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Alternative process to manufacture austenitic–ferritic stainless steel wires
Materials Letters 59 (2005) 1192 – 1194 www.elsevier.com/locate/matlet
Alternative process to manufacture austenitic–ferritic stainless steel wires A. Itman Filho*, J.M.D.A. Rollo, R.V. Silva, G. Martinez Department of Materials, Aeronautics and Automotive Engineering; Engineering School of Sa˜o Carlos-University of Sa˜o Paulo, CEP: 13566-590, Sa˜o Carlos, SP, Brazil Received 26 March 2004; received in revised form 9 December 2004; accepted 11 December 2004 Available online 5 January 2005
Abstract In a general manner, the austenitic stainless steels are used in the final phases of orthodontic treatments because of the remarkable stiffness and ultimate strength, in addition to its good corrosion resistance. Nowadays, a new class of materials, the austenic–ferritic stainless steels, is substituting the austenitic stainless steels in several industrial applications, where these properties are necessary. This work supports the hypothesis that orthodontic wires can be made with austenic–ferritic stainless steels. The advantages are the cost reduction of the raw material and the nickel hypersensitivity effect in patients undergoing orthodontic treatments. The proposal of this study was to manufacture wires from the austenitic–ferritic stainless steel SEW 410 Nr. 14517 (Cr26Ni6Mo3Cu3) and to evaluate the microstructure, mechanical properties and fracture morphology. The wires were obtained through swaging and drawing process and presented ultimate strength as required in the standard for medical devices. One other advantage is that the ordinary process to manufacture austenitic stainless steels wires can be used for the austenitic–ferritic steel due to its high ductility as seen in the tensile testing and fracture morphology analysis. D 2004 Elsevier B.V. All rights reserved. Keywords: Orthodontic wires; Mechanical properties of duplex steels; Deformation and fracture; Wires process
1. Introduction The austenitic stainless steels are used for a wide range of applications including the orthodontic treatments. In this case, the wires are applied in the final phases of the treatments such as finishing and retention. Finishing involves many processes to produce a stable and esthetic result. The goal of retention is to stabilize the new configuration and to avoid the teeth moving back to their original position. Finishing and retention require a wire with specifications such as high strength, stiffness and corrosion resistance, typical mechanical properties of the austenitic stainless steels [1]. Nowadays, the austenitic–ferritic stainless steels are replacing the austenitic ones, due to the optimum compromise between mechanical properties, corrosion resistance and economical advantages. The main applications exist in
* Corresponding author. Tel.: +55 16 2739592; fax: +55 16 2739590. E-mail address: [email protected] (A. Itman Filho). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.12.026
the chemical and oil industries as cast material [2]. According to Cigada et al. [3], the duplex stainless steel can also be regarded as a convenient substitute of austenitic ASTM F 138, however, biocompatibility tests would be necessary for orthopedic and osteosynthesis devices. Itman et al. carried out biocompatibility in vitro tests of cellular adhesion and proliferation, besides osteoblast differentiation expressed as alkaline phosphatase (ALP) activity [4]. The results showed that the duplex stainless steel could be considered biocompatible as is the austenitic ASTM F 138. In this way, it was proposed that the duplex stainless steels could substitute the austenitic stainless steels in orthodontic treatments, with reduction of the manufacturing cost and the nickel hypersensitivity effect in patients undergoing treatments, because in these steels, the nickel content is lower than the austenitic ones [5,6]. Once there is no common technology to manufacture austenic–ferritic stainless steel wires, the objective of this work was to develop a new process to manufacture it. In order to evaluate the mechanical properties tensile tests were carried out. The microstructure and fracture morphology of
A. Itman Filho et al. / Materials Letters 59 (2005) 1192–1194
1193
Fig. 1. (a) Automatic swaging machine FENN 3F. (b) Wire drawing machine.
tensile specimens were also analyzed. To accomplish this work, the SEW 410 Nr. 14517 (Cr26Ni6Mo3Cu3) stainless steel was chosen due to its considerable use in industrial applications.
machine. Wires with 2.5 and 0.9 mm diameter were pulled with 1 mm/min constant strain speed [7]. The fracture surface morphology of the 2.5 mm diameter wires, as annealed condition, were analyzed by scanning electronic microscope DSM960-Zeiss.
