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Effects of vanadium addition on microstructure, mechanical properties and wear resistance of Ni-Hard4 white cast iron
Materials and Design 49 (2013) 888–893
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Effects of vanadium addition on microstructure, mechanical properties and wear resistance of Ni-Hard4 white cast iron M. Mohammadnezhad a,⇑, V. Javaheri b,c, M. Shamanian a, M. Naseri b, M. Bahrami d a
Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Research and Development unit of Isfahan Casting Industrial (ICI), Isfahan 83551-1111, Iran c Iran University of Industries and Mines , Tehran 14395518, Iran d Department of Materials Science and Engineering, Islamic Azad University Najafabad Branch, Najafabad 85141-43131, Iran b
a r t i c l e
i n f o
Article history: Received 19 November 2012 Accepted 13 February 2013 Available online 26 February 2013
a b s t r a c t Ni-Hard4 white cast iron is commonly used in applications requiring excellent abrasion resistance as in the mining and mineral ore processing industry. In this study, the effects of vanadium on the microstructure, mechanical properties and wear-resistance of Ni-Hard4 white cast iron were investigated. This study was conducted in six laboratory-made alloys with different vanadium contents. The microstructure of the samples was characterized using the optical microscopy, the scanning electron microscopy, and the energy dispersive X-ray spectrometry. The impact energy, hardness and wear resistance of the samples were determined. The results indicated that with an increase in vanadium concentration, the chromium carbides were refined, and the volume fraction of carbides was decreased. After increasing the vanadium content by 2%, the microstructure of Ni-Hard4 white cast iron became finer and the hardness and wear resistance were improved without reduction of fracture toughness. The result of on-line service (AG Mill) showed that the wear resistance of Ni-Hard4 white cast iron was modified by 2%, and vanadium liners were 40% better than the basic Ni-Hard4. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Ni-Hard4 white cast iron (NHWCI) is extensively used in applications where good resistance to abrasion wear is required; such as in mineral processing, cement, copper and iron manufacturing. NHWCI is used to produce the liners of the AG Mill in Chadormalu mining & Industrial Company in Iran, (the most dominant iron ore concentrate (Pellet Feed) producer in the Middle East). In mining and mineral processing environments, appropriate impact resistance and excellent wear resistance are required [1]. NHWCI can be described as in situ composites with massive and hard chromium carbides in a martensitic matrix. The discontinuous distribution of rod-like chromium carbide in martensitic matrix increases impact toughness and wear resistance [2]. Hence, the requests for materials which can tolerate harsh service conditions are on the rise. The tendency to decrease common shutdown of the apparatus for replacement of worn components, and a decrease in costs and an increase in productivity have encouraged engineers to evaluate candidate alloys, which provide much better abrasive wear resistance along with adequate toughness. Researchers have reported improvements in the wear resistance and mechanical properties of cast iron without reduction in impact toughness by adding ⇑ Corresponding author. Tel.: +98 311 3802385; fax: +98 311 3802384. E-mail address: [email protected] (M. Mohammadnezhad). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.02.043
strong carbide-forming elements, such as vanadium, tungsten, titanium and niobium [3,4]. The aim of these studies was to refine and modify chromium carbides by increasing the carbide nucleation rate to obtain dispersed fine carbides and to improve the hardness of the matrix. Alloying elements such as titanium, vanadium, and niobium could act as a site for heterogeneous nucleation of carbides [5]. At present, there are a few reports about the research and application of the wear resistant NHWCI. Chung et al. [5] reported the effects of titanium addition on microstructure and wear resistance of hypereutectic high chromium cast iron. Xiaojun et al. [6] researched the effects of titanium on the morphology of primary M7C3 carbides in hypereutectic high chromium white iron. Mousavi Anijdan et al. [7] reported the effects of tungsten on erosion-corrosion behavior of high chromium white cast iron. Xiaohui et al. [8] researched the effect of niobium on the microstructure and mechanical properties of hypereutectic high chromium cast irons. Development and use of the NHWCI have been considered in recent years in comparison to high chromium cast irons because they exhibit higher hardness and better wear resistance [9]. However, no systematic work has been performed to study the effects of vanadium addition on properties of NHWCI. In the present study, various percentages of vanadium were added to NHWCI, and the effect of vanadium addition on the carbide characteristics (size, morphology, distribution, and volume fraction), mechanical properties and wear resistance of NHWCI were investigated.
