- Email: [email protected]
Cementite nano-crystallization in cold drawn pearlitic wires instigated by low temperature annealing
Scripta Materialia 120 (2016) 5–8
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular Article
Cementite nano-crystallization in cold drawn pearlitic wires instigated by low temperature annealing Lichu Zhou a, Feng Fang a,b,⁎, Xuefeng Zhou a, Yiyou Tu a, Zonghan Xie c,d, Jianqing Jiang a,e a
Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing 211189, China Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing 211167, China School of Mechanical Engineering, University of Adelaide, SA 5005, Australia d School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China e Nanjing University of Information Science and Technology, Nanjing 210044, China b c
a r t i c l e
i n f o
Article history: Received 6 February 2016 Received in revised form 31 March 2016 Accepted 1 April 2016 Available online xxxx Keyword: Cold drawn Pearlitic wire Cementite Annealing Crystallization
a b s t r a c t The effects of low temperature annealing on the cementite structure and mechanical properties of cold drawn pearlitic steel wires were investigated. It was found that the thermal annealing at a temperature as low as 483 K could transform cementite in the as-drawn steel wires (ε = 2.0) from amorphous to nano-crystalline state. Against the conventional belief, the low temperature annealing treatment could simultaneously enhance the tensile strength and ductility of the cold drawn wires, for example, by 10% through the annealing treatment at 483 K. The underlying mechanisms responsible for enhanced mechanical properties were discussed, with a focus on the effects of solute carbon diffusion and cementite nanostructure. © 2016 Elsevier Ltd. All rights reserved.
High strength pearlitic steel wires have been used for suspension bridges, automobiles and springs [1]. They are typically produced through drawing hot-rolled steel rods. To increase the strength of cold drawn steel wires, the microstructure change during the drawing process has been extensively studied over the past decades. Pearlite is a two-phased, lamellar structure composed of alternating layers of ferrite and cementite. Although hard cementite phase only represents approximately one-ninth of the total volume in pearlite, its presence is critical to the strength of cold drawn wires [2–3]. Moreover, post-mortem evidence has recently emerged revealing that cementite lamellae also exhibited remarkable ductile behavior during drawing [1,4–5]. Cementite platelets were found to change into numerous nanometer-sized grains as a result of dislocation initiation, propagation and entanglement, before turning into amorphous cementite at a drawing strain of 0.93 [6–7]. Under further drawing, cementite lamellae would start to decompose giving rise to carbon-rich regions between ferrite grains [8–11]. It is known that cold-drawn pearlitic wires are unstable and sensitive to heat treatment [12]. While annealing at relatively low temperatures, the re-distribution of carbon could occur in partially dissolved cementite, thereby affecting the mechanical properties of heavily cold drawn pearlitic wires [13–14]. For example, cementite lamellae could ⁎ Corresponding author at: School of Materials Science and Engineering, Southeast University, Jiangning District, Nanjing 211189, China. E-mail address: [email protected] (F. Fang).
http://dx.doi.org/10.1016/j.scriptamat.2016.04.002 1359-6462/© 2016 Elsevier Ltd. All rights reserved.
assume a spherical shape when heated up to 400 °C [15], and consequently the torsional properties of the heat-treated wires may decrease [16]. However, structural evolution of cementite and resultant mechanical property change in cold drawn pearlitic wires when subjected to low temperature annealing remains elusive. Such a deficit in understanding has rendered it difficult to develop ultrahigh strength pearlitic wires vital for safety-critical applications. The present study endeavors to elucidate the structural change of cementite within cold drawn pearlitic wires at low temperature annealing. Furthermore, its impact upon the mechanical properties of the wires was discussed among other factors. Material used here was hot-rolled rods (Fe-0.83C-0.19Mn-0.20Si0.01S + P in wt%). After pickling and phosphating [17], the rods (13.5 mm in diameter) were cold drawn to 5.0 mm in diameter (ε = 2.0). The temperature of pearlitic wire in drawing is about 373 K. Heat treatment of cold drawn wires was carried out in oil bath for 60 min at 483 K and 553 K, respectively. Then the wire was cooled in air to room temperature. Thin-foil specimens were sectioned along the wire's longitudinal direction and then examined with a JEOL-2100F TEM operated at the accelerating voltage of 200 kV. Bright field (BF) image and dark field (DF) image was taken at same observation place. The DF Image was generated by sing diffraction spot of cementite. X-ray diffraction (XRD) analysis was also performed with Rigaku D-max2100. Tensile tests were determined at room temperature using CMT5105 universal materials tester machine, operating at a speed of 4 mm/min.
