The Bauschinger Effect Application on Cold Micro-Laminated Low- Carbon Wire

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

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

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

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.

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.

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. The mechanical properties of the cold-rolled wire (as received) and after processing using the ATAM machine, were determined by the tensile test (minimum of 7 replicates) according to the ASTM A370 standard.

3 RESULTS AND DISCUSSION

The mechanical properties of the cold-rolled wire (as received) and after processing using the ATAM machine (operational test condition: winding speed of 10,1 m/min, a total of 11 pulleys and an angle of the wire between the pulleys of 47,4 °), are shown in Table 2.

Table 2 – Variation of the mechanical properties of the wire before and after test applying on the CBA machine. Properties As Received After Test ΔP ΔP(%)

Diameter (mm) 1,29 ± 0,005 1,27 ± 0,003 -0,02 -1,3

σe (0,2%) (MPa) 846,7 ± 21 732,3 ± 58 -114,4 -13,5

σr (MPa) 922,8 ± 6 875,9 ± 5 -46,9 -5,1

σr/σe 1,09 ± 0,03 1,20 ± 0,09 +0,11 +10,2

Elongation (%) 1,9 ± 0,4 2,6 ± 0,3 +0,7 +40,1

Thus, through the results shown in Table 2, it is possible to determine the

difference between the mechanical properties (Yield Limit (σe), Tensile Strength Limit

(σr), Elastic Ratio (σr/σe) and Elongation), of the steel wire before and after passing through the ATAM machine. The results are shown in Table 2, where ΔP is the property variation (property values after passing through the ATAM machine subtracted from the steel property values as received) in absolute values and ΔP (%) is the property difference in values percentages.

I South Florida Congress of Development, Miami, 2021.

The test results (Table 2) show a significant reduction of 13,5% (-114,4MPa) in the Yield Strength and 5,1% (-46,9MPa) in the Tensile Strength. There was also an increase in the Elastic Ratio of 10,2% and a considerable increase in Elongation of 40,1%. Furthermore, the diameter of the wire during the test remained almost unchanged (a small reduction of 1,3%).

Figure 7 – Comparison of mechanical properties before and after the wire passes through the ATAM machine.

Figure 7 shows the mechanical properties in “Box Plots” (MONTGOMERY and RUNGER, 2018), representing the variation of experimental data through quartiles. Note that there is no interaction between the interquartile ranges of mechanical properties. “Hypothesis Tests on the Mean” (MONTGOMERY and RUNGER, 2018) was also performed on all mechanical properties tested. It can be said with 95% certainty that the averages are statistically different. Thus, it is possible to affirm that there was a change in relation to mechanical properties, that is, there was a reduction in the Flow Limit and Resistance to Tensile Limit and an increase in the Elastic Ratio and Percentage Elongation.

I South Florida Congress of Development, Miami, 2021.

The Stress-Strain curves for the wire as received (blue curve) and after the test on the CBA machine (red curve), are shown in Figure 8. It is possible to graphically verify the expressive increase in the plasticity of the material and also the reduction of the Yield and Tensile Strength Limit. The plastic deformation field (amount of deformation from the yield limit to the rupture of the specimen) increased by 72%. This mechanical behavior reaffirms that the ATAM machine, through the processing and control of operational variables (winding speed, wire angle between the pulleys, and the number of pulleys), is capable of producing the Bauschinger Effect on the steel wire and, therefore, partially recover the mechanical properties of the hardened wire.

Figure 8 – Stress-strain curve for the wire as received and after testing with the ATAM machine.

4 CONCLUSION

This paper demonstrated the application of the Bauschinger Effect in cold-rolled, low-carbon steel wire, to promote partial stress relief and, therefore, a partial recovery of the mechanical properties of the hardened wire.

At the end of the analysis, it was found that, although the Bauschinger Effect is considered a deleterious effect, the properties of the metal, when controlled, can be used for stress relief in hardened steel wires. The results obtained showed a 72% recovery in the plasticity of low carbon steel wire and an increase in ductility of around 40%.

5 ACKNOWLEDGMENTS 9

I South Florida Congress of Development, Miami, 2021.

To the post-graduation program of Federal University of Ouro Preto (UFOP) - REDEMAT and the Federal Institute of Education, Science and Technology of Minas Gerais (IFMG).

REFERENCES

AMERICAN SOCIETY FOR TESTING AND MATERIALS. ASTM A370: Standard Test Methods and Definitions for Mechanical Testing of Steel Products. West Conshohocken, p. 49. 2017.

AMERICAN SOCIETY FOR TESTING AND MATERIALS. ASTM A1040: Standard Guide for Specifying Harmonized Standard Grade Compositions for Wrought Carbon, Low-Alloy, and Alloy Steels. West Conshohocken, p. 13. 2015.

DAVID, F. F. Influence of thermomechanical treatment on microalloyed steel and common carbon in the manufacture of CA-60 steel wire. Dissertation (Master in Mechanical Engineering) - Federal University of Pernambuco. Recife, p. 102. 2014.

DAVID, Felipe Farage. Máquina para alívio de tensão em arames metálicos. Depositors: Federal Institute of Education Science and Technology of Minas Gerais and Gerdau S.A. BR 10 2020 009155 7. Deposit: 08 Mar. 2020.

ENGHAG, P. Steel Wire Technology. 4ª. ed. Orebro: Applied Materials Technology, 2009.

HU, J. et al. On the evaluation of the Bauschinger effect in an austenitic stainless steel - The role of multi-scale residual stresses. International Journal of Plasticity, 24 Maio 2016. 203-223.

KOSTRYZHEV, A. G. Bauschinger effect in Nb and V microalloyed line pipe steels. Tese (Doutorado em Filosofia) – University of Birmingham. Birmingham, p. 206. 2009.

MONTGOMERY, D. C.; RUNGER, G. C. Applied Statistics and Probability for Engineers. 7ª. ed. United States of America: Wiley, 2018.

PEREIRA, T. S.; CHIU, Y. L.; JONES, I. P. Microstructure characterization of an X70 grade pipeline steel and its dislocation development during Bauschinger testing.

I South Florida Congress of Development, Miami, 2021.

Electron Microscopy and Analysis Group Conference 2013. York: Journal of Physics. 2014. p. 1-4.

SOWERBY, R.; UKO, D. K.; TOMITA, Y. A Review of Certain Aspects of the Bauschinger Effect in Metals. Materials Science and Engineering, Lausanne, 7 Maio 1979. 43-58.