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Microstructural aspects of energy absorption of high manganese steels
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Procedia Manufacturing 27 (2019) 91–97 Procedia Manufacturing 00 (2017) 000–000 www.elsevier.com/locate/procedia
ICAFT/SFU/AutoMetForm ICAFT/SFU/AutoMetForm 2018 2018
Microstructural Microstructural aspects aspects of of energy energy absorption absorption of high manganese steels Manufacturing Engineering Society International Conference 2017, of high manganese steels MESIC 2017, 28-30 June 2017, Vigo (Pontevedra), Spain
* M. M. B. B. Jabłońska Jabłońska*,, K. K. Kowalczyk Kowalczyk
Costing models Silesian for capacity optimization in Industry 4.0: Trade-off University of Technology, Krasinskiego 8, Katowice 40-019, Poland Silesian University of Technology, Krasinskiego 8, Katowice 40-019, Poland between used capacity and operational efficiency Abstract Abstract
A. Santanaa, P. Afonsoa,*, A. Zaninb, R. Wernkeb
The study presents the results of investigation on two grades of fully austenitic high manganese steels subjected to dynamic The study presents the results of investigation on oftwo grades of fully austenitic high manganese steels subjected to dynamic a University Minho, 4800-058 Guimarães, Portugal compression with usage of the Split Hopkinson Pressure Bar (SHPB). The SHPB test takes only few milliseconds to complete, b compression with usage of the Split Hopkinson Pressure89809-000 Bar (SHPB). TheSC, SHPB test takes only few milliseconds to complete, Unochapecó, Chapecó, Brazil and during this time it is impossible to transfer the excess heat out of the specimen, therefore the test has to be carried out in and during this time it is impossible to transfer the excess heat out of the specimen, therefore the test has to be carried out in adiabatic conditions, and the increase of the temperature caused by the work of plastic deformation must be calculated. adiabatic conditions, and the increase of the temperature caused by the work of plastic deformation must be calculated. Compression behaviour of high Mn-Al-C steel in the solid solution state was correlated to the microstructures developed during Compression behaviour of high Mn-Al-C steel in the solid solution state was correlated to the microstructures developed during plastic deformation in order to clarify the dominant deformation mechanisms. The stacking fault energy of tested steels was Abstract plastic deformation in order to clarify the dominant deformation mechanisms. The stacking fault energy of tested steels was between 25 – 50 mJ/m2. The compression behaviour of the steels was manifested by an excellent combination of strength and between 25 – 50 mJ/m2. The compression behaviour of the steels was manifested by an excellent combination of strength and ductility as well as with continuous strain hardening to the high strain rate. There has been performed a comparison of results of ductilitythe as well as with strain to theprocesses high strainwill rate. be There has been a comparison of results of Under concept ofcontinuous "Industry 4.0",hardening production pushed to performed beofincreasingly specific energy absorption during dynamic deformation in the same conditions on two grades MnAl steels. interconnected, The consequence specific energy absorption during dynamic deformation in the much same conditions on twoIngrades of MnAl capacity steels. Theoptimization consequence information based on a real time basis and, necessarily, more efficient. this context, of continuous work hardening, while perfectly combining strength and ductility, is a great ability to absorb energy of plastic of continuous work hardening, while perfectlymaximization, combining strength and ductility, is aorganization’s great ability to profitability absorb energy of value. plastic goes beyond the traditional aim of capacity contributing also for and deformation. deformation. Indeed, lean management and continuous improvement approaches suggest capacity optimization instead of
maximization. The study of capacity optimization and costing models is an important research topic that deserves © 2018 The Authors. Published by Elsevier B.V. 2019 The © 2018 Authors. Published by Elsevier B.V. B.V. This is an openfrom accessboth article the CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) license (https://creativecommons.org/licenses/by-nc-nd/4.0/) contributions theunder practical and theoretical perspectives. This paper presents and discusses a mathematical This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) under responsibility of the scientific committee of ICAFT/SFU/AutoMetForm 2018. model has been Selection and peer-review model for capacity management based on different costing models and TDABC). A generic Selection and peer-review under responsibility of the scientific committee of(ABC ICAFT/SFU/AutoMetForm 2018. developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s Keywords: high strength steels, deformation, microstructure value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity Keywords: high strength steels, deformation, microstructure optimization might hide operational inefficiency. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017. Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency
