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Growth Characteristics and Reinforcing Behavior of In-situ NbCp in Laser Cladded Fe-based Composite Coating
Journal of Materials Science & Technology 31 (2015) 766e772
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Journal of Materials Science & Technology journal homepage: www.jmst.org
Growth Characteristics and Reinforcing Behavior of In-situ NbCp in Laser Cladded Fe-based Composite Coating Qingtang Li*, Yongping Lei**, Hanguang Fu School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
a r t i c l e i n f o Article history: Received 18 February 2014 Received in revised form 13 May 2014 Accepted 18 June 2014 Available online 10 December 2014 Key words: Laser cladding In-situ NbC particle Formation mechanism Mechanical property
Over the past decade, researchers demonstrated much interest in laser cladded metal matrix composite coatings for its good wear resistance. In this paper, in-situ NbCp reinforced Fe-based wear-resistance coatings with different designed NbC contents were produced by laser cladding. The formation mechanism of NbC particle was analyzed. The effects of NbC content on the microstructure and mechanical properties of coatings were investigated. It was revealed that a-(Fe, Cr), (Fe, Cr)7C3, NbC existed in all coatings. NbC particles formed by the chemical reaction of Nb and C dissolved in the molten pool. The increase of designed NbC content led to the growth of the porosity amount, and the increase of the size and the area ratio of the particle, as well as the transformation of the NbCp morphology from quadrangle to petaloid shape. Moreover, the micro-hardness and the wear resistance of the coating were improved with the increase of NbC. When NbCp content was 30 wt%, the mechanical properties decreased slightly. Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
1. Introduction Laser cladding metal matrix composite (MMC) coating possesses excellent characteristics, such as good adhesion, fine microstructure, small heat-affected zone, as well as high hardness and outstanding abrasive wear property under lower load[1e4]. At present, metallic carbides, such as TiC[5], WC[6,7] and NbC, are preferred reinforcing phases applied in MMC laser coating. As a common hard alloy additive, NbC possesses high hardness (>235 GPa), high melting point (2537 K), excellent modulus of elasticity (580 103 MPa) and moderate thermal expansion coefficient (6.65 106/K)[8]. For these advantages, NbC has been used in MMC laser coatings as the reinforcement. For examples, colaço and Vilar [9] investigated the microstructure and wear resistance of the FeeCreC/NbC laser coating. The results showed that the wear resistance of coating presented a non-monotonous variation, and reached a maximum when the volume fraction of NbCp was 20%. Niu et al.[10] investigated the microstructure and mechanical properties of the in-situ NbC particle reinforced Ni-based laser coating. The microstructure consisted of g-(Ni, Fe) matrix, carbide dendrites and NbC particles. The composite coating showed high
average hardness of 1200 HV0.3 and excellent wear resistance, which was 2.5 times as high as that of Ni-based coatings without NbC reinforcement. It is known that the size, volume fraction and the distribution of the carbide particle have important impacts on the mechanical properties of the laser coating[7,9,11]. The focus of the current work was to research the growth and distribution characteristics of NbCp, and then to investigate the effects of designed NbC content on the mechanical properties of the Fe/NbC laser coating. It was revealed that in-situ NbC particles formed by the chemical reaction of Nb and C dissolved in the molten pool. The increase of designed NbC content led to the growth of the porosity amount, and the increase of the size and the ratio of the total area of particles to that of the coating cross-section, as well as the transformation of the NbCp morphology from quadrangle to petaloid shape. The microhardness and the wear resistance of the coating were improved with the increase of NbC. When NbCp content was 30 wt%, the mechanical properties decreased slightly. 2. Experimental 2.1. Materials
* Corresponding author. Ph.D. ** Corresponding author. Prof.; Tel.: þ86 10 67391759; Fax: þ86 10 67396093. E-mail addresses: [email protected] (Q. Li), [email protected] (Y. Lei).
A high carbon steel plate with the dimensions of 100 mm 50 mm 12 mm was used as the substrate, whose nominal composition is listed in Table 1. The surface of substrates
http://dx.doi.org/10.1016/j.jmst.2014.06.012 1005-0302/Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
Q. Li et al. / Journal of Materials Science & Technology 31 (2015) 766e772
767
Table 1 Composition of the substrate (wt%) C
Si
Mn
S
P
Cr
Ni
Cu
V
Mo
1.45
0.40
0.40
0.03
0.03
11.00e12.50
0.25
0.30
0.15e0.30
0.40e0.60
2.3. Characterization
Table 2 Composition of the alloy mixture (wt%) No.
Designed NbC content
C
Nb
Cr
Fe
1 2 3 4
5% 10% 20% 30%
0.57 1.14 2.28 3.43
4.42 8.85 17.71 26.57
18.0 18.0 18.0 18.0
Bal. Bal. Bal. Bal.
