Haobo Lin, Qiang Li, Weigang Hu

Transcription

1 doi: / Tensile Mechanical Properties and Hardness Test of Pre-torsion Test on Axle Material EA4T Haobo Lin, Qiang Li, Weigang Hu Engineering Research Center of Structure Reliability and Operation Measurement Technology of Rail Guided Vehicles, Ministry of Education, Beijing Jiaotong University, Beijing, China Abstract In this paper, the pre torsion test of EA4T axle steel was carried out, and the mechanical properties of the sample before and after the experiment were compared. After torsion, the yield strength of the sample can be improved greatly, and the tensile strength of the specimen is only within a small range. At the same time, the plasticity of the material decreases with the increase of the yield strength. Through the test of the hardness of the sample, it is known that the hardness of the core sample is lower than that of the peripheral samples. For the core and peripheral samples, the strength of the sample can be improved by twisting, and the hardness is increased with the increase of the torsion angle. The hardness and strength of materials is the corresponding relationship, so the increase in hardness and strength is consistent. Keywords: Pre-torsion, Tensile strength, Hardness, EA4T steel. 1. INTRODUCTION Axles made of EA4T axle steel are widely used in the current railway applications. In order to ensure the safety of the axle. The strength of the axle need to design and calculate. Generally, there are two kinds of railway vehicle axle strength calculation methods(mi and Li, 2002). The authoritative design specifications of the axle strength are (JIS E4501, 1995) form Japan, (EN13103, 2001) and (EN13104, 2001) from Europe. The railway department in China also established the axle design standard: (TB/T , 2008) and (TB/T , 2010). In order to make the use of axle use safety, this paper studies make pre-torsion of railway EA4T axle steel(sang and Lloyd, 1979; Li, 1994), in order to improve the mechanical properties of the axle(laukonis andghosh, 1978; Ranta-eskola, 1998). Mechanical properties of materials, also known as mechanical properties, refers to the deformation, damage and other characteristics of material under external forces. It should be determined by experiment and the static load tensile test is the most commonly used method to study the mechanical properties of materials(gb/t , 2010). A section of l 0 is taken as the test section which is called standard distance. As is shown in Figure1. Figure 1. Specimen. 260

2 Static tensile test of bar specimens is carried out at room temperature with cross sectional area of A 0 (Liu, 2004). Stress σ and strain ε are two basic parameters of static load tensile test. = P/ A (1) l l l (2) The tensile curve of low carbon steel is roughly divided into: elastic stage, yield stage, strengthening stage and local deformation stage. The yield stage will produce plastic deformation, when the material turn into the plastic deformation stage, the cross section of the specimen will change. The calculation of the stress is still using the original crosssectional area, which has a certain error with the real stress. Under uniaxial tensile stress, the exponential relationship between the true stress and true strain after entering the plastic deformation stage can be expressed by the following formula. n s C e (3) Take the logarithm of both sides of the formula can get a logarithmic equation. ln s lnc nlne (4) ln Sandln C are linear relationship(gao et al., 2007). The formula for calculating the engineering stress-strain of the tensile strain is as follows. P A 0 ll0 l l l 0 0 (5) The engineering stress-strain is transformed into real stress-strain as expressed by the following formula. P s 1 A l e ln ln 1 l 0 (6) Hardness is an important indicator to measure the degree of hardness of the metal material(yu and Tian, 1982). Hardness is the ability to resist elastic deformation, plastic deformation or damage, which is the ability of the material to resist deformation and damage. Hardness is not a simple physical concept, but a composite index of elasticity, plasticity, strength, toughness and other mechanical properties. According to the hardness testing method, it can be divided into various methods, as like static pressure method, rebound method, scratch test and high temperature hardness and so on(yang, 2010). 2. PRE-TORSION TEST OF EA4T AXLE STEEL 261

3 2.1 Test material The material used in the test is made of EA4T axle steel material, and its chemical composition is shown in Table 1. Table 1 EA4T steel chemical composition (%) C Si Mn P S Cr Mo V Cu Ni Due to surface heat treatment, the micro-structure and properties of the core and surface of EA4T steel are different. In this paper, the core and peripheral samples of EA4T steel were pretreated and studied separately, and the sampling method is shown in Figure Test method Figure 2. Sketch map of the samples. According to the different microstructure and mechanical properties of EA4T axle steel, core and peripheral samples were taken from the blank. Peripheral number is 1-1 to 1-6, and the core number is 2-1 to 2-4. Static torsion tests were carried out on MTS809 torsion test machine. Twist angle and standard distance are shown in Table 2. Table 2 EA4T steel pre-torsion values No. Pre twist angle / d 0 /mm l 0 /mm π π π π π π π π TEHSILE MECHANICAL TEST OF EA4T STEEL AFTER PRE-TORSION 3.1 Tensile test method 262