1. Introduction * Corresponding author. Tel.: +48 32 603 43 50; fax: +48 32 603 44 00 * The Corresponding author. Tel.: +48 603 43 50; fax: information +48 32 603 44 00 cost of idle capacity is 32 a fundamental for companies and their management of extreme importance E-mail address: [email protected] E-mail address: [email protected]
in modern production systems. In general, it is defined as unused capacity or production potential and can be measured 2351-9789 2018 The Authors. Published by Elsevier B.V.hours of manufacturing, etc. The management of the idle capacity in several©ways: tons of production, available 2351-9789 © 2018 The Authors. Published by Elsevier B.V. This is anAfonso. open access under the761; CC fax: BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) * Paulo Tel.:article +351 253 510 +351 253license 604 741 This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection peer-review under responsibility of the scientific committee of ICAFT/SFU/AutoMetForm 2018. E-mailand address: [email protected] Selection and peer-review under responsibility of the scientific committee of ICAFT/SFU/AutoMetForm 2018.
2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under of the scientificbycommittee the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2019responsibility The Authors. Published Elsevier of B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of ICAFT/SFU/AutoMetForm 2018. 10.1016/j.promfg.2018.12.049
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M.B. Jabłońska et al. / Procedia Manufacturing 27 (2019) 91–97 Author name / Procedia Manufacturing 00 (2018) 000–000
1. Introduction A profitable combination of high strength and good plastic properties, as well as good ability to absorb deformation energy, suggests that the possibilities to use high Mn steels will be increasingly more common. The use of specific mechanism of plastic deformation, namely mechanical twinning, may lead to an important change in possibilities of designing and production of structural elements that play a key role in protection in the case of collision: in means of transport, in protective structures – during penetration with high speeds in military or aerospace industry [1-3]. Currently, changes in approach to the design of modern steels are observed. These changes include the FeMnAlC steels out of the group of steels hardened in the result of structural effects induced by plastic deformation. Contents the main alloying elements, i.e. manganese, silicon and aluminum influence the stacking fault energy (SFE) of the steel, and thus are decisive in the definition of a prevailing deformation mechanism [3-5]. It is known that SFE control has turned out to be a process for purposeful activation of the preferred deformation mechanism [1,5,6]. Particularly selected groups of high Mn steels exhibit characteristic effects during deformation, i.e. the ones caused by hardening by mechanical twinning of the austenite – the so-called TWIP effect (twinning induced plasticity) – and hardening by formation of shear micro-bands in the austenite – the so-called MBIP effect (micro-bands inducted plasticity) [5,6]. The principal decisive factor for both possibility and propensity for deformation hardening of manganese steels is deformation rate, since these materials belong to the group of alloys being particularly sensitive to this parameter [7-9]. Deformation rate is one of parameters which may vary in a broad range, while the other parameters are constant or negligibly low. In this context, studies on materials may be carried out using a broad spectrum of deformation rates starting from the range of very low rates, up to the range of deformation rates higher than 104 s-1. The latter values correspond to tests with usage of very high deformation rates obtained by generating plane waves. It is extremely important to determine methods for deformation enabling realization of the deformation in a broad range of rates, from static conditions to very high strain rates [9-17]. Characteristically, steels from the FeMnAlC group, especially