Table 3 Laser processing parameters Power (kW)
Scanning velocity (mm/s)
Beam size (mm)
Powder feeding rate (g/min)
Carrier gas flow (L/min)
Shielding gas flow (L/min)
2.3
4
55
8.0
10
10
were polished and rinsed with acetone prior to cladding. Fe-based alloy powder (45e106 mm), niobium-iron alloy powder (66 wt% Nb, 75e106 mm), high carbon ferrochrome (7 wt% C, 75e106 mm) and low carbon ferrochrome powders (C content<0.04 wt%, 75e106 mm) were blended to prepare the cladding mixtures with different designed NbC contents. The atomic ratio between Nb and C always remained 1:1 in each cladding mixture. The compositions of the mixture are listed in Table 2.
2.2. Laser process An IPG fiber laser system with a maximum power of 6 kW was used to product single and multi-track (7 tracks with 30% overlapping rate) coatings for the observation of the microstructure and wear test, respectively. The laser beam spot was fixed to a 5 mm 5 mm square spot. The laser head was integrated with an ABB robot. DPSF-2 powder feeding system and a coaxial nozzle were used to feed the cladding powders into the molten pool by argon gas. Meanwhile, high-purity argon shielding gas was supplied through the same coaxial nozzle to protect the molten pool. The detailed processing parameters are listed in Table 3.
Single track specimens were sectioned in the transverse direction (10 mm 10 mm 12 mm). Transverse sections were mounted and etched with FeCl3 solution for the examination. The phase identification of the coatings was carried out on a Shimadzu XRD-7000 X-ray diffractometer with CuKa radiation operating at 40 kV. The microstructure characterization of coatings was performed with an OLYMPUS BX51 optical microscope, an HITACHI S-3400 scanning electron microscope (SEM) equipped with an energy-dispersive spectroscopy (EDS) and a JEM2100 transmission electron microscope (TEM). The microhardness was measured with a load of 200 gf using an HX-200 micro-hardness tester. The abrasive wear property was evaluated using an MLS-225 wet sand rubber wheel tester. The structural diagram of the tester is shown in Fig. 1, and the detailed measuring parameters are listed in Table 4. Each value of the micro-hardness and wear rate is the average of three measurements. 3. Results and Discussion 3.1. Microstructure of coatings 3.1.1. Porosities in coatings Fig. 2 illustrates the macro-shape of the laser coatings from transverse cross section. It is found that both the size and the amount of the porosity in the coating increase when NbC content increases. This is because in order to increase the designed NbC content in the coating, more niobium-iron alloy powders with higher melting point (1850 K) were added into the mixture, and the mass fraction of Fe-based alloy with lower melting point (1523 K) decreased accordingly, which led to the growth of the average melting point of the mixture. Therefore, the higher liquid viscosity resulted from the increased melting point in the molten pool, making air bubble difficult to float up and leading to the increase of the porosities. 3.1.2. Phase identification The XRD spectra for four samples are shown in Fig. 3. It is found that a-(Fe, Cr), (Fe, Cr)7C3, NbC exist in all coatings, which means that NbC particle can be in-situ formed. The intensity of NbC diffraction peak enhances with the increase of NbC content. Moreover, g-Fe is found in the coatings with higher NbC content. It is probably caused by the increase of C element in the cladding powder, which can improve the stability of g-Fe during solidification. The typical microstructure of the laser coating is shown in Fig. 4. It is clear that the coating is composed of dispersed white particles
Table 4 Measuring parameters of wear test Hardness Abrasive Rotating Wheel of rubber Size (mm) diameter speed of wheel (r/min) (HS) (mm) Fig. 1. Structural diagram of the wear tester.
176
240
60
Load Wear Sample (N) time dimension (min) (mm)
<400(Al2O3) 200
30
57 25 12
768
Q. Li et al. / Journal of Materials Science & Technology 31 (2015) 766e772
Fig. 2. Macrostructure of the laser coatings with different designed NbC contents: (a) 5 wt% NbC; (b) 30 wt% NbC (porosities are marked by white arrows).
(A), dendritic and equiaxial matrix (B), and compound (C) separating out of the matrix as shown in Fig. 4(a). The compositional analysis of them is shown in Table 5. Three phases are identified as NbC particle, a-(Fe, Cr) matrix, (Fe, Cr)7C3 in alphabetical order. Fig. 4(b) and (c) illustrate the TEM bright-field image and the corresponding selected area diffraction pattern (SADP) of the particle phase, respectively. The result of SADP shows that NbC has the cubic structure, and the calculated lattice parameter is approximately 0.448 nm. The interplanar spacing of (111) and (022) are 0.2590 and 0.1592 nm, respectively, both of which are larger slightly than standard interplanar spacing of them (0.2581 and 0.1580 nm, respectively). Combining with the results of EDS, it can be said that NbCp is a multiple carbide with Fe, Cr and Si elements dissolving in the NbC lattice. This is probably because the growth process of the NbCp was rapid due to the sharp solidification rate (103 K/s)[12]. Fig. 3. XRD patterns of the coatings with different designed NbC contents.
