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Microstructure and mechanical properties of austempered ductile iron with different strength grades
Materials Letters 119 (2014) 47–50
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microstructure and mechanical properties of austempered ductile iron with different strength grades Jiwang Zhang a,n, Ning Zhang a, Mintang Zhang b, Liantao Lu a, Dongfang Zeng a, Qingpeng Song a a b
State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China Henan Aoudi Co., Ltd., Hebi 456750, China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 November 2013 Accepted 20 December 2013 Available online 31 December 2013
Austempered ductile irons (ADIs) with three strength grades were produced successfully by different two-stage heat treatments. The microstructure and mechanical properties of ADIs such as tensile strength, yield strength, elongation and impact toughness were studied. The results show that the strengths of the three grades ADIs well satisfy the requirement of ASTM standard 897M-06 grade 900/ 650/09, grade 1050/750/07 and grade 1200/850/04. Meanwhile, the ductile and impact toughness of the ADIs are larger than those required by the ASTM standard. The microstructure has obvious influence on mechanical properties and fracture behavior. With the decrease of austempering temperature, yield strength and tensile strength of the ADIs increase, while both the impact energy and elongation of them decrease. & 2013 Elsevier B.V. All rights reserved.
Keywords: Austempered ductile iron Microstructure Mechanical properties Impact toughness
1. Introduction Austempered ductile iron (ADI) is considered as a new kind of engineering material and exhibits excellent combinations of high strength, ductility, toughness, fatigue strength and wear resistance [1–5].The attractive properties of ADI are related to the unique microstructure that consists of high carbon austenite and acicular bainitic ferrite with graphite nodules dispersed in the matrix [6]. In recent years, it has been applied to many engineering components such as gears and crankshafts [7,8]. Considerable works have also been conducted on microstructural characteristics as well as mechanical properties and the influences of factors such as austempering temperature, austempering time and alloying elements on microstructure and mechanical properties have been investigated [1–3,9–13]. With the development of ADI, standards for ADI have also been published [14]. In ASTM standard 897M-06, there are 6 grades of ADI according to the ultimate tensile strength. In later years, the research and application of ADI have developed rapidly in China and the standard GB/T 24733-2009 has been published in 2009 [15]. However, researches on ADIs are related primarily to the austempering process parameters rather than to the standard ADI strength grade, which can be confusing to the designer of ADI structures [4]. So the microstructure and properties of ADI with different strength grades should be studied in detail to supply
n
Corresponding author. Tel.: þ 86 28 87600843; fax: þ 86 28 87600868. E-mail address: [email protected] (J. Zhang).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.086
useful information for the ADI makers and component designers. In this study, ADIs with three strength grades were produced; the microstructure, tensile tests and impact tests were conducted; the influences of microstructure on mechanical properties and fracture behavior during impact testing were discussed.
2. Experimental procedure Experimental material and heat treatment: The composition of the ductile iron used in this study was (wt%) 3.70 C, 2.60 Si, 0.19 Mn, 0.62 Ni, 0.20 Mo, 0.61 Cu, 0.013 S, 0.025 P, 0.035 Mg, and balance Fe. The ductile iron was cast in the shape of 25 mm Y-blocks as shown in Fig. 1. In order to obtain ADIs with different strength grades, the cast irons were treated by different heat treatments. The first kind of samples, shortened as ADI1, were austenitized at 910 1C for 120 min and transferred rapidly to a salt bath held at a preselected austempering temperature of 380 1C for 60 min. The second kind of samples, shortened as ADI2, were austenitized at 900 1C for 110 min and austempered at 340 1C for 60 min. The third kind of samples, shortened as ADI3, were austenitized at 910 1C for 150 min and austempered at 300 1C for 120 min. After the heat treatments, samples for microstructure observation, hardness test, tension test and Charpy impact test were taken from the bottom of those blocks as indicated in Fig. 1. Microstructure: The samples were etched in 4% nital solution at room temperature after being polished, then they were observed by an optical microscope. The graphite morphology was rated for
48
J. Zhang et al. / Materials Letters 119 (2014) 47–50
Fig. 1. Dimensions of Y-blocks (unit: mm).
