- Email: [email protected]
low carbon steel composite material by cast and hot rolling process
Materials and Design 31 (2010) 3062–3066
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Fabrication of high chromium cast iron/low carbon steel composite material by cast and hot rolling process Guoliang Xie, Hui Sheng, Jingtao Han, Jing Liu * School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 24 November 2009 Accepted 9 January 2010 Available online 15 January 2010
a b s t r a c t A sandwich-structured composite blank containing a high chromium cast iron (HCCI) and low carbon steel (LCS) claddings was successfully fabricated by casting and hot rolling, and then a series of quenching and tempering treatments were employed. The evolution of microstructures and microhardness of ascast, hot-rolled and heat-treated specimens were investigated. The microstructures of hot-rolled HCCI are refined and significant variations of carbides are observed. A perfect metallurgical bonding between HCCI and LCS is revealed by the continuous distributions of alloy elements. The microhardness of hot-rolled HCCI after quenching and tempering is found to be close to that of as-cast one. The hardness of HCCI can reach up to HV 750 or above after oil quenching. The hardness of HCCI reduced to HV 600–750 after tempering due to the tempering of martensite. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction High chromium cast irons (HCCIs) are excellent wear-resistant materials, which have been widely used for wear-affected equipment operated under extreme conditions, such as facilities in the slurry pumping systems of the oil sands handling and mineral processing [1,2]. The typical microstructures of HCCIs consist of chromium carbides of high hardness dispersed in a matrix which still contains a sufficient concentration of carbon to allow hardening of transformation of austenite to martensite after quenching [3– 6]. The exceptional wear resistance of HCCIs results primarily from the high volume fraction of hard chromium carbides, which helps to prevent the formation of graphite and stabilize the carbides [7,8]. By changing the microstructure and composition of the matrix, a wide variety of ductile irons forging abilities can be obtained between 850 °C and 1000 °C [9,10]. However, the shaping of HCCIs and coatings or cladding materials containing HCCIs is generally carried out by means of casting techniques or spray forming in liquid state [11]. In this research, a composite blank containing HCCI and LCS claddings was prepared by mold casting, and then hot-rolled into composite plates. The hot deformation and heat treatment characteristics of the blank were investigated.
tion of the HCCI material is listed in Table 1. The LCS material consists of 0.2 wt.% carbon. The dimensions of the blank and HCCI core were 120 100 80 mm and 80 60 40 mm, respectively. The melting and casting process were carried out in a vacuum of 0.1 kPa till the blank was cooled to around 500 °C. The sandwichstructured blank was homogenized at 1150 °C for 1.5 h and then hot-rolled at 900–1150 °C, with the pass reduction of 10–15%. The detailed variation of thickness and pass reduction of the composite plate was listed in Table 2. The heat treatments were conducted using a 45 kW chamber furnace. The specimens were homogenized at 950 °C, 1000 °C and 1050 °C for 0.5 h, and then oil quenched to room temperature. The specimens oil quenched at 1000 °C were then tempered at 250 °C, 350 °C, 450 °C, 550 °C, and 650 °C. Hot-rolled and heat-treated specimens were machined, polished and etched by a 2% nitric acid alcohol solution for 20–30 s. The evolution of microstructures during hot rolling and heat treatments were analyzed using a LEO-1450 scanning electron microscope (SEM). The distributions of alloy elements contents across interface of hot-rolled specimens were investigated by energy dispersive spectroscopy analysis (EDS). Vickers microhardness of the low carbon steel, interface area, and HCCI regions were examined using HV-1000 Vickers hardness gauge, with a load of 300 g and lasting time of 15 s.