Fig. 4. Typical morphology of the microstructure of laser coating: (a) SEM micrograph of phases in the coating; (b) bright-field TEM micrograph of the NbC particle; (c) the selected area diffraction pattern (SADP) of the NbC particle in (b).
Q. Li et al. / Journal of Materials Science & Technology 31 (2015) 766e772 Table 5 Chemical composition of phases in Fig. 4 (at. %)
A B C
Fe
Cr
B
Si
C
Nb
6.02 64.10 39.47
3.24 19.14 25.13
e e e
1.20 3.08 1.83
38.87 13.68 33.39
50.67 e 0.18
Fig. 5. EDS results of the NbC particles: (a) atomic ratio of Nb and C in the particle; (b) line scanning result of Nb and C in the particle.
769
3.1.3. Formation of NbC particles In order to understand the characteristics (particle size, amount and distribution) of in-situ NbC particles in the microstructure, the formation mechanism of NbC and the solidification process of the coating should be studied firstly. The melting points of Fe-based alloy, niobium-iron alloy, high carbon ferrochrome, and low carbon ferrochrome are 1523, 1850, 1700, and 1800 K, respectively. The formation of NbC particle requires the carbon atom to be released from high carbon ferrochrome. Therefore, the temperature of molten pool should be higher than the melting point of high carbon ferrochrome (1700 K). Under this condition, there are two possibilities to form NbC particle. One is that some NbeFe particles with larger size do not melt completely due to the rapid solidification of the molten pool. Nb atoms at the surface of niobium-iron particle react with the surrounding C atoms. With the development of reaction process, C atoms diffuse to the core gradually and the NbC particle is formed at last[13]. Another is that the niobiumeiron particle melts completely; Nb dissolves in the molten pool and reacts with carbon atom, and then NbC particle precipitates out in the molten pool[14,15]. The EDS results of the NbC particles are shown in Fig. 5(a) and (b). It can be found that not only Nb and C elements distribute evenly (line scanning result), but also the Nb/C atomic ratio always keeps 1:1 approximately in the whole particle. The carbon content even is more than that of Nb at the particle core. Moreover, a considerable amount of NbC particles present regular morphology (rectangle, cross and petaloid shape); the particle size is smaller obviously compared with the alloy particle in the cladding mixture; and more importantly, there is no NbCp with coreeshell structure observed in the coatings as illustrated in Fig. 6. Apparently, it would be very difficult for NbC particles to meet the characteristics mentioned above if they have been formed around the unmelted Nb particles in the molten pool. Therefore, the main formation mechanism of the NbC particle may be that the NbC particle forms by the chemical reaction of Nb and carbon in the molten pool. Whereas, the binary phase diagram of NbCeFe (Fig. 7)[16] confirms that there is a large phase field, where both liquid iron and solid NbC exist in a semi-solid state. It indicated that there is plenty of time for nucleation and growth of NbC, even though the temperature of molten pool is under the melting point of niobiumeiron alloy. Therefore, it could be said that the first possibility of the NbC formation mechanism may exist simultaneously. 3.1.4. Effects of designed NbC content The morphology of NbC particles and the microstructure of the coatings with different NbC contents are shown in Figs. 6 and 8,
Fig. 6. Morphology of the NbC particle with different designed NbC contents: (a) 5 wt% NbC; (b) 20 wt% NbC; (c) 30 wt% NbC.
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Q. Li et al. / Journal of Materials Science & Technology 31 (2015) 766e772
It is obvious that when designed NbC content is lower, the amount of Nb and C atoms is fewer in the molten pool. Hence, the nucleation and the growth of the particle are limited. When NbC content increases, it is easier for NbC6 octahedral growth units to grow along the [100] direction due to the growth habit of the NbC (the lattice structure of NbC is rock salt type)[17]. At last, the cross and petaloid shaped NbCp with larger size are formed. 3.2. Micro-hardness and wear resistance
Fig. 7. Binary phase diagram of NbCeFe.
respectively. It is worth noting that with the increase of NbC content, both the size and the amount of NbCp increase markedly, and the particle morphology transforms from quadrangle to the petaloid shape gradually.
The variations of the micro-hardness of four coatings along the depth direction are plotted in Fig. 9. It can be found that all microhardness curves change slightly from the top to the bottom of the coatings, and increase sharply within the heat-affected zone. It indicated that the martensitic transformation of the substrate with the high carbon content (1.45 wt%) occurred in this area due to high cooling rate[18]. With the increase of NbC content, the average micro-hardness increases from 819.3 HV0.2 (5 wt%) to 900.5 HV0.2 (20 wt%), and decreases slightly (887.3 HV0.2) when NbC content is 30 wt%. It is obvious that the increase of NbC content can improve the micro-hardness because of the increased size and volume fraction of hard NbCp. However, excess NbCp may cause the rise of the structural stress and the brittleness of the coating, which led to the inapparent change of the micro-hardness when NbC content was more than 20 wt%, as depicted in Fig. 9.