Fig. 2. Microstructures of ADIs observed by optical microscope. (a) ADI1; (b) ADI2; and (c) ADI3.
J. Zhang et al. / Materials Letters 119 (2014) 47–50
the nodularity and nodule count in accordance with the ASTM standard A247 [16]. Tensile tests: The diameter of the tensile sample was 14 mm and the tensile tests were performed based on ASTM standard E-8 [17].
49
Five identical samples were tested for each group and the values were determined by averaging the values of them. Hardness tests: Brinell hardness tests were carried out in the Brinell hardness tester with a load of 300 kg. For each specimen
Table 1 Mechanical properties and impact energy of ADIs. Sample number Ultimate tensile strength (MPa) Yield strength (MPa) Young0 s modulus (GPa) Elongation, (%) Brinell hardness Microhardness Impact energy, (J) ADI1 ADI2 ADI3 n
963(16)* 1140(15) 1290(27)
712(14) 830(12) 952(16)
166(6) 166(3) 169(7)
11.2(0.6) 10.2(0.8) 7.5(0.6)
278(6) 340(10) 383(11)
The values in the brackets are the standard deviation values.
Fig. 3. Fracture surface observations of ADIs for impact tests. (a) ADI1; (b) ADI2; and (c) ADI3.
294(10) 359(8) 403(9)
125(3) 115(2) 98(5)
50
J. Zhang et al. / Materials Letters 119 (2014) 47–50
condition, 10 readings were taken and averaged to represent the hardness values (BHN). Impact tests: Impact toughness of the samples was measured by the Charpy impact test. Tests were carried out at room temperature with unnotched specimens. 3. Results and discussions Microstructure: Fig. 2 shows the microstructures of the samples observed by optical microscope. As shown in Fig. 2, all the austempered microstructures show a matrix consisting of a two-phase mixture of a dark etching bainitic ferrite, which is needle shaped, and the bright etching retained austenite with graphite nodules dispersed in it. The microstructure of ADI1 consists of coarse upper bainite and coarse blocky austenite. The microstructure of ADI2 consists of upper and lower bainite with relatively coarse austenite. However, the microstructure of ADI3 consists of lower bainite in the form of fine needles and fine austenite. The length of the ferrite needles was generally found to increase with the increasing of austenitizing temperature [11]. Though the austenitizing temperature of ADI2 is a little lower than those of the others, the ferrite needle lengths of them show no obvious difference. Therefore, the difference of the microstructure is considered to be mainly attributed to the austempering temperatures. Besides, the metallographic results reveal that the ADIs have similar average nodularity of 91% and the average nodule count is about 126 nodules/mm2. Mechanical properties: The tensile test results of the ADIs are summarized in Table 1. It can be found that with the increase of the strength of ADIs the elongations of them decrease from 11.2% to 7.5%. Besides, it can be seen that the properties of ADI1, ADI2 and ADI3 satisfy the strength and elongation requirement of ASTM standard 897M-06 grade 900/650/09, grade 1050/750/07 and grade 1200/850/04. The decrease of yield strength and tensile strength is due to the increase of retained austenite as well as the coarsening of the microstructure with the increase of austempering temperature [12]. Therefore, the fairly low strength of ADI1 is attributed to the presence of coarse upper bainite and a large amount of blocky austenite in the austempered structure, while the fairly high strength values of ADI3 are attributed to the presence of lower bainite and the finer size of retained austenite in the austempered structure. Microhardness and impact toughness: The hardness and impact energy of ADI1, ADI2 and ADI3 are given in Table 1. It can be seen that with the increase of austempering temperature the hardness of ADI decreases, while the average impact energies of it increases. The change in microstructure morphology from lower bainite to upper bainite and the retained austenite contents increase are largely responsible for the change. Austenite is a softer phase, therefore as the retained austenite content increases the hardness decreases and the impact toughness increases [12]. Meanwhile, it can be seen that both the hardness and impact toughness of ADI1, ADI2 and ADI3 satisfy the requirement of ASTM 897M-06 grade 900/650/09, grade 1050/750/07 and grade 1200/850/04. Fracture behavior: Fig. 3 shows the fractured surfaces of the samples for impact tests. The fracture surface of ADI1 with the highest values of retained austenite volume fraction and impact energy shows a large number of dimples on the entire surface. As shown in Fig. 3b, the fracture surface of ADI2 shows that the plastic deformation spreads over the matrix but the dimple size decreases compared to that of ADI1. However, there are many small-sized dimples on the fracture surface of ADI3 with the lowest values of retained austenite volume fraction and impact energy.
