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Bainite kinetic transformation of austempered AISI 6150 steel
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Original Article
Bainite kinetic transformation of austempered AISI 6150 steel Xue Han a,b , Zhenpu Zhang b , Yanhao Rong b , Steven J. Thrush b,c , Gary C. Barber b , Hongyu Yang a,b,∗ , Feng Qiu a,b,∗ a
Key Laboratory of Automobile Materials, Ministry of Education and School of Materials Science and Engineering, Jilin University, Renmin Street NO. 5988, Changchun 130025, China b Automotive Tribology Center, Department of Mechanical Engineering, School of Engineering and Computer Science, Oakland University, Rochester, Michigan, 48309, USA c U.S. Army Combat Capabilities Development Command Ground Vehicle System Center, 6501 East 11 Mile Rd, Warren, Michigan, 48397, USA
a r t i c l e
i n f o
a b s t r a c t
Article history:
This research studies the kinetic transformation and austempering temperature effect on
Received 13 November 2019
the morphologies of bainite for AISI 6150 steel. A minimum of 10 different holding times for
Accepted 23 November 2019
each austempering temperature were utilized. All the AISI 6150 steel samples with original
Available online xxx
spheroidal pearlite were austenized at 855 ◦ C for 20 min, followed by a lower temperature
Keywords:
temperature and holding time was measured, and the microstructures were observed by
Bainite morphologies
optical microscopy. The kinetic energy for needle like lower bainite and granular like upper
salt bath for austempering at various holding times. The hardness for each austempered
Austempering process
bainite was analyzed by using kinetic transformation equations. The aim of this research
Microstructures
was to study the transformation of bainite, in particular lower bainite, to obtain improved
Transformation kinetics
mechanical properties and higher ductility of AISI 6150 steel, and then apply AISI 6150 steel
Phase activation energy
to more applications. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1.
Introduction
AISI 6150 chromium-vanadium steel is used in many industrial applications including gears, pinions, forged crankshafts, steering knuckles, connecting rods, spindles, pumps, and gear shafts [1,2], due to its high strength, high fatigue strength, and
good hardenability. There are many investigators who have studied austempered AISI 6150 steel, quenched and tempered AISI 6150 steel, and tempered AISI 6150 steel to improve the mechanical properties of AISI 6150 steel. The austempering process offers advantages over quenching and tempering in some applications [3–5].
∗ Corresponding authors at: School of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, P.R. China. E-mails: [email protected] (H. Yang), [email protected] (F. Qiu). https://doi.org/10.1016/j.jmrt.2019.11.062 2238-7854/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Han X, et al. Bainite kinetic transformation of austempered AISI 6150 steel. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.062
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Table 1 – AISI 6150 steel with the chemical composition. C
Cr
Si
Mn
V
P
S
0.45 %-0.83 %
0.8 %-1.1 %
0.15 %-0.3 %
0.7 %-0.9 %
≥0.15 %
≤0.035 %
≤0.04 %
Austempering is a high-performance isothermal heat treatment which can impart superior performance to ferrous metals [6]. Many researchers have suggested that the isothermal heat treatment significantly improves the mechanical properties of materials such as the toughness, strength, and wear resistance, since it produces the bainite microstructure [7,8]. Upper bainite and lower bainite both contain aggregates of small plates or laths of ferrite, however, upper bainite forms at high temperature which allows the excess carbon to partition before it can precipitate in the ferrite. Some of the carbon of lower bainite precipitates in the supersaturated ferrite due to the reduced transformation temperature resulting in slower diffusion [9]. It is well known that material mechanical performances depend on the microstructural modification [10]. Due to the differences in microstructure, therefore, lower bainite and upper bainite have different mechanical properties [11]. Lower bainite results in higher ductility than upper bainite, hence lower bainite has more applications in industry than upper bainite. Bakhtiari et al. [12] studied the mechanical properties of lower bainite and upper bainite of AISI 4340 steel. Their results showed that lower bainite produced with 300 ◦ C austempering temperature or mixed bainite produced with 350 ◦ C austempering temperature resulted in higher yield and ultimate tensile strength, hardness, total elongation and impact energy than upper bainite produced with 400 ◦ C austempering temperature. However, upper bainite of 450 ◦ C austempering temperature resulted in higher yield and tensile strength and hardness compared with other austempering temperatures. Abbaszadeh et al. [13] investigated the mechanical properties of mixed bainite-martensite microstructure in D6AC steel. Results showed that the fully martensitic microstructure results in lower yield and tensile strengths than the mixed microstructure containing martensite and 28 vol.% of lower bainite. The fully martensitic microstructure had higher tensile and Charpy V-notch impact properties than the mixed upper bainite-martensite microstructure. Saeidi et al. [14] studied the different microstructures of heat-treated 4340 steel. They found that steels with martensite–ferrite and full bainite microstructures produced lower ductility and charpy impact energy than steel with bainite–ferrite microstructure. Recently, the mechanical properties of bainite have been widely studied by many investigators [10–16]. However, few researchers have studied the details of bainite transformation and activation energy of AISI 6150 steel. The aim of this research is to study the transformation of bainite, in particular lower bainite, to obtain improved mechanical properties of AISI 6150 steel, and then apply AISI 6150 steel to more applications. For this research, ten holding times were studied for each austempering temperature. Seven different temperatures were utilized to obtain the different bainite morphologies which can be used to obtain
Fig. 1 – The microstructure of as received AISI 6150 steel.
