In the paper, results of impact bending tests of a high-manganese steel of Fe – 30 wt.%, Mn – 9 wt.%, Al – 0,65 wt.%, C grade are presented. The tests were carried out using a flywheel machine, suitable for dynamic stretching and impact bending tests in the range of linear velocity of the forcing element from 5 ÷ 40 m/s. The obtained test results were compared with the results of impact resistance of the studied steel determined using Charpy machine. Structural investigations were carried out using scanning transmission electron microscopy. Surfaces of fractures formed in the break point during bending tests were analyzed, and they indicate a presence of mixed transcrystalline fractures with a predominance of plastic fractures

A. Śmiglewicz

M. B. Jabłońska

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The effect of strain rate on the impact strength of the high-Mn steel

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631METALURGIJA 54 (2015) 4, 631-634A. ŚMIGLEWICZ, M.B. JABŁOŃSKATHE EFFECT OF STRAIN RATE ON THE IMPACT STRENGTH OF THE HIGH-Mn STEELReceived – Primljeno: 2014-08-26Accepted – Prihvaćeno: 2015-02-11Original Scientific Paper – Izvorni znanstveni radISSN 0543-5846METABK 54(4) 631-634 (2015)UDC – UDK 669.74.15:539.55.375:531.76=111A. Śmiglewicz, M.B. Jabłońska, Silesian University of Technology, Faculty of Materials Science and Metallurgy, Katowice, PolandIn the paper, results of impact bending tests of a high-manganese steel of Fe – 30 wt.%, Mn – 9 wt.%, Al – 0,65 wt.%, C grade are presented. The tests were carried out using a flywheel machine, suitable for dynamic stretching and impact bending tests in the range of linear velocity of the forcing element from 5 ÷ 40 m/s. The obtained test results were compared with the results of impact resistance of the studied steel determined using Charpy machine. Struc-tural investigations were carried out using scanning transmission electron microscopy. Surfaces of fractures formed in the break point during bending tests were analyzed, and they indicate a presence of mixed transcrystalline frac-tures with a predominance of plastic fractures.Key words: high manganese steels, Charpy impact test, impact bending, high deformation rateINTRODUCTIONImpact loads of steel, or as a matter of fact, the mode of changes in structure and mechanical properties under the influence of high deformation rates, is the subject of studies in many centers working on strength of structures and material studies worldwide. High and very high de-formation rates lead to changes in the structure and me-chanical properties of materials and especially steels by increasing, among others, the value of yield point. A plas-tic deformation of materials caused by dynamic loads dif-fers significantly from deformations under static and quasi-static conditions. A dynamic deformation is, most of all, an effect of wave phenomena occurring in the ma-terial being deformed. Among important factors affecting the plastic properties of most metallic materials, defor-mation rate may be named [1-8].Deformation rate is the parameter of the plastic working process which may vary in a very wide range, while the other parameters are constant or negligibly low. Studies of materials may be carried out in a very broad range of deformation rates. That is why it is im-possible to use a measurement apparatus of a single type. At present, the most frequently used dynamic methods include: tests using a rotary hammer, tests us-ing a split Hopkinson pressure bar, Taylor impact test, plate impact test and expanding ring test [5, 7, 9, 10].In the literature pertaining to high-manganese steels, there are relatively few reports on tests of steel proper-ties using high deformation rates as well as microstruc-ture analysis. The most publications discuss the analysis of behavior of high-manganese steels subjected to tests with the deformation rates ranging from 102 to 104 s–1, which allows for carrying out the impact compression, tension and torsion tests [10-15]. Many authors [2, 5, 6, 11, 13] analyzed properties of high-manganese steels after tension tests using high deformation rates, also us-ing the split Hopkinson pressure bar and flywheel ma-chine in the range of dynamic rates 102÷103 s–1. Every measurement technique provides detailed information only in a specific rate range. Costs, significantly higher than in case of standard tests, are the major problem in dynamic tests.TEST MATERIALS AND METHODSA high-manganese steel Fe – 30 wt.