In order to expand the application of ADI, the ductility and toughness of it should be improved further [10]. In this study ADIs with three strength grades were produced by different heat treatments and the strengths of them satisfy the requirement of the ASTM standard. Meanwhile, the ductile and impact toughness are larger than those required by the ASTM standard. So it is expected that ADI will be applied to other components with high requirement of ductility. The designers can select ADIs with different strengths according to the requirements of the structure loadings. Besides, good properties of wear and fatigue are also required for components during service, so it is necessary to investigate the fatigue property and wear resistance of ADI. Thus, the research on them will be carried out in the future work.
4. Conclusions Based on the results of this study, the following conclusions can be drawn: 1. ADIs with three strength grades were produced successfully and the strengths of them satisfy the requirement of ASTM standard 897M-06 grade 900/650/09, grade 1050/750/07 and grade 1200/850/04. Meanwhile, the ductile and impact toughness of the ADIs are larger than those required by the ASTM standard. 2. The austempering temperature has significant influence on microstructures of ADIs. When the austempering temperature decreases from 380 1C to 300 1C, the morphology of bainitic ferrite changes from upper to lower and the volume fraction of retained austenite decreases. 3. As the austempering temperature decreases from 380 1C to 300 1C, the yield strength and tensile strength of the ADIs increase, while both the impact energy and elongation of them decrease.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51305363) and the State Key Laboratory Independent Research of China (No. 2012TPL_T13). References [1] Putatunda SK. Mater Sci Eng—Struct Mater Prop Microstruct Process 2001;315:70–80. [2] Eric O, Sidjanin L, Miskovic Z, Zec S, Jovanovic MT. Mater Lett 2004;58:2707–11. [3] Kim YJ, Shin H, Park H, Lim JD. Mater Lett 2008;62:357–60. [4] Lerner YS, Kingsbury GR. J Mater Eng Perform 1998;7:48–52. [5] Chapetti MD. Int J Fatigue 2007;29:860–8. [6] Eric O, Rajnovic D, Zec S, Sidjanin L, Jovanovic MT. Mater Charact 2006;57:211–7. [7] Magalhaes L, Martins R, Seabra J. Tribol Int 2012;46:97–105. [8] Lefevre J, Hayrynen KL. J Mater Eng Perform 2013;22:1914–22. [9] Liu SF, Chen Y, Chen X, Miao HM. J Iron Steel Res Int 2012;19:36–42. [10] Putatunda SK, Gadicherla PK. Mater Sci Eng—StructMater Prop Microstruct Process 1999;268:15–31. [11] Rao PP, Putatunda SK. Mater Sci Eng–Struct Mater Prop Microstruct Process 2003;349:136–49. [12] Putatunda SK, Kesani S, Tackett R, Lawes G. Mater Sci Eng—Struct Mater Prop Microstruct Process 2006;435:112–22. [13] Hsu CH, Lin KT. Mater Sci Eng—Struct Mater Prop Microstruct Process 2011;528:5706–12. [14] A 897M. Annual book of ASTM Standards. vol. 01.02; 2006. [15] GB/T 24733. Austempered ductile iron (ADI) castings. 2009. [16] ASTM A247. Annual book of ASTM Standards. vol. 01.02; 1990. [17] ASTM E-8. Annual book of ASTM Standards. vol. 3.01; 1993.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microstructure and mechanical properties of austempered ductile iron with different strength grades Jiwang Zhang a,n, Ning Zhang a, Mintang Zhang b, Liantao Lu a, Dongfang Zeng a, Qingpeng Song a a b
State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China Henan Aoudi Co., Ltd., Hebi 456750, China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 November 2013 Accepted 20 December 2013 Available online 31 December 2013
Austempered ductile irons (ADIs) with three strength grades were produced successfully by different two-stage heat treatments. The microstructure and mechanical properties of ADIs such as tensile strength, yield strength, elongation and impact toughness were studied. The results show that the strengths of the three grades ADIs well satisfy the requirement of ASTM standard 897M-06 grade 900/ 650/09, grade 1050/750/07 and grade 1200/850/04. Meanwhile, the ductile and impact toughness of the ADIs are larger than those required by the ASTM standard. The microstructure has obvious influence on mechanical properties and fracture behavior. With the decrease of austempering temperature, yield strength and tensile strength of the ADIs increase, while both the impact energy and elongation of them decrease. & 2013 Elsevier B.V. All rights reserved.