2. Experimental 3. Results and discussion The sandwich-structured composite blank containing low carbon steel (LCS) claddings surrounding the high chromium cast iron (HCCI) core was prepared by composite casting. The main composi* Corresponding author. Tel.: +86 10 62332572; fax: +86 10 82381466. E-mail address: [email protected] (J. Liu). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.01.014
3.1. Hot deformation performance During hot rolling process, the compatible deformation of HCCI layer together with the LCS claddings is observed. According to hot rolling experiments, the total thickness of composite plates is
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3.3. Diffusion zone between HCCI and LCS
Table 1 Main chemical composition of HCCI alloy (wt.%). C
Cr
Si
Mn
Ni
Structure
2.4
12.77
0.79
1.02
0.99
Hypo-eutectic
reduced from 80 mm to 5 mm, while the HCCI from 40 mm to around 3.5 mm. The lower reduction of HCCI reveals a slower flowing rate during hot rolling than LCS. Meanwhile, with low strength and good flow ability at high temperatures, the LCS claddings could exhibit a ‘‘lubricating effect” in hot rolling process, which is also in favor of the deformation of HCCI.
Alloy elements such as Cr and Ni would diffuse across the interface between HCCI and LCS during the hot rolling process. The distribution of Cr and Ni elements across the interface zone are determined through EDS, which are shown in Fig. 2. The counting number reflects the relative contents of Ni and Cr elements at different position across the interface. The diffusion
3.2. Microstructure characteristics after hot rolling Fig. 1a shows the typical microstructure of the as-cast billet. The HCCI consists of primarily austenite with a small amount of martensite and a lot of dendritic eutectic carbides of (Fe, Cr)7C3 type [12–14]. These dendrites are surrounded by intergranular eutectic carbides of the M7C3 type [15]. On the LCS side, a mixture of pearlite and ferrite is observed. Less amount of martensite is found in hot-rolled specimens, and the austenite grains are refined. According to Fig. 1b and c, the diameters of austenite grains are measured to be 15–20 lm and 10–15 lm, respectively, which is mainly attributed to the dynamic recrystallization. The shape of carbides changed significantly after hot rolling: rod shaped carbides are observed in specimens with rolling reduction of 80%, and granular carbides are observed in specimens with rolling reduction of 95%, the size of which are refined to 10–14 lm and 6–10 lm, respectively. The variation of carbides is partly attributed to the heavy plastic deformation, which could break the carbides into smaller ones and change the shape of them. Thus some micro gaps are observed between the matrix and carbides due to their brittlement, seen in Fig. 1b and c. Meanwhile, the carbides are partly dissolved during hot rolling, leading to the precipitation of secondary carbides in smaller size and higher carbides volume fraction in hot-rolled samples. The carbides volume fraction measured under the light optical microscope by manual point counting are found to be 12.7% for the cast state sample and 14.4% for the hot-rolled one. The microstructures of LCS consist of ferrite and pearlite, although the amount of pearlite increases near the interface due to the diffusion of carbon.
Table 2 Pass reductions of the hot-rolled composite plate. Pass 0 1 2 3 4 5
Thickness (mm)
Reduction (%)
80 70 60 51 44 38
0 12.5 14.3 15.0 13.7 13.6
Reheated up to 1150 °C for 0.5 h 6 33 7 28 8 24 9 20 10 17 11 14
13.1 15.1 14.3 16.7 15.0 17.6
Reheated up to 1150 °C for 0.5 h 12 12 13 10 14 8.5 15 7 16 6 17 5
14.3 16.7 15.0 17.6 14.3 16.7
Fig. 1. Microstructures of composite blank: (a) as-cast HCCI; (b) and (c) hot-rolled blank with rolling reduction of 80% and 95%, respectively. The left side is LCS, and the right side is HCCI.
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zone of Cr and Ni is determined to be 10–14 lm wide, which is much smaller than the total thickness of the specimen. Due to the diffusion of alloy elements and continuous variation of microstructures, a perfect metallurgical bonding between HCCI and LCS is obtained, which suggests a high bonding strength between the two component layers. The small width of diffusion zone also ensures the retainment of alloying elements such as Cr and Ni that could lead to the excellent wear ability in HCCI core material.