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M. Mohammadnezhad et al. / Materials and Design 49 (2013) 888–893
2. Experimental procedure The alloys were prepared in a 200 kg medium frequency induction furnace. The melting temperature was about 1500 °C, after removal of any dross and slag, casting was started using CO2-silicate molds. The size of the specimen was 100 mm 100 mm 200 mm. The final chemical compositions of the alloys are listed in Table 1. Samples were heat treated in an electric furnace with the ambient atmosphere at 820 °C for 2 h, followed by air-quenching to room temperature. The microstructures of the specimens are characterized using the Olympus optical microscope (OM), scanning electron microscopy (SEM) in a Philips XL30 at an accelerating voltage of 30 kV equipped with an energy dispersive X-ray spectrometer (EDS). The volume fraction of carbides and average carbide diameter (D) were measured by using Clemex Image Analyzer software from 20 OM images of each specimen (randomly selected) at magnification of 200 times. The following equation used to determine carbide diameter as the refining effect of vanadium on the carbides:
rffiffiffiffiffiffiffi A D¼2 Np
ð1Þ 2
In this equation A is the area of the carbides (lm ) and N is the number of the carbides [8]. Impact toughness tests were performed on a Galdabini Charpy impact universal testing machine; all tests were carried out on three samples. The samples hardness was measured on EMCO Rockwell hardness tester. The microhardness of samples was measured by Akashi hardness testing machine at a load of 500 g and dwell time of 10 s. Ten measurements were taken across each sample to obtain the average value of hardness and microhardness. The wear resistance of NHWCI was evaluated using a dry sand/ rubber wheel abrasion wear test method in accordance with ASTM: G65-04 specifications. AFS 50/70 sand was used, and the sand flow rate was adjusted to 300–400 g/min. A testing load of 130 N was selected. The total wear distance and rotation speed were 4500 m and 250 rpm, respectively. The wear resistance of NHWCI was ranked in terms of the weight loss [5–11]. 3. Results and discussion Fig. 1 shows the installation and working parts position, which were optimized in this study. The microstructures of the NHWCI with different amounts of vanadium are illustrated in Fig. 2. The effect of vanadium concentration on the volume fraction and the average diameters of all carbides are shown in Fig. 3. The carbide areas (bell curve) shown in Fig. 4 are obtained from OM images; it can be observed that the carbide areas distribution was decreased gradually with the increase of vanadium concentration. As illustrated, the morphology of chromium carbides was refined gradually, the shape of the carbides became more isotropic, and there was a more homogeneous distribution when vanadium concentration increased. A major effect can be seen on the morphology of chromium carbides. Vanadium is widely used in steel and cast
Fig. 1. Large AG Mill.
iron manufacturing as an effective alloying element for partitioning to the matrix as well as modification of the carbides. Vanadium is one of the strong carbide forming elements; therefore, VC can be easily put into the molten alloy [12]. During the solidification process, the first precipitated compounds have the maximum melting point. The melting points for the compounds vanadium carbides (VC) and chromium carbides (M7C3) are 2830 °C and 1710 °C, respectively. As a result, it is inevitable for small high melting point particles (VC) to act as heterogeneous nucleation nuclei for chromium carbides in the reaction, and they lead to significant refinement of the carbides and improvement of chromium carbides distribution and morphology [11–14]. In (Fig. 5), it has been proved based on results of SEM micrograph and EDS analysis that VC can act as the heterogeneous nuclei of chromium carbides. On the other hand, the addition of alloying elements also influences the eutectic transition temperature range in the phase diagram. Vanadium addition was reported to narrow the solidification temperature interval; it is believed that when the solidification temperature interval is narrower, it shortens the co-existence time of liquid and solid phases in the same crystal growth conditions, and refines the microstructure. According to Eq. (1) the eutectic colonies diameter Ew as a function of eutectic transition temperature range DTE is expressed as [15]:
Ew ¼ 1:68 103 DT E
ð2Þ
This means that the fact that vanadium addition narrows the eutectic transition temperature range leads to a decrease in diameter of the eutectic colonies and refinement of the microstructure. Fig. 6 shows the BSE (backscattered) micrographs of the sample with 2 and 2.5% vanadium. A comparison of Fig. 6a–d shows that the microstructures of the sample with 2% V (Fig. 6a) consist of hard carbides embedded in a martensite matrix whereas the sample with 2.5% V (Fig. 6b) consists of carbides embedded in a ferrite
Table 1 Chemical composition of alloys in this experiment. Alloy element (wt%)
C
Si
Mn
Cr
Mo
Ni
S&P
V
ASTM A532 NV0 NV5 NV10 NV15 NV20 NV25
2.5–3.6 3.12 3.02 3.18 2.97 3.21 3.09
Max2 1.83 1.75 1.64 1.92 1.69 1.79
Max2 0.71 0.62 0.69 0.85 0.92 0.81
7-11 8.21 9.02 8.69 8.96 9.15 8.98
Max2 0.75 0.89 0.69 0.81 0.91 0.79
4.5-7 5.52 5.30 5.48 5.11 5.61 5.34
Max0.15 0.07 0.08 0.06 0.08 0.07 0.09
– 0 0.48 1.02 1.52 1.96 2.58
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Fig. 2. Optical micrographs of samples with different amounts of vanadium addition.
