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Isothermal versus non-isothermal hot compression process: A comparative study on phase transformations and structure–property relationships
Materials and Design 45 (2013) 1–5
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Isothermal versus non-isothermal hot compression process: A comparative study on phase transformations and structure–property relationships M. Abbasi a,⇑, M. Naderi a, A. Saeed-Akbari b a b
Department of Mining and Metallurgy, Amirkabir University of Technology, Tehran, Iran Department of Ferrous Metallurgy, RWTH-Aachen University, Aachen, Germany
a r t i c l e
i n f o
Article history: Received 25 May 2012 Accepted 24 August 2012 Available online 6 September 2012 Keywords: Hot compression Isothermal Non-isothermal Mechanical properties
a b s t r a c t It is known that the direct hot stamping process in which the specimen is deformed and quenched simultaneously results in a high-strength product without the occurrence of springback. In the current work, the effects of isothermal and non-isothermal thermo-mechanical processes on the phase transformations and the resultant microstructure and mechanical properties of 22MnB5 steel are investigated. For the non-isothermal processing route which is similar to direct hot stamping, the specimens were simultaneously compressed and quenched, while in the isothermal route, the specimens were isothermally deformed and subsequently quenched. The results indicated that higher forming loads as well as Ms and Mf temperatures are the characteristics of the former process over the latter one. Additionally, following the isothermal compression process by quenching resulted in a fully martensitic microstructure. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Reductions of weight and air pollution, increment of safety as well as improvement of fuel consumption are some goals of automobile industries [1]. Application of hot stamping process to produce high strength specimens without the occurrence of springback is a solution to meet these requirements [2]. Two kinds of hot stamping processes are known: direct and in-direct. During the popular direct hot stamping process, the blank is initially austenized, then deformed and quenched simultaneously. Indirect hot stamping process is characterized by a room-temperature deformation followed by austenitization in the press tool and the subsequent quenching and calibration [3]. Materials, process modification regarding tools and variables as well as simulation of hot stamping process have been the point of focus of many researchers. Schiebl et al. [4] studied the microstructure, corrosion behavior and mechanical properties of 22MnB5 steel and two non-boron alloyed steels, MS-W 1200 and CP-W 800, after being hot stamped and semi-hot stamped. The material 22MnB5 reached component strength levels over 1500 MPa at elongations of 5–8% while with MS-W 1200 strength value of at most 1200 MPa were obtained. Naderi et al. [5] analyzed the different characteristics of non-boron alloyed steels after being hot stamped. Ambrogio et al. [6] proposed a novel manufacturing process by supplying a continuous current in order to generate heat. Mori et al. [7] used a resistance heating to elevate the temperature ⇑ Corresponding author. Tel.: +98 21 64542949; fax: +98 21 66405846. E-mail address: [email protected] (M. Abbasi). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.08.062
of sheet during forming and studied the effects of warm stamping on the springback of ultra high tensile strength steel sheets. The springback in hat-shaped bending of the high tensile strength steel sheets was eliminated by heating the sheets. Mori and Ito [8] evaluated the effect of different oils for prevention of oxide scale formation occurs immediately when the steel is in contact with air. Xing et al. [9] set up a material model under hot stamping condition of quenchable steel, based on the experimental data of mechanical and physical properties. They also simulated the whole hot stamping process by ABAQUS software. Their simulation results were basically in agreement with experimental results. Another kind of hot stamping process which can be assumed as a modification to direct hot stamping process is targeted in this research. This process consists of an isothermal deformation at high temperature together with a subsequent quenching process. Abbasi et al. [10] studied the effect of strain rate and deformation temperature on the properties resulted from 22MnB5 steel after being isothermally hot compressed and subsequently quenched. In the current work, different properties of a boron-alloyed steel, namely microstructure, hardness, forming load, work hardening rate as well as dilatation data, after being isothermally and nonisothermally compressed are compared. 2. Materials and methods 2.1. As-received material The chemical composition of the studied steel which is known as 22MnB5 is represented in Table 1. CCT diagram of the studied
2
M. Abbasi et al. / Materials and Design 45 (2013) 1–5
Table 1 Chemical composition of the studied steel. C
Si
Mn
P
S
Cr
Ti
B
0.24
0.27
1.14
0.015
0.001
0.17
0.036
0.003
Fig. 1. The CCT diagram of the studied steel.