3.4.2. Effect of tempering treatment on microstructure and hardness The tempering treatment is generally used following the quenching treatment to transform retained austenite remaining in the structure [18]. The microstructures of composite blank specimens tempered at 250–650 °C are shown in Fig. 4a–e. For the hot-rolled samples subjected to tempering treatments at 250–350 °C, tempering of martensite is ought to take place (seen in Fig. 4a and b). Between 400 °C and 550 °C, it is suggested that further destabilization of the retained austenite occurred, produc-
3.4. Microstructures and hardness after heat treatments 3.4.1. Effect of homogenizing and quenching heat treatment on microstructure and hardness Fig. 3 shows the microstructures of the specimens homogenized and quenched at different temperatures, indicating a matrix of martensite and a small amount of austenite in HCCI side. According to the equilibrium phase diagram, the (Cr, Fe)7C3 type eutectic carbides in HCCI could partly dissolved in matrix at high temperature, resulting in a higher carbon content of austenite grains [16,17]. Thus, more secondary carbides were precipitated in quenched samples, as shown in Fig. 3. The average diameters of (Cr, Fe)7C3 type carbides in Fig. 3a–c are 8 lm, 9 lm, and 15 lm, respectively. The carbides volume fraction of samples quenched at 950 °C, 1000 °C and 1050 °C are calculated to be around 35%, 43% and 37%, respectively. Fig. 5a shows the variation of Vickers hardness values at different quenching temperatures. The hardness of HCCI could reach up to HV 750 or above when quenched at 950–1050 °C. The samples quenched at 1000 °C show the highest hardness of HV 850–900, which could be attributed to the variations of carbides volume fraction at different temperatures. As more eutectic carbides are found dissolved in austenitic matrix at higher temperatures, the average alloying element contents of austenite shall increase with increasing temperatures. Thus more secondary carbides are precipitated at 1000 °C than at 950 °C. Meanwhile, more stabilized austenite would be formed at higher homogenizing temperatures, leading to an increase of retained austenite which will decrease the hardness after oil quenching. Therefore, the hardness of HCCI is determined by the hardening effect of secondary carbides and the softening effect of retained austenite. For the LCS side, the size of ferrite and pearlite microstructures grows bigger as temperature increases, with little change of the hardness in the range of HV 250–300.
Fig. 2. Energy dispersive spectrometry diagram of hot-rolled specimen containing HCCI and LCS claddings.
Fig. 3. SEM photographs of hot rolling specimens after homogenizing and quenching heat treatments. (a–c) Quenched at 950 °C, 1000 °C and 1050 °C, respectively.
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Fig. 4. SEM photographs of hot rolling specimens after tempering treatments. (a–e) Quenched at 1000 °C and tempered at 250 °C, 350 °C, 450 °C, 550 °C and 650 °C, respectively.
ing transformation to martensite on cooling [19]. Thus a larger amount of martensite is found in the sample tempered at 450 °C, as shown in Fig. 4c. For samples tempered at 550– 650 °C, a large number of very fine carbides is found precipitated in the matrix, while the greater attack of the matrix by the etching suggests the matrix may be ferritic, (seen in Fig. 4d and e). As the tempering temperatures are below the phase transition temperature, little change of eutectic carbides is found in tempered samples. Tempering treatments on the destabilized material give a significant drop in hardness due to the tempering of martensite. The hardness of HCCI reduces to HV 600–750 after tempering, as shown in Fig. 5b. The hardness of samples shows little change with tempering treatment at 250 °C, 350 °C and 550 °C. The sample tempered at 450 °C shows the highest hardness values of HV 713.4 for
HCCI layers probably due to the decomposition of retained austenite and the transformation to martensite on cooling. Compared with former research on HCCI, it is suggested that secondary hardening of the hot-rolled sample might occur when tempered at round 450 °C. It is also noticed that the hardness increased when tempered at 650 °C, which is quite different from other reported results [20]. This might be explained by the fine carbides with the size of 200–400 nm observed in the matrix (seen in Fig 4e). Different from the casting high chromium irons in which the ferrite may reduce its hardness, fine ferrite grains transformed from retained austenite is found in hot-rolled samples tempered at 650 °C, showing less affect on hardness. The hardness of LCS side decreases at higher tempering temperatures, and shows little difference between the quenched specimens and tempering specimens.
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Acknowledgements We would like to acknowledge the experimental center in University of Science and Technology Beijing and Professor Huiping Feng for experimental advices. References
Fig. 5. Microhardness across the interface of HCCI/LCS composite plates after heat treatments. (a) Quenching at 950–1150 °C; (b) quenched at 1000 °C and tempered at 250–650 °C.