Fig. 3. Effect of vanadium concentration on the volume fraction and average diameters of all carbides.
matrix. By increasing the V content, the volume fraction of chromium carbides in NHWCI decreased (Fig. 3), while the chromium concentration in the matrix increased since more chromium and less carbon promoted change from martensite structure to ferrite,
and the formation of the ferrite matrix. Results obtained by Vickers microhardness of the matrix in tested NHWCI with 2% and 2.5% vanadium illustrated that the hardness of the samples with 2% and 2.5% vanadium were 780 ± 50 HV and 280 ± 42 HV, respectively. The decrease in the hardness of the matrix can be attributed to the existence of ferrite matrix. Results obtained by EDS analysis of the matrix in tested NHWCI with different amounts of vanadium are given in Table 2. The carbon content was calculated by balancing with other elements. These results (Table 2) indicated that with an increase in the vanadium content, the concentration of carbon decreases in the matrix. This is due to the low amount of carbon that increases Ms, whereas the amount of retained austenite is decreased. As a consequence, with an increase in vanadium content in the examined NHWCI, the volume fraction of martensite increases [16,18]. The increase in the martensitic matrix can be correlated qualitatively to the improvement of wear resistance and hardness [17]. The effects of vanadium on the hardness and impact toughness of NHWCI are shown in Fig. 7. By adding vanadium and formation of VC, the hardness of the samples slightly increases because the Vickers hardness of VC is 2800 HV, which is higher than that of chromium carbides (1200–1800 HV) [12,18]. In general, the refinements of carbides positively affect the mechanical properties such as hardness and impact toughness [19]. The hardness value is strongly re-
M. Mohammadnezhad et al. / Materials and Design 49 (2013) 888–893
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Fig. 4. Area fraction distribution of carbides; x-axis: mean area carbides in lm2; y-axis number of the carbides.
duced in the sample with 2.5% vanadium. This is generally attributed to the formation of the ferritic matrix, which is relatively softer than the fine martensite structure. The impact energy relatively increased with an increase in vanadium up to 2%, and strongly increased at 2.5% vanadium alloy. Increase in bulk hardness could imply a decrease in fracture toughness; however, concurrent with increasing hardness, effective parameters such as particle dispersion, decrease in volume fraction carbide, and structure refinement prevented the reduction of impact toughness of NHWCI. The existence of ferrite phase resulted in a sharp increase in impact strength. The weight loss of the NHWCI samples with different concentrations of vanadium during abrasive wear testing are measured, and the results are illustrated in Fig. 8. As demonstrated, the wear resistance of the NHWCI was dominated by the concentration of
vanadium. The weight loss decreased strongly as the %V increased up to a critical value of 2 wt%, above which the weight loss was increased. The variation in the wear behavior should be attributed to two factors that may influence the microstructure and properties of the NHWCI. One of the factors is the presence of hard VC carbides, which are harder than chromium carbides. This hard primary carbide is not easily scratched by natural minerals [20–23]. As %V increases, the matrix changes from a retained austenite structure to a martensite one. The other factor is the refinement of chromium carbides, which appears to be the predominant factor that controls the weight loss of the alloy [22–24]. The fine microstructure consists of fine chromium carbides with a more homogeneous distribution along with VC. This microstructure should be mainly responsible for the highest wear resistance and hardness. In the final sample (2.5 wt% V), due to the formation of ferrite
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Fig. 5. SEM micrographs and EDS patterns showing VC particles and chromium carbide in martensite mixture in 2% V concentration sample.