steel which was obtained by using dilatometry tests, metallographic investigations as well as hardness measurements is represented in Fig. 1. It should be paid attention that although CCT diagram is an effective tool to characterize phase transformations happen during thermo-mechanical processes, but movement of phase domains due to deformation processes is inevitable [11]. 2.2. Thermo-mechanical processes In Fig. 2, two studied processes are schematically illustrated. As thermo-mechanical processes vary differently and there are no standard test methods for applying, experimental condition of these processes varies based on researchers’ goals. In the current research, cylindrical specimens were first austenized at 900 °C for 5 min, then compressed and quenched based on Fig. 2. The total strain and the applied strain rate for both processes were 0.5 and 0.1 s1, respectively. All the experiments took place in a Baehr DIL 805 deformation dilatometer with the cylindrical samples of 5.0 ± 0.1 mm diameter and 10.0 ± 0.1 mm height. The specimens were heated up to the austenization temperature through resistance heating method. Additionally, the compression tests were carried out by SiN2 anvils while argon and helium shower were employed for a controlled cooling with 50 °C/s rate. Molybdenum foils used to prevent the specimen sticking to the anvils and glass powder was added for lubrication. 2.3. Hardness The hardness was evaluated by using Vickers hardness test (HV0.8) method. Hardness tests were performed by using a programmable hardness test machine with uncertainty of about ±5 HV0.8, and the step-size of 300 lm for the measured points. The polished samples, taken from the vertically cut cross section of the cylindrical samples, were utilized for the hardness measurements. More information about the machine used to assess the hardness is represented in Ref. [12].
Fig. 2. Schematic illustration of procedures followed for isothermal (a) and nonisothermal (b) thermo-mechanical processes.
3. Results and discussion 3.1. Microstructure The microstructures of specimens after the mentioned thermo-mechanical processes are represented in Fig. 3. While the pronounced martensitic transformation is the characteristic of isothermal route (Fig. 3a), presence of phases other than martensite is observable in microstructure of non-isothermally deformed sample (Fig. 3b). 3.2. Hardness The hardness mapping technique was used to support the phase identification and characterization (Fig. 4). It is observed that hardness values of isothermally deformed and subsequently quenched specimen are higher than 400 HV0.8, while hardness values of simultaneously deformed and quenched specimen are equal or more than 300 HV0.8. Abbasi et al. [10] denoted that in the investigated steel, the hardness values more than 400–450 and 250–300 HV0.8 were related to martensite and bainite phases, respectively, while the hardness values less than 250 HV0.8 were attributed to ferrite phase. Observed results in Figs. 3 and 4 can be related to the effect of forming process. During forming, different defects such as dislocations are produced [13]. While during isothermal compression process these defects annihilate due to high temperature, specimens which experience simultaneous forming and quenching processes do not get this opportunity [14]. It is well established that martensitic transformation involves the coordinated movement of atoms [15,16]. Additionally, it has been pointed out by different researchers [17–19] that such movements cannot be sustained against defects, such as grain boundaries and dislocations. As a result, presence of dislocations may mechanically stabilize austenite and retard or even impede the martensitic transformation [20].
M. Abbasi et al. / Materials and Design 45 (2013) 1–5
Fig. 3. Microstructure of (a) isothermally compressed and subsequently quenched specimen and (b) simultaneously compressed and quenched specimen.
Moreover, the dislocations and point defects make preferred sites for the nucleation of non-thermal phases with hardness values less than 400 HV0.8 [21]. Therefore, presence of phases other than martensite in microstructure of simultaneously deformed and quenched specimens is predictable. 3.3. Flow curves Flow curves of the studied processing routes during the compression tests are represented in Fig. 5. It is observed that flow curve of isothermally compressed specimen has a plateau curve and stress is lower than 250 MPa, while the flow curve of simultaneously deformed and quenched specimen has an ascending characteristic and stress value increases up to 500 MPa. High temperature isothermal forming brings the possibility for dislocations and defects which are responsible for strain hardening to annihilate and consequently the forming load is low [14,22]. However, due to continuous temperature decrease during the simultaneous forming and quenching process, defects grow continuously and the increase of forming load is resulted. This behaviour is illustrated clearly in Fig. 6 where the changes of work hardening rate for isothermally deformed and subsequently quenched specimen as well as simultaneously deformed and quenched specimen are given. It comes from Fig. 6 that work hardening rate for the former process has a descending behaviour, while for the latter one the work hardening rate initially descends and ascends finally. For a constant strain rate, flow stress is a function of temperature and strain (r = r(T, e)). So work hardening rate is:
@r @ r @T @r @e ¼ þ @e @T e @ e @e T @e
ð1Þ
3
Fig. 4. Hardness maps of (a) isothermally hot compressed and (b) non-isothermally hot compressed specimens.