TWIP steels, are characterized by a strong relationship between properties and the rate of deformation [8,14,15,18,19]. They demonstrate the ability to strengthen as the deformation rate increases without losing good plastic properties [16]. The existing works in this area concern mainly on the behavior of Mn-Al steel in the range of deformation rates from 102 to 104 s-1with usage of the Split Hopkinson Pressure Bar method. It has been shown that when the rate of deformation increases from value 10-4 do 103 in TWIP steel the yield strength increases from about 300 MPa to 600 MPa, and a tensile strength increases from 500 MPa to 950 MPa. As the strength properties increase, good plasticity can be observed and is measured by the deformation value , which reaches a value above 0.4. The aim of this paper was to describe microstructural aspects of energy absorption of fully austenitic high manganese steels. The knowledge of such phenomena would be useful e.g. in optimization of the application process of this material and the effective strengthening the role of these materials in transport implementations. 2. Material and experiment The original objectives were to calculate the specific energy absorption and study the microstructure of two grades of FeMnAlC steels. Steels with the average chemical composition X29 – Fe – 0,3 wt.% C – 26 wt.% Mn – 5 wt.% Al – 3 wt.% Si and X55 – Fe – 0,55 wt.% C – 25 wt.% Mn – 5 wt.% Al – 0,6 wt.% Si served as experimental material. Both steels have a fully austenitic structure after the supersaturation thermal treatment. The samples used in all of the performed tests were machined from round bars with machining. The cylindrical samples with diameter of 5 mm and height of 5 mm were subjected to dynamic deformation with the Split Hopkinson Bar. Surfaces were lubricated using MoS2 in order to reduce friction between the anvil and the specimen. The strain rate value in the compression test was equal to 1.5 x103, 2.5 x103 and 4.0 x103 s-1. Based on the waveforms recorded by a digital oscilloscope for transmitted εT (t) and reflected εR (t) waves and knowing cross-sectional area of the bars A and the specimen AS, and the speed of the elastic wave propagation in the material of the bars C0, and the specimen length L, it is possible to determine stress σ (t), strain ε (t) and strain rate in the specimen [16]. For the given parameters of the compression tests, the value of the SEA index was calculated, i.e. the plastic deformation work LU relative to the sample volume [16]. The calculations have been corrected for the value of elastic deformation work. Scanning electron microscopy was used for analysis of the steel microstructure after dynamic deformation tests.
M.B. Jabłońska et al. / Procedia Manufacturing 27 (2019) 91–97 Author name / Procedia Manufacturing 00 (2018) 000–000
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3. Results 3.1 Influence of strain rate on the properties The stress-strain characteristics of the X55 and X26 steel were obtained after the cold compression process with different deformation velocity. On this basis, a list of properties has been developed (Table 1). Offset yield strength of X29 steel determined at strain rate equal to at 1.5x103s-1 is equal to 663 MPa, whereas in the case of X55 steel its value increases up to 700 MPa. Both steel grades reveal similar positive strain rate sensitivity effect. However, there may be found some differences. Table 1. Selected properties of X29 and X55 steels after SHPB dynamic plastic deformation. Steel grade X29 X55 X29 X55 X29 X55
Strain rate s-1 1.5x103 2.5 x103 4.0 x103
Yield stress, MPa 663 700 720 715 790 775
p, MPa ( = 0,35) 990 1010 1170 1220 1210 1260
The energy absorbed during dynamic compression test was measured due to possible application of tested steels for manufacturing of parts that absorb the energy of plastic deformation in automotive industry. The value of the plastic deformation work relative to the volume of the sample was defined as the specific energy absorption value SEA. The SEA was calculated using the formula given in work [16]. The results of the SEA calculation are shown in Fig. 1. It can be noted that for given deformation parameters the SEA value for steel X55 is higher than for steel X29. X55 steel shows a faster increase of p value under dynamic deformation conditions than the X29 steel. In the area of dynamic behavior X29 steel shows more typical behavior of TWIP steel [6], whereas X55 properties are closer to mechanical response of structural metallic alloys.
Fig. 1. Value of specific energy absorption (SEA) calculated at dynamic deformation conditions in a compression test for X29 and X55 steels.