desired bainite morphologies for AISI 6150 steel applications.
2.
Materials and experimental procedure
2.1.
Chemical composition
The chromium-vanadium steel samples used in this investigation were 30 mm in diameter with a thickness of 20 mm. The chemical composition of AISI 6150 steel is shown in Table 1. The as received microstructure was ferrite and pearlite as shown in Fig. 1. The black granular matrix is the spheroidal pearlite, and the bright area is the ferrite matrix.
2.2.
Heat treatment process
To study the details of bainite transformation and activation energy of AISI 6150 steel, all the samples were austenized at 855 ◦ C for 20 mins. in a high temperature salt bath followed by austempering at seven different temperature in a lower temperature salt bath: 288 ◦ C, 316 ◦ C, 343 ◦ C, 371 ◦ C, 399 ◦ C, 427 ◦ C, or 454 ◦ C, respectively. The various holding times for each temperature were: 20 s, 30 s, 50 s, 60 s, 120 s, 3 mins., 10 mins., 30 mins., 60 mins., and 120 mins., respectively. Then the samples were immersed into oil and cooled to room temperature. The diagram of the heat treat process is shown in Fig. 2. After heat-treatment, all the austempered samples were cut into small pieces for microstructure observations. Before using the optical microscope, the small pieces of each sample were mounted and polished using sandpaper from 240-grit to 1200-grit, followed by polishing with a suspension of Al2 O3 particles, until the surfaces are mirror like. The samples for microstructure studies were etched with 3% nital. The different bainite morphologies were observed by optical microscope observations.
Please cite this article in press as: Han X, et al. Bainite kinetic transformation of austempered AISI 6150 steel. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.062
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3.2.
Fig. 2 – Sketch of heat treatment process.
2.3.
Rockwell hardness measurement
The hardness of all samples was measured using a standard Rockwell C hardness tester with a 150 kg load. Before the hardness tests, all the samples were ground and polished to obtain flat and smooth surfaces. Each sample was measured at least three times, and the average data were reported.
3.
Results
3.1.
Rockwell hardness
Fig. 3 shows the hardness of different temperatures and various holding times of the austempered AISI 6150 steel samples. The beginning point represents the beginning of bainite transformation. The hardness of austempered 288 ◦ C, 316 ◦ C, and 343 ◦ C disks are almost constant at a holding time of 300 s, see Fig. 3a. However, for the austempered 371 ◦ C, 399 ◦ C, 427 ◦ C, and 454 ◦ C disks, the hardness is almost constant at approximately 200 s, see Fig. 3b. The point where the hardness becomes constant is regarded as the point where the austenite to bainite transformation is complete. As the austempering temperature increased, the slope of the hardness versus time curve increases. This means the bainite transformation is faster. This is because the high temperature enhanced the diffusion of carbon, which results in the high transformation rate of bainite.
3
Microstructures of austempered samples
The optical microscope observations of different bainite morphologies are shown in Fig. 4. The dark black color represents the matrix of bainite, the brown color and the bright areas show the martensite microstructure, and matrix of retained austenite, respectively. The needle like microstructure of lower bainite was obtained at 288 ◦ C, 316 ◦ C, and 343 ◦ C austempering temperature, see Fig. 4a-c. Figs. 4d and 4e show the upper bainite observations. At the beginning of the 371 ◦ C, 399 ◦ C austempering temperatures, coarse bainite was obtained, however, as the holding time increased, granular bainite was obtained. 427 ◦ C, and 454 ◦ C produced granular bainite at the beginning stage, the bainite transformation is almost complete as the holding time increased, see Figs. 4f and 4 g. As seen is Fig. 4 the matrix of bainite becomes coarse and the laths become shorter as the austempering temperature increased. Also, higher temperature results in more carbide precipitates. The difference between upper and lower bainite is shown in Fig. 5. Higher temperatures produced upper bainite, because it permits the excess carbon to partition before it can precipitate in the ferrite. However, for the lower bainite, some of the carbon precipitates in the supersaturated ferrite due to the lower temperature. The ferrite is free of precipitates in upper bainite. In the lower bainite formation, carbide particles precipitate from the supersaturated bainitic ferrite.