The dependency of strength to strain at constant temperature and strain rate is normally represented by the following relation [22]:
r ¼ ken
ð2Þ
where n is the strain-hardening exponent and k is the strength coefficient. On the other hand, the temperature dependence of flow stress at constant strain and strain rate generally is presented by [23]:
r ¼ C Exp
Q RT
ð3Þ
where Q is an activation energy for plastic flow, R is the universal gas constant, T is the temperature, and C is the constant. Combining Eqs. (1)–(3) the following equations for isothermal and non-isothermal deformation processes are derived, respectively.
n @r ¼r @e e
ð4Þ
00 @r C n ¼r 2þ @e e T
ð5Þ
It should be mentioned that as strain rate (0.1 s1) and cooling rate (50 °C/s) in the current work are definite, oT/oe is a constant value which it is summarized with other constant values in C 00 constant (Eqs. (4) and (5)). In other word, C 00 ¼ C:Q : @T and as @T is a R @e @e 00 minus number, C has a positive value. Based on Eqs. (4) and (5) , as strain increases, work hardening rate decreases; although, for non-isothermally deformed sample
4
M. Abbasi et al. / Materials and Design 45 (2013) 1–5
Fig. 5. Flow curves of (a) isothermally deformed and subsequently quenched specimen and (b) simultaneously deformed and quenched specimen.
Fig. 7. Comparison between the dilation curves of (a) isothermally and (b) nonisothermally deformed specimens.
sample does not decrease uniformly. This behaviour can also be explained based on Eq. (5) and intrinsic characteristic of 1/T2 function. While the value of this function for high temperatures is very low, it increases dramatically in low temperatures. In this regard, it is predicted that during non-isothermal deformation which strain and temperature respectively increases and decreases, at the final steps of process, the increasing effect of C 00 =T 2 overcomes the decreasing effect of n/e and work hardening rate enhances. 3.4. Dilatation data
Fig. 6. Comparison between work hardening rate curves of (a) isothermally hot compressed and subsequently quenched specimen and (b) non-isothermally hot compressed specimen.
due to positive C 00 =T 2 term, work hardening rate is higher. This prediction is in agreement with experimental observation in Fig. 6. It is also observed in Fig. 6 that unlike isothermally deformed sample which monotonic decrease of work hardening rate is its characteristic, work hardening rate of non-isothermally deformed
Dilatation data relating to the studied processes are represented in Fig. 7. Fig. 7 shows that Ms and Mf temperatures of isothermally compressed sample are lower than the related values for the nonisothermally compressed specimen. In addition, it comes from Fig. 7 that the volume fraction of martensite in the former specimen, due to higher dilation change (Fig. 7), to be more than the other one. This is also highlighted by the metallographic pictures represented in Fig. 3 and hardness maps of Fig. 4. These can be related to the virtue of martensitic transformation which is a diffusionless shear-type transformation and demands a definite driving force [24]. The driving force for martensite transformation during quenching process is achieved by undercooling, but the mechanical driving force supplied during deformation reduces the demanded undercooling and the Ms temperature will be high [25,26]. This correlates well with Fig. 7 that for the simultaneously compressed
M. Abbasi et al. / Materials and Design 45 (2013) 1–5
and quenched sample, the Ms temperature is higher than that of isothermally compressed and subsequently quenched sample. 4. Conclusions In the current research different properties, namely microstructure, hardness, flow curves and dilatation data, of two kinds of specimens are compared: isothermally compressed and subsequently quenched specimens over simultaneously deformed and quenched specimens. For all the experiments the strain and strain rate values were the same and all the specimens were austenized at 900 °C for 5 min. The results indicated that the force applied for isothermal deformation was lower than that of simultaneous deformed and quenched process. It was also concluded that: 1. Fully martensitic microstructure was the characteristic of isothermally deformed and subsequently quenched specimens, while the presence of phases other than martensite was detected for simultaneously deformed and quenched specimens. 2. Ms and Mf temperatures of isothermally compressed and subsequently quenched specimens were lower than those of simultaneously deformed and quenched specimens. 3. Due to the advantages of isothermally deformed process at high temperature over simultaneously deformed and quenched process, application of this process is recommended. References [1] Abbasi M, Ketabchi M, Shakeri HR, Hasanniya MH. Formability enhancement of galvanized IF-steel TWB by modification of forming parameters. J Mater Eng Perform 2012;21:564–71. [2] Naderi M, Ketabchi M, Abbasi M, Bleck W. Semi-hot stamping as an improved process of hot stamping. J Mater Sci Technol 2011;27:369–76. [3] Karbasian H, Tekkaya AE. A review on hot stamping. J Mater Process Technol 2010;210:2103–18. [4] Schiebl G, Possehn T, Heller T, Sikora S. Manufacturing a roof frame from ultrahigh strength steel materials by hot stamping. In: IDDRG international deep drawing group 2004 conference, Sindelfingen, Germany; 2004. [5] Naderi M, Ketabchi M, Abbasi M, Bleck W. Analysis of microstructure and mechanical properties of different high strength carbon steels after hot stamping. J Mater Process Technol 2011;211:1117–25.
