a (Ti,W)C rim structure that increases the rigidity of the ceramic phase, 6
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Fig. 7. TRS and IM of the cermets as a function of Cr content.
4. Conclusions
(2) The particle size of the cermet using FeeMo pre-alloyed powder is much larger than that of the cermet with Fe–Mo–Cr pre-alloyed powder. However, the particle sizes of the Cr-containing cermets are increased slightly with increases in Cr. The particles in the Crfree cermet are angular and polygonal, while those of the Cr-containing cermets are rounded and ellipsoidal. With compositional
(1) The preparation of experimental cermets is optimized with a sintering temperature exceeding that used for conventional cermets with similar chemical compositions because the pre-alloyed powder is less active than pure metallic powders.
Fig. 8. Fracture surface morphologies of the experimental cermets. 7
International Journal of Refractory Metals & Hard Materials 84 (2019) 105031
G. Li, et al.
variations in Mo and Cr in the pre-alloyed powder, the grain sizes of the Cr-containing cermets are increased slightly, unlike the trends observed in conventional cermets with Cr or Cr carbide additives. (3) Core–rim structures appear in the experimental cermets, which is consistent with data from similar conventional cermets. However, the formation mechanism or process of the rim structure is not clarified by the presented observations. (4) The TRS and IM reach the maximum values of 2355 MPa and 13.42 J/cm2, respectively, at the Cr content of 1.5 wt%. These values exceed those measured in similar conventional cermets because of the strengthening effect of Cr and Mo.
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