Fig. 7. Changes in the hardness and impact toughness of NHWCI with different amounts of added vanadium.
Fig. 6. SEM secondary electron image (a) 2% V concentration sample and (b) 2.5% V concentration sample.
Table 2 Chemical composition of the matrix in NHWCI. Alloy
C
Si
Mn
Cr
Mo
Ni
V
Fe
NV0 NV5 NV10 NV15 NV20 NV25
1.92 1.56 1.21 0.97 0.62 0.51
1.74 1.68 1.71 1.65 1.76 1.78
0.62 0.63 0.61 0.59 0.61 0.57
2.31 2.56 3.12 3.46 3.87 4.98
0.46 0.51 0.38 0.58 0.34 0.30
4.48 5.21 5.01 4.95 4.78 5.12
0 0.02 0.23 0.64 1.02 1.53
88.47 87.83 87.73 87.16 87.00 85.21 Fig. 8. Effect of vanadium on wear resistance of NHWCI cast iron.
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Acknowledgements The authors would like to thank the financial support for this work from the Isfahan Casting Industrial (ICI) of Iran. References
Fig. 9. Liner of Ni-Hard4 white cast iron (a) before working and (b) after working.
phase, wear resistance has reduced. These wear results assume that the matrix microstructure plays an important role in wear loss. The result of on-line service in the iron ore AG Mill investigation indicated that the wear resistance for the liner with NHWCI, modified by 2% vanadium, was 40% better than the basic analysis of NHWCI (Fig. 9). 4. Conclusions This paper was carried out to investigate the effect of vanadium concentration on the microstructure, mechanical properties and wear resistance in Ni-Hard4 white cast iron. The following conclusions can be derived from experiments: VC particles can act as substrates for heterogeneous nucleation of chromium carbides, resulting in the significant refinement of the final microstructure. With 2 wt% V added, a fine microstructure with dispersed VC carbide was achieved. This microstructure exhibited the highest hardness and wear resistance without any decreases in impact toughness. The result of on-line service investigation indicated that the wear resistance for the liner with NHWCI, modified by 2% vanadium, was 40% better than the basic NHWCI.
[1] Jinzhu L, Shizhuo L, Yongfa M. Wear resistance of Ni-hard 4 and high chromium cast iron re-evaluated. Wear 1993;166:37–40. [2] Karantzalis AE, Lekatou A, Mavros H. Microstructural modifications of as-cast high-chromium white iron by heat treatment. J Mater Eng Perform 2009;18:174–81. [3] Bedolla-Jacuinde A, Correa R, Quezada JG, Maldonado C. Effect of titanium on the as-cast microstructure of a 16%chromium white iron. Mater Sci Eng A 2005;398:297–308. [4] Radulovic M, Fiset M, Peev K, Tomovic M. The influence of vanadium on fracture toughness and abrasion resistance in high chromium white cast irons. J Mater Sci 1994;29:5085–94. [5] Chung RJ, Tang X, Li DY, Hinckley B, Dolman K. Effects of titanium addition on microstructure and wear resistance of hypereutectic high chromium cast iron Fe–25 wt%Cr–4 wt%C. Wear 2009;267:356–61. [6] Xiaojun W, Jiandong X, Hanguang F, Xiaohui Z. Effect of titanium on the morphology of primary M7C3 carbides in hypereutectic high chromium white iron. Mater Sci Eng A 2007;457:180–5. [7] Mousavi Anijdan SH, Bahrami A, Varahram N, Davami P. Effects of tungsten on erosion–corrosion behavior of high chromium white cast iron. Mater Sci Eng A 2007;454:623–8. [8] Xiaohui Z, Jiandong X, Hanguang F, Bing X. Effect of niobium on the as-cast microstructure of hypereutectic high chromium cast iron. Mater Lett 2008;62:857–60. [9] Jinzhu Liu L, Yongfa M. Development of abrasion-resistant Ni-hard 4 cast irons. Wear 1993;162:833–6. [10] Razavi M, Rahimipour MR, Rajabi-Zamani AH. Effect of nanocrystalline TiC powder addition on the hardness and wear resistance of cast iron. Mater Sci Eng A 2007;454:144–7. [11] Scandian C, Boher C, de Mello JDB, Rézaï-Aria F. Effect of molybdenum and chromium contents in sliding wear of high-chromium white cast iron: the relationship between microstructure and wear. Wear 2009;267:401–8. [12] Pierson HO. Handbook of refractory carbides and nitrides. 1st ed. Sandia National Laboratories; 1996. [13] Xiang C, Yanxiang L. Effect of heat treatment on microstructure and mechanical properties of high boron white cast iron. Mater Sci Eng A 2010;528:770–5. [14] Xiaohui Z, Jiandong X, Yimin G. Effect of heat treatment on microstructure and mechanical properties of a Ti-bearing hypereutectic high chromium white cast iron. Mater Sci Eng A 2008;487:171–9. [15] Ogi K, Matsubara Y, Matsuda K. Eutectic solidification of high chromium cast iron-eutectic mechanism of eutectic growth. AFS Trans 1982;89:197–204. [16] Wiengmoon A, Chairuangsri T, Brown A, Brydson R, Edmonds DV, Pearce JTH. Microstructural and crystallographical study of carbides in 30 wt%Cr cast irons. Acta Mater 2005;53:4143–5. [17] Vadiraj A, Balachandran