6
L. Zhou et al. / Scripta Materialia 120 (2016) 5–8
Four samples were measured for each curve. The tensile tests were performed according to the Chinese National Standard GB/T228-2002. Fig. 1a shows the microstructure typical of cold drawn pearlitic steel wires (ε = 2.0). The spacing between cementite and ferrite lamellae is about 40 nm and the thickness of cementite plates is about 4 nm. Dislocation clusters can also be seen within ferrite. Fig. 1b shows HRTEM micrograph of cold drawn pearlite nanostructure. The ferrite exhibits distinguishable lattice fringes, which are not apparent in cementite. In addition, discrete bright spots do not appear from the diffraction pattern of the cementite component using the Fast Fourier Transformation (FFT) method. IFFT image of the region area shows a disordered structure of cementite, while the cementite shows a weak short range order structure. It indicates that the single cementite crystal layer has been damaged after heavily cold drawn deformation. Cementite in heavily cold drawn pearlite should be contained nano-crystal and amorphous structure [6]. After annealing respectively at 483 K and 553 K for 60 min, the layered assembly remained for the pearlite. Fig. 1c shows the fine structure of the cementite and ferrite following the annealing at 483 K for 60 min. Lattice fringes can be clearly observed in the cementite, whose
discontinuous feature indicates that the cementite may stay in a polycrystalline state. The size of cementite crystal is about 2 nm. FFT pattern of the cementite, as shown in Fig. 1d, was revealed both (210) and (220) crystal planes. The diffraction signal of (210) contains three bright spots positioned close to each other, indicating that lattice defects may exist in the cementite grains. Fig. 1e shows the IFFT image of the cementite with lattice defects, obtained from the marked block in (a). The spacing between cementite atomic layers is 0.206 nm, confirming the existence of the (210) planes. According to Fig. 1b, the lattice defects might be generated during the annealing process when the crystallization of the amorphous cementite took place. Similar results were also obtained after annealing at 553 K for 60 min (not shown here). Fig. 2 shows the X-ray diffraction spectra of cementite obtained from cold drawn steel pearlitic wires subjected to different annealing temperature for 60 min. There are no apparent cementite peaks in the asdrawn wires. It indicates that single cementite crystal layer may have been transformed into nano-crystalline and/or amorphous cementite [6]. With the increase of annealing temperature, both (121) and (221) cementite peaks appeared with increasing intensity, meaning that the cementite in cold drawn pearlite has transformed from amorphous
Fig. 1. TEM micrographs of cold drawn pearlitic wires (ε = 2.0). (a) Microstructure of cold drawn pearlitic wires; (b) cementite within pearlitic structure viewed under HRTEM; inset showing the selected area FFT and IFFT images obtained inside the boxed region; (c) HRTEM image of cold drawn pearlite after annealing at 483 K for 60 min; (d) FFT pattern of cementite in the marked block in (c); and (e) IFFT pattern of (210) cementite in the marked block as seen in (c).