3.3.
Transformation kinetics
3.3.1.
Fraction of bainite transformation
The bainite transformation kinetics was studied using hardness analysis, see Equation 1. Fig. 6 shows the rate of transformation for each austempering temperature. 288 ◦ C to 343 ◦ C austempering temperatures take approximately 600 s to complete the transformation. However, 371 ◦ C to 454 ◦ C austempering temperatures take around 350 s to finish the transformation. The transformation becomes faster as the austempereing temperature increased.
X(t) =
H0 − H(t) H0 − Hf
× 100%
(1)
Where: X(t) – The fraction of transformation;
Fig. 3 – Hardness of austempered AISI 6150 steel samples versus holding time. Please cite this article in press as: Han X, et al. Bainite kinetic transformation of austempered AISI 6150 steel. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.062
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Fig. 4 – Micrographs of various holding times for: a) 288 ◦ C austempered AISI 6150 steel, b) 316 ◦ C austempered AISI 6150 steel, c) 343 ◦ C austempered AISI 6150 steel, d) 371 ◦ C austempered AISI 6150 steel, e) 399 ◦ C austempered AISI 6150 steel, f) 427 ◦ C austempered AISI 6150 steel, g) 454 ◦ C austempered AISI 6150 steel.
Please cite this article in press as: Han X, et al. Bainite kinetic transformation of austempered AISI 6150 steel. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.062
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Fig. 5 – Schematic of the formation of upper and lower bainite.
H0 – The initial hardness, which corresponds with the beginning of bainite transformation after a specific holding time; H(t) – The hardness obtained after a holding time at the austempering temperature; Hf – The final hardness which corresponds with the last transformation of the bainitic reaction. The relationship between the transformed fraction and different temperature at various holding times was determined by the “Avrami” equation [17–21], see Eq. 2. X(t)= 1-exp(-ktn )
(2)
Where: X(t) - Transformation fraction at a certain time k – Rate constant which depends on temperature n - Slope of “Avrami” plot By rearrangement of equation 2, the values of “k” and “n” can be determined by Eq. 3 [20,21]. log [−log (1 − X)] = (n log k + loglog e) + n log t
(3)
The diagram of “log [−log(1 − X)]” versus “log t(s)” and using the “Avrami” equation are used to obtain the values of “k” and “n” for each austempering temperature. The “n” is the slope of the “Avrami” plot and “k” is the y-intercept of the best fit line. The regression equations for each austempering temperature are shown in Fig. 7. After solving the “Avrami” equation, the values of “k” and “n” were summarized in Table 2.
3.3.2.
Phase activation energy
The activation energy Q is the minimum energy which allows the atoms to begin a chemical reaction. Using the “Arrhenius” equation, the equation of activation energy and specific reaction rate is shown as follows [17]: K = A ∗ e−Q/RT
Table 2 – The values of “n” and “k” for bainite formation. Temperature (◦ C)
n
k [1/s]
288 316 343 371 399 427 454
1.7233 1.2822 0.5006 0.7036 1.4616 0.535 0.7241
1.1263 × 10−2 6.035 × 10−3 1.2505 × 10−2 1.4394 × 10−2 8.353 × 10−3 2.8507 × 10−2 1.2509 × 10−2
Table 3 – The values of activation energy Q and reaction frequency factor A. J Q ( mol )
Material ◦
◦
6150 (288 C – 343 C) 6150 (371 ◦ C – 454 ◦ C)
A (1/s)
4.27 × 10 1.15 × 104 3
2.3 × 10−2 1.08 × 10−1
Where: K – Rate constant which depends on temperature A - Reaction frequency factor [1/s] R- General gas constant 8.31 [J/mol*K] Q - Activation energy T- Temperature [◦ C] To obtain the values of Q and A, the Arrhenius equation is rewritten, see Eq. 5. Then, plots of log k versus 1/T are constructed, see Fig. 8. The values of Q (activation energy) and A (reaction frequency factor) are determined by the slope of the regression line and intercept with the Y axis, see Table 3. The results show the upper bainite transformation needs more activation energy than the lower bainite transformation. This is because higher temperature provides more energy to allow the excess carbon to partition before it can precipitate in the ferrite. logk = −loge
Q + logA RT
(5)
(4)
Fig. 6 – Fraction of transformation of AISI 6150 steel. Please cite this article in press as: Han X, et al. Bainite kinetic transformation of austempered AISI 6150 steel. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.062
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Fig. 7 – The plot of “log [−log(1 − X)] versus “log t(s)” for: a) 288 ◦ C austempered temperature, b) 316 ◦ C austempered temperature, c) 343 ◦ C austempered temperature, d) 371 ◦ C austempered temperature, e) 399 ◦ C austempered temperature, f) 427 ◦ C austempered temperature, g) 454 ◦ C austempered temperature.