G, Kamaraj M. Effect of misch metal inoculation on microstructure, mechanical and wear properties of hypoeutectic gray cast irons. Mater Des 2009;30:4488–92. [18] Chenje TW, Simbi DJ, Navara E. Relationship between microstructure, hardness, impact toughness and wear performance of selected grinding media for mineral ore milling operations. Mater Des 2004;25:11–8. [19] Tang XH, Chung R, Li DY, Hinckley B, Dolman K. Variations in microstructure of high chromium cast irons and resultant changes in resistance to wear, corrosion and corrosive wear. Wear 2009;267:116–21. [20] Wang J, Zuo RL, Sun ZP, Li C, Liu HH, Yang HS, et al. Influence of secondary carbides precipitation and transformation on hardening behavior of a 15 Cr–1 Mo–1.5 V white iron. Mater Charact 2005;55:234–40. [21] Xiaohui Z, Jiandong X, Hanguang F, Yiming G. Effect of titanium on the as-cast microstructure of hypereutectic high chromium cast iron. Mater Charact 2008;59:1221–6. [22] Jensen RD, Fundal E, Moller P, Jespersen M. Wear mechanism of abrasion resistant wear parts in raw material vertical roller mills. Wear 2011;271:2707–19. [23] Nieminen R, Autio J, Suomalainen E, Husu N, Kauppi M. Wear mechanism of Ni-Hard4 rollers in chromite ore crushing. Wear 1994;179:95–100. [24] Yao M, Page NW. Friction measurement on Ni-Hard 4 during high pressure crushing of silica. Wear 2001;249:117–26.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Effects of vanadium addition on microstructure, mechanical properties and wear resistance of Ni-Hard4 white cast iron M. Mohammadnezhad a,⇑, V. Javaheri b,c, M. Shamanian a, M. Naseri b, M. Bahrami d a
Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Research and Development unit of Isfahan Casting Industrial (ICI), Isfahan 83551-1111, Iran c Iran University of Industries and Mines , Tehran 14395518, Iran d Department of Materials Science and Engineering, Islamic Azad University Najafabad Branch, Najafabad 85141-43131, Iran b
a r t i c l e
i n f o
Article history: Received 19 November 2012 Accepted 13 February 2013 Available online 26 February 2013
a b s t r a c t Ni-Hard4 white cast iron is commonly used in applications requiring excellent abrasion resistance as in the mining and mineral ore processing industry. In this study, the effects of vanadium on the microstructure, mechanical properties and wear-resistance of Ni-Hard4 white cast iron were investigated. This study was conducted in six laboratory-made alloys with different vanadium contents. The microstructure of the samples was characterized using the optical microscopy, the scanning electron microscopy, and the energy dispersive X-ray spectrometry. The impact energy, hardness and wear resistance of the samples were determined. The results indicated that with an increase in vanadium concentration, the chromium carbides were refined, and the volume fraction of carbides was decreased. After increasing the vanadium content by 2%, the microstructure of Ni-Hard4 white cast iron became finer and the hardness and wear resistance were improved without reduction of fracture toughness. The result of on-line service (AG Mill) showed that the wear resistance of Ni-Hard4 white cast iron was modified by 2%, and vanadium liners were 40% better than the basic Ni-Hard4. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Ni-Hard4 white cast iron (NHWCI) is extensively used in applications where good resistance to abrasion wear is required; such as in mineral processing, cement, copper and iron manufacturing. NHWCI is used to produce the liners of the AG Mill in Chadormalu mining & Industrial Company in Iran, (the most dominant iron ore concentrate (Pellet Feed) producer in the Middle East). In mining and mineral processing environments, appropriate impact resistance and excellent wear resistance are required [1]. NHWCI can be described as in situ composites with massive and hard chromium carbides in a martensitic matrix. The discontinuous distribution of rod-like chromium carbide in martensitic matrix increases impact toughness and wear resistance [2]. Hence, the requests for materials which can tolerate harsh service conditions are on the rise. The tendency to decrease common shutdown of the apparatus for replacement of worn components, and a decrease in costs and an increase in productivity have encouraged engineers to evaluate candidate alloys, which provide much better abrasive wear resistance along with adequate toughness. Researchers have reported improvements in the wear resistance and mechanical properties of cast iron without reduction in impact toughness by adding ⇑ Corresponding author. Tel.: +98 311 3802385; fax: +98 311 3802384. E-mail address: [email protected] (M. Mohammadnezhad). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.02.043