Keywords: Austempered ductile iron Microstructure Mechanical properties Impact toughness
1. Introduction Austempered ductile iron (ADI) is considered as a new kind of engineering material and exhibits excellent combinations of high strength, ductility, toughness, fatigue strength and wear resistance [1–5].The attractive properties of ADI are related to the unique microstructure that consists of high carbon austenite and acicular bainitic ferrite with graphite nodules dispersed in the matrix [6]. In recent years, it has been applied to many engineering components such as gears and crankshafts [7,8]. Considerable works have also been conducted on microstructural characteristics as well as mechanical properties and the influences of factors such as austempering temperature, austempering time and alloying elements on microstructure and mechanical properties have been investigated [1–3,9–13]. With the development of ADI, standards for ADI have also been published [14]. In ASTM standard 897M-06, there are 6 grades of ADI according to the ultimate tensile strength. In later years, the research and application of ADI have developed rapidly in China and the standard GB/T 24733-2009 has been published in 2009 [15]. However, researches on ADIs are related primarily to the austempering process parameters rather than to the standard ADI strength grade, which can be confusing to the designer of ADI structures [4]. So the microstructure and properties of ADI with different strength grades should be studied in detail to supply
n
Corresponding author. Tel.: þ 86 28 87600843; fax: þ 86 28 87600868. E-mail address: [email protected] (J. Zhang).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.086
useful information for the ADI makers and component designers. In this study, ADIs with three strength grades were produced; the microstructure, tensile tests and impact tests were conducted; the influences of microstructure on mechanical properties and fracture behavior during impact testing were discussed.
2. Experimental procedure Experimental material and heat treatment: The composition of the ductile iron used in this study was (wt%) 3.70 C, 2.60 Si, 0.19 Mn, 0.62 Ni, 0.20 Mo, 0.61 Cu, 0.013 S, 0.025 P, 0.035 Mg, and balance Fe. The ductile iron was cast in the shape of 25 mm Y-blocks as shown in Fig. 1. In order to obtain ADIs with different strength grades, the cast irons were treated by different heat treatments. The first kind of samples, shortened as ADI1, were austenitized at 910 1C for 120 min and transferred rapidly to a salt bath held at a preselected austempering temperature of 380 1C for 60 min. The second kind of samples, shortened as ADI2, were austenitized at 900 1C for 110 min and austempered at 340 1C for 60 min. The third kind of samples, shortened as ADI3, were austenitized at 910 1C for 150 min and austempered at 300 1C for 120 min. After the heat treatments, samples for microstructure observation, hardness test, tension test and Charpy impact test were taken from the bottom of those blocks as indicated in Fig. 1. Microstructure: The samples were etched in 4% nital solution at room temperature after being polished, then they were observed by an optical microscope. The graphite morphology was rated for
48
J. Zhang et al. / Materials Letters 119 (2014) 47–50
Fig. 1. Dimensions of Y-blocks (unit: mm).
