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Influence Of Temperatures And Strain Rates On Tensile Deformation Behaviour Of DP 590 Steel
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ScienceDirect Materials Today: Proceedings 18 (2019) 2603–2610
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ICMPC-2019
Influence Of Temperatures And Strain Rates On Tensile Deformation Behaviour Of DP 590 Steel Sandeep Pandrea*, Prathamesh Takalkara, Nitin Kotkundea, Swadesh Kumar Singhb, Ahsan Ul Haqc a Department of Mechanical Engineering, BITS Pilani Hyderabad Campus, Jawahar Nagar,Hyderabad-500078, Telangana, India Department of Mechanical Engineering, Gokaraju Rangaraju Institute of Engineering and Technology, Hyderabad-500090, Telangana, India c Department of Mechanical Engineering VNR Vignana Jyothi Institute of Engineering and Technology, Pragathi Nagar, Nizampet, Hyderabad500090, Telangana, India b
Abstract In this study, tensile deformation behaviour and material properties has been investigated at different temperatures and strain rates. The isothermal uniaxial tensile tests were conducted from room temperature to 7000C at an interval of 1000C at different quasi-static strain rates (0.01, 0.001, 0.0001 s-1). The flow stress is significantly influenced by temperature and strain rate change. Various important material properties namely: yield strength (σy), ultimate tensile strength (σut), % elongation and strain hardening exponent (n) are determined at different temperatures and strain rates. In order to understand the fracture behaviour of a DP steel, microstructural characteristics were studied using Scanning Electron Microscope (SEM). Predominately ductile failure has been reported at higher temperatures. The individual phases (ferrite + martensite) play an important role in defining the ultimate failure mode in tensile testing. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: DP590 steel, tensile deformation behaviour, strain hardening, fracture morphology.
1. Introduction Nowadays, high strength and low weight advanced structural steels are gaining a special attention in various performance critical applications such as automotive and aerospace sectors[1]. This lead to the extensive development of Advanced High Strength Steels (AHSS) [2]. In AHSS steels, DP-590/600 grade is extensively used for automotive applications. * Corresponding author. Tel.: Tel.: +91 8340055252; Fax: +91 40 66303998. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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The microstructure of DP steel consists of two phases, with hard martensitic phase embedded in is a soft ferritic phase. The hard-martensitic phase adds strength to the material and soft ferritic phase gives it ductility [3]. DP 590 steel is primarily used for exposed body panels (doors, hoods and fenders) [3]. Its excellent formability, high work hardening and bake hardening behaviours permit designers to reduce outer panel gauge and weight substantially, while maintaining or improving dent resistance. It offers designers the opportunity to substantially reduce closure weight, and possibly avoid substitution of more costly, lower density materials [4]. The amount of volume fraction of martensite and ferrite present in it plays a vital role in deciding the overall mechanical properties of the material [5], and also the fineness of the phases i.e., the finer grains exhibit good mechanical properties [6]. The deformation behaviour of the material can be studied by performing various types of tests like uniaxial tensile, biaxial tensile and compressive tests [7]. The deformation behaviour of Dual Phase (DP) steels was predicted by Avramovic-Cingara et.al., [8] by considering the effect of martensite morphology, and its distribution in a ferrite matrix on the mechanical properties. They found that, steel with more uniform distribution of martensite showed a slower rate of damage growth and a continuous void nucleation during the deformation process. Al-Abbasi [6] has investigated the effect of ultrafine ferrite on the deformation behaviour of DP steels for the volume fractions of martensite ranging from 6.8% to 31.4% with a step of 6.8%, and at different ferrite grain sizes of 3, 5, 10,15, 20, 25, 30, 40, 50 and 60µ using micromechanical modelling of cells. They found that, finer grain size of ferrite provides more restriction for deformation. The effects of martensite phase fraction, morphology, and phase distribution were investigated experimentally on the mechanical and fracture behaviours of the dual phase steel. The tensile test experiments performed by Leidl et.al., [9] revealed that, the amount of martensitic phase in a commercially produced dual-phase steel showed unexpected dependence of their initial yield behaviour on the content of martensite at room temperature. Other than experimental work, finite element simulations were also done to find the deformation behaviour of DP steels. The effect of temperature and strain rates on the deformation behaviour of DP steels were investigated by Hokka et.al., [10]. They have conducted the tensile tests