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Effect of austempering on microstructure and mechanical properties of a GCr18Mo steel
Materials Science and Engineering A 438–440 (2006) 251–253
Effect of austempering on microstructure and mechanical properties of a GCr18Mo steel Jiuba Wen ∗ , Qian Li, Yongqiang Long School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, China Received 21 March 2005; received in revised form 9 February 2006; accepted 28 February 2006
Abstract The microstructure and mechanical properties of a GCr18Mo steel were investigated. The martensite/lower bainite (BL /M) duplex structure with various BL volume fractions (fBL ) or that full of BL were produced by austenitizing at 870 ◦ C, quenching in nitrates at 230 ◦ C and holding for different times. The steels with such structures exhibit higher strength–toughness than that of full martensite. When fBL is approximately 37.5% obtained by holding for about 60 min, the duplex-structured steel gives rise to the best combination of mechanical properties. © 2006 Elsevier B.V. All rights reserved. Keywords: GCr18Mo steel; BL /M duplex structure; Austempering
1. Introduction It is well known that the steel GCr15 with lower bainite (BL ) structure acquired by austempering possesses favorable mechanical properties [1,2]. Recent studies [3–5] further revealed that a better combination of properties could be achieved when a martensite/lower bainite (BL /M) duplex microstructure, in which the lower bainite volume fraction (fBL ) ranges from 20 to 40%, is produced via austempering, compared with that of full BL or the others. This is because of that during isothermal holding the prior precipitated BL cuts austenite grains and, thus, makes the subsequently formed martensite colonies be finer. Meanwhile, aiming to meet the increasing requirement in hardenability, several high-carbon chromium bearing steels appeared, such as SKF25 (Sweden) and 100CrMo7 (Germany) [6]. Based on 100CrMo7, a GCr18Mo steel was developed in China in the 1990s [7]. The higher contents of Cr and Mo hinder the grain growth, enhance the hardness, strength and the hardenability [8–10]. Besides, the GCr18Mo steel has a lowered Ms temperature that benefits the progress of bainitic transformation. The present work, the strength–toughness of GCr18Mo steel, in austempered state, with BL /M duplex and full BL structures ∗
Corresponding author. Tel.: +86 379 64231846; fax: +86 379 64230597. E-mail address: [email protected] (J. Wen).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.199
is investigated and the optimized fBL in duplex is explored for a best combination of the mechanical properties. 2. Experimental The chemical composition of the investigated GCr18Mo steel is listed in Table 1. The as-received material was in spheroidizing annealed state. The samples were austenized at 870 ◦ C for 30 min. Then, some of them were directly quenched in oil at 80 ◦ C, while the others were austempered at 230 ◦ C in salt bath for 15, 20, 25, 30, 38, 45, 60 and 90 min, respectively, followed by oil cooling. Tempering was carried out at 200 ◦ C for 4 h and then cooling by air. In addition, there is a sample austempered at 230 ◦ C for 4 h and cooled by air but without tempering followed. By using different heat treatments, a series of BL /M duplex microstructures was produced, in which the lower bainite volume fraction (fBL ) varies from 0 to 100% as shown in Table 2. The Rockwell hardness was measured at a HR-150A tester and the impact value (αK ) at a JB30A tester with specimen dimensions 10 mm × 10 mm × 55 mm. The bending strength was tested at a W3-515 hydraulic machine, using the specimens 10 mm × 10 mm × 110 mm with 100 mm span. For the type I fracture toughness (KIC ) tests, the 3-point bending specimens B × W × L = 10 mm × 10 mm × 110 mm with 100 mm span, on which a 9 mm deep notch was spark-eroded by using a Mo wire
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Table 1 Chemical composition of the GCr18Mo steel (wt.%)
3. Results and discussion
C Si Mn P Cr Mo S Ni Cu
Fig. 1 shows the optical microstructures of the samples austempered at 230 ◦ C for different times, resulting in fBL = 29.4, 37.5, 73.3 and 100%, respectively (refer to Table 2). It was found that at the beginning, e.g. the holding time (t) shorter than 30 min, the fBL slowly increases with increasing t and lower bainite are mainly formed as isolated fine needles. When t = 38 min (Fig. 1(a)), bainite starts to form quickly in lobate or acicular shape. At 60 min (Fig. 1(b)), the BL needles grow up gradually and start to form in hassocks. The fBL reaches to 73.3% at t = 90 min and bainite becomes mostly in connected hassocks (Fig. 1(c)). By holding for 240 min, the steel contains full of BL and no single bainite needle was detected as seen in Fig. 1(d). As seen in Table 2, by tempering at the same temperature (200 ◦ C), the hardness of GCr18Mo steel quenched into full bainite and BL /M duplex is slightly lower than that of full martensite. As the austempering time increases, the hardness decreases but they are all over HRC 58.5, satisfying the service requirement of the high speed bearing steels.