L. Zhou et al. / Scripta Materialia 120 (2016) 5–8
Fig. 2. X-ray diffraction of cold drawn wire as-drawn and after annealed at different temperature.
into crystalline state. Notably, the peaks of the annealed cementite are broad, suggesting that the size of cementite crystals is small, as seen in Fig. 1c. It may also deduce that the crystal size of cementite after annealing at 553 K is greater than that obtained at 483 K. Fig. 3 shows the microstructure features of the pearlite steel wire (ε = 2.0), which was annealed at 483 K for 60 min and then plastically deformed to a drawing strain of 0.3. Four dislocation bundles (marked ‘1’ to ‘4’) can be seen in lamellar ferrite in the BF field image (left part
7
of Fig. 3). To reveal the interaction of dislocations and newly formed cementite grains, the DF field image was captured at the same position (the right part of Fig. 3). Bright spots represent cementite nanocrystals formed during the annealing treatment. It indicates that the dislocation groups in ferrite were pinned by newly formed cementite nano-crystals. Fig. 4 shows representative engineering stress-strain curves of cold drawn pearlitic steel wires (ε = 2.0) subjected to different annealing temperatures for 60 min. In case of as-drawn wires, both the tensile strength and the strain-to-failure (i.e., ductility) were measured and found to be 1880 ± 20 MPa and 5.9% ± 0.2%, respectively. After annealing at 483 K for 60 min, the tensile strength of the wires increased to 2080 ± 26 MPa, and surprisingly, the ductility also rose up to 6.3% ± 0.1%. As the annealing temperature further increased to553 K, the tensile strength of the wires decreased slightly to 2030 ± 23 MPa, while its ductility moved up marginally to about 6.6% ± 0.1%. Now an intriguing question comes up: how could the distinct strengthening happen in the annealed wires? In other words, what underlying processes were responsible for the strengthening of low temperature annealed pearlitic wires (ε = 2.0)? During the cold drawing process, carbon atoms dissolved from the cementite could be driven into the ferrite. For heavily cold drawn pearlitic steel wires (ε N 4), the oversaturated solute carbon in ferrite could decorate the dislocations, leading to the increases of the wire strength [18–21]. In this study, the drawing strain was set to be ε = 2.0; as such, the dissolution of cementite was believed to be insignificant during cold drawing [10]. However, subsequent annealing treatment might lead to rapid solute carbon decoration of the ferrite dislocations, for instance, by pipe diffusion, thereby increasing the strength of the wires. Moreover, the cold drawing often leads to the formation of amorphous cementite [6], which would create
Fig. 3. TEM micrographs of cold drawn pearlitic wires annealed at 483 K followed by plastic deformation (ε = 0.3). Left: Bright field image of deformed structure; right: dark field image of the same region. Note that the numbers ‘1’ to ‘4’ in BF mark four dislocation lines lying near each other within a ferrite platelet, while the numbers in DF indicate the cementite nanocrystals impeding the motion of these dislocations.
8
L. Zhou et al. / Scripta Materialia 120 (2016) 5–8
tensile strength and ductility of annealed pearlitic wires was found to increase simultaneously by about 10% when the annealing was set at 483 K. The strength increase is believed to result from solute carbon decoration of the ferrite dislocations coupled with cementite nanostructuring. The enhanced ductility may originate from the rotation/sliding of newly formed cementite nano-crystal.
Acknowledgements
Fig. 4. Representative engineering stress-strain curves of annealed versus as-drawn pearlitic wires.
