The Bauschinger Effect Application on Cold Micro-Laminated Low- Carbon Wire
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I South Florida Congress of Development, Miami, 2021. The Bauschinger Effect Application on Cold Micro-laminated Low- Carbon Wire Felipe Farage David M. Sc. in Mechanical Engineering - Federal University of Pernambuco Federal Institute of Education, Science and Technology of Minas Gerais Michael Pereira de Souza Avenue, 3007. Campinho. Congonhas, MG, Brazil. Zip code: 36417-050. E-mail: [email protected] Luan Marcel Costa Vasconcelos Bachelor Student in Mechanical Engineering - Federal Institute of Education, Science and Technology of Minas Gerais Federal Institute of Education, Science and Technology of Minas Gerais Michael Pereira de Souza Avenue, 3007. Campinho. Congonhas, MG, Brazil. Zip code: 36417-050. E-mail: [email protected] Frank de Mello Liberato Ph. D. in Metallurgic Engineering – Federal University of Minas Gerais Federal Institute of Education, Science and Technology of Minas Gerais Michael Pereira de Souza Avenue, 3007. Campinho. Congonhas, MG, Brazil. Zip code: 36417-050. E-mail: [email protected] Adilson Rodrigues da Costa Ph. D. in Métalurgie Structurale - Université Paris-Sud Federal University of Ouro Preto Tiradentes Square, 20 Ouro Preto, Minas Gerais, Brazil. Zip code:35400-000 E-mail: [email protected] ABSTRACT The Bauschinger Effect is a phenomenon that occurs in metals to reduce mechanical strength and increase ductility. In general, the mechanical properties of steel are considered to be a deleterious effect. The present work seeks to control this phenomenon in low carbon steel wires, as an alternative to the production of annealed or galvanized wire without the use of annealing heat treatment for stress relief. For this purpose, the "Metal Wire Stress Relief” machine (“Alívio de Tensão em Arames Metálicos” ATAM 1 I South Florida Congress of Development, Miami, 2021. machine, patent application number BR 10 2020 009155 7) was designed and built, which will apply cyclical flexing efforts through pulleys. These cyclical efforts will trigger the Bauschinger Effect which is the phenomenon responsible for the cold relief of stresses. The variables used to promote this effect were the number of pulleys, the angle between the pulleys, and the speed of the wire during its processing. The results showed that the ATAM machine is capable of relief considerably the Yield Strength, the Tensile Strength, and increase the ductility of cold-rolled, low-carbon steel wire. Keywords: Bauschinger Effect, Low-carbon steel wire, Stress relief. 1 INTRODUCTION In the production of steel wires, the Bauschinger Effect demonstrates significant potential for adapting the final mechanical properties of steel. The Bauschinger Effect is defined by the reduction of the yield limit of a polycrystalline metal after a pre-strain in the opposite direction to the conformation (KOSTRYZHEV, 2009). According to Hu et al. (2016), this phenomenon increases the ductility and reduces the yield and resistance limit. These changes in properties prove to be beneficial for recovering ductility in steel wires that have been subjected to high rates of cold deformation. Also, according to Pereira et al. (2014), after the Bauschinger Effect, there is a rearrangement of discordances in a sub-grain structure, which reduces the material yield limit. The reduction of the yield and strength limit is beneficial in the manufacture of annealed wires and generates a possibility of reducing the annealing cycle time or using less expensive common carbon steel alloys. The “Metal Wire Stress Relief” machine (“Alívio de Tensão em Arames Metálicos” ATAM machine, patent application number BR 10 2020 009155 7) was inspired by the working principle of the conventional straightening process, as shown schematically by Figure 1. This process promotes cyclical and plastic deformations well- defined flexures to remove all the stresses that caused a spiral curvature (Figure 2), to introduce a new deformation pattern, corresponding to that of a straight wire. Each flexion must be applied to produce a tension in the material above its yield limit, or the wire will return to its original position. (ENGHAG, 2009). 