Fig. 8. Microstructure of laser coatings with: (a) 5 wt% NbC; (b) 10 wt% NbC; (c) 20 wt% NbC; (d) 30 wt% NbC.
Q. Li et al. / Journal of Materials Science & Technology 31 (2015) 766e772
Fig. 9. Variations of the micro-hardness along the depth direction of the coatings.
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Fig. 11. It can be found in Fig. 11(a) that there are numerous furrows parallel to each other on the substrate surface, which is caused by micro-plowing and grooving of Al2O3 abrasive particles in the gap between the sample surface and the rotating rubber wheel. It can be seen in Fig. 11(b) and (c) that when NbC content are more than 20 wt%, the width, depth and continuity of the furrows are diminished, the porosities and cracks are found on the worn surface. During the wear process, Al2O3 abrasive particles can easily scratch and plow the substrate surface due to the large difference of their hardness. By contrast, the laser coatings have lower wear rate benefiting from the higher hardness and dispersed in-situ NbCp in the layer, which protrudes gradually from the surface during the initial stage of the wear, and prevents the surface from further abrasion[19]. In addition, the interaction of the NbCp and abrasive Al2O3 particles may lead to the fragmentation and blunting of the abrasive particles. Thus the severity of the microcutting action of the blunted abrasive particles is considerably reduced[20]. With the increase of NbC content, the protective effect of the NbCp is more and more significant, and therefore the wear rate of coating decreased as depicted in Fig. 10. But meantime, the increase of the size and the volume fraction of the NbCp may also cause the rise of the structural stress and the brittleness of the coating, which makes NbCp more fragile and easy to spall during the abrasion[9,21]. As a consequence, the wear rate of the coating with 30 wt% NbC increases slightly. Moreover, the rise of the structural stress induces the generation and propagation of the crack in the abrasive process as illustrated in Fig. 11(b) and (c).
4. Conclusions
Fig. 10. Bar graph of the wear rate of substrate and coatings with different NbC contents.
The wear rate of the substrate and the coatings with different NbC contents are shown in Fig. 10. It can be seen that the wear resistance of each coating is higher than that of the substrate. The wear rate of the coatings with 20 wt% NbCp is just a quarter to that of substrate. Moreover, the wear rate decreases with increasing the NbC content, and increases slightly when NbC content is 30 wt%. The worn surfaces of the substrate and coatings are illustrated in
(1) The size and amount of porosity in the coating increase with the increase of designed NbC content. (2) a-(Fe, Cr), (Fe, Cr)7C3, NbC exist in all coatings, and g-Fe is found in the coatings containing higher NbC content. NbC is a multiple carbide with the Fe, Cr and Si elements dissolving in the NbC lattice. (3) NbC particles form by the chemical reaction of Nb and C elements dissolving in the molten pool. When the designed NbC content increases, the size and the amount of the particle increase, and the particle morphology transforms from the quadrangle to the petaloid shape gradually. (4) With the increase of the NbC, the micro-hardness and the wear resistance of the coating are improved. When NbC content is 30 wt%, both of them decrease slightly.
Fig. 11. Wear appearance of the substrate and the coatings: (a) substrate; (b) coating with 20 wt% NbC; (c) coating with 30 wt% NbC.
Q. Li et al. / Journal of Materials Science & Technology 31 (2015) 766e772
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Acknowledgment The authors would like to thank the National Natural Science Foundation of China under grant (10009012201203) for financial support for this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
L. Dubourg, D. Ursescu, F. Hlawka, A. Cornet, Wear 258 (2005) 1745e1754. K. Das, T.K. Bandyopadhyay, S. Das, J. Mater. Sci. 37 (2002) 3881e3892. D.Q. Dai, G.Y. Liang, X.Q. Tian, Hot Working Technol. 8 (2004) 19e51. D. Ravnikar, N.B. Dahotre, J. Grum, Appl. Surf. Sci. 282 (2013) 914e922. S. Yang, M.L. Zhong, W.J. Liu, Mater. Sci. Eng. A. 343 (2002) 71e78. C. Guo, J. Chen, J.S. Zhou, Surf. Coat. Technol. 206 (2012) 2064e2071. C.F. Wu, M.X. Ma, W.J. Liu, M.L. Zhong, Mater. Lett. 62 (2008) 3077e3080. F.Y. Hung, Z.Y. Yan, L.H. Chen, Surf. Coat. Technol. 200 (2006) 6881e6887. R. Colaço, R. Vilar, Wear 255 (2003) 643e650.