within the temperature range of -100oC to 235oC and at strain rates ranging from 10-3 to 1250 s-1. Finally, found that the DP steels are insensitive in that temperature range, and sensitive towards strain rates. Cao et.al., [11] have also worked on finding the effect of temperature and strain rate on the deformation behaviour of DP steels. The tensile properties like yield, ultimate tensile strength and ductility were found at different test temperatures (-60 oC to 100 oC) and strain rates (10–4 to 102 s-1). They finally described the tensile flow behaviour of the material by using Voce equation along with the Kocks - Mecking model. A microstructure based micromechanical model was developed by Surajit Kumar Paul [12] to capture the deformation behavior, plastic strain localization and plastic instability of DP 590 steel. He used a microstructurebased approach by means of representative volume element (RVE) to find the deformation behaviour. Uthaisangsuk and sodjit [13] also developed a finite element based micromechanical model to predict the stress-strain behaviour of DP steels. They described the RVE model based on 2D microstructure. The flow curves of the individual phases were explained based on the dislocation theory and local chemical composition. It has been observed from the literature that, very few studies are reported about the comprehensive understanding of deformation behaviour and microstructure characteristics of DP steel at different temperatures and strain rates. The aim of this present work is to investigate the effect of deformation temperature and strain rate on the mechanical properties, flow behaviour and fracture morphology of the material. 2. Experimental Study The 1 mm DP 590 sheet was used to conduct tensile tests. The chemical composition of the as received DP-590 steel is mentioned in Table 1. Table.1 Chemical composition of the studied DP590 steel (wt.%) Element C Si Mn P Weight %
0.075
0.26
2.29
0.007
S
Cr
Mo
Ni
0.003
0.45
0.3
0.006
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The microstructure of the as received samples was studied using Light Optical Microscopy (LOM). The specimen for metallographic study was prepared by polishing and etching it as per ASTM E3 and E407 standards [14,15]. The ferritic matrix is represented by grey colour and the martensitic islands by bright spots as shown in Fig. 1.
Fig. 1. Microstructure of the DP590 steel at 500x magnification. Isothermal tensile tests were conducted on a computer controlled Universal Testing Machine (UTM), as shown in Fig. 2. It has a maximum load capacity of 50 kN, and heating capacity of two zone split furnace was from room temperature up to 12000C with ± 3°C accuracy. Samples are first heated to their deformation temperature at 20°C/min, and preserved for 5 minutes at that temperature in order to obtain a uniform distribution of temperature throughout the sample. Three samples were tested in each test setting, and average values were reported. High temperature contact type extensometer was used for tensile test experiments. A computer control system was used to record the load-displacement data, further converted into true stress-true strain curves. Universal tensile machine (UTM) is equipped with a feedback control system to execute exponential increase of the actuator speed to obtain constant true strain rates. The experiments were performed from RT to 7000C at an interval of 1000C, and wide range of slow strain rates from 0.01 s-1 to 0.0001 s-1. The true stress vs true strain data is attained from experimental test setup. Modifications in Software have been ingeniously done to have exponentially increasing crosshead speed for constant strain rate.
Fig. 2. Schematic Diagram of UTM of 50kN capacity with two zone split furnace heating furnace.
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3. Results and Discussion 3.1 Hot Deformation Behaviour The representative true stress strain curve at four different setting i.e. (a & b) variation of temperatures at slow and high strain rates respectively and (c & d) variation of strain rate at room temperature and 7000C respectively. It is noticed from Fig. 3 (a & b) that the flow stress is significantly influenced by the variation of temperatures. Influence of strain rate change at a particular temperature on flow stress is negligible at room temperature and lower elevated temperature (3000C) as shown in Fig. 3 (c). However, the variation in flow stress is considerable above 4000C as shown in Fig. 3 (d). For a particular strain rate, higher strength (yield and ultimate) and lower percentage elongation is observed at higher strain rate condition. Flow stress are sharply increasing with the rise of small strain (up to 0.1), then subsequent slow increasing in the flow stress till ultimate tensile strength (σuts). The sharp increase of tensile strength is because of uniform macroscopic deformation. This uniform deformation is due to mobility of dislocation during initial stage of deformation. It is noteworthy to mention that serrations (Dynamic strain aging) are reported in DP 590 steel at 7000C and all strain rate conditions.