a thick transition zone between ferrite. While being subjected to annealing at relatively low temperatures (for example, below 553 K), the amorphous cementite could be converted into ultra-fine nanocrystallites (as seen in Fig. 1e), which served as physical barriers against dislocation motion, as showed in Fig. 3. It is believed that such cementite re-crystallization (or nano-structuring) played a critical role in strengthening the annealed pearlitic wires and, at the same time, slightly enhancing their ductility presumably due to the rotation and/ or sliding of newly formed cementite nano-grains. The uniform elongation actually decreased following annealing at 483 K (Fig. 4). Such a decrease in ductility might be partly due to solute carbon dissolved from the cementite that decorated and held dislocations within adjacent ferrite, thereby limiting their mobility [18,22–23]. With a further increase of annealing temperature, an opposite trend observed for the strength and ductility values, which may attribute to the agglomeration of cementite grain size (reducing the dislocation barrier) and recovery/recrystallization of deformed ferrite. In addition, the oversaturated solute carbon inside ferrite may also move out to form crystal cementite. As a result, the strength of wires decreased but the ductility increased. In-situ mechanical testing and/or computer simulation may be required to pinpoint exact steps that governed the change of mechanical properties. In summary, cementite platelets in pearlitic steel rods converted into an amorphous state after cold drawing (ε = 2.0). After low temperature annealing; namely, 483 K and 553 K for 60 min, the amorphous cementite turned into a fine nano-crystal structure with lattice defects. Both
This work was supported by the Natural Science Foundation of China (grant nos. 51371050 and 51301038), the Industry-University Strategic Research Program, Jiangsu Province, China (grant no. BA2014088) and the Key Research Project, Jiangsu Province, China (grant no. BE2015097), and was also supported by Key Laboratory for Advanced Metallic Materials, Jiangsu Province (BM2007204) and Open Project of Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology (ASMA201402). Dr. Z. Xie acknowledges the support provided by the Australian Research Council (DP15). The authors also thank Dr. K. Zhang and C.W. Jin for their assistance with TEM analysis.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
M. Zelin, Acta Mater. 50 (2002) 4431–4447. J. Languillaume, G. Kapelski, B. Baudelet, Acta Mater. 45 (1997) 1201–1212. W.J. Nam, C.M. Bae, Mater. Sci. Eng. A 203 (1995) 278–285. A. Inoue, T. Ogura, T. Masumoto, Scr. Metall. 11 (1977) 1. J. Toribio, E. Ovejero, Scr. Mater. 39 (1998) 323. F. Fang, Y. Zhao, P. Liu, L. Zhou, X. Hu, X. Zhou, Z. Xie, Mater. Sci. Eng. A 608 (2014) 11–15. C. Borchers, T. Al-Kassab, S. Goto, R. Kirchheim, Mater. Sci. Eng. A 502 (2009) 131–138. V.G. Gavriljuk, Scr. Mater. 45 (2001) 1469–1472. C.W. Bang, J.B. Seol, Y.S. Yang, C.G. Park, Scr. Mater. 108 (2015) 151–155. Y.J. Li, P. Choi, C. Borchers, S. Westerkamp, S. Goto, D. Raabe, R. Kirchheim, Acta Mater. 59 (2011) 3965–3977. Y.J. Li, D. Raabe, M. Herbig, P.P. Choi, S. Goto, A. Kostka, R. Kirchheim, Phys. Rev. Lett. 113 (2014) 106104. J. Languillaume, G. Kapelski, B. Baudelet, Mater. Lett. 33 (1997) 241–245. Y.J. Li, P.P. Choi, S. Goto, C. Borchers, D. Raabe, R. Kirchheim, Acta Mater. 60 (2012) 4005–4016. J. Takahashi, M. Kosaka, K. Kawakami, T. Tarui, Acta Mater. 60 (2012) 387–395. F. Fang, X. Hu, S. Chen, Z. Xie, J. Jiang, Mater. Sci. Eng. A 547 (2012) 51–54. X.J. Hu, L. Wang, F. Fang, Z.Q. Ma, Z.H. Xie, J.Q. Jiang, J. Mater. Sci. 48 (2013) 5528. F. Fang, J.H. Jiang, S.Y. Tan, A.B. Ma, J.Q. Jiang, Surf. Coat. Technol. 204 (2010) 2381–2385. M. Herbig, D. Raabe, Y.J. Li, P. Choi, S. Zaefferer, S. Goto, Phys. Rev. Lett. 112 (2014) 126103. S.W. Joung, U.G. Kang, S.P. Hong, Y.W. Kim, W.J. Nam, Mater. Sci. Eng. A 586 (2013) 171–177. Y.J. Li, A. Kostka, P.P. Choi, S. Goto, D. Ponge, R. Kirchheim, D. Raabe, Acta Mater. 84 (2015) 110–123. R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103–189. M. Umemoto, Z.G. Liu, K. Masuyama, K. Tsuchiy, Scr. Mater. 45 (2001) 391–397. M. Herbig, P. Chio, D. Raabe, Ultramicroscopy 153 (2015) 32–39.
