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Effect of “additional shear strain layer” on tensile strength and microstructure of fine drawn wire
Journal of Materials Processing Technology 177 (2006) 704–708
Effect of “additional shear strain layer” on tensile strength and microstructure of fine drawn wire S. Kajino ∗ , M. Asakawa School of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku, Tokyo 169-8555, Japan
Abstract Fine wires of the order of 0.1 mm in diameter have become popular for mechanical and electrical applications. In the drawing process, a large shear deformation zone with a hardened layer, referred to as the “additional shear strain layer”, is generated beneath the surface layer of the wire. This study clarified that the depth of this additional shear strain layer was about 0.04 mm for various diameter fine wires. As the diameter decreases, the area ratio of additional shear layer increases. Hence, the additional shear strain layer is a factor for the improvement in high strength and ductility of the wires. In order to discuss the cause of the increase in the surface layer’s tensile strength, the crystal orientation was measured via the electron back-scatter diffraction (EBSD) method. It was ascertained that the crystal of surface layer was subdivided more easily than the center layer, and therefore the tensile strength of surface layer increased. © 2006 Elsevier B.V. All rights reserved. Keywords: Additional shear strain layer; Crystal orientation; Tensile strength; EBSD; Crystal subdivision
1. Introduction Fine wires on the order of 0.1 mm in diameter have become popular for mechanical and electrical applications, such as micro springs, micro pins, printer mesh, and cutting wire for electrical discharge machining (EDM), and wire for cutting silicon, quartz, and other semiconductor materials. To determine the cause of this combination of high strength and high ductility, several factors are considered. The first is size effect. The area ratio of the crystal grain for wire increases because the diameter is thinner. It is believed that the physical properties of fine wire are different from that of large wire. The second factor is the heat treatment effect. Fine wire heats and cools from the surface to the center evenly. So, the microstructure of the surface seems to be the same to that of the center. The third factor is rapid cooling effect during the fine wire drawing process, as compared to that of large wire. In wire drawing, heat is generated. However, this heat is released rapidly in the case of fine wire. The strain aging is small, which is believed to be the key to maintaining ductility.
∗
Corresponding author. Tel.: +81 3 5286 3270; fax: +81 3 5286 3270. E-mail address: [email protected] (S. Kajino).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.04.108
In the drawing process, a large shear deformation zone with a hardened layer, referred to as the “additional shear strain layer”, is generated beneath the surface layer of the wire [1]. This study found that the depth of this additional shear strain layer was about 0.04 mm [2], and constant for various diameter fine wires with the same reduction. As the diameter decreases, the area ratio of additional shear layer increases. Hence, the additional shear strain layer is a factor for the improvement in high strength and ductility of the wires. The purpose of this paper is to investigate the cause of increase in tensile strength. 2. Observation of crystal orientation by EBSD In order to investigate the crystal subdivision, the crystal orientation was measured by SEM–EBSD along longitudinal direction. The 0.300 mm low carbon steel wire was annealed at 1073 K during 30 min. After that the wire was drawn to 0.275 mm with 16% reduction at low speed 0.1 m/min. The kinematic viscosity of lubricant was 1023 cSt. Fig. 1 shows the material coordinate system and measurement areas. The EBSD measurement areas were taken at the surface layer and the center of wires.
S. Kajino, M. Asakawa / Journal of Materials Processing Technology 177 (2006) 704–708
705
Fig. 1. Material coordinate system and measurement areas.
Fig. 4. Crystal grain maps of 1 pass drawn wire.
Fig. 2. Crystal grain maps of 1 pass drawn wire. Fig. 5. Ratio of crystal grain number in annealed wire.
The length of measurement was 0.1 mm as enough length to measure the crystal orientations. The width was 0.04 mm corresponded to the depth of the additional shear strain layer. The measurement step was 0.5 m. The measurement spot diameter was 0.5 m. In this measurement, the grain boundary was decided when the misorientation angle between each spots was more than 2◦ , 5◦ , 8◦ and 15◦ , respectively. Fig. 2 shows the crystal grain maps of 1 pass drawn wire in 5◦ and 15◦ misorientation angle boundary. In the case of surface layer, the crystal grain size was getting small, as the boundary angle became small. On the other hand, the crystal grain size was almost the same in 8◦ and 5◦ boundary at center layer. Fig. 3 shows a ratio of crystal grain number of each angle boundary compared with that of 15◦ . The ratio increased, as the misorientation angle was getting smaller. Fig. 4 shows the crystal grain maps of annealed wire. The crystal grain size was almost constant in both surface and center
Fig. 3. Ratio of crystal grain number in 1 pass drawn wire.
layer. Fig. 5 shows the ratio of crystal grain number. As shown here, the ratio was almost same except 15◦ boundary in both surface and center layer. Fig. 6 shows the misorientation boundary maps of center layer. In annealed wire, a boundary greater than 15◦ was mainly observed, with no misorientation in the grain. On the other hand, in the case of the first pass drawn wire, a misorienta-
Fig. 6. Misorientation map of center layer.