5
[6] Ambrogio G, Filice L, Gagliardi F. Formability of lightweight alloys by hot incremental sheet forming. Mater Des 2012;34:501–8. [7] Mori K, Maki S, Tanaka Y. Warm and hot stamping of ultra tensile strength steel sheets using resistance heating. CIRP Ann Manuf Technol 2009;54:209–12. [8] Mori K, Ito D. Prevention of oxidation in hot stamping of quenchable steel sheet by oxidation preventive oil. CIRP Ann Manuf Technol 2009;58:267–70. [9] Xing ZW, Bao J, Yang YY. Numerical simulation of hot stamping of quenchable boron steel. Mater Sci Eng A 2009;499:28–31. [10] Abbasi M, Saeed-Akbari A, Naderi M. The effect of strain rate and deformation temperature on the characteristics of isothermally hot compressed boronalloyed steel. Mater Sci Eng A 2012;538:356–63. [11] Taherian R, Najafzadeh A, Shateri R. Drawing of CCCT diagrams by static deformation and consideration deformation effect on martensite and bainite transformation in NiCrMoV steel. J Mater Process Technol 2008;196:321–31. [12] Naderi M, Saeed-Akbari A, Bleck W. Quantitative and qualitative investigation of the heterogeneous microstructure using surface hardness mapping and dilatation data. Mater Lett 2008;62:1132–5. [13] Abbasi M, Ketabchi M, Ramazani A, Abbasi M, Prahl U. Investigation into the effects of weld zone and geometric discontinuity on the formability reduction of tailor welded blanks. Comput Mater Sci 2012;59:158–64. [14] Fan DW, Park RB, Cho YR, Cooman BCD. Influence of isothermal deformation conditions on the mechanical properties of 22MnB5 HPF steel. Steel Res Int 2010;81:292–8. [15] Porter DA, Easterling KE. Phase transformation in metals and alloys. Cheltenham: Nelson Thornes Ltd.; 1992. [16] Krauss G. Steels: heat treatment and processing principles. New York: ASM International; 1990. [17] Bhadeshia HKDH. The bainite transformation: unresolved issues. Mater Sci Eng A 1999;273–275:58–66. [18] Maalekian M, Kozeschink E, Chatterjee S, Bhadeshia HKDH. Mechanical stabilization of eutectoid steel. Mater Sci Technol 2007;23:610–2. [19] Naderi M, Saeed-Akbari A, Bleck W. The effects of non-isothermal deformation on martensitic transformation in 22nB5 steel. Mater Sci Eng A 2008;487:445–55. [20] Garcia-Mateo C, Caballero FG, Chao J, Capdevila C, Garcia de Andres C. Mechanical stability of retained austenite during plastic deformation of super high strength carbide free bainite steels. J Mater Sci 2009;44:4617–24. [21] Reed-Hill RE, Abbaschian R. Physical metallurgy principles. Boston: PWS Publishing Company; 1991. [22] Dieter GE. Mechanical metallurgy. Singapore: McGraw-Hill Book Company; 1988. [23] Gottstein G. Physical foundations of materials science. Germany: SpringerVerlag; 2004. [24] Raghavan V, Antia DP. The driving force for martensitic transformations in low alloy steels. Metall Mater Trans A 1996;27A:1127–32. [25] Bleck W. Materials Science of Steel, Textbook for RWTH students. Aachen: Verlag Mainz; 2007. [26] Guimarães JRC. Stress assisted martensite: pre-strain, grain-size and strainrate effects. Mater Sci Eng A 2008;475:343–7.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Isothermal versus non-isothermal hot compression process: A comparative study on phase transformations and structure–property relationships M. Abbasi a,⇑, M. Naderi a, A. Saeed-Akbari b a b
Department of Mining and Metallurgy, Amirkabir University of Technology, Tehran, Iran Department of Ferrous Metallurgy, RWTH-Aachen University, Aachen, Germany
a r t i c l e
i n f o
Article history: Received 25 May 2012 Accepted 24 August 2012 Available online 6 September 2012 Keywords: Hot compression Isothermal Non-isothermal Mechanical properties