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular Article
Cementite nano-crystallization in cold drawn pearlitic wires instigated by low temperature annealing Lichu Zhou a, Feng Fang a,b,⁎, Xuefeng Zhou a, Yiyou Tu a, Zonghan Xie c,d, Jianqing Jiang a,e a
Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing 211189, China Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing 211167, China School of Mechanical Engineering, University of Adelaide, SA 5005, Australia d School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China e Nanjing University of Information Science and Technology, Nanjing 210044, China b c
a r t i c l e
i n f o
Article history: Received 6 February 2016 Received in revised form 31 March 2016 Accepted 1 April 2016 Available online xxxx Keyword: Cold drawn Pearlitic wire Cementite Annealing Crystallization
a b s t r a c t The effects of low temperature annealing on the cementite structure and mechanical properties of cold drawn pearlitic steel wires were investigated. It was found that the thermal annealing at a temperature as low as 483 K could transform cementite in the as-drawn steel wires (ε = 2.0) from amorphous to nano-crystalline state. Against the conventional belief, the low temperature annealing treatment could simultaneously enhance the tensile strength and ductility of the cold drawn wires, for example, by 10% through the annealing treatment at 483 K. The underlying mechanisms responsible for enhanced mechanical properties were discussed, with a focus on the effects of solute carbon diffusion and cementite nanostructure. © 2016 Elsevier Ltd. All rights reserved.
High strength pearlitic steel wires have been used for suspension bridges, automobiles and springs [1]. They are typically produced through drawing hot-rolled steel rods. To increase the strength of cold drawn steel wires, the microstructure change during the drawing process has been extensively studied over the past decades. Pearlite is a two-phased, lamellar structure composed of alternating layers of ferrite and cementite. Although hard cementite phase only represents approximately one-ninth of the total volume in pearlite, its presence is critical to the strength of cold drawn wires [2–3]. Moreover, post-mortem evidence has recently emerged revealing that cementite lamellae also exhibited remarkable ductile behavior during drawing [1,4–5]. Cementite platelets were found to change into numerous nanometer-sized grains as a result of dislocation initiation, propagation and entanglement, before turning into amorphous cementite at a drawing strain of 0.93 [6–7]. Under further drawing, cementite lamellae would start to decompose giving rise to carbon-rich regions between ferrite grains [8–11]. It is known that cold-drawn pearlitic wires are unstable and sensitive to heat treatment [12]. While annealing at relatively low temperatures, the re-distribution of carbon could occur in partially dissolved cementite, thereby affecting the mechanical properties of heavily cold drawn pearlitic wires [13–14]. For example, cementite lamellae could ⁎ Corresponding author at: School of Materials Science and Engineering, Southeast University, Jiangning District, Nanjing 211189, China. E-mail address: [email protected] (F. Fang).
http://dx.doi.org/10.1016/j.scriptamat.2016.04.002 1359-6462/© 2016 Elsevier Ltd. All rights reserved.
assume a spherical shape when heated up to 400 °C [15], and consequently the torsional properties of the heat-treated wires may decrease [16]. However, structural evolution of cementite and resultant mechanical property change in cold drawn pearlitic wires when subjected to low temperature annealing remains elusive. Such a deficit in understanding has rendered it difficult to develop ultrahigh strength pearlitic wires vital for safety-critical applications. The present study endeavors to elucidate the structural change of cementite within cold drawn pearlitic wires at low temperature annealing. Furthermore, its impact upon the mechanical properties of the wires was discussed among other factors. Material used here was hot-rolled rods (Fe-0.83C-0.19Mn-0.20Si0.01S + P in wt%). After pickling and phosphating [17], the rods (13.5 mm in diameter) were cold drawn to 5.0 mm in diameter (ε = 2.0). The temperature of pearlitic wire in drawing is about 373 K. Heat treatment of cold drawn wires was carried out in oil bath for 60 min at 483 K and 553 K, respectively. Then the wire was cooled in air to room temperature. Thin-foil specimens were sectioned along the wire's longitudinal direction and then examined with a JEOL-2100F TEM operated at the accelerating voltage of 200 kV. Bright field (BF) image and dark field (DF) image was taken at same observation place. The DF Image was generated by sing diffraction spot of cementite. X-ray diffraction (XRD) analysis was also performed with Rigaku D-max2100. Tensile tests were determined at room temperature using CMT5105 universal materials tester machine, operating at a speed of 4 mm/min.