0.9–1.05 0.2–0.4 0.25–0.45 ≤0.015 1.65–1.95 0.15–0.25 ≤0.015 ≤0.3 ≤0.3
of 0.18 mm diameter. The KIC was calculated by Y (a/W)P␦ BW 1/2 where P␦ is the ultimate load, i.e. the load at fracture (kN); a the crack size (mm) and Y is the factor of crack size (a/W). The quenched sample was etched by LEPERA reagent while the tempered ones by 4% nital. The quantitative measurement of duplex structure was carried out with a ISA-4 image analysis system. KIC =
Table 2 Relationship between volume fraction of lower bainite and hardness Austempering time, t (min)
0
30
38
45
60
90
240
fBL (%)
0
0.5
1.0
4.5
10.2
29.4
32.7
37.5
73.3
100
66.8 61.1
66.4 60.9
66.4 60.6
66.1 61.7
65.4 60.6
63.4 59.9
64.6 61.0
63.2 59.2
60.2 58.6
Quenched state (HRC) Tempered state (HRC)
15
20
25
Fig. 1. Optical micrographs of the GCr18Mo steel austempered for different times: (a) t = 38 min; (b) t = 60 min; (c) t = 90 min; (d) t = 240 min.
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The fracture toughness KIC of the GCr18Mo steel was improved obviously by austempering treatment. It is 20.5 MPa/ m2 in the quenched and tempered state with full of martensite, while it increases to 45.0 and 52.4 MPa/m2 when fBL is 32.7 (t = 45 min) and 100% (240 min), respectively. As it is well known that the fracture mechanism and the path of crack propagation strongly depend on the microstructure of materials. In the investigated steel containing small amount of lower bainite, e.g. fBL less than 20%, intergranular fracture rarely occurs. When the cleavage crack meets BL /M interface, the cracking can change its propagation path, the stress concentration will be effectively released and thus shortens the length of single cracks. In the meantime, the incoherent ferrite in lower bainite makes the stress at the crack tip relaxed which is favorable for the ductile mechanism being undertaken instead of the brittle ones [11,12]. Fig. 2. Relationship between the amount of lower bainite and impact toughness.
4. Conclusion The GCr18Mo steel in conventional quenched state exhibits high hardness (66.8 HRC) but low strength–toughness (αK = 60 × 104 J/m2 and σ bb = 2805 MPa). Austempering treatment can dramatically improve its αK , σ bb and KIC to about 1.5–2.5 times high, respectively. The steel composed of BL /M duplex structure with 37.5% volume fraction of lower bainite produced by austempering at 230 ◦ C for 60 min and tempering at 200 ◦ C for 4 h gives rise to an optimized combination of mechanical properties, that is, αK = 108 × 104 J/m2 , σ bb = 4485 MPa and KIC = 45.0 MPa/m2 together with a rather high hardness 59.2 HRC. References
Fig. 3. Relationship between the amount of lower bainite and bending strength.