2. Material and methods 3. Results The SEW 410 Nr. 14517 (Cr26Ni6Mo3Cu3) stainless steel was melted in an electrical induction furnace with vacuum-protection. Cylinders of 200 mm length and 12 mm diameter were elaborated and due to of its small dimension, the best option was to reduce the material to the final diameter by swaging and drawing. Initially, the cylinders were annealed at 1050 8C during 1 h. Then, they were milled to 10 mm diameter, swaged to 3.5 mm diameter in the automatic forming machine FENN 3F and drawn to 2.5 mm diameter. Fig. 1 shows the equipment used to manufacture the steel to 2.5 mm diameter. The wire with 2.5 mm diameter was drawn again to 0.9 mm diameter by industrial processing of cold drawing. Samples for microstructural characterization and tensile testing were cut and prepared by conventional means. The microstructures were analyzed through an optical microscope Leco RT 240. The tensile testing was realized according to the ASTM E8-00 in an Emic DL1000 universal Table 1 Steps to manufacture the austenitic–ferritic stainless steel wires Step
Processing
1 2 3 4 5 6 7 8 9 10
Cylinder F 12200 mm as cast Annealed at 1050 8C during 1 h Drilled to F 10 mm Swaged to F 6.3 mm Annealed at 1050 8C during 1 h Swaged to F 3.5 mm Annealed at 1050 8C during 1 h Drawing to F 2.5 mm Annealed at 1050 8C during 1 h Drawing to F 0.9 mm
Generally, austenitic stainless steel wires are obtained through forging and rolling of large ingots. In this work, a specific process was developed using small cast ingots in a small research furnace. The process was realized only by cold worked once manufacturing by hot worked could precipitate the sigma phase, which is harmful to the mechanical properties of the austenitic–ferritic stainless steel wires [8]. The swaging and drawing process were realized gradually using wire gages with different diameters. Due to the cold worked hardness increase was verified so heat softening was carried out, as seen in Table 1. Step-by-step percentages of the reduction area are presented in Table 2. The values indicate the high ductility of the austenitic–ferritic stainless steel. The microstructure of the wire with 2.5 mm after annealing is showed in Fig. 2. It is observed an aligned microstructure with thin and irregular grains. The heat treatment was not enough to remove the drawing texture. It is known from the literature that the deformation by cold worked favors the alignment so it is expected that the drawing to 0.9 mm diameter increases the grain stretching [9].
Table 2 The percentages of reduction of area step by step in the swaging and drawing process Processing
FInitial
Swaging Swaging Drawing Drawing
10.0 6.3 3.5 2.5
(mm)
FFinal
6.3 3.5 2.5 0.9
(mm)
Reduction of area (%) 60 69 49 87
1194
A. Itman Filho et al. / Materials Letters 59 (2005) 1192–1194
Fig. 2. Aligned grains in the wire with 2.5 mm diameter (annealed condition). Etched: HNO3+HCl.
The tensile testing results are shown in Table 3. The wire presented ultimate strength as specified in the standard ISO 5832-1 (Implants for Surgery—Metallic Materials: Part 1— Wrought Stainless Steel) [10]. The fracture morphology of the tensile specimens is presented in Fig. 3. The fracture occurred in a ductile way by microvoids coalescence ratifying the high ductility of the wire. The fracture began in the center and ended at the edge by shearing, typical characteristics of the bcup and coneQ fracture [11]. Summarizing, the austenitic–ferritic stainless steel wires presented ultimate strength conformity required in the standard for surgical implants and enough ductility to hold up high strain in the cold worked. Furthermore, this characteristic makes the wire easy to handle for orthodontic devices. As the steel presented high ductility even with the cold worked, there is expectation that it is possible to manufacture wires according to the austenitic stainless steel process, however, it is necessary to avoid the sigma phase precipitation. In this case, quench annealing should be used.
Fig. 3. SEM images showing the representative fracture morphology of tensile specimens of 2.5 mm diameter as annealed condition.