strong carbide-forming elements, such as vanadium, tungsten, titanium and niobium [3,4]. The aim of these studies was to refine and modify chromium carbides by increasing the carbide nucleation rate to obtain dispersed fine carbides and to improve the hardness of the matrix. Alloying elements such as titanium, vanadium, and niobium could act as a site for heterogeneous nucleation of carbides [5]. At present, there are a few reports about the research and application of the wear resistant NHWCI. Chung et al. [5] reported the effects of titanium addition on microstructure and wear resistance of hypereutectic high chromium cast iron. Xiaojun et al. [6] researched the effects of titanium on the morphology of primary M7C3 carbides in hypereutectic high chromium white iron. Mousavi Anijdan et al. [7] reported the effects of tungsten on erosion-corrosion behavior of high chromium white cast iron. Xiaohui et al. [8] researched the effect of niobium on the microstructure and mechanical properties of hypereutectic high chromium cast irons. Development and use of the NHWCI have been considered in recent years in comparison to high chromium cast irons because they exhibit higher hardness and better wear resistance [9]. However, no systematic work has been performed to study the effects of vanadium addition on properties of NHWCI. In the present study, various percentages of vanadium were added to NHWCI, and the effect of vanadium addition on the carbide characteristics (size, morphology, distribution, and volume fraction), mechanical properties and wear resistance of NHWCI were investigated.
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M. Mohammadnezhad et al. / Materials and Design 49 (2013) 888–893
2. Experimental procedure The alloys were prepared in a 200 kg medium frequency induction furnace. The melting temperature was about 1500 °C, after removal of any dross and slag, casting was started using CO2-silicate molds. The size of the specimen was 100 mm 100 mm 200 mm. The final chemical compositions of the alloys are listed in Table 1. Samples were heat treated in an electric furnace with the ambient atmosphere at 820 °C for 2 h, followed by air-quenching to room temperature. The microstructures of the specimens are characterized using the Olympus optical microscope (OM), scanning electron microscopy (SEM) in a Philips XL30 at an accelerating voltage of 30 kV equipped with an energy dispersive X-ray spectrometer (EDS). The volume fraction of carbides and average carbide diameter (D) were measured by using Clemex Image Analyzer software from 20 OM images of each specimen (randomly selected) at magnification of 200 times. The following equation used to determine carbide diameter as the refining effect of vanadium on the carbides:
rffiffiffiffiffiffiffi A D¼2 Np
ð1Þ 2
In this equation A is the area of the carbides (lm ) and N is the number of the carbides [8]. Impact toughness tests were performed on a Galdabini Charpy impact universal testing machine; all tests were carried out on three samples. The samples hardness was measured on EMCO Rockwell hardness tester. The microhardness of samples was measured by Akashi hardness testing machine at a load of 500 g and dwell time of 10 s. Ten measurements were taken across each sample to obtain the average value of hardness and microhardness. The wear resistance of NHWCI was evaluated using a dry sand/ rubber wheel abrasion wear test method in accordance with ASTM: G65-04 specifications. AFS 50/70 sand was used, and the sand flow rate was adjusted to 300–400 g/min. A testing load of 130 N was selected. The total wear distance and rotation speed were 4500 m and 250 rpm, respectively. The wear resistance of NHWCI was ranked in terms of the weight loss [5–11]. 3. Results and discussion Fig. 1 shows the installation and working parts position, which were optimized in this study. The microstructures of the NHWCI with different amounts of vanadium are illustrated in Fig. 2. The effect of vanadium concentration on the volume fraction and the average diameters of all carbides are shown in Fig. 3. The carbide areas (bell curve) shown in Fig. 4 are obtained from OM images; it can be observed that the carbide areas distribution was decreased gradually with the increase of vanadium concentration. As illustrated, the morphology of chromium carbides was refined gradually, the shape of the carbides became more isotropic, and there was a more homogeneous distribution when vanadium concentration increased. A major effect can be seen on the morphology of chromium carbides. Vanadium is widely used in steel and cast
Fig. 1. Large AG Mill.