Fig.3. True stress-strain curves at different temperatures and strain rates for DP590 steel. The material properties are evaluated at different temperatures and quasi static strain rates condition. For each setting, experiment was performed at 3 times and average stress values were reported. Thus, total 72 experiments were performed to evaluate the average material properties of DP 590 steel. The yield point determines the limits of performance for mechanical components, since it represents the upper limit to forces that can be applied without permanent deformation. The average 0.2% offset yield strength (σys) and ultimate tensile strength are determined at different temperatures as shown in Fig. 4 (a). As expected, yield and ultimate tensile strength decreases as temperature increases. The yield and ultimate tensile strength variation are also determined for different strain rates as shown in Fig. 4 (b & c). The yield and ultimate tensile strength are comparatively higher in case of high strain rate (0.01 s-1) than slow strain rate condition (0.0001s-1). The gradual decrease of yield and ultimate tensile strength
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is reported from room temperature to 4000C in all strain rate conditions. However, significant decrease in the strength is found above 4000C. This could be because of increasing in stacking fault energy in the material. Conversely, the percentage elongation increases with temperature increases for all the strain rates as shown in Fig. 5(a). The significant increase in % elongation is reported above 4000C. The effect of strain rate variation at a particular temperature is not consistent as shown in Fig. 5(b). But, in most of the cases, slower strain rate has prolonged elongation than the higher strain rates.
(a)
(b)
(c) Fig. 4. Variation of (a) yield & ultimate tensile strength at different temprtaures (b) Yield strength at different strain rates (c) Ultimate tensile strength at different strain rates
(a)
(b)
Fig. 5. Variation of % elongation (a) different temperatures (b) different strain rates
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Strain-hardening coefficient (n) is determined by the dependence of the flow (yield) stress on the level of strain. In materials with a high n value, the flow stress increases rapidly with strain. This tends to distribute further strain to regions of lower strain and flow stress. A high n value leads to a large difference between yield strength and ultimate tensile strength which is an indication of good workability of the material. The n value is calculated based on the Hollomon power law. Generally, for many structural materials, it has been seen that n value is largely dependent on temperature and strain rate change. Fig. 6 (a & b) shows the variation of n value with respect to temperature and strain rates. As expected, n value decreases as temperature increases which indicate predominant strain softening in the material at higher temperature. Also, strain hardening value is higher in case of higher strain rate than the lower strain rate. Thus, based on the above material properties variations at different temperatures and strain rates, it is observed that workability of the material substantially improved at higher temperatures.
(a)
(b)
Fig. 6. Variation of strain hardening exponent (n) (a) different temperatures (b) different strain rates 3.2 Fracture Morphologies The fracture surfaces are observed at different magnifications to determine the macroscopic fracture mode and to concurrently characterize the intrinsic features on the tensile fracture surface during uniaxial tensile deformation. The fracture morphologies of the tensile specimens at different temperatures i.e., RT, 200oC, 400oC, 600oC and 7000C were observed using Scanning Electron Microscopy (SEM) at a magnification of 5000x as shown in Fig. 7 (a -e). The observed sample is selected parallel to the fracture surface. The fracture surface at RT reveals small dimples and cleavage facets which indicate brittle mode of fracture. The sufficient amount of multiple fracture sites, voids are found in fracture surface. As the temperature increases, sufficient amount of various size of small dimples are observed up to 4000C. However, the size of dimples is significantly increase above 4000C which can also reflect in sudden drop in yield and ultimate tensile strength and substantial increase in % elongation. Ductile dimples with minimum display of brittle failure through cleavage were observed at higher temperatures. This reflects that stresses were fairly transferred from ferrite to martensite particles during plastic deformation and ferrite remained soft to promote ductile failure. On the other hand, at RT and lower temperature region, the process of work hardening would have developed dislocations and resulting stresses in the ferrite were accumulated till the fracture by the development of cleavage facets. These observations clearly indicate the fact that in DP-590 steel near fracture morphologies of individual phases (ferrite + martensite) play an important role in defining the ultimate failure mode in tensile testing.
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Fig. 7. Scanning Electron Microscope (SEM) images of fractured specimens at (a) RT, (b) 200oC, (c) 400oC (d) 600oC and (e) 700oC. 4. Conclusions The tensile deformation behaviour of DP590 steel at different temperatures and strain rates have been investigated. The following conclusions were drawn on the influence of temperatures and strain rates on the flow behaviour, material properties and fracture morphology: i.