Please cite this article in press as: Han X, et al. Bainite kinetic transformation of austempered AISI 6150 steel. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.062
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Fig. 8 – The linear relationship between log k and 1/T: a) lower bainite of 288 ◦ C to 343 ◦ C austempering temperatures, b) upper bainite of 371 ◦ C to 454 ◦ C austempering temperatures.
4.
Conclusions
In this study, the bainite activation kinetic energy, hardness, and microstructures of different bainite morphologies have been investigated. These results can be used to obtain optimum microstructures of austempered AISI 6150 steel and enhance the mechanical properties of AISI 6150 steel. The results are as follows: 1 371 ◦ C to 454 ◦ C austempering temperatures results in constant hardness faster than the 288 ◦ C to 343 ◦ C austempering temperatures, due to the high temperature enhancing the diffusion of carbon. The higher diffusion rate of carbon increased the transformation rate of bainite. 2 As the austempering temperature increased, the hardness of AISI 6150 steel decreased for the same holding time. 3 Upper bainite produced higher transformation rate than lower bainite. 4 288 ◦ C to 343 ◦ C austempering temperatures result in needle like bainite, however, the 371 ◦ C to 454 ◦ C austempering temperatures produced two morphologies for upper bainite: coarse bainite and granular bainite. Also, higher temperature results in more carbide precipitates. 5 Upper bainite needs higher activation energy than lower bainite to start the reaction. Higher temperature provides more energy to allow the excess carbon to partition before it can precipitate in the ferrite. We declare no financial and personal relationships with organizations and other people. Also, there is no conflicts of interest to this work.
Conflict of interest The authors declare no conflicts of interest.
Acknowledgements This work was supported by the Science and Technology Development Program of Jilin Province, China (20190302004GX) and the Automotive Tribology Center at Oakland University, USA.
references
[1] Li H, Hu J, Li J, Chen G, Sun X. Effect of tempering temperature on microstructure and mechanical properties of AISI 6150 steel. J Cent South Univ 2013;20(4):866–70, doi:10.1007/s11771-013-1559-y. [2] Li C, Liu X, Wang G. Simulation on temperature field of 50CrV4 automobile gear bar steel in continuous rolling by FEM. J Mater Process Technol 2002;120(1-3):26–9, http://dx.doi.org/10.1016/s0924-0136(01)01050-0. [3] Tartaglia JM, Hayrynen KL. A comparison of fatigue properties of austempered versus quenched and tempered 4340 steel. J Mater Eng Perform 2011;21(6):1008–24, http://dx.doi.org/10.1007/s11665-011-9951-y. [4] Lefevre J, Hayrynen KL. Austempered materials for powertrain applications. J Mater Eng Perform 2013;22(7):1914–22, http://dx.doi.org/10.1007/s11665-013-0557-4. [5] Edmonds D, He K, Rizzo F, Cooman BD, Matlock D, Speer J. Quenching and partitioning martensite—a novel steel heat treatment. Mater Sci Eng A 2006;438-440:25–34, http://dx.doi.org/10.1016/j.msea.2006.02.133. [6] Keough JR, Hayrynen KL. Carbidic austempered ductile Iron (CADI), ductile Iron news. Issue 2000;3. [7] Bayati H, Elliott R. Effect of microstructural features on the austempering heat treatment processing window. Mater Sci Forum 2000:73–8, http://dx.doi.org/10.4028/www, scientific.net/msf.329-330.73. [8] Shen FS, Krauss G. The effect of phosphorous content and proeutectoid carbide distribution on the fracture behavior of 52100 steel. Htm J Heat Treat Mater 1982;2(3):238–49, http://dx.doi.org/10.1007/bf02833224. [9] Qiao Z, Liu Y, Yu L, Gao Z. Formation mechanism of granular bainite in a 30CrNi3MoV steel. J Alloys Compd 2009;475(1-2):560–4, http://dx.doi.org/10.1016/j.jallcom.2008.07.110. [10] Bhadeshia HKDH. Bainite in steel. Institute of Materials; 1992. [11] Aglan H, Liu Z, Hassan M, Fateh M. Mechanical and fracture behavior of bainitic rail steel. J Mater Process Technol 2004;151(1-3):268–74, http://dx.doi.org/10.1016/j.jmatprotec.2004.04.073. [12] Wang H, Lü P, Cai X, Zhai B, Zhao J, Wei B. Rapid solidification kinetics and mechanical property characteristics of Ni–Zr eutectic alloys processed under electromagnetic levitation state. Mater Sci Eng A 2019;138660, http://dx.doi.org/10.1016/j.msea.2019.138660. [13] Bakhtiari R, Ekrami A. The effect of bainite morphology on the mechanical properties of a high bainite dual phase (HBDP) steel. Mater Sci Eng A 2009;525(1-2):159–65, http://dx.doi.org/10.1016/j.msea.2009.07.042.