a b s t r a c t It is known that the direct hot stamping process in which the specimen is deformed and quenched simultaneously results in a high-strength product without the occurrence of springback. In the current work, the effects of isothermal and non-isothermal thermo-mechanical processes on the phase transformations and the resultant microstructure and mechanical properties of 22MnB5 steel are investigated. For the non-isothermal processing route which is similar to direct hot stamping, the specimens were simultaneously compressed and quenched, while in the isothermal route, the specimens were isothermally deformed and subsequently quenched. The results indicated that higher forming loads as well as Ms and Mf temperatures are the characteristics of the former process over the latter one. Additionally, following the isothermal compression process by quenching resulted in a fully martensitic microstructure. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Reductions of weight and air pollution, increment of safety as well as improvement of fuel consumption are some goals of automobile industries [1]. Application of hot stamping process to produce high strength specimens without the occurrence of springback is a solution to meet these requirements [2]. Two kinds of hot stamping processes are known: direct and in-direct. During the popular direct hot stamping process, the blank is initially austenized, then deformed and quenched simultaneously. Indirect hot stamping process is characterized by a room-temperature deformation followed by austenitization in the press tool and the subsequent quenching and calibration [3]. Materials, process modification regarding tools and variables as well as simulation of hot stamping process have been the point of focus of many researchers. Schiebl et al. [4] studied the microstructure, corrosion behavior and mechanical properties of 22MnB5 steel and two non-boron alloyed steels, MS-W 1200 and CP-W 800, after being hot stamped and semi-hot stamped. The material 22MnB5 reached component strength levels over 1500 MPa at elongations of 5–8% while with MS-W 1200 strength value of at most 1200 MPa were obtained. Naderi et al. [5] analyzed the different characteristics of non-boron alloyed steels after being hot stamped. Ambrogio et al. [6] proposed a novel manufacturing process by supplying a continuous current in order to generate heat. Mori et al. [7] used a resistance heating to elevate the temperature ⇑ Corresponding author. Tel.: +98 21 64542949; fax: +98 21 66405846. E-mail address: [email protected] (M. Abbasi). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.08.062
of sheet during forming and studied the effects of warm stamping on the springback of ultra high tensile strength steel sheets. The springback in hat-shaped bending of the high tensile strength steel sheets was eliminated by heating the sheets. Mori and Ito [8] evaluated the effect of different oils for prevention of oxide scale formation occurs immediately when the steel is in contact with air. Xing et al. [9] set up a material model under hot stamping condition of quenchable steel, based on the experimental data of mechanical and physical properties. They also simulated the whole hot stamping process by ABAQUS software. Their simulation results were basically in agreement with experimental results. Another kind of hot stamping process which can be assumed as a modification to direct hot stamping process is targeted in this research. This process consists of an isothermal deformation at high temperature together with a subsequent quenching process. Abbasi et al. [10] studied the effect of strain rate and deformation temperature on the properties resulted from 22MnB5 steel after being isothermally hot compressed and subsequently quenched. In the current work, different properties of a boron-alloyed steel, namely microstructure, hardness, forming load, work hardening rate as well as dilatation data, after being isothermally and nonisothermally compressed are compared. 2. Materials and methods 2.1. As-received material The chemical composition of the studied steel which is known as 22MnB5 is represented in Table 1. CCT diagram of the studied
2
M. Abbasi et al. / Materials and Design 45 (2013) 1–5
Table 1 Chemical composition of the studied steel. C
Si
Mn
P
S
Cr
Ti
B
0.24
0.27
1.14
0.015
0.001
0.17
0.036
0.003
Fig. 1. The CCT diagram of the studied steel.