706
S. Kajino, M. Asakawa / Journal of Materials Processing Technology 177 (2006) 704–708 Table 1 Estimated tensile strength of surface layer
Fig. 7. Nominal stress–strain diagram of original wire.
Misorentation angle θ (◦ )
Estimated tensile strength by Hall–Petch law σ B (MPa)
15 8 5 2
469 478 524 532
Table 2 Estimated tensile strength of center layer
tion less than 15◦ , as indicated by arrow, was observed in the grain.
Misorentation angle θ (◦ )
Estimated tensile strength by Hall–Petch law σ B (MPa)
3. Discussion
15 8 5 2
362 393 409 499
3.1. The subdivision of the crystal grain The number of crystal grains increased as the boundary angle decreased in 1 pass of the drawn wire. This meant that the crystal grains were subdivided by misorientation angle less than 8◦ . On the other hand, the crystal grain was not subdivided in annealed wire because the number of crystal grains was constant. It was suggested that misorienation less than 8◦ were generated during drawing process. Considering the difference between surface and center layer, 5◦ boundary was generated at surface layer, while only 2◦ boundary was generated at center layer. It was estimated that misorientation angle of surface layer was more than that of center layer. The crystal was subdivided at surface layer more easily than at center. Work hardening and crystal grain subdivision was considered as a cause of increase in tensile strength. Fig. 7 shows the stress–strain diagram of annealed 0.3 mm wire. Fig. 8 shows the equivalent strain after drawing in the FEM simulation result. The equivalent plastic strain of center layer was 0.20, while that of surface layer was 0.30 and 1.5 times the value of the center layer. The work hardening was different between surface and center. The increase of tensile strength at center layer was 30 MPa, and that at surface layer was 50 MPa in the stress–strain diagram. The tensile strength was calculated with using Hall–Petch law as follow in each misorientation angle. The difference of work hardening was considered in following formulas. Tables 1 and 2 show the calculated results. σ = 7.5d −1/2 + 50
Fig. 8. FEM result of equivalent plastic strain after drawing.
σ = 7.5d −1/2 + 30
(2)
where σ is the tensile strength and d is the diameter of crystal grain. Formula (1) was used for surface layer, and formula (2) was used for center layer. Tensile test was performed as the surface layer was thinned. Fig. 9 shows the tensile strength of removed area after the wire was thinned to diameter D. The tensile strength of surface layer was the strongest. The tensile strength decreased from surface to center. The tensile strength of surface layer was 530 MPa, compared to that of center layer of 400 MPa. In the case of 5◦ boundary, the calculated result was closest to the experimental result at both surface and center layer. It is considered that the 5◦ boundary contributed to the tensile strength as a grain boundary. Fig. 10 shows the relationship between misorientation angle and boundary energy. It was found that the boundary energy of 5◦ misorientation angle was almost half as much as maximum boundary energy [3]. In addition, it has been found out that misorientation boundary more than 5◦ were etched effectively by Nital [4]. It was considered as a grain boundary. In this research, it was possible to consider 5◦ misorientation as a grain boundary and this affected the tensile strength in the Hall–Petch law.
(1)
Fig. 9. Tensile strength variation as surface layer was thinned.
S. Kajino, M. Asakawa / Journal of Materials Processing Technology 177 (2006) 704–708
707
Fig. 10. Relationship between boundary energy and misorientation angle θ.
Two-degrees boundary was observed at both surface and center layer. We considered that this misorientation angle caused work hardening, however, this was not a grain boundary. The 2◦ boundary did not contribute to the tensile strength in the Hall–Petch law.
Fig. 13. Tensile strength of removed area in low friction.