[10] X. Niu, M.J. Chao, W.L. Wang, B. Yuan, E.J. Liang, Chin. J. Laser. (7) (2006) 987e992 (in Chinese). [11] X. Tong, F.H. Li, M. Kuang, W.Y. Ma, X.H. Chen, M. Liu, Appl. Surf. Sci. 258 (2012) 3214e3220. ski, Mater. Sci. Eng. A 379 (2004) [12] R. Jendrzejewski, I. Kreja, G. Sliwi n 313e320. [13] S.J. Xie, G. Dong, Appl. Laser. 32 (2012) 459e463. [14] A. Emamian, S.F. Corbin, A. Khajepour, Surf. Coat. Technol. 205 (2010) 2007e2015. [15] A.M. Kanury, Metall. Trans. A. 23 (1992) 2349e2356. [16] H. Frey, H. Holleck, Thermal Anal. 1 (1975) 339e348. [17] Y.X. Jin, H. Zhang, S.Y. Zeng, E.L. Zhang, Q.F. Li, Acta Metall. Sin. 38 (2002) 1223e1227 (in Chinese). [18] Q.B. Liu, H.B. Liu, J. Mater. Process. Technol. 88 (1999) 77e82. [19] M.S. Yang, X.B. Liu, J.W. Fan, X.M. He, S.H. Shi, G.Y. Fu, M.D. Wang, S.F. Chen, Appl. Surf. Sci. 258 (2012) 3757e3762. [20] A.K. Srivastava, K. Das, J. Mater. Eng. Perform. 21 (2012) 2438e2445. [21] H.B. Wang, X.Y. Song, C.B. Wei, Y. Gao, G.S. Guo, J. Mater. Sci. Technol. 29 (2013) 1067e1073.
Contents lists available at ScienceDirect
Journal of Materials Science & Technology journal homepage: www.jmst.org
Growth Characteristics and Reinforcing Behavior of In-situ NbCp in Laser Cladded Fe-based Composite Coating Qingtang Li*, Yongping Lei**, Hanguang Fu School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
a r t i c l e i n f o Article history: Received 18 February 2014 Received in revised form 13 May 2014 Accepted 18 June 2014 Available online 10 December 2014 Key words: Laser cladding In-situ NbC particle Formation mechanism Mechanical property
Over the past decade, researchers demonstrated much interest in laser cladded metal matrix composite coatings for its good wear resistance. In this paper, in-situ NbCp reinforced Fe-based wear-resistance coatings with different designed NbC contents were produced by laser cladding. The formation mechanism of NbC particle was analyzed. The effects of NbC content on the microstructure and mechanical properties of coatings were investigated. It was revealed that a-(Fe, Cr), (Fe, Cr)7C3, NbC existed in all coatings. NbC particles formed by the chemical reaction of Nb and C dissolved in the molten pool. The increase of designed NbC content led to the growth of the porosity amount, and the increase of the size and the area ratio of the particle, as well as the transformation of the NbCp morphology from quadrangle to petaloid shape. Moreover, the micro-hardness and the wear resistance of the coating were improved with the increase of NbC. When NbCp content was 30 wt%, the mechanical properties decreased slightly. Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
1. Introduction Laser cladding metal matrix composite (MMC) coating possesses excellent characteristics, such as good adhesion, fine microstructure, small heat-affected zone, as well as high hardness and outstanding abrasive wear property under lower load[1e4]. At present, metallic carbides, such as TiC[5], WC[6,7] and NbC, are preferred reinforcing phases applied in MMC laser coating. As a common hard alloy additive, NbC possesses high hardness (>235 GPa), high melting point (2537 K), excellent modulus of elasticity (580 103 MPa) and moderate thermal expansion coefficient (6.65 106/K)[8]. For these advantages, NbC has been used in MMC laser coatings as the reinforcement. For examples, colaço and Vilar [9] investigated the microstructure and wear resistance of the FeeCreC/NbC laser coating. The results showed that the wear resistance of coating presented a non-monotonous variation, and reached a maximum when the volume fraction of NbCp was 20%. Niu et al.[10] investigated the microstructure and mechanical properties of the in-situ NbC particle reinforced Ni-based laser coating. The microstructure consisted of g-(Ni, Fe) matrix, carbide dendrites and NbC particles. The composite coating showed high
average hardness of 1200 HV0.3 and excellent wear resistance, which was 2.5 times as high as that of Ni-based coatings without NbC reinforcement. It is known that the size, volume fraction and the distribution of the carbide particle have important impacts on the mechanical properties of the laser coating[7,9,11]. The focus of the current work was to research the growth and distribution characteristics of NbCp, and then to investigate the effects of designed NbC content on the mechanical properties of the Fe/NbC laser coating. It was revealed that in-situ NbC particles formed by the chemical reaction of Nb and C dissolved in the molten pool. The increase of designed NbC content led to the growth of the porosity amount, and the increase of the size and the ratio of the total area of particles to that of the coating cross-section, as well as the transformation of the NbCp morphology from quadrangle to petaloid shape. The microhardness and the wear resistance of the coating were improved with the increase of NbC. When NbCp content was 30 wt%, the mechanical properties decreased slightly. 2. Experimental 2.1. Materials
* Corresponding author. Ph.D. ** Corresponding author. Prof.; Tel.: þ86 10 67391759; Fax: þ86 10 67396093. E-mail addresses: [email protected] (Q. Li), [email protected] (Y. Lei).