4. Conclusions In this work, a sandwich-structured HCCI/LCS composite blank was successfully fabricated through casting and then hot-rolled into composite plates, with a perfect metallurgical bonding between them. The plastic deformation of HCCI material was realized through this method. The microstructures of hot-rolled HCCI are refined and significant variations of carbides are observed. The hardness of HCCI can reach up to HV 750 or above after oil quenching. The sample quenched at 1000 °C shows the highest hardness of HV 850–900 due to the high carbides volume fraction. The sample tempered at 450 °C shows the highest hardness value of HV 713.4 for HCCI layers, probably due to the decomposition of retained austenite and transformation to martensite on cooling. Based on the experimental results and discussions above, it is suggested that the properties of the two components are retained after the hot rolling and heat treatments, which can be deduced by their microstructures and hardness. Thus the composite may have more advanced mechanical properties than the two components.
[1] 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. [2] Llewellyn RJ, Yick SK, Dolman KF. Scouring erosion resistance of metallic materials used in slurry pump service. Wear 2004;256:592–9. [3] Weber K, Regener D, Mehner H, Menzel M. Characterization of the microstructure of high-chromium cast irons using Mössbauer spectroscopy. Mater Charact 2001;46:399–406. [4] Zhi XH, Xing JD, Fu HG, Xiao B. Effect of niobium on the as-cast microstructure of hypereutectic high chromium cast iron. Mater Lett 2008;62:857–60. [5] 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. [6] Zhi XH, Xing JD, Fu HG, Gao YM. Effect of titanium on the as-cast microstructure of hypereutectic high chromium cast iron. Mater Charact 2008;59:1221–6. [7] Wu XJ, Xing JD, Fu HG, Zhi XH. Effect of titanium on the morphology of primary M7C3 carbides in hypereutectic high chromium white iron. Mater Sci Eng A 2007;457:180–5. [8] Arnold BK, Heijkoop T, Lloyd PG, Rubenis G, Sare IR. Wear of cast-bonded components in a coal pulvesier mill. Wear 1997;203-204:663–70. [9] El-Bitar T, El-Banna E. Contribution of forming parameters on the properties of hot rolled ductile cast iron alloys. Mater Lett 1997;31:145–50. [10] Qi K, Yu FY, Bai FD, Yan ZM, Wang ZX, Li TJ. Research on the hot deformation behavior and graphite morphology of spheroidal graphite cast iron at high strain rate. Mater Des 2009;30:4511–5. [11] Li D, Liu LG, Zhang YK, Ye CL, Ren XJ, Yang YL, Yang QX, Lu BT. Phase diagram calculation of high chromium cast irons and influence of its chemical composition. Mater Des 2009;30:340–5. [12] Kambakas K, Tsakiropoulos P. Solidification of high-Cr white cast iron-WC particle reinforced composites. Mater Sci Eng A 2005;413-414:538–44. [13] Luo JL, Chiovelli S. Corrosion and wear resistance of chrome white irons – a correlation to their composition and microstructure. Metall Mater Trans A 2006;37A:3029–38. [14] Qian M, Harada SJ, Kuroshima Yoshihito, Nagayoshi Hideaki. Surface hardening of ductile cast iron using stainless steel. Mater Sci Eng A 1996;208:88–92. [15] Wiengmoon A, Chairuangsri T, Poolthong N. Pearce Electron microscopy and hardness study of a semi-solid processed 27 wt.% Cr cast iron. Mater Sci Eng A 2008;480:333–41. [16] Aso S, Goto S, Komatsu Y, Hartono W. Sliding wear of graphite crystallized chromium white cast iron. Wear 2001;250:511–7. [17] Pintaude G, Tschiptschin AP, Tanaka DK, Sinatora A. The particle size effect on abrasive wear of high chromium white cast iron mills balls. Wear 2001;250:66–70. [18] Gundlach RB, Parks JL. Influence of abrasive hardness on the wear resistance of high chromium irons. Wear 1978;46:97–108. [19] Zum Gahr Karl Heinz, Eldis George T. Abrasive wear of white cast irons. Wear 1980;64:175–94. [20] Tabrett CP, Sare IR. The effect of heat treatment on the abrasion resistance of alloy white irons. Wear 1997;203-204:206–19.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Fabrication of high chromium cast iron/low carbon steel composite material by cast and hot rolling process Guoliang Xie, Hui Sheng, Jingtao Han, Jing Liu * School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 24 November 2009 Accepted 9 January 2010 Available online 15 January 2010