%, Mn – 9 wt.%, Al – 0,65 wt.%, C grade was the test material. The steel was prepared in Institute for Ferrous Metallurgy, Gli-wice, Poland, in a vacuum induction furnace from Balz-ers. The casting process was carried out under an argon atmosphere, via a hot tundish, to a water-cooled copper concast mould properly prepared earlier. The concast mould feedstock was rolled into square section with a dimension of 45 mm. The so-prepared material was used for preparation of samples for the impact tests. The test samples with V-notch was made according to PN-EN 10045-1 standard. The impact tests were carried out using a Charpy machine and a flywheel machine of RSO type. The flywheel machine owned by Silesian University of Technology allows for deforming, stretch-ing and bending samples with a linear impact rate in the range of 5 to 40 m/s, corresponding to deformation rates in the range of 102 to 104 s–1. The impact bending tests using a flywheel machine was carried out with various deformation rates in the range of linear velocity of the working element of 7.5 m/s, 15 m/s, and 30 m/s. During impact bending, the sample is fractured on an anvil us-ing a ram (claw). The interchangeable claw is installed on a flywheel with a very high moment of inertia [3]. 632A. ŚMIGLEWICZ et al.: THE EFFECT OF STRAIN RATE ON THE IMPACT STRENGTH OF THE HIGH-Mn STEEL METALURGIJA 54 (2015) 4, 631-634Fractographic studies of the fractures formed after the Charpy impact test and the impact bending test were carried out by scanning electron microscopy on a Hi-tachi S-4200 microscope.RESULTS AND DISCUSSIONThe investigations were carried out for high-manga-nese steels with an austenitic-ferritic structure. The Charpy impact test was carried out using a hammer with an initial energy of 150 J, which turned out to be too low to break the sample. For next measurements, a hammer with an initial energy of 300 J was used. Doubled ener-gy allowed for breaking next samples. Impact resist-ance by Charpy method was determined using the for-mula (1):  (1)where: KV – impact work used for breaking the sample / J; S – surface area of the initial cross-section of the sample / cm2.After Charpy impact tests, fractographic studies of the obtained fractures were carried out – Figure 1a,bThe dynamic bending test was carried out at the ram linear velocity of 7,5 m/s, 15 m/s, and 30 m/s. Based on the data recorded by the Next View 4,2 software, the following quantities were obtained: duration, force, ve-locity, which were used for calculations of work and impact resistance. The bending force vs. the test dura-tion for each of the deformation rates studied is plotted in Figure 2.The result presented in the form of a combined graph of bending force vs. time was used for calculation of work needed to break the sample and impact resistance.Based on the path travelled and the bending force, corresponding graphs for the individual velocities were prepared, with the surface area below the curve consti-tuted the work needed to break the sample. An exem-plary graph is shown in Figure 3.The surface area below the curve was divided into a finite amount of rectangles with the sum of surface ar-eas constituting the whole surface area below the curve, and thus the work LU needed to break the sample. The impact resistance in the impact bending test was deter-mined based on the dependence (2):  (2)where: LU – deformation work at the break / J, S – sur-face area of the initial cross-section of the sample / cm2.Comparative results from the test carried out are gathered in Table 1. The graph of bending force vs. time shows a characteristics of the course of the dynamic bending test. It was ascertained that the higher deforma-tion rate, the higher the stress required to break the sam-ple, and thus the higher deformation rate, the shorter the time to break.Table 1  Results of impact resistance tests of Fe – 30 wt.