3.2. Effect of friction on crystal subdivision at surface layer It is considered that the main cause of the crystal grain subdivision is the additional shear deformation generated by friction between the material and the die. Therefore, the effect of friction was investigated. Figs. 11 and 12 show relationship between the ratio of the crystal grain number and boundary of misorientation angle θ as shown here, at low friction, there was no difference between surface and center layer. On the other hand, a thigh friction, the crystal number of surface layer increased as the mis-
Fig. 11. Relationship between ratio of crystal grain umber and misorientation angle θ in low friction.
Fig. 14. Tensile strength of removed area in high friction.
orientation angle. Therefore, the crystal grains were subdivided. Especially, the 5◦ misorientation angle boundary was observed in only the surface layer. The tensile test was performed as the surface layer was thinned. Figs. 13 and 14 show the tensile strength of removed area. As it can be observed, at low friction, the tensile strength was almost constant. On the other hand, in the case of high friction drawing, tensile strenght decreased from surface to center. In this result, it was estimated that 5◦ boundary contributed to the tensile strength. We considered that friction was a important factor for generation of the 5◦ boundary and subdivision of crystal grain. 4. Conclusions The crystal grain subdivision of surface layer was investigated. The following conclusions were made.
Fig. 12. Relationship between ratio of crystal grain umber and misorientation angle θ in high friction.
(1) The crystal grain of the surface layer was subdivided earlier than that of the center layer due to an increase in the misorientation angle. (2) A misorientation angle greater than 5◦ was considered as a grain boundary, and this significantly contributed to the tensile strength. (3) Friction is an important factor for generation of the 5◦ boundary and subdivision of crystal grain.
708
S. Kajino, M. Asakawa / Journal of Materials Processing Technology 177 (2006) 704–708
References [1] N. Inakazu, Metal Drawing and Fiber Texture, 1st ed. Kindai Hensyu Ltd., 1985. [2] S. Kajino, M. Asakawa, The effect of additional shear strain layer on mechanical properties of fine drawn wire, in: Proceedings of the Metal Forming 2004, 10th International Conference on Metal Forming, Krakow, 2004, pp. 635–639.
[3] S. Kouda, Introduction to Metal Physics, Corona Publishing, Co. Ltd., 1964. [4] S. Torizuka, et al., Effect of strain on the microstructure and mechanical properties of multi-pass warm caliber rolled low carbon steel, Scripta Mater. (2006).
Effect of “additional shear strain layer” on tensile strength and microstructure of fine drawn wire S. Kajino ∗ , M. Asakawa School of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku, Tokyo 169-8555, Japan
Abstract Fine wires of the order of 0.1 mm in diameter have become popular for mechanical and electrical applications. In the drawing process, a large shear deformation zone with a hardened layer, referred to as the “additional shear strain layer”, is generated beneath the surface layer of the wire. This study clarified that the depth of this additional shear strain layer was about 0.04 mm for various diameter fine wires. As the diameter decreases, the area ratio of additional shear layer increases. Hence, the additional shear strain layer is a factor for the improvement in high strength and ductility of the wires. In order to discuss the cause of the increase in the surface layer’s tensile strength, the crystal orientation was measured via the electron back-scatter diffraction (EBSD) method. It was ascertained that the crystal of surface layer was subdivided more easily than the center layer, and therefore the tensile strength of surface layer increased. © 2006 Elsevier B.V. All rights reserved. Keywords: Additional shear strain layer; Crystal orientation; Tensile strength; EBSD; Crystal subdivision
1. Introduction Fine wires on the order of 0.1 mm in diameter have become popular for mechanical and electrical applications, such as micro springs, micro pins, printer mesh, and cutting wire for electrical discharge machining (EDM), and wire for cutting silicon, quartz, and other semiconductor materials. To determine the cause of this combination of high strength and high ductility, several factors are considered. The first is size effect. The area ratio of the crystal grain for wire increases because the diameter is thinner. It is believed that the physical properties of fine wire are different from that of large wire. The second factor is the heat treatment effect. Fine wire heats and cools from the surface to the center evenly. So, the microstructure of the surface seems to be the same to that of the center. The third factor is rapid cooling effect during the fine wire drawing process, as compared to that of large wire. In wire drawing, heat is generated. However, this heat is released rapidly in the case of fine wire. The strain aging is small, which is believed to be the key to maintaining ductility.