iron manufacturing as an effective alloying element for partitioning to the matrix as well as modification of the carbides. Vanadium is one of the strong carbide forming elements; therefore, VC can be easily put into the molten alloy [12]. During the solidification process, the first precipitated compounds have the maximum melting point. The melting points for the compounds vanadium carbides (VC) and chromium carbides (M7C3) are 2830 °C and 1710 °C, respectively. As a result, it is inevitable for small high melting point particles (VC) to act as heterogeneous nucleation nuclei for chromium carbides in the reaction, and they lead to significant refinement of the carbides and improvement of chromium carbides distribution and morphology [11–14]. In (Fig. 5), it has been proved based on results of SEM micrograph and EDS analysis that VC can act as the heterogeneous nuclei of chromium carbides. On the other hand, the addition of alloying elements also influences the eutectic transition temperature range in the phase diagram. Vanadium addition was reported to narrow the solidification temperature interval; it is believed that when the solidification temperature interval is narrower, it shortens the co-existence time of liquid and solid phases in the same crystal growth conditions, and refines the microstructure. According to Eq. (1) the eutectic colonies diameter Ew as a function of eutectic transition temperature range DTE is expressed as [15]:
Ew ¼ 1:68 103 DT E
ð2Þ
This means that the fact that vanadium addition narrows the eutectic transition temperature range leads to a decrease in diameter of the eutectic colonies and refinement of the microstructure. Fig. 6 shows the BSE (backscattered) micrographs of the sample with 2 and 2.5% vanadium. A comparison of Fig. 6a–d shows that the microstructures of the sample with 2% V (Fig. 6a) consist of hard carbides embedded in a martensite matrix whereas the sample with 2.5% V (Fig. 6b) consists of carbides embedded in a ferrite
Table 1 Chemical composition of alloys in this experiment. Alloy element (wt%)
C
Si
Mn
Cr
Mo
Ni
S&P
V
ASTM A532 NV0 NV5 NV10 NV15 NV20 NV25
2.5–3.6 3.12 3.02 3.18 2.97 3.21 3.09
Max2 1.83 1.75 1.64 1.92 1.69 1.79
Max2 0.71 0.62 0.69 0.85 0.92 0.81
7-11 8.21 9.02 8.69 8.96 9.15 8.98
Max2 0.75 0.89 0.69 0.81 0.91 0.79
4.5-7 5.52 5.30 5.48 5.11 5.61 5.34
Max0.15 0.07 0.08 0.06 0.08 0.07 0.09
– 0 0.48 1.02 1.52 1.96 2.58
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Fig. 2. Optical micrographs of samples with different amounts of vanadium addition.
Fig. 3. Effect of vanadium concentration on the volume fraction and average diameters of all carbides.
matrix. By increasing the V content, the volume fraction of chromium carbides in NHWCI decreased (Fig. 3), while the chromium concentration in the matrix increased since more chromium and less carbon promoted change from martensite structure to ferrite,
and the formation of the ferrite matrix. Results obtained by Vickers microhardness of the matrix in tested NHWCI with 2% and 2.5% vanadium illustrated that the hardness of the samples with 2% and 2.5% vanadium were 780 ± 50 HV and 280 ± 42 HV, respectively. The decrease in the hardness of the matrix can be attributed to the existence of ferrite matrix. Results obtained by EDS analysis of the matrix in tested NHWCI with different amounts of vanadium are given in Table 2. The carbon content was calculated by balancing with other elements. These results (Table 2) indicated that with an increase in the vanadium content, the concentration of carbon decreases in the matrix. This is due to the low amount of carbon that increases Ms, whereas the amount of retained austenite is decreased. As a consequence, with an increase in vanadium content in the examined NHWCI, the volume fraction of martensite increases [16,18]. The increase in the martensitic matrix can be correlated qualitatively to the improvement of wear resistance and hardness [17]. The effects of vanadium on the hardness and impact toughness of NHWCI are shown in Fig. 7. By adding vanadium and formation of VC, the hardness of the samples slightly increases because the Vickers hardness of VC is 2800 HV, which is higher than that of chromium carbides (1200–1800 HV) [12,18]. In general, the refinements of carbides positively affect the mechanical properties such as hardness and impact toughness [19]. The hardness value is strongly re-
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Fig. 4. Area fraction distribution of carbides; x-axis: mean area carbides in lm2; y-axis number of the carbides.