4 Tensile tests were carried out on MTS810 tensile testing machine, as shown in Figure 3. Based on the collected data, the engineering stress-strain curves are drawn, which is further converted to the true stress-strain curves of the samples. The plastic index such as the ratio of the section shrinkage and elongation of the sample are calculated by measuring the diameter and standard distance after fracture(xu and Yang, 1999; Liu and Ma, 2014). Figure 3. MTS810 tensile testing machine. 3.2 Test data collection According to the plane hypothesis theory of circular shaft torsion (Shen, 2010), it can be considered that the diameter and the standard distance of EA4T axle steel specimen are not changed after pre-torsion. The force and displacement data of the specimen were collected after uniaxial tensile test of EA4T axle steel specimen on MTS810. Force deformation curves are shown in Figure 4 and Figure 5. Figure 4. Outside force - deformation curve. Figure 5. Core force-deformation curve. 263

5 As can be seen from the figure 4 and figure 5, the force deformation curves of the outer and core samples after torsion are higher than that of the non-torsion curve, which shows that the load level of the specimen can be improved. No matter Peripheral and core samples, non-torsion specimens and the torsion specimens have similar bearing capacity at deformation displacement 5mm. For the peripheral samples, the tensile load of the specimens after torsion is increased. And with the increase of the twist angle, the tensile load level of the sample increases. When the torsion angle is π, continue to increase the torsion angle, the maximum tensile strength of the specimen remains unchanged basically. For core samples, the rules are basically the same. In addition to uniaxial tensile testing of EA4T axle steel, measuring force-displacement, cross section diameter and standard distance after fracture of specimen which are used to calculate the plastic index of the material are also measured in test. Specific data is shown in Table 3. Table 3 Core and periphery specimens under different torsion angle measurement data No. maximum force/kn d /mm l /mm Test data processing Transform the force-deformation curve into the engineering stress-strain curve, which is the σ ε curve, shown in Figure 6 and Figure 7. Figure 6. Outside samples engineering stress-strain curve. 264

6 Figure 7. Core samples engineering stress-strain curve. Turn engineering stress-strain curves into true stress-strain curves are shown in Figure 8 and Figure 9. We can see from figures that there are different stress-strain curves of core and peripheral samples. The curves of is higher than that of core samples, which means the mechanical properties peripheral material is higher than the core. In the elastic stage, the slope of the periphery and core samples are close, that is, the elastic modulus of the material remains unchanged basically. The yield strength is improved with the increase of the torsion angle. And the yield strength are significantly higher than specimen with non-torsional for both outer and core specimens. When the torsion angle is up to 2π, the performance of the material is increased to a maximum value. Yield strength and tensile strength of the material is basically unchanged if torsion angle continues to increase, but the plastic property of the material is reduced. When the nontorsion specimen reaches the yield point, stress does have little fluctuation with the obvious increases of strain, and it has an obvious yield phenomenon. When the torsion specimen reaches the yield point, the stress increases with the increase of strain. From the latter part of the curve, it can tells that the torsion samples entering into the local degeneration stage is earlier than non-torsion sample. Figure 8. Outside true stress-strain curve. 265

7 Figure 9. Core true stress-strain curve. There is no obvious yield phenomenon in the true stress-strain curves of the core specimens. So the yield strength of the material need to be calculated according to the test data(wang et al., 2004). The elastic modulus is obtained by fitting the tensile curve of elastic segment. Translate the straight line which passing through the origin point and which slope is elastic modulus to the right of 0.2 unit. As shown in Figure 10 and Figure 11. Figure 10. Outside yield strength calculation diagram. Figure 11. Core yield strength calculation diagram. 266

8 From the Figure 12 and Figure 13, it can be seen that the yield strength increase with the increase of the torsion angle, which can be increased by about 15%. The yield strength increased at a faster rate between 0-π. The increase rate of yield strength slows down when twist angle is beyond π. The tensile strength of the peripheral samples has a 5% reduction when the torsion angle isπ/2. With the twist angle continued increase to 2π, the tensile strength of non-torsion and torsion specimens are basically the same. When the twist angle is beyond 2π, the tensile strength continue to decline. The yield strength of core samples increases with the increase of torsion angle, which can be increased to about 25% and higher than peripheral samples. The yield strength of core samples can be increased by about 15% when the twist angle isπ/2. As the torsion angle continues to increase, the yield strength of the material continues to rise. The tensile strength of the core samples float a little with the increase of twist angle, which is almost the same vale with non-torsion specimen. Figure 12. Outside sample torsion angle-strength curve. Figure 13. Core sample torsion angle-strength curve. Figure 14 and Figure 15 show torsion angle-section shrinkage curves and torsion angleelongation curves of peripheral and core sample. The section shrinkage and elongation rate of samples are smaller with the increase of the twist angle, but the change is small. When the torsion angle is small, the yield stage and hardening stage of material accounted for a larger proportion. A large degree of plastic deformation is occurred in the specimen and showed the characteristics of plastic fracture. With the increase of torsion angle, plastic deformation becomes smaller in the process of tensile specimen. 267