a b s t r a c t A sandwich-structured composite blank containing a high chromium cast iron (HCCI) and low carbon steel (LCS) claddings was successfully fabricated by casting and hot rolling, and then a series of quenching and tempering treatments were employed. The evolution of microstructures and microhardness of ascast, hot-rolled and heat-treated specimens were investigated. The microstructures of hot-rolled HCCI are refined and significant variations of carbides are observed. A perfect metallurgical bonding between HCCI and LCS is revealed by the continuous distributions of alloy elements. The microhardness of hot-rolled HCCI after quenching and tempering is found to be close to that of as-cast one. The hardness of HCCI can reach up to HV 750 or above after oil quenching. The hardness of HCCI reduced to HV 600–750 after tempering due to the tempering of martensite. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction High chromium cast irons (HCCIs) are excellent wear-resistant materials, which have been widely used for wear-affected equipment operated under extreme conditions, such as facilities in the slurry pumping systems of the oil sands handling and mineral processing [1,2]. The typical microstructures of HCCIs consist of chromium carbides of high hardness dispersed in a matrix which still contains a sufficient concentration of carbon to allow hardening of transformation of austenite to martensite after quenching [3– 6]. The exceptional wear resistance of HCCIs results primarily from the high volume fraction of hard chromium carbides, which helps to prevent the formation of graphite and stabilize the carbides [7,8]. By changing the microstructure and composition of the matrix, a wide variety of ductile irons forging abilities can be obtained between 850 °C and 1000 °C [9,10]. However, the shaping of HCCIs and coatings or cladding materials containing HCCIs is generally carried out by means of casting techniques or spray forming in liquid state [11]. In this research, a composite blank containing HCCI and LCS claddings was prepared by mold casting, and then hot-rolled into composite plates. The hot deformation and heat treatment characteristics of the blank were investigated.
tion of the HCCI material is listed in Table 1. The LCS material consists of 0.2 wt.% carbon. The dimensions of the blank and HCCI core were 120 100 80 mm and 80 60 40 mm, respectively. The melting and casting process were carried out in a vacuum of 0.1 kPa till the blank was cooled to around 500 °C. The sandwichstructured blank was homogenized at 1150 °C for 1.5 h and then hot-rolled at 900–1150 °C, with the pass reduction of 10–15%. The detailed variation of thickness and pass reduction of the composite plate was listed in Table 2. The heat treatments were conducted using a 45 kW chamber furnace. The specimens were homogenized at 950 °C, 1000 °C and 1050 °C for 0.5 h, and then oil quenched to room temperature. The specimens oil quenched at 1000 °C were then tempered at 250 °C, 350 °C, 450 °C, 550 °C, and 650 °C. Hot-rolled and heat-treated specimens were machined, polished and etched by a 2% nitric acid alcohol solution for 20–30 s. The evolution of microstructures during hot rolling and heat treatments were analyzed using a LEO-1450 scanning electron microscope (SEM). The distributions of alloy elements contents across interface of hot-rolled specimens were investigated by energy dispersive spectroscopy analysis (EDS). Vickers microhardness of the low carbon steel, interface area, and HCCI regions were examined using HV-1000 Vickers hardness gauge, with a load of 300 g and lasting time of 15 s.
2. Experimental 3. Results and discussion The sandwich-structured composite blank containing low carbon steel (LCS) claddings surrounding the high chromium cast iron (HCCI) core was prepared by composite casting. The main composi* Corresponding author. Tel.: +86 10 62332572; fax: +86 10 82381466. E-mail address: [email protected] (J. Liu). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.01.014
3.1. Hot deformation performance During hot rolling process, the compatible deformation of HCCI layer together with the LCS claddings is observed. According to hot rolling experiments, the total thickness of composite plates is
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3.3. Diffusion zone between HCCI and LCS
Table 1 Main chemical composition of HCCI alloy (wt.%). C
Cr
Si
Mn
Ni
Structure
2.4
12.77
0.79
1.02
0.99
Hypo-eutectic
reduced from 80 mm to 5 mm, while the HCCI from 40 mm to around 3.5 mm. The lower reduction of HCCI reveals a slower flowing rate during hot rolling than LCS. Meanwhile, with low strength and good flow ability at high temperatures, the LCS claddings could exhibit a ‘‘lubricating effect” in hot rolling process, which is also in favor of the deformation of HCCI.