duced in the sample with 2.5% vanadium. This is generally attributed to the formation of the ferritic matrix, which is relatively softer than the fine martensite structure. The impact energy relatively increased with an increase in vanadium up to 2%, and strongly increased at 2.5% vanadium alloy. Increase in bulk hardness could imply a decrease in fracture toughness; however, concurrent with increasing hardness, effective parameters such as particle dispersion, decrease in volume fraction carbide, and structure refinement prevented the reduction of impact toughness of NHWCI. The existence of ferrite phase resulted in a sharp increase in impact strength. The weight loss of the NHWCI samples with different concentrations of vanadium during abrasive wear testing are measured, and the results are illustrated in Fig. 8. As demonstrated, the wear resistance of the NHWCI was dominated by the concentration of
vanadium. The weight loss decreased strongly as the %V increased up to a critical value of 2 wt%, above which the weight loss was increased. The variation in the wear behavior should be attributed to two factors that may influence the microstructure and properties of the NHWCI. One of the factors is the presence of hard VC carbides, which are harder than chromium carbides. This hard primary carbide is not easily scratched by natural minerals [20–23]. As %V increases, the matrix changes from a retained austenite structure to a martensite one. The other factor is the refinement of chromium carbides, which appears to be the predominant factor that controls the weight loss of the alloy [22–24]. The fine microstructure consists of fine chromium carbides with a more homogeneous distribution along with VC. This microstructure should be mainly responsible for the highest wear resistance and hardness. In the final sample (2.5 wt% V), due to the formation of ferrite
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Fig. 5. SEM micrographs and EDS patterns showing VC particles and chromium carbide in martensite mixture in 2% V concentration sample.
Fig. 7. Changes in the hardness and impact toughness of NHWCI with different amounts of added vanadium.
Fig. 6. SEM secondary electron image (a) 2% V concentration sample and (b) 2.5% V concentration sample.
Table 2 Chemical composition of the matrix in NHWCI. Alloy
C
Si
Mn
Cr
Mo
Ni
V
Fe
NV0 NV5 NV10 NV15 NV20 NV25
1.92 1.56 1.21 0.97 0.62 0.51
1.74 1.68 1.71 1.65 1.76 1.78
0.62 0.63 0.61 0.59 0.61 0.57
2.31 2.56 3.12 3.46 3.87 4.98
0.46 0.51 0.38 0.58 0.34 0.30
4.48 5.21 5.01 4.95 4.78 5.12
0 0.02 0.23 0.64 1.02 1.53
88.47 87.83 87.73 87.16 87.00 85.21 Fig. 8. Effect of vanadium on wear resistance of NHWCI cast iron.
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Acknowledgements The authors would like to thank the financial support for this work from the Isfahan Casting Industrial (ICI) of Iran. References
Fig. 9. Liner of Ni-Hard4 white cast iron (a) before working and (b) after working.
phase, wear resistance has reduced. These wear results assume that the matrix microstructure plays an important role in wear loss. The result of on-line service in the iron ore AG Mill investigation indicated that the wear resistance for the liner with NHWCI, modified by 2% vanadium, was 40% better than the basic analysis of NHWCI (Fig. 9). 4. Conclusions This paper was carried out to investigate the effect of vanadium concentration on the microstructure, mechanical properties and wear resistance in Ni-Hard4 white cast iron. The following conclusions can be derived from experiments: VC particles can act as substrates for heterogeneous nucleation of chromium carbides, resulting in the significant refinement of the final microstructure. With 2 wt% V added, a fine microstructure with dispersed VC carbide was achieved. This microstructure exhibited the highest hardness and wear resistance without any decreases in impact toughness. The result of on-line service investigation indicated that the wear resistance for the liner with NHWCI, modified by 2% vanadium, was 40% better than the basic NHWCI.
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