6
L. Zhou et al. / Scripta Materialia 120 (2016) 5–8
Four samples were measured for each curve. The tensile tests were performed according to the Chinese National Standard GB/T228-2002. Fig. 1a shows the microstructure typical of cold drawn pearlitic steel wires (ε = 2.0). The spacing between cementite and ferrite lamellae is about 40 nm and the thickness of cementite plates is about 4 nm. Dislocation clusters can also be seen within ferrite. Fig. 1b shows HRTEM micrograph of cold drawn pearlite nanostructure. The ferrite exhibits distinguishable lattice fringes, which are not apparent in cementite. In addition, discrete bright spots do not appear from the diffraction pattern of the cementite component using the Fast Fourier Transformation (FFT) method. IFFT image of the region area shows a disordered structure of cementite, while the cementite shows a weak short range order structure. It indicates that the single cementite crystal layer has been damaged after heavily cold drawn deformation. Cementite in heavily cold drawn pearlite should be contained nano-crystal and amorphous structure [6]. After annealing respectively at 483 K and 553 K for 60 min, the layered assembly remained for the pearlite. Fig. 1c shows the fine structure of the cementite and ferrite following the annealing at 483 K for 60 min. Lattice fringes can be clearly observed in the cementite, whose
discontinuous feature indicates that the cementite may stay in a polycrystalline state. The size of cementite crystal is about 2 nm. FFT pattern of the cementite, as shown in Fig. 1d, was revealed both (210) and (220) crystal planes. The diffraction signal of (210) contains three bright spots positioned close to each other, indicating that lattice defects may exist in the cementite grains. Fig. 1e shows the IFFT image of the cementite with lattice defects, obtained from the marked block in (a). The spacing between cementite atomic layers is 0.206 nm, confirming the existence of the (210) planes. According to Fig. 1b, the lattice defects might be generated during the annealing process when the crystallization of the amorphous cementite took place. Similar results were also obtained after annealing at 553 K for 60 min (not shown here). Fig. 2 shows the X-ray diffraction spectra of cementite obtained from cold drawn steel pearlitic wires subjected to different annealing temperature for 60 min. There are no apparent cementite peaks in the asdrawn wires. It indicates that single cementite crystal layer may have been transformed into nano-crystalline and/or amorphous cementite [6]. With the increase of annealing temperature, both (121) and (221) cementite peaks appeared with increasing intensity, meaning that the cementite in cold drawn pearlite has transformed from amorphous
Fig. 1. TEM micrographs of cold drawn pearlitic wires (ε = 2.0). (a) Microstructure of cold drawn pearlitic wires; (b) cementite within pearlitic structure viewed under HRTEM; inset showing the selected area FFT and IFFT images obtained inside the boxed region; (c) HRTEM image of cold drawn pearlite after annealing at 483 K for 60 min; (d) FFT pattern of cementite in the marked block in (c); and (e) IFFT pattern of (210) cementite in the marked block as seen in (c).