Alloy elements such as Cr and Ni would diffuse across the interface between HCCI and LCS during the hot rolling process. The distribution of Cr and Ni elements across the interface zone are determined through EDS, which are shown in Fig. 2. The counting number reflects the relative contents of Ni and Cr elements at different position across the interface. The diffusion
3.2. Microstructure characteristics after hot rolling Fig. 1a shows the typical microstructure of the as-cast billet. The HCCI consists of primarily austenite with a small amount of martensite and a lot of dendritic eutectic carbides of (Fe, Cr)7C3 type [12–14]. These dendrites are surrounded by intergranular eutectic carbides of the M7C3 type [15]. On the LCS side, a mixture of pearlite and ferrite is observed. Less amount of martensite is found in hot-rolled specimens, and the austenite grains are refined. According to Fig. 1b and c, the diameters of austenite grains are measured to be 15–20 lm and 10–15 lm, respectively, which is mainly attributed to the dynamic recrystallization. The shape of carbides changed significantly after hot rolling: rod shaped carbides are observed in specimens with rolling reduction of 80%, and granular carbides are observed in specimens with rolling reduction of 95%, the size of which are refined to 10–14 lm and 6–10 lm, respectively. The variation of carbides is partly attributed to the heavy plastic deformation, which could break the carbides into smaller ones and change the shape of them. Thus some micro gaps are observed between the matrix and carbides due to their brittlement, seen in Fig. 1b and c. Meanwhile, the carbides are partly dissolved during hot rolling, leading to the precipitation of secondary carbides in smaller size and higher carbides volume fraction in hot-rolled samples. The carbides volume fraction measured under the light optical microscope by manual point counting are found to be 12.7% for the cast state sample and 14.4% for the hot-rolled one. The microstructures of LCS consist of ferrite and pearlite, although the amount of pearlite increases near the interface due to the diffusion of carbon.
Table 2 Pass reductions of the hot-rolled composite plate. Pass 0 1 2 3 4 5
Thickness (mm)
Reduction (%)
80 70 60 51 44 38
0 12.5 14.3 15.0 13.7 13.6
Reheated up to 1150 °C for 0.5 h 6 33 7 28 8 24 9 20 10 17 11 14
13.1 15.1 14.3 16.7 15.0 17.6
Reheated up to 1150 °C for 0.5 h 12 12 13 10 14 8.5 15 7 16 6 17 5
14.3 16.7 15.0 17.6 14.3 16.7
Fig. 1. Microstructures of composite blank: (a) as-cast HCCI; (b) and (c) hot-rolled blank with rolling reduction of 80% and 95%, respectively. The left side is LCS, and the right side is HCCI.
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zone of Cr and Ni is determined to be 10–14 lm wide, which is much smaller than the total thickness of the specimen. Due to the diffusion of alloy elements and continuous variation of microstructures, a perfect metallurgical bonding between HCCI and LCS is obtained, which suggests a high bonding strength between the two component layers. The small width of diffusion zone also ensures the retainment of alloying elements such as Cr and Ni that could lead to the excellent wear ability in HCCI core material.
