Effect of the Twins on Mechanical Properties of AISI 304 Stainless Steel Wire Using Electrical Current Method

Transcription

1 Materials Transactions, Vol. 52, No. 1 (2011) pp. 25 to 30 #2011 The Japan Institute of Metals Effect of the Twins on Mechanical Properties of AISI 304 Stainless Steel Wire Using Electrical Current Method Hsu-Chi Chuang 1, Fei-Yi Hung 2; *, Truan-Sheng Lui 1; * and Li-Hui Chen 1 1 Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701, R. O. China 2 Institute of Nanotechnology and Microsystems Engineering, Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan 701, R. O. China Electrical current process has been used in the thin plate metals and the fine wires because of its higher efficiency (rapidly recrystallization) and the advantage of avoiding oxidation without inert gas. This study investigated the transformation of twin structure and mechanical properties of 304 stainless steel after electrical current test by direct current. The results show the microstructure of matrix transported from preferred structure to equiaxed grains and a few annealed twins with increasing the current density of joule from 32 J to 188 J. The hardness of matrix remained stable under 83 J because of the number of twins increased, and the tensile properties were similar to the wire using furnace heating. After electrical current test at 188 J, the strain hardening exponent of wires was 0.44, was suitable to be used for cold work procedure. [doi: /matertrans.m ] (Received August 4, 2010; Accepted October 22, 2010; Published December 25, 2010) Keywords: AISI 304, electrical current, tensile property, current density of joule 1. Introduction Table 1 Chemical composition of wire. The electrical current heating treatment procedure (EHT procedure) includes two phenomena: joule heat and electromigration of metal ions. 1 4) This procedure does not need inert gas to avoid oxidation, and it owns higher efficiency thermal energy. Many materials, such as iron, copper and aluminum plate or wire product processes use it to replace the furnace heating treatment. As for application of cold work, the conventional furnace heating cannot be used in deep drawing and stretching processes for metal materials. 5) For EHT process, the matrix has more twins and less unusual grains that can offer the reliability in thin thickness working, but a few references discuss the electrical current mechanism. According to the literature, 5,6) the aluminum billets can be homogenized rapidly by EHT procedure. In addition, the efficiency of EHT procedure increased with increasing the current density in the structural transformation. Some studies 7,8) showed that the microstructure of aluminum plate had refine the grains and the reliability of cold work was raised using EHT procedure. It showed a close correlation between the grain growth and electrical current conditions. 9) The report of EHT in stainless steel wires was not many. 10,11) They only analyzed the surface characteristics and mechanical properties. However, the recrystallization of metal wire using electrical current method has still not been examined, and in particular, the strain hardening exponent of the electrical current 304 wire is worthy of further investigation. Therefore, this study not only discusses the microstructure of wires, but also understands the electrical twins how to affect the mechanical properties. In addition, the relation between the strain-hardening exponent and the twin content was explained and clarified the contribution of EHT method. *Corresponding author, fyhung@mail.ncku.edu.tw; z @ .ncku.edu.tw Component C Si Mn P S Cr Ni Cu Mo (mass%) Experiment Procedure The experimental material is AISI 304 stainless steel wire of cold drawn wire. Table 1 shows the chemical composition of specimen (mass%). The diameter of the wire was drawn from 1 mm to 0.5 mm (the cross section area is mm 2 ). Figure 1 displays the EHT instrument revealing consists of direct current power supply and two copper wheels. The voltage value and current value can be modulated by controlling the knob on the power supply unit. Actual working temperature (joule heat) was measured by thermocouple. A length of 200 mm was the working distance between two centers of the copper wheels and the holding time of all samples is 3 s. Figure 2 shows the trend of joule heating capacity value variation with increasing voltage in EHT process. In the stabilized linear zone, the electrical energy totally transferred to the joule heat. The equation how to calculate the joule heating capacity was following: P ¼ I V s, where P is the electric power, I is the working current (A), V is the working voltage (V), and s is the time (s). The joule heating capacity (J) was used to represent every different experimental condition. The temperature was induced and the wire surface was about 530 C at the transition point (188 J). In addition, this study chose two experimental conditions for EHT process: (1) Four the current density of 32, 54, 83 and 188 J and an identical the duration of electrical current. (2) Identical duration of electrical current (3 s) varied the current density of 32, 54, 83 and 188 J. Before and after EHT process, the microstructures of the wires were determined using an image analyzer and X-ray