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3.2 Influence of strain rate on the microstructure Considering the properties of materials under dynamic compression conditions, it is necessary to analyse the structural changes that accompany this process. The dynamic deformation affected plays a significant role in this case. During compression at the speed of 1.5x103 s-1 structure effects related to glide activity in two systems were registered for both steels. (Fig. 2 a-d). The structure has been strongly defected. On the background of emerging cellular dislocation structure there have been disclosed systems of narrow bands of deformation twins. In X55 steel two individual shear bands are visible (Fig. 2d). At deformation with strain rate of 2.5x103 s-1 in the X29steel together with twins creation in two twinning systems the fine cellular dislocation structure (Fig. 3 a, b) has been observed. In the X55 steel the situation is similar, however, next to the structure containing mechanical twins a dislocation structure with not well-formed dislocation cells is visible. It was confirmed by higher values of p in X55 in comparison to the X29 steel.
Fig. 2. Microstructure of X29 a), b) and X55 c), d) after SHPB dynamic deformation at 1.5x103 s-1, STEM.
Deformation with the highest strain rate leads to the activation of strong mechanisms of the evolution of the dislocation structure of both steels. Mechanical twins still dominate (Fig. 4 a, c). However, changes have occurred in the matrix. Non-crystallographic shear bands are formed demonstrating high local deformation and high heterogeneity of plastic deformation (Fig. 4 b, d). The presence of large shear bands indicates greater involvement of the slip mechanism in the deformation process than before. In the dynamic deformation conditions in FeMnAlC steels structure evolution during imposed strain is mainly relaxed by means of dislocation cell formation and non-crystallographic micro-bands are often formed at high strains. Cells with low dislocation density surrounded by cell walls with high density of dislocations under a high strain rate, the dislocations cross-slipped mutually and dislocation cells formed are shown in Fig. 7(b–d). The high entanglement of the dislocations impeded the motion of other dislocations, and lowered the density of the movable dislocations per volume unit. All these effects are certainly of great importance in influencing the properties of FeMnAlC steel, and it especially pertains to those with high ability to absorb strain energy.
M.B. Jabłońska et al. / Procedia Manufacturing 27 (2019) 91–97 Author name / Procedia Manufacturing 00 (2018) 000–000
Fig. 3. Microstructure of X29 a), b) and X55 c), d) after SHPB dynamic deformation at 2.5x103 s-1, STEM.
Fig. 4. Microstructure of X29 a), b) and X55 c), d) after SHPB dynamic deformation at 4.0x103 s-1, STEM.
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4. Conclusion 1.
The tested fully austenitic steels exhibited an excellent combination of strength and high strain value over 0.35 during dynamic SHPB deformation at room temperature. The strain hardening rate of the steel during compression deformation continuously increased with increasing strain before yielding plastic instability. The steels have exhibited high specific energy absorption value SEA measured at dynamic deformation conditions. The SEA value for the X29 steel at strain rate 4.0 x103 for = 0.35 is 0.3 J/mm3, while SEA for the X55 steel at the same conditions is 0.38 J/mm3. The mainly deformation mechanism in both steels during dynamic compression process is mechanical twinning. However, it also plays a significant role dislocation glide, which occurs at given deformation parameters through the evolution of the dislocation structure. This takes place as a result of the cellular structure forming and as a result of creation of micro-bands as well as non-crystallographic micro-bands. Continuous strain hardening causes an excellent combination of strength and ductility and as a consequence also very high capacity to absorb the plastic deformation energy.
2. 3.
4.
Acknowledgements The research was supported by the BK221/RM0/2018
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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M.B. Jabłońska et al. / Procedia Manufacturing 27 (2019) 91–97 Author name / Procedia Manufacturing 00 (2018) 000–000
[18] A. Śmiglewicz, W. Moćko, K. Rodak, I. Bednarczyk, M. B. Jabłońska, Study of dislocation substructures in High-Mn steels after dynamic deformation tests, Acta Physica Polonica A, 130 (4), (2016), 942-945. [19] A. Grajcar, D. Woźniak, A. Kozłowska, Non-metallic inclusions and hot-working behaviour of advanced high-strength medium-Mn steels, Archives of Metallurgy And Materials, 61, (2016) 2, 811-820.