steel which was obtained by using dilatometry tests, metallographic investigations as well as hardness measurements is represented in Fig. 1. It should be paid attention that although CCT diagram is an effective tool to characterize phase transformations happen during thermo-mechanical processes, but movement of phase domains due to deformation processes is inevitable [11]. 2.2. Thermo-mechanical processes In Fig. 2, two studied processes are schematically illustrated. As thermo-mechanical processes vary differently and there are no standard test methods for applying, experimental condition of these processes varies based on researchers’ goals. In the current research, cylindrical specimens were first austenized at 900 °C for 5 min, then compressed and quenched based on Fig. 2. The total strain and the applied strain rate for both processes were 0.5 and 0.1 s1, respectively. All the experiments took place in a Baehr DIL 805 deformation dilatometer with the cylindrical samples of 5.0 ± 0.1 mm diameter and 10.0 ± 0.1 mm height. The specimens were heated up to the austenization temperature through resistance heating method. Additionally, the compression tests were carried out by SiN2 anvils while argon and helium shower were employed for a controlled cooling with 50 °C/s rate. Molybdenum foils used to prevent the specimen sticking to the anvils and glass powder was added for lubrication. 2.3. Hardness The hardness was evaluated by using Vickers hardness test (HV0.8) method. Hardness tests were performed by using a programmable hardness test machine with uncertainty of about ±5 HV0.8, and the step-size of 300 lm for the measured points. The polished samples, taken from the vertically cut cross section of the cylindrical samples, were utilized for the hardness measurements. More information about the machine used to assess the hardness is represented in Ref. [12].
Fig. 2. Schematic illustration of procedures followed for isothermal (a) and nonisothermal (b) thermo-mechanical processes.
3. Results and discussion 3.1. Microstructure The microstructures of specimens after the mentioned thermo-mechanical processes are represented in Fig. 3. While the pronounced martensitic transformation is the characteristic of isothermal route (Fig. 3a), presence of phases other than martensite is observable in microstructure of non-isothermally deformed sample (Fig. 3b). 3.2. Hardness The hardness mapping technique was used to support the phase identification and characterization (Fig. 4). It is observed that hardness values of isothermally deformed and subsequently quenched specimen are higher than 400 HV0.8, while hardness values of simultaneously deformed and quenched specimen are equal or more than 300 HV0.8. Abbasi et al. [10] denoted that in the investigated steel, the hardness values more than 400–450 and 250–300 HV0.8 were related to martensite and bainite phases, respectively, while the hardness values less than 250 HV0.8 were attributed to ferrite phase. Observed results in Figs. 3 and 4 can be related to the effect of forming process. During forming, different defects such as dislocations are produced [13]. While during isothermal compression process these defects annihilate due to high temperature, specimens which experience simultaneous forming and quenching processes do not get this opportunity [14]. It is well established that martensitic transformation involves the coordinated movement of atoms [15,16]. Additionally, it has been pointed out by different researchers [17–19] that such movements cannot be sustained against defects, such as grain boundaries and dislocations. As a result, presence of dislocations may mechanically stabilize austenite and retard or even impede the martensitic transformation [20].
M. Abbasi et al. / Materials and Design 45 (2013) 1–5
Fig. 3. Microstructure of (a) isothermally compressed and subsequently quenched specimen and (b) simultaneously compressed and quenched specimen.
Moreover, the dislocations and point defects make preferred sites for the nucleation of non-thermal phases with hardness values less than 400 HV0.8 [21]. Therefore, presence of phases other than martensite in microstructure of simultaneously deformed and quenched specimens is predictable. 3.3. Flow curves Flow curves of the studied processing routes during the compression tests are represented in Fig. 5. It is observed that flow curve of isothermally compressed specimen has a plateau curve and stress is lower than 250 MPa, while the flow curve of simultaneously deformed and quenched specimen has an ascending characteristic and stress value increases up to 500 MPa. High temperature isothermal forming brings the possibility for dislocations and defects which are responsible for strain hardening to annihilate and consequently the forming load is low [14,22]. However, due to continuous temperature decrease during the simultaneous forming and quenching process, defects grow continuously and the increase of forming load is resulted. This behaviour is illustrated clearly in Fig. 6 where the changes of work hardening rate for isothermally deformed and subsequently quenched specimen as well as simultaneously deformed and quenched specimen are given. It comes from Fig. 6 that work hardening rate for the former process has a descending behaviour, while for the latter one the work hardening rate initially descends and ascends finally. For a constant strain rate, flow stress is a function of temperature and strain (r = r(T, e)). So work hardening rate is:
@r @ r @T @r @e ¼ þ @e @T e @ e @e T @e
ð1Þ
3
Fig. 4. Hardness maps of (a) isothermally hot compressed and (b) non-isothermally hot compressed specimens.