Please cite this article in press as: Han X, et al. Bainite kinetic transformation of austempered AISI 6150 steel. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.062
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[14] Abbaszadeh K, Saghafian H, Kheirandish S. Effect of bainite morphology on mechanical properties of the mixed bainite-martensite microstructure in D6AC steel. J Mater Sci Technol 2012;28(4):336–42, http://dx.doi.org/10.1016/s1005-0302(12)60065-6. [15] Saeidi N, Ekrami A. Comparison of mechanical properties of martensite/ferrite and bainite/ferrite dual phase 4340 steels. Mater Sci Eng A 2009;523(1-2):125–9, http://dx.doi.org/10.1016/j.msea.2009.06.057. [16] Shi L, Yan Z, Liu Y, Yang X, Qiao Z, Ning B, et al. Development of ferrite/bainite bands and study of bainite transformation retardation in HSLA steel during continuous cooling. Met Mater Int 2014;20(1):19–25, http://dx.doi.org/10.1007/s12540-014-1006-0. [17] Qiao Z, Liu Y, Ning B, Li H, Yan Z, Zhou X, et al. Bainitic transformation behavior of ultra-high strength 30CrNi3MoV
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Available online at www.sciencedirect.com
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Original Article
Bainite kinetic transformation of austempered AISI 6150 steel Xue Han a,b , Zhenpu Zhang b , Yanhao Rong b , Steven J. Thrush b,c , Gary C. Barber b , Hongyu Yang a,b,∗ , Feng Qiu a,b,∗ a
Key Laboratory of Automobile Materials, Ministry of Education and School of Materials Science and Engineering, Jilin University, Renmin Street NO. 5988, Changchun 130025, China b Automotive Tribology Center, Department of Mechanical Engineering, School of Engineering and Computer Science, Oakland University, Rochester, Michigan, 48309, USA c U.S. Army Combat Capabilities Development Command Ground Vehicle System Center, 6501 East 11 Mile Rd, Warren, Michigan, 48397, USA
a r t i c l e
i n f o
a b s t r a c t
Article history:
This research studies the kinetic transformation and austempering temperature effect on
Received 13 November 2019
the morphologies of bainite for AISI 6150 steel. A minimum of 10 different holding times for
Accepted 23 November 2019
each austempering temperature were utilized. All the AISI 6150 steel samples with original
Available online xxx
spheroidal pearlite were austenized at 855 ◦ C for 20 min, followed by a lower temperature
Keywords:
temperature and holding time was measured, and the microstructures were observed by
Bainite morphologies
optical microscopy. The kinetic energy for needle like lower bainite and granular like upper
salt bath for austempering at various holding times. The hardness for each austempered
Austempering process
bainite was analyzed by using kinetic transformation equations. The aim of this research
Microstructures
was to study the transformation of bainite, in particular lower bainite, to obtain improved
Transformation kinetics
mechanical properties and higher ductility of AISI 6150 steel, and then apply AISI 6150 steel
Phase activation energy
to more applications. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1.
Introduction
AISI 6150 chromium-vanadium steel is used in many industrial applications including gears, pinions, forged crankshafts, steering knuckles, connecting rods, spindles, pumps, and gear shafts [1,2], due to its high strength, high fatigue strength, and
good hardenability. There are many investigators who have studied austempered AISI 6150 steel, quenched and tempered AISI 6150 steel, and tempered AISI 6150 steel to improve the mechanical properties of AISI 6150 steel. The austempering process offers advantages over quenching and tempering in some applications [3–5].