The fracture surface at RT reveals small dimples and cleavage facets which indicate brittle mode of fracture. Flow stress behaviour is influenced by temperature and strain rate change. The flow stress variation is significant above 4000C. DSA phenomena is observed at 7000C in all the strain rates. The gradual decrease of yield, ultimate tensile strength and strain hardening exponent is observed from room temperature to 4000C in all strain rates condition. Conversely, significant decrease in the strength and n value and increase in % elongation is found above 4000C. The strain softening mechanism is more predominant at higher temperature.
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ii.
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RT fracture surface reveals small dimples and cleavage facets which indicate predominately brittle mode of fracture. Various size and shapes of dimples with minimum display of cleavage facets have been seen at higher temperatures which indicates predominately ductile failure. Thus, fracture morphologies of individual phases (ferrite + martensite) plays a crucial role in defining the ultimate failure mode in tensile testing. Future work involves analysis of correlation of mechanical properties with dimple size and shape measurement.
References [1]
G. Davies, 1 - Introduction BT - Materials for Automobile Bodies, in: Butterworth-Heinemann, Oxford, 2003: pp. 1–9.
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Q. Lai, L. Brassart, O. Bouaziz, M. Gouné, M. Verdier, G. Parry, A. Perlade, Y. Bréchet, T. Pardoen, Influence of martensite volume
Advance Automobile Material for Light Weight Future – A Review, (2012) 15–22.
fraction and hardness on the plastic behavior of dual-phase steels: Experiments and micromechanical modeling, Int. J. Plast. 80 (2016) 187–203. doi:10.1016/j.ijplas.2015.09.006. [6]
F.M. Al-Abbasi, Predicting the effect of ultrafine ferrite on the deformation behavior of DP-steels, Comput. Mater. Sci. 119 (2016) 90–
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107. doi:10.1016/j.commatsci.2016.03.048. DP 780-HY and TRIP 780) using the uniaxial tensile and the biaxial Viscous Pressure Bulge (VPB) tests, J. Mater. Process. Technol. 210 (2010) 429–436. doi:10.1016/j.jmatprotec.2009.10.003. [8]
G. Avramovic-cingara, Y. Ososkov, M.K. Jain, D.S. Wilkinson, Effect of martensite distribution on damage behaviour in DP600 dual phase steels, Mater. Sci. Eng. A. 516 (2009) 7–16. doi:10.1016/j.msea.2009.03.055.
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U. Liedl, S. Traint, E.A. Werner, An unexpected feature of the stress-strain diagram of dual-phase steel, Comput. Mater. Sci. 25 (2002) 122–128. doi:10.1016/S0927-0256(02)00256-2.
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S. Curtze, V.T. Kuokkala, M. Hokka, P. Peura, Deformation behavior of TRIP and DP steels in tension at different temperatures over a
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S.K. Paul, Real microstructure based micromechanical model to simulate microstructural level deformation behavior and failure
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ScienceDirect Materials Today: Proceedings 18 (2019) 2603–2610
www.materialstoday.com/proceedings
ICMPC-2019
Influence Of Temperatures And Strain Rates On Tensile Deformation Behaviour Of DP 590 Steel Sandeep Pandrea*, Prathamesh Takalkara, Nitin Kotkundea, Swadesh Kumar Singhb, Ahsan Ul Haqc a Department of Mechanical Engineering, BITS Pilani Hyderabad Campus, Jawahar Nagar,Hyderabad-500078, Telangana, India Department of Mechanical Engineering, Gokaraju Rangaraju Institute of Engineering and Technology, Hyderabad-500090, Telangana, India c Department of Mechanical Engineering VNR Vignana Jyothi Institute of Engineering and Technology, Pragathi Nagar, Nizampet, Hyderabad500090, Telangana, India b
Abstract In this study, tensile deformation behaviour and material properties has been investigated at different temperatures and strain rates. The isothermal uniaxial tensile tests were conducted from room temperature to 7000C at an interval of 1000C at different quasi-static strain rates (0.01, 0.001, 0.0001 s-1). The flow stress is significantly influenced by temperature and strain rate change. Various important material properties namely: yield strength (σy), ultimate tensile strength (σut), % elongation and strain hardening exponent (n) are determined at different temperatures and strain rates. In order to understand the fracture behaviour of a DP steel, microstructural characteristics were studied using Scanning Electron Microscope (SEM). Predominately ductile failure has been reported at higher temperatures. The individual phases (ferrite + martensite) play an important role in defining the ultimate failure mode in tensile testing. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: DP590 steel, tensile deformation behaviour, strain hardening, fracture morphology.