The dependency of strength to strain at constant temperature and strain rate is normally represented by the following relation [22]:
r ¼ ken
ð2Þ
where n is the strain-hardening exponent and k is the strength coefficient. On the other hand, the temperature dependence of flow stress at constant strain and strain rate generally is presented by [23]:
r ¼ C Exp
Q RT
ð3Þ
where Q is an activation energy for plastic flow, R is the universal gas constant, T is the temperature, and C is the constant. Combining Eqs. (1)–(3) the following equations for isothermal and non-isothermal deformation processes are derived, respectively.
n @r ¼r @e e
ð4Þ
00 @r C n ¼r 2þ @e e T
ð5Þ
It should be mentioned that as strain rate (0.1 s1) and cooling rate (50 °C/s) in the current work are definite, oT/oe is a constant value which it is summarized with other constant values in C 00 constant (Eqs. (4) and (5)). In other word, C 00 ¼ C:Q : @T and as @T is a R @e @e 00 minus number, C has a positive value. Based on Eqs. (4) and (5) , as strain increases, work hardening rate decreases; although, for non-isothermally deformed sample
4
M. Abbasi et al. / Materials and Design 45 (2013) 1–5
Fig. 5. Flow curves of (a) isothermally deformed and subsequently quenched specimen and (b) simultaneously deformed and quenched specimen.
Fig. 7. Comparison between the dilation curves of (a) isothermally and (b) nonisothermally deformed specimens.
sample does not decrease uniformly. This behaviour can also be explained based on Eq. (5) and intrinsic characteristic of 1/T2 function. While the value of this function for high temperatures is very low, it increases dramatically in low temperatures. In this regard, it is predicted that during non-isothermal deformation which strain and temperature respectively increases and decreases, at the final steps of process, the increasing effect of C 00 =T 2 overcomes the decreasing effect of n/e and work hardening rate enhances. 3.4. Dilatation data
Fig. 6. Comparison between work hardening rate curves of (a) isothermally hot compressed and subsequently quenched specimen and (b) non-isothermally hot compressed specimen.
due to positive C 00 =T 2 term, work hardening rate is higher. This prediction is in agreement with experimental observation in Fig. 6. It is also observed in Fig. 6 that unlike isothermally deformed sample which monotonic decrease of work hardening rate is its characteristic, work hardening rate of non-isothermally deformed
Dilatation data relating to the studied processes are represented in Fig. 7. Fig. 7 shows that Ms and Mf temperatures of isothermally compressed sample are lower than the related values for the nonisothermally compressed specimen. In addition, it comes from Fig. 7 that the volume fraction of martensite in the former specimen, due to higher dilation change (Fig. 7), to be more than the other one. This is also highlighted by the metallographic pictures represented in Fig. 3 and hardness maps of Fig. 4. These can be related to the virtue of martensitic transformation which is a diffusionless shear-type transformation and demands a definite driving force [24]. The driving force for martensite transformation during quenching process is achieved by undercooling, but the mechanical driving force supplied during deformation reduces the demanded undercooling and the Ms temperature will be high [25,26]. This correlates well with Fig. 7 that for the simultaneously compressed
M. Abbasi et al. / Materials and Design 45 (2013) 1–5
and quenched sample, the Ms temperature is higher than that of isothermally compressed and subsequently quenched sample. 4. Conclusions In the current research different properties, namely microstructure, hardness, flow curves and dilatation data, of two kinds of specimens are compared: isothermally compressed and subsequently quenched specimens over simultaneously deformed and quenched specimens. For all the experiments the strain and strain rate values were the same and all the specimens were austenized at 900 °C for 5 min. The results indicated that the force applied for isothermal deformation was lower than that of simultaneous deformed and quenched process. It was also concluded that: 1. Fully martensitic microstructure was the characteristic of isothermally deformed and subsequently quenched specimens, while the presence of phases other than martensite was detected for simultaneously deformed and quenched specimens. 2. Ms and Mf temperatures of isothermally compressed and subsequently quenched specimens were lower than those of simultaneously deformed and quenched specimens. 3. Due to the advantages of isothermally deformed process at high temperature over simultaneously deformed and quenched process, application of this process is recommended. References [1] Abbasi M, Ketabchi M, Shakeri HR, Hasanniya MH. Formability enhancement of galvanized IF-steel TWB by modification of forming parameters. J Mater Eng Perform 2012;21:564–71. [2] Naderi M, Ketabchi M, Abbasi M, Bleck W. Semi-hot stamping as an improved process of hot stamping. J Mater Sci Technol 2011;27:369–76. [3] Karbasian H, Tekkaya AE. A review on hot stamping. J Mater Process Technol 2010;210:2103–18. [4] Schiebl G, Possehn T, Heller T, Sikora S. Manufacturing a roof frame from ultrahigh strength steel materials by hot stamping. In: IDDRG international deep drawing group 2004 conference, Sindelfingen, Germany; 2004. [5] Naderi M, Ketabchi M, Abbasi M, Bleck W. Analysis of microstructure and mechanical properties of different high strength carbon steels after hot stamping. J Mater Process Technol 2011;211:1117–25.