%, Mn – 9 wt.%, Al – 0,65 wt.%, C steel after the impact bending testlinear velocityv = 7,5 m/s v = 15 m/s v = 30 m/sFmax / kN 35 35 36 41 41 42 53 54 55LU / J 163 164 166 173 175 177 192 193 195S / cm2 0,8 0,8 0,8 0,8 0,8 0,8 0,8 0,8 0,8U / J/cm2 207 208 201 219 220 223 240 242 244Comparative results from the test carried out are gathered in Table 2.Table 2  Results of impact resistance tests of the studied Fe – 30 wt.%, Mn – 9 wt.%, Al – 0,65 wt.%, C steelImpact resistance / J/cmCharpy impact testImpact bending tests using a fl ywheel machine7,5 / m/s 15 / m/s 30 / m/s200 207 222 242The lowest impact resistance characterizes the mate-rial after the Charpy impact test. While comparing the initial Charpy impact test 200 / J/cm2 to the dynamic Figure 2  Graph of bending force vs. time for velocities: v = 7,5 m/s; v = 15 m/s; v = 30m/sFigure 3  Graph of bending force vs. path for velocity v = 7,5 m/s a) b)Figure 1  Fractures of the tested steel after the Charpy impact test, SEM images; a) mat fracture typical for plastic type; b) surface of transcrystalline ductile fractures633A. ŚMIGLEWICZ et al.: THE EFFECT OF STRAIN RATE ON THE IMPACT STRENGTH OF THE HIGH-Mn STEELMETALURGIJA 54 (2015) 4, 631-634CONCLUSIONSThe studied carried out proved an influence of the deformation rates on the ability to transfer dynamic loads of the studied Fe – 30 wt.%, Mn – 9 wt.%, Al – 0,65 wt.%, C steel, confirming an increase in the impact resistance under the influence of increasing deforma-tion rate. For instance, in case of a velocity of 7,5 m/s, the impact resistance amounted to 209,5 / J/cm2, and for 30 m/s it increased by 15,5 %. The impact resistance determined in the paper expresses the amount of energy per a unit surface area, absorbed by the material at breaking in the result of an high-velocity impact.The results of studies of the macrostructure of frac-tures formed in the result of the tests carried out, showed a plastic form of the fractures. Structural analysis re-vealed void characteristic for plastic transcrystalline breaking on the observed surfaces of the studied steel. Fractographic studies showed a presence of mixed frac-tures with plastic fractures with crater structure pre-vailed.AcknowledgmentsWork conducted within the Research and Develop-ment Project of National Centre for Research and de-velopment No. 15 0012 06 as well as BK-220/RM3/2015REFERENCES[1] J. Malinowski, Z. Kowalewski, L. Kruszka, The experi-mental method and the study of plastic flow of metals in the range of very high strain rate, Institute of Fundamental Technological Research Polish Academy of Sciences, Warsaw 10 (2007).[2] W. Bleck P. Larour, A. Bäumer, High Strain Rate Tensile Testing Of Modern Car Body Steels. Materials Forum 29 (2005) 21-28.[3] Research Project KBN Nr N507 069 32/1932. Structural and mechanical aspects of the dynamic deformation of me-tallic materials for the automotive industry, Silesian Uni-versity of Technology, Katowice (2005).[4] A. Niechajowicz, A. Tobota, Application of flywheel ma-chine for sheet metal dynamic tensile tests, Archives of Civil and Mechanical Engineering 8 (2008) 2, 129-137.[5] M.B. Jabłońska, Mechanical properties and fractographic analysis of high manganese steels after dynamic deforma-tion tests, Arch. of Metal. And Mater. 59 (2014) 3, 1193-1197.[6] S.T Chiou, C. Chengb, W.S Lee, Strain rate effects on the mechanical properties of a Fe–Mn–Al alloy under dyna-mic impact deformations Materials Science and Enginee-ring A, 392 (2005), 156-162.