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Available Availableonline onlineatatwww.sciencedirect.com www.sciencedirect.com Procedia Manufacturing 00 (2018) 000–000 Procedia Manufacturing 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Manufacturing 27 (2019) 91–97 Procedia Manufacturing 00 (2017) 000–000 www.elsevier.com/locate/procedia
ICAFT/SFU/AutoMetForm ICAFT/SFU/AutoMetForm 2018 2018
Microstructural Microstructural aspects aspects of of energy energy absorption absorption of high manganese steels Manufacturing Engineering Society International Conference 2017, of high manganese steels MESIC 2017, 28-30 June 2017, Vigo (Pontevedra), Spain
* M. M. B. B. Jabłońska Jabłońska*,, K. K. Kowalczyk Kowalczyk
Costing models Silesian for capacity optimization in Industry 4.0: Trade-off University of Technology, Krasinskiego 8, Katowice 40-019, Poland Silesian University of Technology, Krasinskiego 8, Katowice 40-019, Poland between used capacity and operational efficiency Abstract Abstract
A. Santanaa, P. Afonsoa,*, A. Zaninb, R. Wernkeb
The study presents the results of investigation on two grades of fully austenitic high manganese steels subjected to dynamic The study presents the results of investigation on oftwo grades of fully austenitic high manganese steels subjected to dynamic a University Minho, 4800-058 Guimarães, Portugal compression with usage of the Split Hopkinson Pressure Bar (SHPB). The SHPB test takes only few milliseconds to complete, b compression with usage of the Split Hopkinson Pressure89809-000 Bar (SHPB). TheSC, SHPB test takes only few milliseconds to complete, Unochapecó, Chapecó, Brazil and during this time it is impossible to transfer the excess heat out of the specimen, therefore the test has to be carried out in and during this time it is impossible to transfer the excess heat out of the specimen, therefore the test has to be carried out in adiabatic conditions, and the increase of the temperature caused by the work of plastic deformation must be calculated. adiabatic conditions, and the increase of the temperature caused by the work of plastic deformation must be calculated. Compression behaviour of high Mn-Al-C steel in the solid solution state was correlated to the microstructures developed during Compression behaviour of high Mn-Al-C steel in the solid solution state was correlated to the microstructures developed during plastic deformation in order to clarify the dominant deformation mechanisms. The stacking fault energy of tested steels was Abstract plastic deformation in order to clarify the dominant deformation mechanisms. The stacking fault energy of tested steels was between 25 – 50 mJ/m2. The compression behaviour of the steels was manifested by an excellent combination of strength and between 25 – 50 mJ/m2. The compression behaviour of the steels was manifested by an excellent combination of strength and ductility as well as with continuous strain hardening to the high strain rate. There has been performed a comparison of results of ductilitythe as well as with strain to theprocesses high strainwill rate. be There has been a comparison of results of Under concept ofcontinuous "Industry 4.0",hardening production pushed to performed beofincreasingly specific energy absorption during dynamic deformation in the same conditions on two grades MnAl steels. interconnected, The consequence specific energy absorption during dynamic deformation in the much same conditions on twoIngrades of MnAl capacity steels. Theoptimization consequence information based on a real time basis and, necessarily, more efficient. this context, of continuous work hardening, while perfectly combining strength and ductility, is a great ability to absorb energy of plastic of continuous work hardening, while perfectlymaximization, combining strength and ductility, is aorganization’s great ability to profitability absorb energy of value. plastic goes beyond the traditional aim of capacity contributing also for and deformation. deformation. Indeed, lean management and continuous improvement approaches suggest capacity optimization instead of