L. Zhou et al. / Scripta Materialia 120 (2016) 5–8
Fig. 2. X-ray diffraction of cold drawn wire as-drawn and after annealed at different temperature.
into crystalline state. Notably, the peaks of the annealed cementite are broad, suggesting that the size of cementite crystals is small, as seen in Fig. 1c. It may also deduce that the crystal size of cementite after annealing at 553 K is greater than that obtained at 483 K. Fig. 3 shows the microstructure features of the pearlite steel wire (ε = 2.0), which was annealed at 483 K for 60 min and then plastically deformed to a drawing strain of 0.3. Four dislocation bundles (marked ‘1’ to ‘4’) can be seen in lamellar ferrite in the BF field image (left part
7
of Fig. 3). To reveal the interaction of dislocations and newly formed cementite grains, the DF field image was captured at the same position (the right part of Fig. 3). Bright spots represent cementite nanocrystals formed during the annealing treatment. It indicates that the dislocation groups in ferrite were pinned by newly formed cementite nano-crystals. Fig. 4 shows representative engineering stress-strain curves of cold drawn pearlitic steel wires (ε = 2.0) subjected to different annealing temperatures for 60 min. In case of as-drawn wires, both the tensile strength and the strain-to-failure (i.e., ductility) were measured and found to be 1880 ± 20 MPa and 5.9% ± 0.2%, respectively. After annealing at 483 K for 60 min, the tensile strength of the wires increased to 2080 ± 26 MPa, and surprisingly, the ductility also rose up to 6.3% ± 0.1%. As the annealing temperature further increased to553 K, the tensile strength of the wires decreased slightly to 2030 ± 23 MPa, while its ductility moved up marginally to about 6.6% ± 0.1%. Now an intriguing question comes up: how could the distinct strengthening happen in the annealed wires? In other words, what underlying processes were responsible for the strengthening of low temperature annealed pearlitic wires (ε = 2.0)? During the cold drawing process, carbon atoms dissolved from the cementite could be driven into the ferrite. For heavily cold drawn pearlitic steel wires (ε N 4), the oversaturated solute carbon in ferrite could decorate the dislocations, leading to the increases of the wire strength [18–21]. In this study, the drawing strain was set to be ε = 2.0; as such, the dissolution of cementite was believed to be insignificant during cold drawing [10]. However, subsequent annealing treatment might lead to rapid solute carbon decoration of the ferrite dislocations, for instance, by pipe diffusion, thereby increasing the strength of the wires. Moreover, the cold drawing often leads to the formation of amorphous cementite [6], which would create
Fig. 3. TEM micrographs of cold drawn pearlitic wires annealed at 483 K followed by plastic deformation (ε = 0.3). Left: Bright field image of deformed structure; right: dark field image of the same region. Note that the numbers ‘1’ to ‘4’ in BF mark four dislocation lines lying near each other within a ferrite platelet, while the numbers in DF indicate the cementite nanocrystals impeding the motion of these dislocations.
8
L. Zhou et al. / Scripta Materialia 120 (2016) 5–8
tensile strength and ductility of annealed pearlitic wires was found to increase simultaneously by about 10% when the annealing was set at 483 K. The strength increase is believed to result from solute carbon decoration of the ferrite dislocations coupled with cementite nanostructuring. The enhanced ductility may originate from the rotation/sliding of newly formed cementite nano-crystal.
Acknowledgements
Fig. 4. Representative engineering stress-strain curves of annealed versus as-drawn pearlitic wires.