There are expectations that it will be possible to manufacture wires according to the austenitic stainless steel process. Acknowledgments The authors wish to thank the Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP) for their financial support of this work through processes numbers 01/09212-3 and 02/01325-6.
4. Conclusions An alternative process was developed to manufacture austenitic–ferritic stainless steel wires through swaging and drawing. The wires showed ultimate strength as required in the standard for medical devices, besides high ductility.
Table 3 Mechanical properties of SEW 410 Nr. 14517 stainless steel wire Processing route
Yield strength (MPa)
Ultimate strength (MPa)
Elongationa (%)
Drawing (F 0.9 mm) ISO 5832-1 (F 0.7–1.0 mm)
1518F37
1726F41
2.2F0.4
a
L o=100 mm.
1500–1750
References [1] S.R. Drake, et al., Am. J. Orthod. Dentofac. Orthop. (1998) 206. [2] C.R. Clayton, K.G. Martin, Conf. Proceedings High Nitrogen Steels, The Institute of Metals, Lille, 1989, p. 256. [3] A. Cigada, S. De Martis, Conf. Proceedings Stainless Steel, ISIJ, Chiaba, 1991, p. 716. [4] A. Itman, J.M.D.A. Rollo, S.M. Rossitti, Anais do XV Cong. Brasileiro de Cieˆncia e Eng. dos Materiais (Natal, Brazil) 2002. [5] M.E. Finn, A.K. Srivastava, Proceedings Int. Conference on Processing and Manufacturing of Advanced Materials (Las Vegas, USA) 2000. [6] E.A. Dainesi, et al., Am. J. Orthod. Dentofac. Orthop. 113 (1998) 655. [7] ASTM E 8-00, Annual Book of ASTM Standards. 03.01 (2000) 56. [8] Y. Maehara, Met. Sci. 17 (1983) 541. [9] R.N. Gunn, Duplex Stainless Steel: Microstructure, Properties and Applications, Abington Publishing, Cambridge, England, 1997. [10] ISO 5832-1, International organization for standardization (1997). [11] D. Hull, Fractography, University Press, Cambridge, 1999.
Alternative process to manufacture austenitic–ferritic stainless steel wires A. Itman Filho*, J.M.D.A. Rollo, R.V. Silva, G. Martinez Department of Materials, Aeronautics and Automotive Engineering; Engineering School of Sa˜o Carlos-University of Sa˜o Paulo, CEP: 13566-590, Sa˜o Carlos, SP, Brazil Received 26 March 2004; received in revised form 9 December 2004; accepted 11 December 2004 Available online 5 January 2005
Abstract In a general manner, the austenitic stainless steels are used in the final phases of orthodontic treatments because of the remarkable stiffness and ultimate strength, in addition to its good corrosion resistance. Nowadays, a new class of materials, the austenic–ferritic stainless steels, is substituting the austenitic stainless steels in several industrial applications, where these properties are necessary. This work supports the hypothesis that orthodontic wires can be made with austenic–ferritic stainless steels. The advantages are the cost reduction of the raw material and the nickel hypersensitivity effect in patients undergoing orthodontic treatments. The proposal of this study was to manufacture wires from the austenitic–ferritic stainless steel SEW 410 Nr. 14517 (Cr26Ni6Mo3Cu3) and to evaluate the microstructure, mechanical properties and fracture morphology. The wires were obtained through swaging and drawing process and presented ultimate strength as required in the standard for medical devices. One other advantage is that the ordinary process to manufacture austenitic stainless steels wires can be used for the austenitic–ferritic steel due to its high ductility as seen in the tensile testing and fracture morphology analysis. D 2004 Elsevier B.V. All rights reserved. Keywords: Orthodontic wires; Mechanical properties of duplex steels; Deformation and fracture; Wires process
1. Introduction The austenitic stainless steels are used for a wide range of applications including the orthodontic treatments. In this case, the wires are applied in the final phases of the treatments such as finishing and retention. Finishing involves many processes to produce a stable and esthetic result. The goal of retention is to stabilize the new configuration and to avoid the teeth moving back to their original position. Finishing and retention require a wire with specifications such as high strength, stiffness and corrosion resistance, typical mechanical properties of the austenitic stainless steels [1]. Nowadays, the austenitic–ferritic stainless steels are replacing the austenitic ones, due to the optimum compromise between mechanical properties, corrosion resistance and economical advantages. The main applications exist in
