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Studying the Effects of Ultrasonic Vibration on Microstructure and Hardness of St14 Steel Sheet
Nima Najafizadeh, Majid Rajabi, Ramin Hashemi, Saeid Amini
To cite this version:
Nima Najafizadeh, Majid Rajabi, Ramin Hashemi, Saeid Amini. Studying the Effects of Ultrasonic Vibration on Microstructure and Hardness of St14 Steel Sheet. The Biennial International Conference on Experimental Solid Mechanics (X-Mech 2018), Feb 2018, Tehran, Iran. �hal-03024527�
IUST
The Biennial International Conference on Experimental Solid Mechanics (X-Mech 2018)
13-14 Feb., 2018, Tehran, Iran
Center of Excellence in Experimental Solid Mechanics and Dynamics, School of Mechanical Engineering, Iran University of Science and Technology
Studying the Effects of Ultrasonic Vibration on Microstructure and Hardness of St14 Steel Sheet
1 Najafizadeh, N., 2*Rajabi, M., 3Hashemi, R. and 4Amini, S.
1 M.Sc. student, Sustainable Manufacturing Systems Laboratory, School of Mechanical Engineering, Iran University of Science and Technology, Tehran, 16846-13114, Iran
2 Assistant Professor, Sustainable Manufacturing Systems Laboratory, School of Mechanical Engineering, Iran University of Science and Technology, Tehran, 16846-13114, Iran
3 Assistant Professor, School of Mechanical Engineering, Iran University of Science and Technology, Tehran, 16846-13114, Iran
4 Department of Manufacturing, Faculty of Mechanical Engineering, Kashan University, Kashan, 87317- 53153, Iran
*(corresponding author: majid_rajabi@iust.ac.ir)
Abstract- Ultrasonic energy is a method used for applying severe plastic deformation on metal surfaces. The objective of this study is to investigate the effect of ultrasonic vibration on the hardness of St14 steel sheet. In the present work, a hemispherical-head tool, controlled by a CNC milling machine, shaped the sheet metal till the necking starts to happen.
Conventional as well as the new ultrasonic test have been performed on a St14 steel sheet and obtained data were used to compare the hardness of the sheets. It is observed that the hardness of the sample shaped by applying ultrasonic vibrations to the tool with an amplitude of 15µm at 20 kHz increased significantly when compared to the sample shaped without using ultrasonic vibration, revealing the efficiency of the ultrasonic operation in increasing the hardness.
Keywords - Ultrasonic Vibration, Hardness, Necking, Deformation Behavior, Grain Size.
Introduction
Technological practice, especially in automotive and aerospace industries, is hardly imaginable without mechanical surface treatment processes which have been utilized to improve tribological properties and to prevent failures of mechanical components and parts [1]. According to Hall- Petch equation, mechanical properties of materials can be improved by grain refining [2]. Ultrasonic energy, which is used as a method to apply severe plastic deformations on metal surfaces, results in mechanical improvements such as hardness, yield stress, and surface smoothness. Karimi et al.
[3]studied the effect of ultrasonic nanocrystalline surface modification (UNSM) process on the hardness of steel using a vibrating tool and observed that the surface structure of steel 7225 became finer at nanometer scale, Moreover with an increase in the number of impacts, the surface hardness also increased. Many researchers have studied the application of
ultrasonic vibration on different manufacturing processes such as upsetting [4], ECAP [5] and press forming [6]. The purpose of the present work is to examine the hardness of two similar St14 steel sheets, shaped by a hemispherical-head tool with and without applying ultrasonic vibrations to the tool respectively, to investigate the effect of ultrasonic vibrations on microstructure and hardness of the sheets.
Material and Method
As received St-14 steel sheet with the chemical composition as indicated in Table 1 was used to prepare two similar samples with a width of 50 mm, length of 50 mm and thickness of 0.7 mm. After preparing the samples, they were placed between a blank-holder and a matrix. In the next step, a hemispherical-head tool, controlled by a CNC milling machine, moved downward with the speed of 2 mm/s and shaped the sheet till the necking occurred, as it is shown in
Fig. 1. The first sample was shaped conventionally and without applying ultrasonic vibrations, but the second sample was formed by applying ultrasonic vibration to the tool using a ultrasonic transducer with the frequency of 20.5 kHz and the amplitude of 15µm. A sudden drop in the load- displacement diagram of the tool was considered as the moment of necking for each sample. After the test was carried out, the samples were ground, polished and etched respectively to obtain a better surface condition for microstructural observation by an optical microscope. Micro- Vickers hardness measuring of samples has been implemented by applying the load of 100 N to the polished surfaces for 10 seconds. The measurements were repeated at least five times to minimize the errors.
TABLE 1. CHEMICALCOMPOSITIONSOFST14STEELSHEET
Fe C Si Mn P S Cr Ni
Base 0.05 0.008 0.20 0.006 0.006 0.005 0.02
Fig.1: Schematic view of the test equipment.
Results and discussion
The hardness of the samples above was also measured. For the example which has been shaped without applying ultrasonic vibrations to the tool, the hardness is 143.9 HV, and for the one shaped using ultrasonic vibrations, the hardness is 184.9, indicating that an increase in hardness was directly proportional to applying ultrasonic vibrations to the tool. By comparing the value of the measured hardness, it could be concluded that hardness increased about 28% due to
ultrasonic vibrations of the tool. Fig. 2 shows the hardness of samples:
Fig. 2: Micro-Vickers hardness of samples
Applying repeated impacts on the surface of sample led to microstructural of it, and obviously, the hardness will increase by microstructural. Fig.3 shows the microstructure of the samples, indicating that the grain size has been decreased by applying ultrasonic vibrations to the tool. Fig.4 shows the grain size of the samples. The measurement has been done in longitudinal and transverse directions.
Fig. 3: Microstructure of two samples; a) shaped without applying ultrasonic vibrations, b) shaped by applying ultrasonic vibrations.
Fig.4: Grain size of the samples
It could be indicated by microstructural observations and Micro-Vickers test that the grain refinement has a direct effect on increasing of the hardness.
Conclusion
a b
Ultrasonic Transducer
Forming Tool
Blank-Holder
Matrix Steel Sheet
Two simple steel St14 samples were shaped using a hemispherical-head tool with and without applying ultrasonic vibrations to the tool respectively. The microstructural observation shows a reduction in the grain size of the sample shaped by ultrasonic vibration. Also, the hardness of the sample increased due to applying ultrasonic vibrations.
References
[1] A. Amanov et al., Wear 286-287(Supplement C) (2012) 136-144.
[2] E. Hall, Proceedings of the Physical Society. Section B 64(9) (1951) 747.
[3] A. Karimi, S. Amini, The International Journal of Advanced Manufacturing Technology 83(5) (2016) 1127-1134.
[4] Y. Liu et al., Ultrasonics 53(3) (2013) 803-807.
[5] F. Djavanroodi et al., Ultrasonics 53(6) (2013) 1089-1096.
[6] Y. Ashida, H. Aoyama, Journal of Materials Processing Technology 187 (2007) 118-122.