a thick transition zone between ferrite. While being subjected to annealing at relatively low temperatures (for example, below 553 K), the amorphous cementite could be converted into ultra-fine nanocrystallites (as seen in Fig. 1e), which served as physical barriers against dislocation motion, as showed in Fig. 3. It is believed that such cementite re-crystallization (or nano-structuring) played a critical role in strengthening the annealed pearlitic wires and, at the same time, slightly enhancing their ductility presumably due to the rotation and/ or sliding of newly formed cementite nano-grains. The uniform elongation actually decreased following annealing at 483 K (Fig. 4). Such a decrease in ductility might be partly due to solute carbon dissolved from the cementite that decorated and held dislocations within adjacent ferrite, thereby limiting their mobility [18,22–23]. With a further increase of annealing temperature, an opposite trend observed for the strength and ductility values, which may attribute to the agglomeration of cementite grain size (reducing the dislocation barrier) and recovery/recrystallization of deformed ferrite. In addition, the oversaturated solute carbon inside ferrite may also move out to form crystal cementite. As a result, the strength of wires decreased but the ductility increased. In-situ mechanical testing and/or computer simulation may be required to pinpoint exact steps that governed the change of mechanical properties. In summary, cementite platelets in pearlitic steel rods converted into an amorphous state after cold drawing (ε = 2.0). After low temperature annealing; namely, 483 K and 553 K for 60 min, the amorphous cementite turned into a fine nano-crystal structure with lattice defects. Both
This work was supported by the Natural Science Foundation of China (grant nos. 51371050 and 51301038), the Industry-University Strategic Research Program, Jiangsu Province, China (grant no. BA2014088) and the Key Research Project, Jiangsu Province, China (grant no. BE2015097), and was also supported by Key Laboratory for Advanced Metallic Materials, Jiangsu Province (BM2007204) and Open Project of Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology (ASMA201402). Dr. Z. Xie acknowledges the support provided by the Australian Research Council (DP15). The authors also thank Dr. K. Zhang and C.W. Jin for their assistance with TEM analysis.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
M. Zelin, Acta Mater. 50 (2002) 4431–4447. J. Languillaume, G. Kapelski, B. Baudelet, Acta Mater. 45 (1997) 1201–1212. W.J. Nam, C.M. Bae, Mater. Sci. Eng. A 203 (1995) 278–285. A. Inoue, T. Ogura, T. Masumoto, Scr. Metall. 11 (1977) 1. J. Toribio, E. Ovejero, Scr. Mater. 39 (1998) 323. F. Fang, Y. Zhao, P. Liu, L. Zhou, X. Hu, X. Zhou, Z. Xie, Mater. Sci. Eng. A 608 (2014) 11–15. C. Borchers, T. Al-Kassab, S. Goto, R. Kirchheim, Mater. Sci. Eng. A 502 (2009) 131–138. V.G. Gavriljuk, Scr. Mater. 45 (2001) 1469–1472. C.W. Bang, J.B. Seol, Y.S. Yang, C.G. Park, Scr. Mater. 108 (2015) 151–155. Y.J. Li, P. Choi, C. Borchers, S. Westerkamp, S. Goto, D. Raabe, R. Kirchheim, Acta Mater. 59 (2011) 3965–3977. Y.J. Li, D. Raabe, M. Herbig, P.P. Choi, S. Goto, A. Kostka, R. Kirchheim, Phys. Rev. Lett. 113 (2014) 106104. J. Languillaume, G. Kapelski, B. Baudelet, Mater. Lett. 33 (1997) 241–245. Y.J. Li, P.P. Choi, S. Goto, C. Borchers, D. Raabe, R. Kirchheim, Acta Mater. 60 (2012) 4005–4016. J. Takahashi, M. Kosaka, K. Kawakami, T. Tarui, Acta Mater. 60 (2012) 387–395. F. Fang, X. Hu, S. Chen, Z. Xie, J. Jiang, Mater. Sci. Eng. A 547 (2012) 51–54. X.J. Hu, L. Wang, F. Fang, Z.Q. Ma, Z.H. Xie, J.Q. Jiang, J. Mater. Sci. 48 (2013) 5528. F. Fang, J.H. Jiang, S.Y. Tan, A.B. Ma, J.Q. Jiang, Surf. Coat. Technol. 204 (2010) 2381–2385. M. Herbig, D. Raabe, Y.J. Li, P. Choi, S. Zaefferer, S. Goto, Phys. Rev. Lett. 112 (2014) 126103. S.W. Joung, U.G. Kang, S.P. Hong, Y.W. Kim, W.J. Nam, Mater. Sci. Eng. A 586 (2013) 171–177. Y.J. Li, A. Kostka, P.P. Choi, S. Goto, D. Ponge, R. Kirchheim, D. Raabe, Acta Mater. 84 (2015) 110–123. R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103–189. M. Umemoto, Z.G. Liu, K. Masuyama, K. Tsuchiy, Scr. Mater. 45 (2001) 391–397. M. Herbig, P. Chio, D. Raabe, Ultramicroscopy 153 (2015) 32–39.