1. Introduction Nowadays, high strength and low weight advanced structural steels are gaining a special attention in various performance critical applications such as automotive and aerospace sectors[1]. This lead to the extensive development of Advanced High Strength Steels (AHSS) [2]. In AHSS steels, DP-590/600 grade is extensively used for automotive applications. * Corresponding author. Tel.: Tel.: +91 8340055252; Fax: +91 40 66303998. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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S. Pandre et al. / Materials Today: Proceedings 18 (2019) 2603–2610
The microstructure of DP steel consists of two phases, with hard martensitic phase embedded in is a soft ferritic phase. The hard-martensitic phase adds strength to the material and soft ferritic phase gives it ductility [3]. DP 590 steel is primarily used for exposed body panels (doors, hoods and fenders) [3]. Its excellent formability, high work hardening and bake hardening behaviours permit designers to reduce outer panel gauge and weight substantially, while maintaining or improving dent resistance. It offers designers the opportunity to substantially reduce closure weight, and possibly avoid substitution of more costly, lower density materials [4]. The amount of volume fraction of martensite and ferrite present in it plays a vital role in deciding the overall mechanical properties of the material [5], and also the fineness of the phases i.e., the finer grains exhibit good mechanical properties [6]. The deformation behaviour of the material can be studied by performing various types of tests like uniaxial tensile, biaxial tensile and compressive tests [7]. The deformation behaviour of Dual Phase (DP) steels was predicted by Avramovic-Cingara et.al., [8] by considering the effect of martensite morphology, and its distribution in a ferrite matrix on the mechanical properties. They found that, steel with more uniform distribution of martensite showed a slower rate of damage growth and a continuous void nucleation during the deformation process. Al-Abbasi [6] has investigated the effect of ultrafine ferrite on the deformation behaviour of DP steels for the volume fractions of martensite ranging from 6.8% to 31.4% with a step of 6.8%, and at different ferrite grain sizes of 3, 5, 10,15, 20, 25, 30, 40, 50 and 60µ using micromechanical modelling of cells. They found that, finer grain size of ferrite provides more restriction for deformation. The effects of martensite phase fraction, morphology, and phase distribution were investigated experimentally on the mechanical and fracture behaviours of the dual phase steel. The tensile test experiments performed by Leidl et.al., [9] revealed that, the amount of martensitic phase in a commercially produced dual-phase steel showed unexpected dependence of their initial yield behaviour on the content of martensite at room temperature. Other than experimental work, finite element simulations were also done to find the deformation behaviour of DP steels. The effect of temperature and strain rates on the deformation behaviour of DP steels were investigated by Hokka et.al., [10]. They have conducted the tensile tests within the temperature range of -100oC to 235oC and at strain rates ranging from 10-3 to 1250 s-1. Finally, found that the DP steels are insensitive in that temperature range, and sensitive towards strain rates. Cao et.al., [11] have also worked on finding the effect of temperature and strain rate on the deformation behaviour of DP steels. The tensile properties like yield, ultimate tensile strength and ductility were found at different test temperatures (-60 oC to 100 oC) and strain rates (10–4 to 102 s-1). They finally described the tensile flow behaviour of the material by using Voce equation along with the Kocks - Mecking model. A microstructure based micromechanical model was developed by Surajit Kumar Paul [12] to capture the deformation behavior, plastic strain localization and plastic instability of DP 590 steel. He used a microstructurebased approach by means of representative volume element (RVE) to find the deformation behaviour. Uthaisangsuk and sodjit [13] also developed a finite element based micromechanical model to predict the stress-strain behaviour of DP steels. They described the RVE model based on 2D microstructure. The flow curves of the individual phases were explained based on the dislocation theory and local chemical composition. It has been observed from the literature that, very few studies are reported about the comprehensive understanding of deformation behaviour and microstructure characteristics of DP steel at different temperatures and strain rates. The aim of this present work is to investigate the effect of deformation temperature and strain rate on the mechanical properties, flow behaviour and fracture morphology of the material. 2. Experimental Study The 1 mm DP 590 sheet was used to conduct tensile tests. The chemical composition of the as received DP-590 steel is mentioned in Table 1. Table.1 Chemical composition of the studied DP590 steel (wt.%) Element C Si Mn P Weight %
0.075
0.26
2.29
0.007
S
Cr
Mo
Ni
0.003
0.45
0.3
0.006
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The microstructure of the as received samples was studied using Light Optical Microscopy (LOM). The specimen for metallographic study was prepared by polishing and etching it as per ASTM E3 and E407 standards [14,15]. The ferritic matrix is represented by grey colour and the martensitic islands by bright spots as shown in Fig. 1.