Fig. 2 shows the dependence of impact ductility on fBL . It is indicated that compared with conventional martensite quenching, austempering dramatically improves the impact ductility of the steel. The αK increases with increasing fBL and reaches to 167 × 104 J/m2 for 100% BL structure. The variation of ultimate bending strength (σ bb ) with fBL is shown in Fig. 3. One can find again that the σ bb of BL /M and full BL steel is superior to that of full martensite structure, especially when fBL exceeds 10.2%. It achieves a maximum 4485 MPa for 37.5% BL and keeps quite high (3885 MPa) even when the steel contains full of bainite.
[1] F. Wang, Bearing 1 (1992) 54–60. [2] Y. Liu, T. Jiang, Bearing 9 (1994) 32–37. [3] T.Y. Hsu (Xu Zuyao), Y. Chen, W. Chen, Metall. Trans. A 18 (1987) 1531–1532. [4] X. Lin, Mech. Eng. Mater. 2 (2000) 40–43. [5] Z. Zhang, Y. Liu, Z. Fan, Trans. Mater. Heat Treatment 1 (2002) 57–60. [6] X. Wang, Z. Zhang, Hot Working Technol. 2 (2000) 34–36. [7] M. Sun, T. Lin, S. Yao, Bearing 8 (2001) 28–30. [8] R.F. Hehemann, K.R. Kinsman, H.I. Aaronson, Metall. Trans. 3 (1972) 1077–1094. [9] H.K.D.H. Bhadeshia, D.V. Edmonds, Metall. Trans. A 10 (1979) 895– 907. [10] S.K. Liu, W.T. Reynolds Jr., H. Hu, G.J. Shiflet, H.I. Aaronson, Metall. Trans. A 16 (1985) 457–468. [11] E. Yajima, T. Miyazaki, T. Sugiyama, H. Terajima, Trans. Jpn. Inst. Met. 15 (1974) 173–179. [12] Z. Li, D. Wu, J. Iron Steel Res. 11 (6) (2004) 40–44.
Effect of austempering on microstructure and mechanical properties of a GCr18Mo steel Jiuba Wen ∗ , Qian Li, Yongqiang Long School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, China Received 21 March 2005; received in revised form 9 February 2006; accepted 28 February 2006
Abstract The microstructure and mechanical properties of a GCr18Mo steel were investigated. The martensite/lower bainite (BL /M) duplex structure with various BL volume fractions (fBL ) or that full of BL were produced by austenitizing at 870 ◦ C, quenching in nitrates at 230 ◦ C and holding for different times. The steels with such structures exhibit higher strength–toughness than that of full martensite. When fBL is approximately 37.5% obtained by holding for about 60 min, the duplex-structured steel gives rise to the best combination of mechanical properties. © 2006 Elsevier B.V. All rights reserved. Keywords: GCr18Mo steel; BL /M duplex structure; Austempering
1. Introduction It is well known that the steel GCr15 with lower bainite (BL ) structure acquired by austempering possesses favorable mechanical properties [1,2]. Recent studies [3–5] further revealed that a better combination of properties could be achieved when a martensite/lower bainite (BL /M) duplex microstructure, in which the lower bainite volume fraction (fBL ) ranges from 20 to 40%, is produced via austempering, compared with that of full BL or the others. This is because of that during isothermal holding the prior precipitated BL cuts austenite grains and, thus, makes the subsequently formed martensite colonies be finer. Meanwhile, aiming to meet the increasing requirement in hardenability, several high-carbon chromium bearing steels appeared, such as SKF25 (Sweden) and 100CrMo7 (Germany) [6]. Based on 100CrMo7, a GCr18Mo steel was developed in China in the 1990s [7]. The higher contents of Cr and Mo hinder the grain growth, enhance the hardness, strength and the hardenability [8–10]. Besides, the GCr18Mo steel has a lowered Ms temperature that benefits the progress of bainitic transformation. The present work, the strength–toughness of GCr18Mo steel, in austempered state, with BL /M duplex and full BL structures ∗