9 Figure 14. Outside samples torsion angle- ratio curve. Figure 15. Core samples torsion angle- ratio curve. According to the true stress and true strain of the samples under different torsion angles, the strain hardening exponent and the strength coefficient of the sample were obtained. The specific values are shown in Table 4. According to the formula 4, the strain hardening exponent and the hardening coefficient of samples before and after pre-torsion can be obtained. The specific values are shown in Table 5. Table 4 The sample under different torsion angle of true stress and true strain No. 2% 2.5% 3% 3.5% 4%

10 Table 5 Sample under different torsion angle of hardening index and coefficient of strength No. n ln C Figure 16 and Figure 17 show that with the increase of the torsion angle, the strain hardening exponent of the sample is gradually reduced, showing the softening phenomenon of the material, and the slope value of the straight line is the hardening exponent in the figure. The enhancement factor decreases with the increase of the torsion angle. The elongation decreases and the plasticity decreases. The value of enhancement factor is linear intercept. Figure 16. The ln s-ln e curve of the outside samples. Figure 17. The ln s-ln e curve of the core samples. To sum up, the conclusion can be obtained. Pre-torsion for EA4T can greatly improve the yield strength of the sample, and the tensile strength of the specimen is only within a small range. The yield strength is improved, but the elongation rate of the specimens is decreased, that is, the plasticity of the material decreases. Non-torsion sample has 269

11 higher elongation, but the corresponding bearing capacity is low. Therefore, in the practical application of engineering, expect meeting the bearing capacity and plastic requirements of EA4T steel, select an appropriate twist angle, it can also strengthen the EA4T axle steel and improve the mechanical properties of the axle steel and the value use. 4. HARDNESS TEST OF EA4T AXLE STEEL SPECIMEN 4.1 Test method Do mechanical polishing of the sample, so that the specimen can be observed under the optical microscope. Using MH-VK image processing micro Vivtorinox hardness tester to study micro hardness test, which measurement range is HV. Using diamond pyramid indenter vertically press sample surface to get an indentation. Moving the cross scale under the microscope to determine the two ends of the two diagonal. The Vivtorinox hardness value of the sample is calculated by the instrument. The test load is 500g, and the loading time is 10s. Pick the value of r/r which is 0.25, 0.5, 0.75 and 1 as unit. Test hardness of specimen along radial direction, take the average value of 10 different test points which are uniform distribution at the same radius as final values(liang et al., 2015) 4.2 Test method Test results are shown in Figure 18 and Figure 19. As can be seen form the figures, the hardness of the core sample is lower than that of the external sample, which is related to the different structure of the material along the radial direction. For the core samples, the hardness of the sample is changed after the torsion, and the hardness of the material is improved. Along the radial direction, the hardness of the material increases linearly with the increase of the torsional strain. The hardness of the position which far from core increased by 9.1%. For the peripheral samples, the hardness is higher than that of the core, and the hardness increases with the increase of the torsion angle. When the twist angle isπ, the hardness of the position which far from core increased by 7.9%. And When the twist angle isπ, the hardness of the position which far from core increased by 16.3%. Hardness and strength of the material is the corresponding relationship, so the hardness increase and the strength increase of the sample is consistent after torsion. Figure 18. Outside samples hardness distribution curve along the radial. 270

12 5. CONCLUSION Figure 19. Core samples hardness distribution curve along the radial. (1) The mechanical properties of the samples before and after torsion were compared. The experimental results show that the yield strength of the samples can be improved greatly after torsion test, and the tensile strength of specimens is only within a small range. With the increase of yield strength, the elongation of specimens decreases, that is, the decrease of the plasticity of the material. (2) Compared the mechanical properties of the peripheral samples and core samples of EA4T axle steel, and the mechanical properties of peripheral samples are better than that of the core samples in terms of yield strength and tensile strength. (3) Through the hardness test of the sample, it is known that the hardness of the core sample is lower than that of the peripheral samples. For both the core and the peripheral samples, the strength of the sample can be improved by twisting, and the hardness is improved with the increase of the torsion angle. Hardness and strength of the material is the corresponding relationship, so the improvement of hardness is consistent with the increase of the strength of the material. 6. ACKNOWLEDGMENTS This paper is supported by The National Key Research and Development Program of China(2016YFB ). 7. REFERENCES EN (2001). Rail way application-wheel sets and bogie-non-powered axles- Design method. EN (2001). Railway application-wheel sets and bogie-powered axles - Design method. Gao Y.J., Wang C., Xu Z.L. (2007). Welding of high strength steel with yield strength of 900 MPa, Transactions of the China Welding Institution, 28(9), GB/T (2010). Metallic materials Tensile testing at ambient temperature. JIS E4501. (1995). Railway rolling stock - design method for strength of axles. Joseph. V.L., Amit K.G. (1978). Effects of Strain Path Changes on the Formability of Sheet Metals, Metallurgical Transactions A, 9(12), Liang C., Qin Z.X., Lu X. (2015). Surface Strengthening Study of EA4T Axle Steel by Ultrasonic Impact Treatment, Journal of Dalian Jiaotong University, 36(4),

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