Fig. 1. Microstructure of the DP590 steel at 500x magnification. Isothermal tensile tests were conducted on a computer controlled Universal Testing Machine (UTM), as shown in Fig. 2. It has a maximum load capacity of 50 kN, and heating capacity of two zone split furnace was from room temperature up to 12000C with ± 3°C accuracy. Samples are first heated to their deformation temperature at 20°C/min, and preserved for 5 minutes at that temperature in order to obtain a uniform distribution of temperature throughout the sample. Three samples were tested in each test setting, and average values were reported. High temperature contact type extensometer was used for tensile test experiments. A computer control system was used to record the load-displacement data, further converted into true stress-true strain curves. Universal tensile machine (UTM) is equipped with a feedback control system to execute exponential increase of the actuator speed to obtain constant true strain rates. The experiments were performed from RT to 7000C at an interval of 1000C, and wide range of slow strain rates from 0.01 s-1 to 0.0001 s-1. The true stress vs true strain data is attained from experimental test setup. Modifications in Software have been ingeniously done to have exponentially increasing crosshead speed for constant strain rate.
Fig. 2. Schematic Diagram of UTM of 50kN capacity with two zone split furnace heating furnace.
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3. Results and Discussion 3.1 Hot Deformation Behaviour The representative true stress strain curve at four different setting i.e. (a & b) variation of temperatures at slow and high strain rates respectively and (c & d) variation of strain rate at room temperature and 7000C respectively. It is noticed from Fig. 3 (a & b) that the flow stress is significantly influenced by the variation of temperatures. Influence of strain rate change at a particular temperature on flow stress is negligible at room temperature and lower elevated temperature (3000C) as shown in Fig. 3 (c). However, the variation in flow stress is considerable above 4000C as shown in Fig. 3 (d). For a particular strain rate, higher strength (yield and ultimate) and lower percentage elongation is observed at higher strain rate condition. Flow stress are sharply increasing with the rise of small strain (up to 0.1), then subsequent slow increasing in the flow stress till ultimate tensile strength (σuts). The sharp increase of tensile strength is because of uniform macroscopic deformation. This uniform deformation is due to mobility of dislocation during initial stage of deformation. It is noteworthy to mention that serrations (Dynamic strain aging) are reported in DP 590 steel at 7000C and all strain rate conditions.
Fig.3. True stress-strain curves at different temperatures and strain rates for DP590 steel. The material properties are evaluated at different temperatures and quasi static strain rates condition. For each setting, experiment was performed at 3 times and average stress values were reported. Thus, total 72 experiments were performed to evaluate the average material properties of DP 590 steel. The yield point determines the limits of performance for mechanical components, since it represents the upper limit to forces that can be applied without permanent deformation. The average 0.2% offset yield strength (σys) and ultimate tensile strength are determined at different temperatures as shown in Fig. 4 (a). As expected, yield and ultimate tensile strength decreases as temperature increases. The yield and ultimate tensile strength variation are also determined for different strain rates as shown in Fig. 4 (b & c). The yield and ultimate tensile strength are comparatively higher in case of high strain rate (0.01 s-1) than slow strain rate condition (0.0001s-1). The gradual decrease of yield and ultimate tensile strength
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is reported from room temperature to 4000C in all strain rate conditions. However, significant decrease in the strength is found above 4000C. This could be because of increasing in stacking fault energy in the material. Conversely, the percentage elongation increases with temperature increases for all the strain rates as shown in Fig. 5(a). The significant increase in % elongation is reported above 4000C. The effect of strain rate variation at a particular temperature is not consistent as shown in Fig. 5(b). But, in most of the cases, slower strain rate has prolonged elongation than the higher strain rates.