* Corresponding author. Tel.: +55 16 2739592; fax: +55 16 2739590. E-mail address: [email protected] (A. Itman Filho). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.12.026
the chemical and oil industries as cast material [2]. According to Cigada et al. [3], the duplex stainless steel can also be regarded as a convenient substitute of austenitic ASTM F 138, however, biocompatibility tests would be necessary for orthopedic and osteosynthesis devices. Itman et al. carried out biocompatibility in vitro tests of cellular adhesion and proliferation, besides osteoblast differentiation expressed as alkaline phosphatase (ALP) activity [4]. The results showed that the duplex stainless steel could be considered biocompatible as is the austenitic ASTM F 138. In this way, it was proposed that the duplex stainless steels could substitute the austenitic stainless steels in orthodontic treatments, with reduction of the manufacturing cost and the nickel hypersensitivity effect in patients undergoing treatments, because in these steels, the nickel content is lower than the austenitic ones [5,6]. Once there is no common technology to manufacture austenic–ferritic stainless steel wires, the objective of this work was to develop a new process to manufacture it. In order to evaluate the mechanical properties tensile tests were carried out. The microstructure and fracture morphology of
A. Itman Filho et al. / Materials Letters 59 (2005) 1192–1194
1193
Fig. 1. (a) Automatic swaging machine FENN 3F. (b) Wire drawing machine.
tensile specimens were also analyzed. To accomplish this work, the SEW 410 Nr. 14517 (Cr26Ni6Mo3Cu3) stainless steel was chosen due to its considerable use in industrial applications.
machine. Wires with 2.5 and 0.9 mm diameter were pulled with 1 mm/min constant strain speed [7]. The fracture surface morphology of the 2.5 mm diameter wires, as annealed condition, were analyzed by scanning electronic microscope DSM960-Zeiss.
2. Material and methods 3. Results The SEW 410 Nr. 14517 (Cr26Ni6Mo3Cu3) stainless steel was melted in an electrical induction furnace with vacuum-protection. Cylinders of 200 mm length and 12 mm diameter were elaborated and due to of its small dimension, the best option was to reduce the material to the final diameter by swaging and drawing. Initially, the cylinders were annealed at 1050 8C during 1 h. Then, they were milled to 10 mm diameter, swaged to 3.5 mm diameter in the automatic forming machine FENN 3F and drawn to 2.5 mm diameter. Fig. 1 shows the equipment used to manufacture the steel to 2.5 mm diameter. The wire with 2.5 mm diameter was drawn again to 0.9 mm diameter by industrial processing of cold drawing. Samples for microstructural characterization and tensile testing were cut and prepared by conventional means. The microstructures were analyzed through an optical microscope Leco RT 240. The tensile testing was realized according to the ASTM E8-00 in an Emic DL1000 universal Table 1 Steps to manufacture the austenitic–ferritic stainless steel wires Step
Processing
1 2 3 4 5 6 7 8 9 10
Cylinder F 12200 mm as cast Annealed at 1050 8C during 1 h Drilled to F 10 mm Swaged to F 6.3 mm Annealed at 1050 8C during 1 h Swaged to F 3.5 mm Annealed at 1050 8C during 1 h Drawing to F 2.5 mm Annealed at 1050 8C during 1 h Drawing to F 0.9 mm
Generally, austenitic stainless steel wires are obtained through forging and rolling of large ingots. In this work, a specific process was developed using small cast ingots in a small research furnace. The process was realized only by cold worked once manufacturing by hot worked could precipitate the sigma phase, which is harmful to the mechanical properties of the austenitic–ferritic stainless steel wires [8]. The swaging and drawing process were realized gradually using wire gages with different diameters. Due to the cold worked hardness increase was verified so heat softening was carried out, as seen in Table 1. Step-by-step percentages of the reduction area are presented in Table 2. The values indicate the high ductility of the austenitic–ferritic stainless steel. The microstructure of the wire with 2.5 mm after annealing is showed in Fig. 2. It is observed an aligned microstructure with thin and irregular grains. The heat treatment was not enough to remove the drawing texture. It is known from the literature that the deformation by cold worked favors the alignment so it is expected that the drawing to 0.9 mm diameter increases the grain stretching [9].