A high carbon steel plate with the dimensions of 100 mm 50 mm 12 mm was used as the substrate, whose nominal composition is listed in Table 1. The surface of substrates
http://dx.doi.org/10.1016/j.jmst.2014.06.012 1005-0302/Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
Q. Li et al. / Journal of Materials Science & Technology 31 (2015) 766e772
767
Table 1 Composition of the substrate (wt%) C
Si
Mn
S
P
Cr
Ni
Cu
V
Mo
1.45
0.40
0.40
0.03
0.03
11.00e12.50
0.25
0.30
0.15e0.30
0.40e0.60
2.3. Characterization
Table 2 Composition of the alloy mixture (wt%) No.
Designed NbC content
C
Nb
Cr
Fe
1 2 3 4
5% 10% 20% 30%
0.57 1.14 2.28 3.43
4.42 8.85 17.71 26.57
18.0 18.0 18.0 18.0
Bal. Bal. Bal. Bal.
Table 3 Laser processing parameters Power (kW)
Scanning velocity (mm/s)
Beam size (mm)
Powder feeding rate (g/min)
Carrier gas flow (L/min)
Shielding gas flow (L/min)
2.3
4
55
8.0
10
10
were polished and rinsed with acetone prior to cladding. Fe-based alloy powder (45e106 mm), niobium-iron alloy powder (66 wt% Nb, 75e106 mm), high carbon ferrochrome (7 wt% C, 75e106 mm) and low carbon ferrochrome powders (C content<0.04 wt%, 75e106 mm) were blended to prepare the cladding mixtures with different designed NbC contents. The atomic ratio between Nb and C always remained 1:1 in each cladding mixture. The compositions of the mixture are listed in Table 2.
2.2. Laser process An IPG fiber laser system with a maximum power of 6 kW was used to product single and multi-track (7 tracks with 30% overlapping rate) coatings for the observation of the microstructure and wear test, respectively. The laser beam spot was fixed to a 5 mm 5 mm square spot. The laser head was integrated with an ABB robot. DPSF-2 powder feeding system and a coaxial nozzle were used to feed the cladding powders into the molten pool by argon gas. Meanwhile, high-purity argon shielding gas was supplied through the same coaxial nozzle to protect the molten pool. The detailed processing parameters are listed in Table 3.
Single track specimens were sectioned in the transverse direction (10 mm 10 mm 12 mm). Transverse sections were mounted and etched with FeCl3 solution for the examination. The phase identification of the coatings was carried out on a Shimadzu XRD-7000 X-ray diffractometer with CuKa radiation operating at 40 kV. The microstructure characterization of coatings was performed with an OLYMPUS BX51 optical microscope, an HITACHI S-3400 scanning electron microscope (SEM) equipped with an energy-dispersive spectroscopy (EDS) and a JEM2100 transmission electron microscope (TEM). The microhardness was measured with a load of 200 gf using an HX-200 micro-hardness tester. The abrasive wear property was evaluated using an MLS-225 wet sand rubber wheel tester. The structural diagram of the tester is shown in Fig. 1, and the detailed measuring parameters are listed in Table 4. Each value of the micro-hardness and wear rate is the average of three measurements. 3. Results and Discussion 3.1. Microstructure of coatings 3.1.1. Porosities in coatings Fig. 2 illustrates the macro-shape of the laser coatings from transverse cross section. It is found that both the size and the amount of the porosity in the coating increase when NbC content increases. This is because in order to increase the designed NbC content in the coating, more niobium-iron alloy powders with higher melting point (1850 K) were added into the mixture, and the mass fraction of Fe-based alloy with lower melting point (1523 K) decreased accordingly, which led to the growth of the average melting point of the mixture. Therefore, the higher liquid viscosity resulted from the increased melting point in the molten pool, making air bubble difficult to float up and leading to the increase of the porosities. 3.1.2. Phase identification The XRD spectra for four samples are shown in Fig. 3. It is found that a-(Fe, Cr), (Fe, Cr)7C3, NbC exist in all coatings, which means that NbC particle can be in-situ formed. The intensity of NbC diffraction peak enhances with the increase of NbC content. Moreover, g-Fe is found in the coatings with higher NbC content. It is probably caused by the increase of C element in the cladding powder, which can improve the stability of g-Fe during solidification. The typical microstructure of the laser coating is shown in Fig. 4. It is clear that the coating is composed of dispersed white particles
Table 4 Measuring parameters of wear test Hardness Abrasive Rotating Wheel of rubber Size (mm) diameter speed of wheel (r/min) (HS) (mm) Fig. 1. Structural diagram of the wear tester.