[7] J. Kansy, A. Hanc, M. Jablonska, E. Bernstock-Kopac zyń-ska, D. Giebel, Point Defect Study in Fe Alloy as a Fun-ction of Thermal Treatment by Positron Lifetime Spectro-scopy, Progress in Positron Annihilation (2011) 666, 50-53. [8] C. Froustey, M. Lambert, J.L. Charles, J.L. Lataillade, De-sign of an Impact Loading Machine Based on a Flywheel Device: Application to the Fatigue Resistance of the High Rate Pre-straining Sensitivity of Aluminium Alloys, Expe-rimental Mechanics (2007) 47, 709-721bending test at the velocity of 7,5 m/s, the impact resist-ance of the material increases by 5,3 %. While increas-ing the deformation rate to 15 m/s and to 30 m/s, the impact resistance increases in relation to the initial test by 11,6 % and 21,6 %, respectively. In the dynamic bending test, the work needed to break the sample at the lowest velocity of 7,5 m/s amounted to 165 / J. When the velocity was increased to 15 m/s and then to 30 m/s, the work needed to break the sample increased by 6 %, and later it increased almost thrice, amounting to 17 %.After the Charpy impact test and the impact bend-ing, fractographic studies of the obtained fractures were carried out Figure 1, Figures 4  6.Irrespective of the deformation rate used, the studied steel was characterized by a mixed fracture in the break point. Mostly areas of characteristic plastic fracture oc-curred, and also areas of were observed. The fracture was characterized by a fibrous structure, presence of interconnected microvoids, as well as mat surface, typi-cal for fractures of plastic type. a) b)Figure 4  Fractures of the tested steel after the impact bending with a velocity of 7.5 m/s, SEM images; a) mat surface with several microcracks; b) surface of transcrystalline ductile fractures with microvoids a) b)Figure 5  Fractures of the tested steel after the impact bending with a velocity of 15 m/s, SEM images a) surface with microcracks; b) surface of transcrystalline ductile fractures with presence a cleavage planes a) b)Figure 6  Fractures of the tested steel after the impact bending with a velocity of 30 m/s, SEM images; a) well mat surface with several microcracks; b) surface of transcrystalline ductile fractures634A. ŚMIGLEWICZ et al.: THE EFFECT OF STRAIN RATE ON THE IMPACT STRENGTH OF THE HIGH-Mn STEEL METALURGIJA 54 (2015) 4, 631-634[9] J. Janiszewski, J. Gacek, Studies of dynamic mechanical properties of steels used for bodies, Institute of Ferrous Metalurgy (2010), 118-123.[10] G. Frommeyer, O. Grassel, L. Kruger, L.W. Meyer, High strength Fe-Mn-(Al, Si) TRIP/ TWIP steel development – properties – application, International Journal of Plasticity, 16 (2000), 1391-1409.[11] G. Frommeyer, U. Brux, P. Neumann, Supra-Ductile and High-Strength Manganese-TRIP/TWIP Steels for High Energy Absorption Purposes. ISIJ International 43 (2003), 438.[12] M. Jabłońska, A. Śmiglewicz, Analysis of substructure of high-Mn steels in the context of dominant stress mechani-sm, Defect and Diffusion Forum 334-335 (2013), 177-181[13] D. Li, Y. Wei, C. Lid, L. Hod, D. Lid, X. In, Effects of High Strain Rate on Properties and Microstructure Evolution of TWIP Steel Subjected to Impact Loading, Journal of Iron and Steel Research, International 17 (2010), 67-73.[14] Z. Gronostajski, S. Polak, Quasi-static and dynamic defor-mation of double-hat thin-walled element of vehicle con-trolled body crushing zones joined by clinching, Archives of Civil and Mechanical Engineering 2 (2008), 57-66.[15] K. T. Park, S. W. Hwang, J. H. Ji, C. S. Lee, Static and Dynamic Deformation of Fully Austenitic High Mn Steels, Procedia Engineering 10 (2011), 1002–1006.Note:  The responsible translator for English language is Dr. Janusz Mrzigod, Katowice, Poland

The effect of strain rate on the impact strength of the high-Mn steel

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