maximization. The study of capacity optimization and costing models is an important research topic that deserves © 2018 The Authors. Published by Elsevier B.V. 2019 The © 2018 Authors. Published by Elsevier B.V. B.V. This is an openfrom accessboth article the CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) license (https://creativecommons.org/licenses/by-nc-nd/4.0/) contributions theunder practical and theoretical perspectives. This paper presents and discusses a mathematical This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) under responsibility of the scientific committee of ICAFT/SFU/AutoMetForm 2018. model has been Selection and peer-review model for capacity management based on different costing models and TDABC). A generic Selection and peer-review under responsibility of the scientific committee of(ABC ICAFT/SFU/AutoMetForm 2018. developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s Keywords: high strength steels, deformation, microstructure value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity Keywords: high strength steels, deformation, microstructure optimization might hide operational inefficiency. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017. Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency
1. Introduction * Corresponding author. Tel.: +48 32 603 43 50; fax: +48 32 603 44 00 * The Corresponding author. Tel.: +48 603 43 50; fax: information +48 32 603 44 00 cost of idle capacity is 32 a fundamental for companies and their management of extreme importance E-mail address: [email protected] E-mail address: [email protected]
in modern production systems. In general, it is defined as unused capacity or production potential and can be measured 2351-9789 2018 The Authors. Published by Elsevier B.V.hours of manufacturing, etc. The management of the idle capacity in several©ways: tons of production, available 2351-9789 © 2018 The Authors. Published by Elsevier B.V. This is anAfonso. open access under the761; CC fax: BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) * Paulo Tel.:article +351 253 510 +351 253license 604 741 This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection peer-review under responsibility of the scientific committee of ICAFT/SFU/AutoMetForm 2018. E-mailand address: [email protected] Selection and peer-review under responsibility of the scientific committee of ICAFT/SFU/AutoMetForm 2018.
2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under of the scientificbycommittee the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2019responsibility The Authors. Published Elsevier of B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of ICAFT/SFU/AutoMetForm 2018. 10.1016/j.promfg.2018.12.049
92 2
M.B. Jabłońska et al. / Procedia Manufacturing 27 (2019) 91–97 Author name / Procedia Manufacturing 00 (2018) 000–000
1. Introduction A profitable combination of high strength and good plastic properties, as well as good ability to absorb deformation energy, suggests that the possibilities to use high Mn steels will be increasingly more common. The use of specific mechanism of plastic deformation, namely mechanical twinning, may lead to an important change in possibilities of designing and production of structural elements that play a key role in protection in the case of collision: in means of transport, in protective structures – during penetration with high speeds in military or aerospace industry [1-3]. Currently, changes in approach to the design of modern steels are observed. These changes include the FeMnAlC steels out of the group of steels hardened in the result of structural effects induced by plastic deformation. Contents the main alloying elements, i.e. manganese, silicon and aluminum influence the stacking fault energy (SFE) of the steel, and thus are decisive in the definition of a prevailing deformation mechanism [3-5]. It is known that SFE control has turned out to be a process for purposeful activation of the preferred deformation mechanism [1,5,6]. Particularly selected groups of high Mn steels exhibit characteristic effects during deformation, i.e. the ones caused by hardening by mechanical twinning of the austenite – the so-called TWIP effect (twinning induced plasticity) – and hardening by formation of shear micro-bands in the austenite – the so-called MBIP effect (micro-bands inducted plasticity) [5,6]. The principal decisive factor for both possibility and propensity for deformation hardening of manganese steels is deformation rate, since these materials belong to the group of alloys being particularly sensitive to this parameter [7-9]. Deformation rate is one of parameters which may vary in a broad range, while the other parameters are constant or negligibly low. In this context, studies on materials may be carried out using a broad spectrum of deformation rates starting from the range of very low rates, up to the range of deformation rates higher than 104 s-1. The latter values correspond to tests with usage of very high deformation rates obtained by generating plane waves. It is extremely important to determine methods for deformation