Corresponding author. Tel.: +86 379 64231846; fax: +86 379 64230597. E-mail address: [email protected] (J. Wen).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.199
is investigated and the optimized fBL in duplex is explored for a best combination of the mechanical properties. 2. Experimental The chemical composition of the investigated GCr18Mo steel is listed in Table 1. The as-received material was in spheroidizing annealed state. The samples were austenized at 870 ◦ C for 30 min. Then, some of them were directly quenched in oil at 80 ◦ C, while the others were austempered at 230 ◦ C in salt bath for 15, 20, 25, 30, 38, 45, 60 and 90 min, respectively, followed by oil cooling. Tempering was carried out at 200 ◦ C for 4 h and then cooling by air. In addition, there is a sample austempered at 230 ◦ C for 4 h and cooled by air but without tempering followed. By using different heat treatments, a series of BL /M duplex microstructures was produced, in which the lower bainite volume fraction (fBL ) varies from 0 to 100% as shown in Table 2. The Rockwell hardness was measured at a HR-150A tester and the impact value (αK ) at a JB30A tester with specimen dimensions 10 mm × 10 mm × 55 mm. The bending strength was tested at a W3-515 hydraulic machine, using the specimens 10 mm × 10 mm × 110 mm with 100 mm span. For the type I fracture toughness (KIC ) tests, the 3-point bending specimens B × W × L = 10 mm × 10 mm × 110 mm with 100 mm span, on which a 9 mm deep notch was spark-eroded by using a Mo wire
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J. Wen et al. / Materials Science and Engineering A 438–440 (2006) 251–253
Table 1 Chemical composition of the GCr18Mo steel (wt.%)
3. Results and discussion
C Si Mn P Cr Mo S Ni Cu
Fig. 1 shows the optical microstructures of the samples austempered at 230 ◦ C for different times, resulting in fBL = 29.4, 37.5, 73.3 and 100%, respectively (refer to Table 2). It was found that at the beginning, e.g. the holding time (t) shorter than 30 min, the fBL slowly increases with increasing t and lower bainite are mainly formed as isolated fine needles. When t = 38 min (Fig. 1(a)), bainite starts to form quickly in lobate or acicular shape. At 60 min (Fig. 1(b)), the BL needles grow up gradually and start to form in hassocks. The fBL reaches to 73.3% at t = 90 min and bainite becomes mostly in connected hassocks (Fig. 1(c)). By holding for 240 min, the steel contains full of BL and no single bainite needle was detected as seen in Fig. 1(d). As seen in Table 2, by tempering at the same temperature (200 ◦ C), the hardness of GCr18Mo steel quenched into full bainite and BL /M duplex is slightly lower than that of full martensite. As the austempering time increases, the hardness decreases but they are all over HRC 58.5, satisfying the service requirement of the high speed bearing steels.
0.9–1.05 0.2–0.4 0.25–0.45 ≤0.015 1.65–1.95 0.15–0.25 ≤0.015 ≤0.3 ≤0.3
of 0.18 mm diameter. The KIC was calculated by Y (a/W)P␦ BW 1/2 where P␦ is the ultimate load, i.e. the load at fracture (kN); a the crack size (mm) and Y is the factor of crack size (a/W). The quenched sample was etched by LEPERA reagent while the tempered ones by 4% nital. The quantitative measurement of duplex structure was carried out with a ISA-4 image analysis system. KIC =
Table 2 Relationship between volume fraction of lower bainite and hardness Austempering time, t (min)
0
30
38
45
60
90
240
fBL (%)
0
0.5
1.0
4.5
10.2
29.4
32.7
37.5
73.3
100
66.8 61.1
66.4 60.9
66.4 60.6
66.1 61.7
65.4 60.6
63.4 59.9
64.6 61.0
63.2 59.2
60.2 58.6
Quenched state (HRC) Tempered state (HRC)
15
20
25
Fig. 1. Optical micrographs of the GCr18Mo steel austempered for different times: (a) t = 38 min; (b) t = 60 min; (c) t = 90 min; (d) t = 240 min.