2 26 H.-C. Chuang, F.-Y. Hung, T.-S. Lui and L.-H. Chen Fig. 1 Schematic illustration of experimental apparatus. Fig. 3 The microstructure of specimens (a) cold drawn wire and (b) furnace heating treatment (F. H.) wire at 1050 C for 7 s (grain size is about 30 mm and the fraction area of twin is about 22%). Fig. 2 The relationship between the heating capacity and working voltage. diffraction. Tensile properties of wires after EHT were obtained using Instron 5560 tester (strain rate is 1: s 1 ). For microhardness testing, each analysis datum is the average of 10 test results. According to the tensile curve Hollomon equation: ¼ K " n, 12) the strain-hardening exponent of wires was calculated. is the true stress and " is actual plastic strain. K means strength constant and n is strain-hardening exponent. So, the workability of cold draw wires after EHT process was investigated using strainhardening exponent. 3. Results and Discussion 3.1 Microstructural variation of electrical current process Microstructure of cold drawn 304 wire is shown in Fig. 3(a). It reveals the flow-drawing structure. The microstructure of stainless steel wire annealed at conventional furnace (F. H.) is shown in Fig. 3(b). Annealed twin boundaries with face-centered cubic lattice (FCC) demonstrated the lowest stacking fault energy. 13,14) Notably, the grain size of phase was about 34 mm and twin, martensite 15) and ferrite were in the matrix. After EHT test (current density of joule was identical at 188 J), the microstructure was shown in Fig. 4. The grain size of phase and area fraction of twin of wires was achieved the stabile state of value at about 15 s of EHT process. Notably, the structures of EHT wires at 3 to 5 s were similar to that of the furnace heating wire. In other words, 304 stainless steel wires underwent 188 J for 35 s that possessed alike microstructure as the furnace heating. Figure 5 showed the grain size and area fraction of twins increased with increasing the current density. At the current density of 32 J, the energy has not enhance recrystallization yet, the structure remained the metal flow of cold-draw process. When the current density of joule was raised to 188 J, the grain size and the area fraction of twins were similar to that of furnace heating wires. X-ray diffraction of electrical current wires and conventional furnace heating wires were shown in Fig. 6. With increasing current density, the initial martensite in matrix had disappeared, the intensity of ferrite reduced, and the intensity of austenite increased. In addition, the peak intensity of phase of some EHT wires was higher than that of the furnace heating wire. The reason is that EHT process is a rapidly heating treatment (overall). However, the thermal condition of the furnace heating wire (F. H.) is from outside to inside. This phenomenon highlights

3 Effect of the Twins on Mechanical Properties of AISI 304 Stainless Steel Wire Using Electrical Current Method 27 Fig. 4 Microstructures of electrical current specimens: (a) 1 s and (b) 30 s at 188 J, respectively. (c) Grain size and the area fraction of twin in the duration of electrical current (188 joule) and (d) micro-hardness of specimens at 54 and 188 J. that increasing current density was enhanced the matrix phase transformation (from 0 and to phase). 3.2 Mechanical properties The average microhardness of non-annealed wire was Hv 511. In Fig. 7, the hardness of EHT wire at 32 J did not decrease significantly and was similar to the cold drawn wire. When the joule heating capacity was more than 54 J (Hv: 237), the microhardness of wire had more stable value (even lower than F. H. wire). In addition, the large energy of EHT process can assist the grains to have a rapid growth within 3 s. Notably, the boundaries of some twins still blocked the motion of dislocation and cannot grow during electrical current. So, the grain size and the area fraction of twins of EHT wire were higher than the value of conventional furnace heated wires. Figure 8 showed the results of tensile test after EHT (the electrical current duration was 3 s). The tensile strength and yield strength of the wires reduced with increasing the current density. To compared with the microstructure of EHT wires, the conventional furnace heating wire needed more thermal energy to perform the phase transformation (from " martensites to phases). In fact, EHT process is a rapidly heating treatment and thermal propagation is overall: E wires absorb ; E supply (absorb is the energy of wires absorbed and supply is the energy of power supply offered). The F. H. (radiation heat) of thermal propagation is from outside to inside (E wires absorb < E supply ). Notably, EHT wires possessed the rapid annealed mechanism and the related properties were similar to that of the conventional furnace heating wires. Sometimes, the furnace heating wires had higher hardness due to the residual "-martensite and small grains in the matrix. Moreover, the twin boundary blocked the motion of dislocations to have higher mechanical strength. 16,17) According to Fig. 4 and Fig. 8, it is obvious that the hardness and flow stress were decreased with increasing the current density of joule. The area friction of twins reduced and the workability of plastic deformation was improved. 3.3 Strain hardening exponent The value of strain hardening exponent (n value) of EHT and conventional furnace heating specimens are shown in Fig. 9. The n value increased with increasing the current density of joule. Notably, n value of 188 J wire had approached to the furnace does. We may say that EHT process was able to obtain the excellent strain hardening property. When the EHT wires underwent the plastic deformation, the inside stress moved toward the grains (not grain boundary) because of the strain occurred in two slip systems. It can be explained by the necking had not easily happen. In