∗ Corresponding authors at: School of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, P.R. China. E-mails: [email protected] (H. Yang), [email protected] (F. Qiu). https://doi.org/10.1016/j.jmrt.2019.11.062 2238-7854/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Han X, et al. Bainite kinetic transformation of austempered AISI 6150 steel. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.062
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Table 1 – AISI 6150 steel with the chemical composition. C
Cr
Si
Mn
V
P
S
0.45 %-0.83 %
0.8 %-1.1 %
0.15 %-0.3 %
0.7 %-0.9 %
≥0.15 %
≤0.035 %
≤0.04 %
Austempering is a high-performance isothermal heat treatment which can impart superior performance to ferrous metals [6]. Many researchers have suggested that the isothermal heat treatment significantly improves the mechanical properties of materials such as the toughness, strength, and wear resistance, since it produces the bainite microstructure [7,8]. Upper bainite and lower bainite both contain aggregates of small plates or laths of ferrite, however, upper bainite forms at high temperature which allows the excess carbon to partition before it can precipitate in the ferrite. Some of the carbon of lower bainite precipitates in the supersaturated ferrite due to the reduced transformation temperature resulting in slower diffusion [9]. It is well known that material mechanical performances depend on the microstructural modification [10]. Due to the differences in microstructure, therefore, lower bainite and upper bainite have different mechanical properties [11]. Lower bainite results in higher ductility than upper bainite, hence lower bainite has more applications in industry than upper bainite. Bakhtiari et al. [12] studied the mechanical properties of lower bainite and upper bainite of AISI 4340 steel. Their results showed that lower bainite produced with 300 ◦ C austempering temperature or mixed bainite produced with 350 ◦ C austempering temperature resulted in higher yield and ultimate tensile strength, hardness, total elongation and impact energy than upper bainite produced with 400 ◦ C austempering temperature. However, upper bainite of 450 ◦ C austempering temperature resulted in higher yield and tensile strength and hardness compared with other austempering temperatures. Abbaszadeh et al. [13] investigated the mechanical properties of mixed bainite-martensite microstructure in D6AC steel. Results showed that the fully martensitic microstructure results in lower yield and tensile strengths than the mixed microstructure containing martensite and 28 vol.% of lower bainite. The fully martensitic microstructure had higher tensile and Charpy V-notch impact properties than the mixed upper bainite-martensite microstructure. Saeidi et al. [14] studied the different microstructures of heat-treated 4340 steel. They found that steels with martensite–ferrite and full bainite microstructures produced lower ductility and charpy impact energy than steel with bainite–ferrite microstructure. Recently, the mechanical properties of bainite have been widely studied by many investigators [10–16]. However, few researchers have studied the details of bainite transformation and activation energy of AISI 6150 steel. The aim of this research is to study the transformation of bainite, in particular lower bainite, to obtain improved mechanical properties of AISI 6150 steel, and then apply AISI 6150 steel to more applications. For this research, ten holding times were studied for each austempering temperature. Seven different temperatures were utilized to obtain the different bainite morphologies which can be used to obtain
Fig. 1 – The microstructure of as received AISI 6150 steel.
desired bainite morphologies for AISI 6150 steel applications.
2.
Materials and experimental procedure
2.1.
Chemical composition
The chromium-vanadium steel samples used in this investigation were 30 mm in diameter with a thickness of 20 mm. The chemical composition of AISI 6150 steel is shown in Table 1. The as received microstructure was ferrite and pearlite as shown in Fig. 1. The black granular matrix is the spheroidal pearlite, and the bright area is the ferrite matrix.
2.2.
Heat treatment process
To study the details of bainite transformation and activation energy of AISI 6150 steel, all the samples were austenized at 855 ◦ C for 20 mins. in a high temperature salt bath followed by austempering at seven different temperature in a lower temperature salt bath: 288 ◦ C, 316 ◦ C, 343 ◦ C, 371 ◦ C, 399 ◦ C, 427 ◦ C, or 454 ◦ C, respectively. The various holding times for each temperature were: 20 s, 30 s, 50 s, 60 s, 120 s, 3 mins., 10 mins., 30 mins., 60 mins., and 120 mins., respectively. Then the samples were immersed into oil and cooled to room temperature. The diagram of the heat treat process is shown in Fig. 2. After heat-treatment, all the austempered samples were cut into small pieces for microstructure observations. Before using the optical microscope, the small pieces of each sample were mounted and polished using sandpaper from 240-grit to 1200-grit, followed by polishing with a suspension of Al2 O3 particles, until the surfaces are mirror like. The samples for microstructure studies were etched with 3% nital. The different bainite morphologies were observed by optical microscope observations.
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3.2.
Fig. 2 – Sketch of heat treatment process.