enabling realization of the deformation in a broad range of rates, from static conditions to very high strain rates [9-17]. Characteristically, steels from the FeMnAlC group, especially TWIP steels, are characterized by a strong relationship between properties and the rate of deformation [8,14,15,18,19]. They demonstrate the ability to strengthen as the deformation rate increases without losing good plastic properties [16]. The existing works in this area concern mainly on the behavior of Mn-Al steel in the range of deformation rates from 102 to 104 s-1with usage of the Split Hopkinson Pressure Bar method. It has been shown that when the rate of deformation increases from value 10-4 do 103 in TWIP steel the yield strength increases from about 300 MPa to 600 MPa, and a tensile strength increases from 500 MPa to 950 MPa. As the strength properties increase, good plasticity can be observed and is measured by the deformation value , which reaches a value above 0.4. The aim of this paper was to describe microstructural aspects of energy absorption of fully austenitic high manganese steels. The knowledge of such phenomena would be useful e.g. in optimization of the application process of this material and the effective strengthening the role of these materials in transport implementations. 2. Material and experiment The original objectives were to calculate the specific energy absorption and study the microstructure of two grades of FeMnAlC steels. Steels with the average chemical composition X29 – Fe – 0,3 wt.% C – 26 wt.% Mn – 5 wt.% Al – 3 wt.% Si and X55 – Fe – 0,55 wt.% C – 25 wt.% Mn – 5 wt.% Al – 0,6 wt.% Si served as experimental material. Both steels have a fully austenitic structure after the supersaturation thermal treatment. The samples used in all of the performed tests were machined from round bars with machining. The cylindrical samples with diameter of 5 mm and height of 5 mm were subjected to dynamic deformation with the Split Hopkinson Bar. Surfaces were lubricated using MoS2 in order to reduce friction between the anvil and the specimen. The strain rate value in the compression test was equal to 1.5 x103, 2.5 x103 and 4.0 x103 s-1. Based on the waveforms recorded by a digital oscilloscope for transmitted εT (t) and reflected εR (t) waves and knowing cross-sectional area of the bars A and the specimen AS, and the speed of the elastic wave propagation in the material of the bars C0, and the specimen length L, it is possible to determine stress σ (t), strain ε (t) and strain rate in the specimen [16]. For the given parameters of the compression tests, the value of the SEA index was calculated, i.e. the plastic deformation work LU relative to the sample volume [16]. The calculations have been corrected for the value of elastic deformation work. Scanning electron microscopy was used for analysis of the steel microstructure after dynamic deformation tests.
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3. Results 3.1 Influence of strain rate on the properties The stress-strain characteristics of the X55 and X26 steel were obtained after the cold compression process with different deformation velocity. On this basis, a list of properties has been developed (Table 1). Offset yield strength of X29 steel determined at strain rate equal to at 1.5x103s-1 is equal to 663 MPa, whereas in the case of X55 steel its value increases up to 700 MPa. Both steel grades reveal similar positive strain rate sensitivity effect. However, there may be found some differences. Table 1. Selected properties of X29 and X55 steels after SHPB dynamic plastic deformation. Steel grade X29 X55 X29 X55 X29 X55
Strain rate s-1 1.5x103 2.5 x103 4.0 x103
Yield stress, MPa 663 700 720 715 790 775
p, MPa ( = 0,35) 990 1010 1170 1220 1210 1260
The energy absorbed during dynamic compression test was measured due to possible application of tested steels for manufacturing of parts that absorb the energy of plastic deformation in automotive industry. The value of the plastic deformation work relative to the volume of the sample was defined as the specific energy absorption value SEA. The SEA was calculated using the formula given in work [16]. The results of the SEA calculation are shown in Fig. 1. It can be noted that for given deformation parameters the SEA value for steel X55 is higher than for steel X29. X55 steel shows a faster increase of p value under dynamic deformation conditions than the X29 steel. In the area of dynamic behavior X29 steel shows more typical behavior of TWIP steel [6], whereas X55 properties are closer to mechanical response of structural metallic alloys.