∗
Corresponding author. Tel.: +81 3 5286 3270; fax: +81 3 5286 3270. E-mail address: [email protected] (S. Kajino).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.04.108
In the drawing process, a large shear deformation zone with a hardened layer, referred to as the “additional shear strain layer”, is generated beneath the surface layer of the wire [1]. This study found that the depth of this additional shear strain layer was about 0.04 mm [2], and constant for various diameter fine wires with the same reduction. As the diameter decreases, the area ratio of additional shear layer increases. Hence, the additional shear strain layer is a factor for the improvement in high strength and ductility of the wires. The purpose of this paper is to investigate the cause of increase in tensile strength. 2. Observation of crystal orientation by EBSD In order to investigate the crystal subdivision, the crystal orientation was measured by SEM–EBSD along longitudinal direction. The 0.300 mm low carbon steel wire was annealed at 1073 K during 30 min. After that the wire was drawn to 0.275 mm with 16% reduction at low speed 0.1 m/min. The kinematic viscosity of lubricant was 1023 cSt. Fig. 1 shows the material coordinate system and measurement areas. The EBSD measurement areas were taken at the surface layer and the center of wires.
S. Kajino, M. Asakawa / Journal of Materials Processing Technology 177 (2006) 704–708
705
Fig. 1. Material coordinate system and measurement areas.
Fig. 4. Crystal grain maps of 1 pass drawn wire.
Fig. 2. Crystal grain maps of 1 pass drawn wire. Fig. 5. Ratio of crystal grain number in annealed wire.
The length of measurement was 0.1 mm as enough length to measure the crystal orientations. The width was 0.04 mm corresponded to the depth of the additional shear strain layer. The measurement step was 0.5 m. The measurement spot diameter was 0.5 m. In this measurement, the grain boundary was decided when the misorientation angle between each spots was more than 2◦ , 5◦ , 8◦ and 15◦ , respectively. Fig. 2 shows the crystal grain maps of 1 pass drawn wire in 5◦ and 15◦ misorientation angle boundary. In the case of surface layer, the crystal grain size was getting small, as the boundary angle became small. On the other hand, the crystal grain size was almost the same in 8◦ and 5◦ boundary at center layer. Fig. 3 shows a ratio of crystal grain number of each angle boundary compared with that of 15◦ . The ratio increased, as the misorientation angle was getting smaller. Fig. 4 shows the crystal grain maps of annealed wire. The crystal grain size was almost constant in both surface and center
Fig. 3. Ratio of crystal grain number in 1 pass drawn wire.
layer. Fig. 5 shows the ratio of crystal grain number. As shown here, the ratio was almost same except 15◦ boundary in both surface and center layer. Fig. 6 shows the misorientation boundary maps of center layer. In annealed wire, a boundary greater than 15◦ was mainly observed, with no misorientation in the grain. On the other hand, in the case of the first pass drawn wire, a misorienta-
Fig. 6. Misorientation map of center layer.
706
S. Kajino, M. Asakawa / Journal of Materials Processing Technology 177 (2006) 704–708 Table 1 Estimated tensile strength of surface layer
Fig. 7. Nominal stress–strain diagram of original wire.
Misorentation angle θ (◦ )
Estimated tensile strength by Hall–Petch law σ B (MPa)
15 8 5 2
469 478 524 532
Table 2 Estimated tensile strength of center layer
tion less than 15◦ , as indicated by arrow, was observed in the grain.
Misorentation angle θ (◦ )
Estimated tensile strength by Hall–Petch law σ B (MPa)
3. Discussion
15 8 5 2
362 393 409 499
3.1. The subdivision of the crystal grain The number of crystal grains increased as the boundary angle decreased in 1 pass of the drawn wire. This meant that the crystal grains were subdivided by misorientation angle less than 8◦ . On the other hand, the crystal grain was not subdivided in annealed wire because the number of crystal grains was constant. It was suggested that misorienation less than 8◦ were generated during drawing process. Considering the difference between surface and center layer, 5◦ boundary was generated at surface layer, while only 2◦ boundary was generated at center layer. It was estimated that misorientation angle of surface layer was more than that of center layer. The crystal was subdivided at surface layer more easily than at center. Work hardening and crystal grain subdivision was considered as a cause of increase in tensile strength. Fig. 7 shows the stress–strain diagram of annealed 0.3 mm wire. Fig. 8 shows the equivalent strain after drawing in the FEM simulation result. The equivalent plastic strain of center layer was 0.20, while that of surface layer was 0.30 and 1.5 times the value of the center layer. The work hardening was different between surface and center. The increase of tensile strength at center layer was 30 MPa, and that at surface layer was 50 MPa in the stress–strain diagram. The tensile strength was calculated with using Hall–Petch law as follow in each misorientation angle. The difference of work hardening was considered in following formulas. Tables 1 and 2 show the calculated results. σ = 7.5d −1/2 + 50
Fig. 8. FEM result of equivalent plastic strain after drawing.