2.3.
Rockwell hardness measurement
The hardness of all samples was measured using a standard Rockwell C hardness tester with a 150 kg load. Before the hardness tests, all the samples were ground and polished to obtain flat and smooth surfaces. Each sample was measured at least three times, and the average data were reported.
3.
Results
3.1.
Rockwell hardness
Fig. 3 shows the hardness of different temperatures and various holding times of the austempered AISI 6150 steel samples. The beginning point represents the beginning of bainite transformation. The hardness of austempered 288 ◦ C, 316 ◦ C, and 343 ◦ C disks are almost constant at a holding time of 300 s, see Fig. 3a. However, for the austempered 371 ◦ C, 399 ◦ C, 427 ◦ C, and 454 ◦ C disks, the hardness is almost constant at approximately 200 s, see Fig. 3b. The point where the hardness becomes constant is regarded as the point where the austenite to bainite transformation is complete. As the austempering temperature increased, the slope of the hardness versus time curve increases. This means the bainite transformation is faster. This is because the high temperature enhanced the diffusion of carbon, which results in the high transformation rate of bainite.
3
Microstructures of austempered samples
The optical microscope observations of different bainite morphologies are shown in Fig. 4. The dark black color represents the matrix of bainite, the brown color and the bright areas show the martensite microstructure, and matrix of retained austenite, respectively. The needle like microstructure of lower bainite was obtained at 288 ◦ C, 316 ◦ C, and 343 ◦ C austempering temperature, see Fig. 4a-c. Figs. 4d and 4e show the upper bainite observations. At the beginning of the 371 ◦ C, 399 ◦ C austempering temperatures, coarse bainite was obtained, however, as the holding time increased, granular bainite was obtained. 427 ◦ C, and 454 ◦ C produced granular bainite at the beginning stage, the bainite transformation is almost complete as the holding time increased, see Figs. 4f and 4 g. As seen is Fig. 4 the matrix of bainite becomes coarse and the laths become shorter as the austempering temperature increased. Also, higher temperature results in more carbide precipitates. The difference between upper and lower bainite is shown in Fig. 5. Higher temperatures produced upper bainite, because it permits the excess carbon to partition before it can precipitate in the ferrite. However, for the lower bainite, some of the carbon precipitates in the supersaturated ferrite due to the lower temperature. The ferrite is free of precipitates in upper bainite. In the lower bainite formation, carbide particles precipitate from the supersaturated bainitic ferrite.
3.3.
Transformation kinetics
3.3.1.
Fraction of bainite transformation
The bainite transformation kinetics was studied using hardness analysis, see Equation 1. Fig. 6 shows the rate of transformation for each austempering temperature. 288 ◦ C to 343 ◦ C austempering temperatures take approximately 600 s to complete the transformation. However, 371 ◦ C to 454 ◦ C austempering temperatures take around 350 s to finish the transformation. The transformation becomes faster as the austempereing temperature increased.
X(t) =
H0 − H(t) H0 − Hf
× 100%
(1)
Where: X(t) – The fraction of transformation;
Fig. 3 – Hardness of austempered AISI 6150 steel samples versus holding time. Please cite this article in press as: Han X, et al. Bainite kinetic transformation of austempered AISI 6150 steel. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.062
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Fig. 4 – Micrographs of various holding times for: a) 288 ◦ C austempered AISI 6150 steel, b) 316 ◦ C austempered AISI 6150 steel, c) 343 ◦ C austempered AISI 6150 steel, d) 371 ◦ C austempered AISI 6150 steel, e) 399 ◦ C austempered AISI 6150 steel, f) 427 ◦ C austempered AISI 6150 steel, g) 454 ◦ C austempered AISI 6150 steel.
Please cite this article in press as: Han X, et al. Bainite kinetic transformation of austempered AISI 6150 steel. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.062
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Fig. 5 – Schematic of the formation of upper and lower bainite.
H0 – The initial hardness, which corresponds with the beginning of bainite transformation after a specific holding time; H(t) – The hardness obtained after a holding time at the austempering temperature; Hf – The final hardness which corresponds with the last transformation of the bainitic reaction. The relationship between the transformed fraction and different temperature at various holding times was determined by the “Avrami” equation [17–21], see Eq. 2. X(t)= 1-exp(-ktn )
(2)
Where: X(t) - Transformation fraction at a certain time k – Rate constant which depends on temperature n - Slope of “Avrami” plot By rearrangement of equation 2, the values of “k” and “n” can be determined by Eq. 3 [20,21]. log [−log (1 − X)] = (n log k + loglog e) + n log t
(3)
The diagram of “log [−log(1 − X)]” versus “log t(s)” and using the “Avrami” equation are used to obtain the values of “k” and “n” for each austempering temperature. The “n” is the slope of the “Avrami” plot and “k” is the y-intercept of the best fit line. The regression equations for each austempering temperature are shown in Fig. 7. After solving the “Avrami” equation, the values of “k” and “n” were summarized in Table 2.