2 I South Florida Congress of Development, Miami, 2021. Figure 1 – A wire straightening via pulleys. (ENGHAG, 2009) Figure 2 – Curvature resulting from the wire drawing process in the (a) horizontal (Cast) and (b) vertical (Helix) (DAVID, 2014). In the straightening process, compression and tensile stresses are applied to the wire surface as shown in Figure 3. The fraction of the plastic and elastic deformation is applied to the material depending on the diameter of the straightening roll. The smaller the roll diameter, the greater the plastic deformation fraction. Figure 3 – Fraction of plastic and elastic deformation in the profile of a straightened wire, adapted (ENGHAG, 2009). In the straightening step of steel wires, the material undergoes bending as shown in Figure 1. Thus, the material is subjected to three stress states: compression, traction, and zero (Figure 3). The wire surface in contact with the pulley is the region of maximum compressive deformation. On the surface opposite the straightening roll, the material is subjected to the maximum tensile stress. Therefore, there is an internal line in which the applied stress 3 I South Florida Congress of Development, Miami, 2021. is zero. The diagram containing the three regions is shown in Figure 3. As the straightening rollers alternate in position (Figure 1), the tension and compression stress regions also alternate in a cyclic mode, promoting a stress relief in the wire, a phenomenon called Bauschinger Effect (SOWERBY, UKO and TOMITA, 1979). It is important to note that this stress relief can be produced using only cyclic plastic deformations of compression and traction, which are produced by straightening the micro-laminated or drawn wire in pulleys. Thus, there is a potential to avoid the stress relief heat treatment, which demands time and temperature (carried out in bell-type ovens or liquid lead vats at 700ºC). This reduces a step in the manufacturing process and consequently the cost of producing micro-laminated or drawn wire. Therefore, the objective of this article is to promote the Bauschinger Effect on the low carbon cold micro-laminated steel wire, with the chemical composition according to the ASTM A1040 class 1006 standard. In this way, the potential to reduce the yield limit, tensile strength, and increase the ductility of the wire with a high degree of cold reduction will be evaluated. This stress relief process will be developed through a cyclic cold bending process applied by the ATAM machine pulleys. 2 METHODOLOGY For the experiments, common carbon steel wire with a diameter of 1.29 mm was used, with a chemical composition equivalent to the ASTM A1040 class 1006 standard. This material is commonly used in the cold microlamination of galvanized and annealed steel wires. The data on the chemical composition of this steel are shown in Table 1, were obtained employing optical spectrometry tests. Table 1: Chemical composition of ASTM A1040 grade 1006 steel. Chemical Composition (% weight) Steel Grade C Mn Si S P N (ppm) 1006 0,05 0,42 0,080 0,014 0,022 29 The steel wire was initially processed on the Metal Wire Stress Relief machine (patent application number BR 10 2020 009155 7, Figure 4). This machine was 4 I South Florida Congress of Development, Miami, 2021. developed, built, and installed at the IFMG Testing and Metallography Laboratory in partnership with Gerdau S.A. The purpose of this machine is to promote and control the Bauschinger Effect, through the following operational variables: wire winding speed, the angle between the pulleys, and the number of pulleys. The ATAM machine (Figure 4) consists of 3 parts: 1. Stocker, 2. Pulley Table and 3. Winder. Figure 4 - Three-dimensional schematic drawing of the ATAM machine. The Stocker is responsible for accommodate and receive the steel wire rolls. The Pulley Table is the place where the steel pulleys will be inserted to determine the wire deformation path. The number, and angle between the pulleys will be alternated there. The winder pulls the steel wire through the system. There it is possible to change the winding speed of the wire. To promote the Bauschinger Effect on the ATAM machine, the following operational test condition was used: winding speed of 10,1 m/min, a total of 11 pulleys, and a wire angle between the pulleys of 47,4 °. The processing of the steel wire by the ATAM machine is shown in Figure 5. The deformation path of the steel wire in the processing is in the sinusoidal format. 5 I South Florida Congress of Development, Miami, 2021. Figure 5 – Positioning of the 11 pulleys at a fixed angle of 47,4º for cold micro-laminated wire. The nominal winding speed of the wire was monitored through the speed sensor of the ATAM machine and it has presented an average value of 10,1 m/min. Note that initially the speed increases and in 3 seconds it reaches the nominal speed as shown in Figure 6. After 51 seconds the test is interrupted and the wire speed reaches its zero value in 5 seconds. The test time at rated speed was 48 seconds, with 8 meters of wire being processed. Figure 6 – Wire winding speed curve during the test on the CBA machine. The samples for the tensile test were taken after 5 seconds, to ensure that the wire was processed at a speed of 10,1 m/min (Figure 6). Also, 11 pulleys were used positioned at a fixed angle of 47,4º, in the central position of the template, as shown in Figure 5. 6 I South Florida Congress of Development, Miami, 2021. Tensile tests were carried out to identify the main mechanical properties (yield strength, elastic ratio, and elongation) of the steel wire. The tests were performed on the EMIC 100kN universal testing machine from IFMG.