176
240
60
Load Wear Sample (N) time dimension (min) (mm)
<400(Al2O3) 200
30
57 25 12
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Fig. 2. Macrostructure of the laser coatings with different designed NbC contents: (a) 5 wt% NbC; (b) 30 wt% NbC (porosities are marked by white arrows).
(A), dendritic and equiaxial matrix (B), and compound (C) separating out of the matrix as shown in Fig. 4(a). The compositional analysis of them is shown in Table 5. Three phases are identified as NbC particle, a-(Fe, Cr) matrix, (Fe, Cr)7C3 in alphabetical order. Fig. 4(b) and (c) illustrate the TEM bright-field image and the corresponding selected area diffraction pattern (SADP) of the particle phase, respectively. The result of SADP shows that NbC has the cubic structure, and the calculated lattice parameter is approximately 0.448 nm. The interplanar spacing of (111) and (022) are 0.2590 and 0.1592 nm, respectively, both of which are larger slightly than standard interplanar spacing of them (0.2581 and 0.1580 nm, respectively). Combining with the results of EDS, it can be said that NbCp is a multiple carbide with Fe, Cr and Si elements dissolving in the NbC lattice. This is probably because the growth process of the NbCp was rapid due to the sharp solidification rate (103 K/s)[12]. Fig. 3. XRD patterns of the coatings with different designed NbC contents.
Fig. 4. Typical morphology of the microstructure of laser coating: (a) SEM micrograph of phases in the coating; (b) bright-field TEM micrograph of the NbC particle; (c) the selected area diffraction pattern (SADP) of the NbC particle in (b).
Q. Li et al. / Journal of Materials Science & Technology 31 (2015) 766e772 Table 5 Chemical composition of phases in Fig. 4 (at. %)
A B C
Fe
Cr
B
Si
C
Nb
6.02 64.10 39.47
3.24 19.14 25.13
e e e
1.20 3.08 1.83
38.87 13.68 33.39
50.67 e 0.18
Fig. 5. EDS results of the NbC particles: (a) atomic ratio of Nb and C in the particle; (b) line scanning result of Nb and C in the particle.
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3.1.3. Formation of NbC particles In order to understand the characteristics (particle size, amount and distribution) of in-situ NbC particles in the microstructure, the formation mechanism of NbC and the solidification process of the coating should be studied firstly. The melting points of Fe-based alloy, niobium-iron alloy, high carbon ferrochrome, and low carbon ferrochrome are 1523, 1850, 1700, and 1800 K, respectively. The formation of NbC particle requires the carbon atom to be released from high carbon ferrochrome. Therefore, the temperature of molten pool should be higher than the melting point of high carbon ferrochrome (1700 K). Under this condition, there are two possibilities to form NbC particle. One is that some NbeFe particles with larger size do not melt completely due to the rapid solidification of the molten pool. Nb atoms at the surface of niobium-iron particle react with the surrounding C atoms. With the development of reaction process, C atoms diffuse to the core gradually and the NbC particle is formed at last[13]. Another is that the niobiumeiron particle melts completely; Nb dissolves in the molten pool and reacts with carbon atom, and then NbC particle precipitates out in the molten pool[14,15]. The EDS results of the NbC particles are shown in Fig. 5(a) and (b). It can be found that not only Nb and C elements distribute evenly (line scanning result), but also the Nb/C atomic ratio always keeps 1:1 approximately in the whole particle. The carbon content even is more than that of Nb at the particle core. Moreover, a considerable amount of NbC particles present regular morphology (rectangle, cross and petaloid shape); the particle size is smaller obviously compared with the alloy particle in the cladding mixture; and more importantly, there is no NbCp with coreeshell structure observed in the coatings as illustrated in Fig. 6. Apparently, it would be very difficult for NbC particles to meet the characteristics mentioned above if they have been formed around the unmelted Nb particles in the molten pool. Therefore, the main formation mechanism of the NbC particle may be that the NbC particle forms by the chemical reaction of Nb and carbon in the molten pool. Whereas, the binary phase diagram of NbCeFe (Fig. 7)[16] confirms that there is a large phase field, where both liquid iron and solid NbC exist in a semi-solid state. It indicated that there is plenty of time for nucleation and growth of NbC, even though the temperature of molten pool is under the melting point of niobiumeiron alloy. Therefore, it could be said that the first possibility of the NbC formation mechanism may exist simultaneously. 3.1.4. Effects of designed NbC content The morphology of NbC particles and the microstructure of the coatings with different NbC contents are shown in Figs. 6 and 8,
Fig. 6. Morphology of the NbC particle with different designed NbC contents: (a) 5 wt% NbC; (b) 20 wt% NbC; (c) 30 wt% NbC.