(a)
(b)
(c) Fig. 4. Variation of (a) yield & ultimate tensile strength at different temprtaures (b) Yield strength at different strain rates (c) Ultimate tensile strength at different strain rates
(a)
(b)
Fig. 5. Variation of % elongation (a) different temperatures (b) different strain rates
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Strain-hardening coefficient (n) is determined by the dependence of the flow (yield) stress on the level of strain. In materials with a high n value, the flow stress increases rapidly with strain. This tends to distribute further strain to regions of lower strain and flow stress. A high n value leads to a large difference between yield strength and ultimate tensile strength which is an indication of good workability of the material. The n value is calculated based on the Hollomon power law. Generally, for many structural materials, it has been seen that n value is largely dependent on temperature and strain rate change. Fig. 6 (a & b) shows the variation of n value with respect to temperature and strain rates. As expected, n value decreases as temperature increases which indicate predominant strain softening in the material at higher temperature. Also, strain hardening value is higher in case of higher strain rate than the lower strain rate. Thus, based on the above material properties variations at different temperatures and strain rates, it is observed that workability of the material substantially improved at higher temperatures.
(a)
(b)
Fig. 6. Variation of strain hardening exponent (n) (a) different temperatures (b) different strain rates 3.2 Fracture Morphologies The fracture surfaces are observed at different magnifications to determine the macroscopic fracture mode and to concurrently characterize the intrinsic features on the tensile fracture surface during uniaxial tensile deformation. The fracture morphologies of the tensile specimens at different temperatures i.e., RT, 200oC, 400oC, 600oC and 7000C were observed using Scanning Electron Microscopy (SEM) at a magnification of 5000x as shown in Fig. 7 (a -e). The observed sample is selected parallel to the fracture surface. The fracture surface at RT reveals small dimples and cleavage facets which indicate brittle mode of fracture. The sufficient amount of multiple fracture sites, voids are found in fracture surface. As the temperature increases, sufficient amount of various size of small dimples are observed up to 4000C. However, the size of dimples is significantly increase above 4000C which can also reflect in sudden drop in yield and ultimate tensile strength and substantial increase in % elongation. Ductile dimples with minimum display of brittle failure through cleavage were observed at higher temperatures. This reflects that stresses were fairly transferred from ferrite to martensite particles during plastic deformation and ferrite remained soft to promote ductile failure. On the other hand, at RT and lower temperature region, the process of work hardening would have developed dislocations and resulting stresses in the ferrite were accumulated till the fracture by the development of cleavage facets. These observations clearly indicate the fact that in DP-590 steel near fracture morphologies of individual phases (ferrite + martensite) play an important role in defining the ultimate failure mode in tensile testing.
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Fig. 7. Scanning Electron Microscope (SEM) images of fractured specimens at (a) RT, (b) 200oC, (c) 400oC (d) 600oC and (e) 700oC. 4. Conclusions The tensile deformation behaviour of DP590 steel at different temperatures and strain rates have been investigated. The following conclusions were drawn on the influence of temperatures and strain rates on the flow behaviour, material properties and fracture morphology: i.
The fracture surface at RT reveals small dimples and cleavage facets which indicate brittle mode of fracture. Flow stress behaviour is influenced by temperature and strain rate change. The flow stress variation is significant above 4000C. DSA phenomena is observed at 7000C in all the strain rates. The gradual decrease of yield, ultimate tensile strength and strain hardening exponent is observed from room temperature to 4000C in all strain rates condition. Conversely, significant decrease in the strength and n value and increase in % elongation is found above 4000C. The strain softening mechanism is more predominant at higher temperature.
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ii.
S. Pandre et al. / Materials Today: Proceedings 18 (2019) 2603–2610
RT fracture surface reveals small dimples and cleavage facets which indicate predominately brittle mode of fracture. Various size and shapes of dimples with minimum display of cleavage facets have been seen at higher temperatures which indicates predominately ductile failure. Thus, fracture morphologies of individual phases (ferrite + martensite) plays a crucial role in defining the ultimate failure mode in tensile testing. Future work involves analysis of correlation of mechanical properties with dimple size and shape measurement.
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