3.3.2.
Phase activation energy
The activation energy Q is the minimum energy which allows the atoms to begin a chemical reaction. Using the “Arrhenius” equation, the equation of activation energy and specific reaction rate is shown as follows [17]: K = A ∗ e−Q/RT
Table 2 – The values of “n” and “k” for bainite formation. Temperature (◦ C)
n
k [1/s]
288 316 343 371 399 427 454
1.7233 1.2822 0.5006 0.7036 1.4616 0.535 0.7241
1.1263 × 10−2 6.035 × 10−3 1.2505 × 10−2 1.4394 × 10−2 8.353 × 10−3 2.8507 × 10−2 1.2509 × 10−2
Table 3 – The values of activation energy Q and reaction frequency factor A. J Q ( mol )
Material ◦
◦
6150 (288 C – 343 C) 6150 (371 ◦ C – 454 ◦ C)
A (1/s)
4.27 × 10 1.15 × 104 3
2.3 × 10−2 1.08 × 10−1
Where: K – Rate constant which depends on temperature A - Reaction frequency factor [1/s] R- General gas constant 8.31 [J/mol*K] Q - Activation energy T- Temperature [◦ C] To obtain the values of Q and A, the Arrhenius equation is rewritten, see Eq. 5. Then, plots of log k versus 1/T are constructed, see Fig. 8. The values of Q (activation energy) and A (reaction frequency factor) are determined by the slope of the regression line and intercept with the Y axis, see Table 3. The results show the upper bainite transformation needs more activation energy than the lower bainite transformation. This is because higher temperature provides more energy to allow the excess carbon to partition before it can precipitate in the ferrite. logk = −loge
Q + logA RT
(5)
(4)
Fig. 6 – Fraction of transformation of AISI 6150 steel. Please cite this article in press as: Han X, et al. Bainite kinetic transformation of austempered AISI 6150 steel. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.062
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Fig. 7 – The plot of “log [−log(1 − X)] versus “log t(s)” for: a) 288 ◦ C austempered temperature, b) 316 ◦ C austempered temperature, c) 343 ◦ C austempered temperature, d) 371 ◦ C austempered temperature, e) 399 ◦ C austempered temperature, f) 427 ◦ C austempered temperature, g) 454 ◦ C austempered temperature.
Please cite this article in press as: Han X, et al. Bainite kinetic transformation of austempered AISI 6150 steel. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.062
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Fig. 8 – The linear relationship between log k and 1/T: a) lower bainite of 288 ◦ C to 343 ◦ C austempering temperatures, b) upper bainite of 371 ◦ C to 454 ◦ C austempering temperatures.
4.
Conclusions
In this study, the bainite activation kinetic energy, hardness, and microstructures of different bainite morphologies have been investigated. These results can be used to obtain optimum microstructures of austempered AISI 6150 steel and enhance the mechanical properties of AISI 6150 steel. The results are as follows: 1 371 ◦ C to 454 ◦ C austempering temperatures results in constant hardness faster than the 288 ◦ C to 343 ◦ C austempering temperatures, due to the high temperature enhancing the diffusion of carbon. The higher diffusion rate of carbon increased the transformation rate of bainite. 2 As the austempering temperature increased, the hardness of AISI 6150 steel decreased for the same holding time. 3 Upper bainite produced higher transformation rate than lower bainite. 4 288 ◦ C to 343 ◦ C austempering temperatures result in needle like bainite, however, the 371 ◦ C to 454 ◦ C austempering temperatures produced two morphologies for upper bainite: coarse bainite and granular bainite. Also, higher temperature results in more carbide precipitates. 5 Upper bainite needs higher activation energy than lower bainite to start the reaction. Higher temperature provides more energy to allow the excess carbon to partition before it can precipitate in the ferrite. We declare no financial and personal relationships with organizations and other people. Also, there is no conflicts of interest to this work.
Conflict of interest The authors declare no conflicts of interest.
Acknowledgements This work was supported by the Science and Technology Development Program of Jilin Province, China (20190302004GX) and the Automotive Tribology Center at Oakland University, USA.
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Please cite this article in press as: Han X, et al. Bainite kinetic transformation of austempered AISI 6150 steel. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.062