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[6] Ambrogio G, Filice L, Gagliardi F. Formability of lightweight alloys by hot incremental sheet forming. Mater Des 2012;34:501–8. [7] Mori K, Maki S, Tanaka Y. Warm and hot stamping of ultra tensile strength steel sheets using resistance heating. CIRP Ann Manuf Technol 2009;54:209–12. [8] Mori K, Ito D. Prevention of oxidation in hot stamping of quenchable steel sheet by oxidation preventive oil. CIRP Ann Manuf Technol 2009;58:267–70. [9] Xing ZW, Bao J, Yang YY. Numerical simulation of hot stamping of quenchable boron steel. Mater Sci Eng A 2009;499:28–31. [10] Abbasi M, Saeed-Akbari A, Naderi M. The effect of strain rate and deformation temperature on the characteristics of isothermally hot compressed boronalloyed steel. Mater Sci Eng A 2012;538:356–63. [11] Taherian R, Najafzadeh A, Shateri R. Drawing of CCCT diagrams by static deformation and consideration deformation effect on martensite and bainite transformation in NiCrMoV steel. J Mater Process Technol 2008;196:321–31. [12] Naderi M, Saeed-Akbari A, Bleck W. Quantitative and qualitative investigation of the heterogeneous microstructure using surface hardness mapping and dilatation data. Mater Lett 2008;62:1132–5. [13] Abbasi M, Ketabchi M, Ramazani A, Abbasi M, Prahl U. Investigation into the effects of weld zone and geometric discontinuity on the formability reduction of tailor welded blanks. Comput Mater Sci 2012;59:158–64. [14] Fan DW, Park RB, Cho YR, Cooman BCD. Influence of isothermal deformation conditions on the mechanical properties of 22MnB5 HPF steel. Steel Res Int 2010;81:292–8. [15] Porter DA, Easterling KE. Phase transformation in metals and alloys. Cheltenham: Nelson Thornes Ltd.; 1992. [16] Krauss G. Steels: heat treatment and processing principles. New York: ASM International; 1990. [17] Bhadeshia HKDH. The bainite transformation: unresolved issues. Mater Sci Eng A 1999;273–275:58–66. [18] Maalekian M, Kozeschink E, Chatterjee S, Bhadeshia HKDH. Mechanical stabilization of eutectoid steel. Mater Sci Technol 2007;23:610–2. [19] Naderi M, Saeed-Akbari A, Bleck W. The effects of non-isothermal deformation on martensitic transformation in 22nB5 steel. Mater Sci Eng A 2008;487:445–55. [20] Garcia-Mateo C, Caballero FG, Chao J, Capdevila C, Garcia de Andres C. Mechanical stability of retained austenite during plastic deformation of super high strength carbide free bainite steels. J Mater Sci 2009;44:4617–24. [21] Reed-Hill RE, Abbaschian R. Physical metallurgy principles. Boston: PWS Publishing Company; 1991. [22] Dieter GE. Mechanical metallurgy. Singapore: McGraw-Hill Book Company; 1988. [23] Gottstein G. Physical foundations of materials science. Germany: SpringerVerlag; 2004. [24] Raghavan V, Antia DP. The driving force for martensitic transformations in low alloy steels. Metall Mater Trans A 1996;27A:1127–32. [25] Bleck W. Materials Science of Steel, Textbook for RWTH students. Aachen: Verlag Mainz; 2007. [26] Guimarães JRC. Stress assisted martensite: pre-strain, grain-size and strainrate effects. Mater Sci Eng A 2008;475:343–7.