3.4.2. Effect of tempering treatment on microstructure and hardness The tempering treatment is generally used following the quenching treatment to transform retained austenite remaining in the structure [18]. The microstructures of composite blank specimens tempered at 250–650 °C are shown in Fig. 4a–e. For the hot-rolled samples subjected to tempering treatments at 250–350 °C, tempering of martensite is ought to take place (seen in Fig. 4a and b). Between 400 °C and 550 °C, it is suggested that further destabilization of the retained austenite occurred, produc-
3.4. Microstructures and hardness after heat treatments 3.4.1. Effect of homogenizing and quenching heat treatment on microstructure and hardness Fig. 3 shows the microstructures of the specimens homogenized and quenched at different temperatures, indicating a matrix of martensite and a small amount of austenite in HCCI side. According to the equilibrium phase diagram, the (Cr, Fe)7C3 type eutectic carbides in HCCI could partly dissolved in matrix at high temperature, resulting in a higher carbon content of austenite grains [16,17]. Thus, more secondary carbides were precipitated in quenched samples, as shown in Fig. 3. The average diameters of (Cr, Fe)7C3 type carbides in Fig. 3a–c are 8 lm, 9 lm, and 15 lm, respectively. The carbides volume fraction of samples quenched at 950 °C, 1000 °C and 1050 °C are calculated to be around 35%, 43% and 37%, respectively. Fig. 5a shows the variation of Vickers hardness values at different quenching temperatures. The hardness of HCCI could reach up to HV 750 or above when quenched at 950–1050 °C. The samples quenched at 1000 °C show the highest hardness of HV 850–900, which could be attributed to the variations of carbides volume fraction at different temperatures. As more eutectic carbides are found dissolved in austenitic matrix at higher temperatures, the average alloying element contents of austenite shall increase with increasing temperatures. Thus more secondary carbides are precipitated at 1000 °C than at 950 °C. Meanwhile, more stabilized austenite would be formed at higher homogenizing temperatures, leading to an increase of retained austenite which will decrease the hardness after oil quenching. Therefore, the hardness of HCCI is determined by the hardening effect of secondary carbides and the softening effect of retained austenite. For the LCS side, the size of ferrite and pearlite microstructures grows bigger as temperature increases, with little change of the hardness in the range of HV 250–300.
Fig. 2. Energy dispersive spectrometry diagram of hot-rolled specimen containing HCCI and LCS claddings.
Fig. 3. SEM photographs of hot rolling specimens after homogenizing and quenching heat treatments. (a–c) Quenched at 950 °C, 1000 °C and 1050 °C, respectively.
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Fig. 4. SEM photographs of hot rolling specimens after tempering treatments. (a–e) Quenched at 1000 °C and tempered at 250 °C, 350 °C, 450 °C, 550 °C and 650 °C, respectively.
ing transformation to martensite on cooling [19]. Thus a larger amount of martensite is found in the sample tempered at 450 °C, as shown in Fig. 4c. For samples tempered at 550– 650 °C, a large number of very fine carbides is found precipitated in the matrix, while the greater attack of the matrix by the etching suggests the matrix may be ferritic, (seen in Fig. 4d and e). As the tempering temperatures are below the phase transition temperature, little change of eutectic carbides is found in tempered samples. Tempering treatments on the destabilized material give a significant drop in hardness due to the tempering of martensite. The hardness of HCCI reduces to HV 600–750 after tempering, as shown in Fig. 5b. The hardness of samples shows little change with tempering treatment at 250 °C, 350 °C and 550 °C. The sample tempered at 450 °C shows the highest hardness values of HV 713.4 for
HCCI layers probably due to the decomposition of retained austenite and the transformation to martensite on cooling. Compared with former research on HCCI, it is suggested that secondary hardening of the hot-rolled sample might occur when tempered at round 450 °C. It is also noticed that the hardness increased when tempered at 650 °C, which is quite different from other reported results [20]. This might be explained by the fine carbides with the size of 200–400 nm observed in the matrix (seen in Fig 4e). Different from the casting high chromium irons in which the ferrite may reduce its hardness, fine ferrite grains transformed from retained austenite is found in hot-rolled samples tempered at 650 °C, showing less affect on hardness. The hardness of LCS side decreases at higher tempering temperatures, and shows little difference between the quenched specimens and tempering specimens.
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Acknowledgements We would like to acknowledge the experimental center in University of Science and Technology Beijing and Professor Huiping Feng for experimental advices. References
Fig. 5. Microhardness across the interface of HCCI/LCS composite plates after heat treatments. (a) Quenching at 950–1150 °C; (b) quenched at 1000 °C and tempered at 250–650 °C.