σ = 7.5d −1/2 + 30
(2)
where σ is the tensile strength and d is the diameter of crystal grain. Formula (1) was used for surface layer, and formula (2) was used for center layer. Tensile test was performed as the surface layer was thinned. Fig. 9 shows the tensile strength of removed area after the wire was thinned to diameter D. The tensile strength of surface layer was the strongest. The tensile strength decreased from surface to center. The tensile strength of surface layer was 530 MPa, compared to that of center layer of 400 MPa. In the case of 5◦ boundary, the calculated result was closest to the experimental result at both surface and center layer. It is considered that the 5◦ boundary contributed to the tensile strength as a grain boundary. Fig. 10 shows the relationship between misorientation angle and boundary energy. It was found that the boundary energy of 5◦ misorientation angle was almost half as much as maximum boundary energy [3]. In addition, it has been found out that misorientation boundary more than 5◦ were etched effectively by Nital [4]. It was considered as a grain boundary. In this research, it was possible to consider 5◦ misorientation as a grain boundary and this affected the tensile strength in the Hall–Petch law.
(1)
Fig. 9. Tensile strength variation as surface layer was thinned.
S. Kajino, M. Asakawa / Journal of Materials Processing Technology 177 (2006) 704–708
707
Fig. 10. Relationship between boundary energy and misorientation angle θ.
Two-degrees boundary was observed at both surface and center layer. We considered that this misorientation angle caused work hardening, however, this was not a grain boundary. The 2◦ boundary did not contribute to the tensile strength in the Hall–Petch law.
Fig. 13. Tensile strength of removed area in low friction.
3.2. Effect of friction on crystal subdivision at surface layer It is considered that the main cause of the crystal grain subdivision is the additional shear deformation generated by friction between the material and the die. Therefore, the effect of friction was investigated. Figs. 11 and 12 show relationship between the ratio of the crystal grain number and boundary of misorientation angle θ as shown here, at low friction, there was no difference between surface and center layer. On the other hand, a thigh friction, the crystal number of surface layer increased as the mis-
Fig. 11. Relationship between ratio of crystal grain umber and misorientation angle θ in low friction.
Fig. 14. Tensile strength of removed area in high friction.
orientation angle. Therefore, the crystal grains were subdivided. Especially, the 5◦ misorientation angle boundary was observed in only the surface layer. The tensile test was performed as the surface layer was thinned. Figs. 13 and 14 show the tensile strength of removed area. As it can be observed, at low friction, the tensile strength was almost constant. On the other hand, in the case of high friction drawing, tensile strenght decreased from surface to center. In this result, it was estimated that 5◦ boundary contributed to the tensile strength. We considered that friction was a important factor for generation of the 5◦ boundary and subdivision of crystal grain. 4. Conclusions The crystal grain subdivision of surface layer was investigated. The following conclusions were made.
Fig. 12. Relationship between ratio of crystal grain umber and misorientation angle θ in high friction.
(1) The crystal grain of the surface layer was subdivided earlier than that of the center layer due to an increase in the misorientation angle. (2) A misorientation angle greater than 5◦ was considered as a grain boundary, and this significantly contributed to the tensile strength. (3) Friction is an important factor for generation of the 5◦ boundary and subdivision of crystal grain.
708
S. Kajino, M. Asakawa / Journal of Materials Processing Technology 177 (2006) 704–708
References [1] N. Inakazu, Metal Drawing and Fiber Texture, 1st ed. Kindai Hensyu Ltd., 1985. [2] S. Kajino, M. Asakawa, The effect of additional shear strain layer on mechanical properties of fine drawn wire, in: Proceedings of the Metal Forming 2004, 10th International Conference on Metal Forming, Krakow, 2004, pp. 635–639.
[3] S. Kouda, Introduction to Metal Physics, Corona Publishing, Co. Ltd., 1964. [4] S. Torizuka, et al., Effect of strain on the microstructure and mechanical properties of multi-pass warm caliber rolled low carbon steel, Scripta Mater. (2006).