4 28 H.-C. Chuang, F.-Y. Hung, T.-S. Lui and L.-H. Chen Fig. 6 Fig. 7 Fig. 5 Microstructures: (a) 32 J, (b) 188 J and (c) the variation of grain size and the area fraction of twin with different heating capacities at 3 s. addition, the high efficiency of joule heat and the electromigration of metal ions enhanced the matrix transfer to phase. One research18) indicated that the average of slip distance was affected by the grain refinement and the dislocation moving. It is clear that the dislocation intertwined of wires was improved using the EHT process. A short duration of electrical current, the grain growth reduced the flow stress and increased the n value. X-ray diffraction of different Joule heating capacity. Micro-hardness of specimens in different heating capacity. 3.4 Strain hardening exponent and electrified twin Figure 10 exhibited the relationship of grain size and hardness in the varied joule heating capacity. In addition, efficiency of the number of electrified twin to n value was showed in Fig. 10. The grain size became bigger and the hardness became more stability with increasing the joule heating capacity. In the other word, the over joule heating capacity could not let the matrix soften. Notably, the most important was that less the current density of 40 J could soften the matrix, and the hardness of electrified (Hv: 220) was lower than that of the furnace heating (Hv: 340). In fact, the area fraction of twin using electrical current treatment was more than furnace heating (Fig. 10). It is clear that the dislocation of matrix moved difficultly when the area fraction of twin increased. This is the reason why the hardness become stability with increasing the number of twin when the grain growth in matrix. According to the research,19) the twin boundaries grow by lattice and grain boundary diffusion mechanisms when the electron passed along grain boundary during electrical current process. It is not to be denied that a few annealing twins were formed by furnace heating process because the major diffusion pass was the grain boundary diffusion. In the relation of strain

5 Effect of the Twins on Mechanical Properties of AISI 304 Stainless Steel Wire Using Electrical Current Method 29 (a) (b) F. H Flow stress, σ / MPa F. H. σ 0.2 σ UTS ε t Total elongation, ε / % 200 F. H Joule heating capacity, J / W S Fig. 10 The trend of microstructure and mechanical properties. Fig. 8 (a) stress-strain curve and (b) flow stress and elongation of wires at different heating capacities. hardening exponent and electrical current twin was found that the area fraction of twin and the n value were higher than furnace heating when joule heating capacity was more than 83 J. It not only get the excellent strain hardening exponent value, but the stainless steel wires also have good workability of uniform deformation. Fig. 9 The n value of specimens in different heating capacities. 4. Conclusions After the electrical current heating treatment (EHT), the 304 wires possessed the similar mechanical properties as that of furnace heating wires. Under the current density of 83 J, the matrix not only occurred the phase transformation obviously, but also stabilized the deformation resistance and hardness. A rapid EHT was able to improve the dislocation mechanism and increased the n value of wires to enhance the workability. Acknowledgements The authors are grateful to National Cheng Kung University, the Center for Micro/Nano Science and Technology (NCKU Project of Promoting Academic Excellence & Developing World Class Research Center: D ) and NSC E MY2/NSC E for the financial support.

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