4. Conclusions In this work, a sandwich-structured HCCI/LCS composite blank was successfully fabricated through casting and then hot-rolled into composite plates, with a perfect metallurgical bonding between them. The plastic deformation of HCCI material was realized through this method. The microstructures of hot-rolled HCCI are refined and significant variations of carbides are observed. The hardness of HCCI can reach up to HV 750 or above after oil quenching. The sample quenched at 1000 °C shows the highest hardness of HV 850–900 due to the high carbides volume fraction. The sample tempered at 450 °C shows the highest hardness value of HV 713.4 for HCCI layers, probably due to the decomposition of retained austenite and transformation to martensite on cooling. Based on the experimental results and discussions above, it is suggested that the properties of the two components are retained after the hot rolling and heat treatments, which can be deduced by their microstructures and hardness. Thus the composite may have more advanced mechanical properties than the two components.
[1] 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. [2] Llewellyn RJ, Yick SK, Dolman KF. Scouring erosion resistance of metallic materials used in slurry pump service. Wear 2004;256:592–9. [3] Weber K, Regener D, Mehner H, Menzel M. Characterization of the microstructure of high-chromium cast irons using Mössbauer spectroscopy. Mater Charact 2001;46:399–406. [4] Zhi XH, Xing JD, Fu HG, Xiao B. Effect of niobium on the as-cast microstructure of hypereutectic high chromium cast iron. Mater Lett 2008;62:857–60. [5] 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. [6] Zhi XH, Xing JD, Fu HG, Gao YM. Effect of titanium on the as-cast microstructure of hypereutectic high chromium cast iron. Mater Charact 2008;59:1221–6. [7] Wu XJ, Xing JD, Fu HG, Zhi XH. Effect of titanium on the morphology of primary M7C3 carbides in hypereutectic high chromium white iron. Mater Sci Eng A 2007;457:180–5. [8] Arnold BK, Heijkoop T, Lloyd PG, Rubenis G, Sare IR. Wear of cast-bonded components in a coal pulvesier mill. Wear 1997;203-204:663–70. [9] El-Bitar T, El-Banna E. Contribution of forming parameters on the properties of hot rolled ductile cast iron alloys. Mater Lett 1997;31:145–50. [10] Qi K, Yu FY, Bai FD, Yan ZM, Wang ZX, Li TJ. Research on the hot deformation behavior and graphite morphology of spheroidal graphite cast iron at high strain rate. Mater Des 2009;30:4511–5. [11] Li D, Liu LG, Zhang YK, Ye CL, Ren XJ, Yang YL, Yang QX, Lu BT. Phase diagram calculation of high chromium cast irons and influence of its chemical composition. Mater Des 2009;30:340–5. [12] Kambakas K, Tsakiropoulos P. Solidification of high-Cr white cast iron-WC particle reinforced composites. Mater Sci Eng A 2005;413-414:538–44. [13] Luo JL, Chiovelli S. Corrosion and wear resistance of chrome white irons – a correlation to their composition and microstructure. Metall Mater Trans A 2006;37A:3029–38. [14] Qian M, Harada SJ, Kuroshima Yoshihito, Nagayoshi Hideaki. Surface hardening of ductile cast iron using stainless steel. Mater Sci Eng A 1996;208:88–92. [15] Wiengmoon A, Chairuangsri T, Poolthong N. Pearce Electron microscopy and hardness study of a semi-solid processed 27 wt.% Cr cast iron. Mater Sci Eng A 2008;480:333–41. [16] Aso S, Goto S, Komatsu Y, Hartono W. Sliding wear of graphite crystallized chromium white cast iron. Wear 2001;250:511–7. [17] Pintaude G, Tschiptschin AP, Tanaka DK, Sinatora A. The particle size effect on abrasive wear of high chromium white cast iron mills balls. Wear 2001;250:66–70. [18] Gundlach RB, Parks JL. Influence of abrasive hardness on the wear resistance of high chromium irons. Wear 1978;46:97–108. [19] Zum Gahr Karl Heinz, Eldis George T. Abrasive wear of white cast irons. Wear 1980;64:175–94. [20] Tabrett CP, Sare IR. The effect of heat treatment on the abrasion resistance of alloy white irons. Wear 1997;203-204:206–19.