Fig. 1. Value of specific energy absorption (SEA) calculated at dynamic deformation conditions in a compression test for X29 and X55 steels.
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M.B. Jabłońska et al. / Procedia Manufacturing 27 (2019) 91–97 Author name / Procedia Manufacturing 00 (2018) 000–000
3.2 Influence of strain rate on the microstructure Considering the properties of materials under dynamic compression conditions, it is necessary to analyse the structural changes that accompany this process. The dynamic deformation affected plays a significant role in this case. During compression at the speed of 1.5x103 s-1 structure effects related to glide activity in two systems were registered for both steels. (Fig. 2 a-d). The structure has been strongly defected. On the background of emerging cellular dislocation structure there have been disclosed systems of narrow bands of deformation twins. In X55 steel two individual shear bands are visible (Fig. 2d). At deformation with strain rate of 2.5x103 s-1 in the X29steel together with twins creation in two twinning systems the fine cellular dislocation structure (Fig. 3 a, b) has been observed. In the X55 steel the situation is similar, however, next to the structure containing mechanical twins a dislocation structure with not well-formed dislocation cells is visible. It was confirmed by higher values of p in X55 in comparison to the X29 steel.
Fig. 2. Microstructure of X29 a), b) and X55 c), d) after SHPB dynamic deformation at 1.5x103 s-1, STEM.
Deformation with the highest strain rate leads to the activation of strong mechanisms of the evolution of the dislocation structure of both steels. Mechanical twins still dominate (Fig. 4 a, c). However, changes have occurred in the matrix. Non-crystallographic shear bands are formed demonstrating high local deformation and high heterogeneity of plastic deformation (Fig. 4 b, d). The presence of large shear bands indicates greater involvement of the slip mechanism in the deformation process than before. In the dynamic deformation conditions in FeMnAlC steels structure evolution during imposed strain is mainly relaxed by means of dislocation cell formation and non-crystallographic micro-bands are often formed at high strains. Cells with low dislocation density surrounded by cell walls with high density of dislocations under a high strain rate, the dislocations cross-slipped mutually and dislocation cells formed are shown in Fig. 7(b–d). The high entanglement of the dislocations impeded the motion of other dislocations, and lowered the density of the movable dislocations per volume unit. All these effects are certainly of great importance in influencing the properties of FeMnAlC steel, and it especially pertains to those with high ability to absorb strain energy.
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Fig. 3. Microstructure of X29 a), b) and X55 c), d) after SHPB dynamic deformation at 2.5x103 s-1, STEM.
Fig. 4. Microstructure of X29 a), b) and X55 c), d) after SHPB dynamic deformation at 4.0x103 s-1, STEM.
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4. Conclusion 1.
The tested fully austenitic steels exhibited an excellent combination of strength and high strain value over 0.35 during dynamic SHPB deformation at room temperature. The strain hardening rate of the steel during compression deformation continuously increased with increasing strain before yielding plastic instability. The steels have exhibited high specific energy absorption value SEA measured at dynamic deformation conditions. The SEA value for the X29 steel at strain rate 4.0 x103 for = 0.35 is 0.3 J/mm3, while SEA for the X55 steel at the same conditions is 0.38 J/mm3. The mainly deformation mechanism in both steels during dynamic compression process is mechanical twinning. However, it also plays a significant role dislocation glide, which occurs at given deformation parameters through the evolution of the dislocation structure. This takes place as a result of the cellular structure forming and as a result of creation of micro-bands as well as non-crystallographic micro-bands. Continuous strain hardening causes an excellent combination of strength and ductility and as a consequence also very high capacity to absorb the plastic deformation energy.
2. 3.
4.
Acknowledgements The research was supported by the BK221/RM0/2018
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