Fig. 2. Microstructures of ADIs observed by optical microscope. (a) ADI1; (b) ADI2; and (c) ADI3.
J. Zhang et al. / Materials Letters 119 (2014) 47–50
the nodularity and nodule count in accordance with the ASTM standard A247 [16]. Tensile tests: The diameter of the tensile sample was 14 mm and the tensile tests were performed based on ASTM standard E-8 [17].
49
Five identical samples were tested for each group and the values were determined by averaging the values of them. Hardness tests: Brinell hardness tests were carried out in the Brinell hardness tester with a load of 300 kg. For each specimen
Table 1 Mechanical properties and impact energy of ADIs. Sample number Ultimate tensile strength (MPa) Yield strength (MPa) Young0 s modulus (GPa) Elongation, (%) Brinell hardness Microhardness Impact energy, (J) ADI1 ADI2 ADI3 n
963(16)* 1140(15) 1290(27)
712(14) 830(12) 952(16)
166(6) 166(3) 169(7)
11.2(0.6) 10.2(0.8) 7.5(0.6)
278(6) 340(10) 383(11)
The values in the brackets are the standard deviation values.
Fig. 3. Fracture surface observations of ADIs for impact tests. (a) ADI1; (b) ADI2; and (c) ADI3.
294(10) 359(8) 403(9)
125(3) 115(2) 98(5)
50
J. Zhang et al. / Materials Letters 119 (2014) 47–50
condition, 10 readings were taken and averaged to represent the hardness values (BHN). Impact tests: Impact toughness of the samples was measured by the Charpy impact test. Tests were carried out at room temperature with unnotched specimens. 3. Results and discussions Microstructure: Fig. 2 shows the microstructures of the samples observed by optical microscope. As shown in Fig. 2, all the austempered microstructures show a matrix consisting of a two-phase mixture of a dark etching bainitic ferrite, which is needle shaped, and the bright etching retained austenite with graphite nodules dispersed in it. The microstructure of ADI1 consists of coarse upper bainite and coarse blocky austenite. The microstructure of ADI2 consists of upper and lower bainite with relatively coarse austenite. However, the microstructure of ADI3 consists of lower bainite in the form of fine needles and fine austenite. The length of the ferrite needles was generally found to increase with the increasing of austenitizing temperature [11]. Though the austenitizing temperature of ADI2 is a little lower than those of the others, the ferrite needle lengths of them show no obvious difference. Therefore, the difference of the microstructure is considered to be mainly attributed to the austempering temperatures. Besides, the metallographic results reveal that the ADIs have similar average nodularity of 91% and the average nodule count is about 126 nodules/mm2. Mechanical properties: The tensile test results of the ADIs are summarized in Table 1. It can be found that with the increase of the strength of ADIs the elongations of them decrease from 11.2% to 7.5%. Besides, it can be seen that the properties of ADI1, ADI2 and ADI3 satisfy the strength and elongation requirement of ASTM standard 897M-06 grade 900/650/09, grade 1050/750/07 and grade 1200/850/04. The decrease of yield strength and tensile strength is due to the increase of retained austenite as well as the coarsening of the microstructure with the increase of austempering temperature [12]. Therefore, the fairly low strength of ADI1 is attributed to the presence of coarse upper bainite and a large amount of blocky austenite in the austempered structure, while the fairly high strength values of ADI3 are attributed to the presence of lower bainite and the finer size of retained austenite in the austempered structure. Microhardness and impact toughness: The hardness and impact energy of ADI1, ADI2 and ADI3 are given in Table 1. It can be seen that with the increase of austempering temperature the hardness of ADI decreases, while the average impact energies of it increases. The change in microstructure morphology from lower bainite to upper bainite and the retained austenite contents increase are largely responsible for the change. Austenite is a softer phase, therefore as the retained austenite content increases the hardness decreases and the impact toughness increases [12]. Meanwhile, it can be seen that both the hardness and impact toughness of ADI1, ADI2 and ADI3 satisfy the requirement of ASTM 897M-06 grade 900/650/09, grade 1050/750/07 and grade 1200/850/04. Fracture behavior: Fig. 3 shows the fractured surfaces of the samples for impact tests. The fracture surface of ADI1 with the highest values of retained austenite volume fraction and impact energy shows a large number of dimples on the entire surface. As shown in Fig. 3b, the fracture surface of ADI2 shows that the plastic deformation spreads over the matrix but the dimple size decreases compared to that of ADI1. However, there are many small-sized dimples on the fracture surface of ADI3 with the lowest values of retained austenite volume fraction and impact energy.