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The fracture toughness KIC of the GCr18Mo steel was improved obviously by austempering treatment. It is 20.5 MPa/ m2 in the quenched and tempered state with full of martensite, while it increases to 45.0 and 52.4 MPa/m2 when fBL is 32.7 (t = 45 min) and 100% (240 min), respectively. As it is well known that the fracture mechanism and the path of crack propagation strongly depend on the microstructure of materials. In the investigated steel containing small amount of lower bainite, e.g. fBL less than 20%, intergranular fracture rarely occurs. When the cleavage crack meets BL /M interface, the cracking can change its propagation path, the stress concentration will be effectively released and thus shortens the length of single cracks. In the meantime, the incoherent ferrite in lower bainite makes the stress at the crack tip relaxed which is favorable for the ductile mechanism being undertaken instead of the brittle ones [11,12]. Fig. 2. Relationship between the amount of lower bainite and impact toughness.
4. Conclusion The GCr18Mo steel in conventional quenched state exhibits high hardness (66.8 HRC) but low strength–toughness (αK = 60 × 104 J/m2 and σ bb = 2805 MPa). Austempering treatment can dramatically improve its αK , σ bb and KIC to about 1.5–2.5 times high, respectively. The steel composed of BL /M duplex structure with 37.5% volume fraction of lower bainite produced by austempering at 230 ◦ C for 60 min and tempering at 200 ◦ C for 4 h gives rise to an optimized combination of mechanical properties, that is, αK = 108 × 104 J/m2 , σ bb = 4485 MPa and KIC = 45.0 MPa/m2 together with a rather high hardness 59.2 HRC. References
Fig. 3. Relationship between the amount of lower bainite and bending strength.
Fig. 2 shows the dependence of impact ductility on fBL . It is indicated that compared with conventional martensite quenching, austempering dramatically improves the impact ductility of the steel. The αK increases with increasing fBL and reaches to 167 × 104 J/m2 for 100% BL structure. The variation of ultimate bending strength (σ bb ) with fBL is shown in Fig. 3. One can find again that the σ bb of BL /M and full BL steel is superior to that of full martensite structure, especially when fBL exceeds 10.2%. It achieves a maximum 4485 MPa for 37.5% BL and keeps quite high (3885 MPa) even when the steel contains full of bainite.
[1] F. Wang, Bearing 1 (1992) 54–60. [2] Y. Liu, T. Jiang, Bearing 9 (1994) 32–37. [3] T.Y. Hsu (Xu Zuyao), Y. Chen, W. Chen, Metall. Trans. A 18 (1987) 1531–1532. [4] X. Lin, Mech. Eng. Mater. 2 (2000) 40–43. [5] Z. Zhang, Y. Liu, Z. Fan, Trans. Mater. Heat Treatment 1 (2002) 57–60. [6] X. Wang, Z. Zhang, Hot Working Technol. 2 (2000) 34–36. [7] M. Sun, T. Lin, S. Yao, Bearing 8 (2001) 28–30. [8] R.F. Hehemann, K.R. Kinsman, H.I. Aaronson, Metall. Trans. 3 (1972) 1077–1094. [9] H.K.D.H. Bhadeshia, D.V. Edmonds, Metall. Trans. A 10 (1979) 895– 907. [10] S.K. Liu, W.T. Reynolds Jr., H. Hu, G.J. Shiflet, H.I. Aaronson, Metall. Trans. A 16 (1985) 457–468. [11] E. Yajima, T. Miyazaki, T. Sugiyama, H. Terajima, Trans. Jpn. Inst. Met. 15 (1974) 173–179. [12] Z. Li, D. Wu, J. Iron Steel Res. 11 (6) (2004) 40–44.