Table 2 The percentages of reduction of area step by step in the swaging and drawing process Processing
FInitial
Swaging Swaging Drawing Drawing
10.0 6.3 3.5 2.5
(mm)
FFinal
6.3 3.5 2.5 0.9
(mm)
Reduction of area (%) 60 69 49 87
1194
A. Itman Filho et al. / Materials Letters 59 (2005) 1192–1194
Fig. 2. Aligned grains in the wire with 2.5 mm diameter (annealed condition). Etched: HNO3+HCl.
The tensile testing results are shown in Table 3. The wire presented ultimate strength as specified in the standard ISO 5832-1 (Implants for Surgery—Metallic Materials: Part 1— Wrought Stainless Steel) [10]. The fracture morphology of the tensile specimens is presented in Fig. 3. The fracture occurred in a ductile way by microvoids coalescence ratifying the high ductility of the wire. The fracture began in the center and ended at the edge by shearing, typical characteristics of the bcup and coneQ fracture [11]. Summarizing, the austenitic–ferritic stainless steel wires presented ultimate strength conformity required in the standard for surgical implants and enough ductility to hold up high strain in the cold worked. Furthermore, this characteristic makes the wire easy to handle for orthodontic devices. As the steel presented high ductility even with the cold worked, there is expectation that it is possible to manufacture wires according to the austenitic stainless steel process, however, it is necessary to avoid the sigma phase precipitation. In this case, quench annealing should be used.
Fig. 3. SEM images showing the representative fracture morphology of tensile specimens of 2.5 mm diameter as annealed condition.
There are expectations that it will be possible to manufacture wires according to the austenitic stainless steel process. Acknowledgments The authors wish to thank the Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP) for their financial support of this work through processes numbers 01/09212-3 and 02/01325-6.
4. Conclusions An alternative process was developed to manufacture austenitic–ferritic stainless steel wires through swaging and drawing. The wires showed ultimate strength as required in the standard for medical devices, besides high ductility.
Table 3 Mechanical properties of SEW 410 Nr. 14517 stainless steel wire Processing route
Yield strength (MPa)
Ultimate strength (MPa)
Elongationa (%)
Drawing (F 0.9 mm) ISO 5832-1 (F 0.7–1.0 mm)
1518F37
1726F41
2.2F0.4
a
L o=100 mm.
1500–1750
References [1] S.R. Drake, et al., Am. J. Orthod. Dentofac. Orthop. (1998) 206. [2] C.R. Clayton, K.G. Martin, Conf. Proceedings High Nitrogen Steels, The Institute of Metals, Lille, 1989, p. 256. [3] A. Cigada, S. De Martis, Conf. Proceedings Stainless Steel, ISIJ, Chiaba, 1991, p. 716. [4] A. Itman, J.M.D.A. Rollo, S.M. Rossitti, Anais do XV Cong. Brasileiro de Cieˆncia e Eng. dos Materiais (Natal, Brazil) 2002. [5] M.E. Finn, A.K. Srivastava, Proceedings Int. Conference on Processing and Manufacturing of Advanced Materials (Las Vegas, USA) 2000. [6] E.A. Dainesi, et al., Am. J. Orthod. Dentofac. Orthop. 113 (1998) 655. [7] ASTM E 8-00, Annual Book of ASTM Standards. 03.01 (2000) 56. [8] Y. Maehara, Met. Sci. 17 (1983) 541. [9] R.N. Gunn, Duplex Stainless Steel: Microstructure, Properties and Applications, Abington Publishing, Cambridge, England, 1997. [10] ISO 5832-1, International organization for standardization (1997). [11] D. Hull, Fractography, University Press, Cambridge, 1999.