In order to expand the application of ADI, the ductility and toughness of it should be improved further [10]. In this study ADIs with three strength grades were produced by different heat treatments and the strengths of them satisfy the requirement of the ASTM standard. Meanwhile, the ductile and impact toughness are larger than those required by the ASTM standard. So it is expected that ADI will be applied to other components with high requirement of ductility. The designers can select ADIs with different strengths according to the requirements of the structure loadings. Besides, good properties of wear and fatigue are also required for components during service, so it is necessary to investigate the fatigue property and wear resistance of ADI. Thus, the research on them will be carried out in the future work.
4. Conclusions Based on the results of this study, the following conclusions can be drawn: 1. ADIs with three strength grades were produced successfully and the strengths of them satisfy the requirement of ASTM standard 897M-06 grade 900/650/09, grade 1050/750/07 and grade 1200/850/04. Meanwhile, the ductile and impact toughness of the ADIs are larger than those required by the ASTM standard. 2. The austempering temperature has significant influence on microstructures of ADIs. When the austempering temperature decreases from 380 1C to 300 1C, the morphology of bainitic ferrite changes from upper to lower and the volume fraction of retained austenite decreases. 3. As the austempering temperature decreases from 380 1C to 300 1C, the yield strength and tensile strength of the ADIs increase, while both the impact energy and elongation of them decrease.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51305363) and the State Key Laboratory Independent Research of China (No. 2012TPL_T13). References [1] Putatunda SK. Mater Sci Eng—Struct Mater Prop Microstruct Process 2001;315:70–80. [2] Eric O, Sidjanin L, Miskovic Z, Zec S, Jovanovic MT. Mater Lett 2004;58:2707–11. [3] Kim YJ, Shin H, Park H, Lim JD. Mater Lett 2008;62:357–60. [4] Lerner YS, Kingsbury GR. J Mater Eng Perform 1998;7:48–52. [5] Chapetti MD. Int J Fatigue 2007;29:860–8. [6] Eric O, Rajnovic D, Zec S, Sidjanin L, Jovanovic MT. Mater Charact 2006;57:211–7. [7] Magalhaes L, Martins R, Seabra J. Tribol Int 2012;46:97–105. [8] Lefevre J, Hayrynen KL. J Mater Eng Perform 2013;22:1914–22. [9] Liu SF, Chen Y, Chen X, Miao HM. J Iron Steel Res Int 2012;19:36–42. [10] Putatunda SK, Gadicherla PK. Mater Sci Eng—StructMater Prop Microstruct Process 1999;268:15–31. [11] Rao PP, Putatunda SK. Mater Sci Eng–Struct Mater Prop Microstruct Process 2003;349:136–49. [12] Putatunda SK, Kesani S, Tackett R, Lawes G. Mater Sci Eng—Struct Mater Prop Microstruct Process 2006;435:112–22. [13] Hsu CH, Lin KT. Mater Sci Eng—Struct Mater Prop Microstruct Process 2011;528:5706–12. [14] A 897M. Annual book of ASTM Standards. vol. 01.02; 2006. [15] GB/T 24733. Austempered ductile iron (ADI) castings. 2009. [16] ASTM A247. Annual book of ASTM Standards. vol. 01.02; 1990. [17] ASTM E-8. Annual book of ASTM Standards. vol. 3.01; 1993.