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It is obvious that when designed NbC content is lower, the amount of Nb and C atoms is fewer in the molten pool. Hence, the nucleation and the growth of the particle are limited. When NbC content increases, it is easier for NbC6 octahedral growth units to grow along the [100] direction due to the growth habit of the NbC (the lattice structure of NbC is rock salt type)[17]. At last, the cross and petaloid shaped NbCp with larger size are formed. 3.2. Micro-hardness and wear resistance
Fig. 7. Binary phase diagram of NbCeFe.
respectively. It is worth noting that with the increase of NbC content, both the size and the amount of NbCp increase markedly, and the particle morphology transforms from quadrangle to the petaloid shape gradually.
The variations of the micro-hardness of four coatings along the depth direction are plotted in Fig. 9. It can be found that all microhardness curves change slightly from the top to the bottom of the coatings, and increase sharply within the heat-affected zone. It indicated that the martensitic transformation of the substrate with the high carbon content (1.45 wt%) occurred in this area due to high cooling rate[18]. With the increase of NbC content, the average micro-hardness increases from 819.3 HV0.2 (5 wt%) to 900.5 HV0.2 (20 wt%), and decreases slightly (887.3 HV0.2) when NbC content is 30 wt%. It is obvious that the increase of NbC content can improve the micro-hardness because of the increased size and volume fraction of hard NbCp. However, excess NbCp may cause the rise of the structural stress and the brittleness of the coating, which led to the inapparent change of the micro-hardness when NbC content was more than 20 wt%, as depicted in Fig. 9.
Fig. 8. Microstructure of laser coatings with: (a) 5 wt% NbC; (b) 10 wt% NbC; (c) 20 wt% NbC; (d) 30 wt% NbC.
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Fig. 9. Variations of the micro-hardness along the depth direction of the coatings.
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Fig. 11. It can be found in Fig. 11(a) that there are numerous furrows parallel to each other on the substrate surface, which is caused by micro-plowing and grooving of Al2O3 abrasive particles in the gap between the sample surface and the rotating rubber wheel. It can be seen in Fig. 11(b) and (c) that when NbC content are more than 20 wt%, the width, depth and continuity of the furrows are diminished, the porosities and cracks are found on the worn surface. During the wear process, Al2O3 abrasive particles can easily scratch and plow the substrate surface due to the large difference of their hardness. By contrast, the laser coatings have lower wear rate benefiting from the higher hardness and dispersed in-situ NbCp in the layer, which protrudes gradually from the surface during the initial stage of the wear, and prevents the surface from further abrasion[19]. In addition, the interaction of the NbCp and abrasive Al2O3 particles may lead to the fragmentation and blunting of the abrasive particles. Thus the severity of the microcutting action of the blunted abrasive particles is considerably reduced[20]. With the increase of NbC content, the protective effect of the NbCp is more and more significant, and therefore the wear rate of coating decreased as depicted in Fig. 10. But meantime, the increase of the size and the volume fraction of the NbCp may also cause the rise of the structural stress and the brittleness of the coating, which makes NbCp more fragile and easy to spall during the abrasion[9,21]. As a consequence, the wear rate of the coating with 30 wt% NbC increases slightly. Moreover, the rise of the structural stress induces the generation and propagation of the crack in the abrasive process as illustrated in Fig. 11(b) and (c).
4. Conclusions
Fig. 10. Bar graph of the wear rate of substrate and coatings with different NbC contents.
The wear rate of the substrate and the coatings with different NbC contents are shown in Fig. 10. It can be seen that the wear resistance of each coating is higher than that of the substrate. The wear rate of the coatings with 20 wt% NbCp is just a quarter to that of substrate. Moreover, the wear rate decreases with increasing the NbC content, and increases slightly when NbC content is 30 wt%. The worn surfaces of the substrate and coatings are illustrated in
(1) The size and amount of porosity in the coating increase with the increase of designed NbC content. (2) a-(Fe, Cr), (Fe, Cr)7C3, NbC exist in all coatings, and g-Fe is found in the coatings containing higher NbC content. NbC is a multiple carbide with the Fe, Cr and Si elements dissolving in the NbC lattice. (3) NbC particles form by the chemical reaction of Nb and C elements dissolving in the molten pool. When the designed NbC content increases, the size and the amount of the particle increase, and the particle morphology transforms from the quadrangle to the petaloid shape gradually. (4) With the increase of the NbC, the micro-hardness and the wear resistance of the coating are improved. When NbC content is 30 wt%, both of them decrease slightly.
Fig. 11. Wear appearance of the substrate and the coatings: (a) substrate; (b) coating with 20 wt% NbC; (c) coating with 30 wt% NbC.
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Acknowledgment The authors would like to thank the National Natural Science Foundation of China under grant (10009012201203) for financial support for this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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