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Influence of solution treatment temperature on mechanical properties of a Fe–Ni–Cr alloy
Materials Letters 57 (2003) 3805 – 3809 www.elsevier.com/locate/matlet
Influence of solution treatment temperature on mechanical properties of a Fe–Ni–Cr alloy Dayong Cai *, Mei Yao, Pulin Nie, Wenchang Liu College of Materials Science and Engineering, Yanshan University, Qinhuangdao 066004, China Received 11 December 2002; received in revised form 18 February 2003; accepted 26 February 2003
Abstract Influence of solution treatment temperature on mechanical properties of a Fe – Ni – Cr alloy was studied in this work. The results indicate that the strength and the ductile properties are optimum after solution treatment at 1000 jC followed by conventional two-step aging, but decrease markedly with the increase of solution temperature. Microstructure analyses show that TiC phase particles in the microstructure partly dissolves into the matrix when the solution treatment temperature is higher than 1100 jC, resulting in the coarsening of austenitic grain. Flake-like M3B2 phase precipitates at the grain boundary in the specimens solution-treated at temperatures higher than 1050 jC and its formation induces the mechanical properties to be worse. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Fe – Ni – Cr alloy; Interstitial phase; Mechanical property; Microstructure; Heat treatment; Grain boundary
1. Introduction The material studied in this work is an iron-base superalloy used in manufacturing fastener and spring, working at elevated temperatures. This alloy is microalloyed with small amount of boron to strengthen the grain boundary. Boron is a common additive to many superalloys since it can significantly increase the high temperature properties, such as creep rupture life and tensile strength. But it is found that the mechanical properties, especially ductility, are quite sensitive to heat treatment process [1 –4]. In this work, influences of solution treatment temperature on the mechanical * Corresponding author. Tel.: +86-335-8074729; fax: +86-3358051148. E-mail address: [email protected] (D. Cai).
properties and the microstructures of Fe –Ni – Cr alloy were studied.
2. Experimental Hot-rolled bars of Fe –Ni – Cr alloy with chemical composition presented in Table 1 were used in this work. Specimens cut from the rolled bars were subjected to the following heat treatments: solution treated at 1000, 1050, 1100 and 1150 jC for 3 h, respectively, followed by water cooling, then aged at 750 jC for 16 h followed by furnace cooling (50 jC/h) to 650 jC, and held at this temperature for 16 h, then air-cooled). Tensile properties of the as heat-treated alloy were measured at room temperature. Microstructure and
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00182-4
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D. Cai et al. / Materials Letters 57 (2003) 3805–3809
Table 1 Chemical composition of the Fe – Ni – Cr alloy (wt.%) C
Cr
Ni
Mo
Al
Ti
B
Mn
Si
P
S
Fe
0.05
11.6
23.4
1.14
0.55
3.06
< 0.01
< 0.09
< 0.60
< 0.02
< 0.01
Bal.
fracture surface were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Composition as well as amount of precipitates was analyzed with energy dispersive spectrum (EDS) and X-ray diffraction (XRD) methods.
3. Results and discussion The influence of solution treatment temperature on the mechanical properties of the alloy is shown in Fig. 1. It can be seen that the specimen solutiontreated at 1000 jC shows optimum mechanical properties. The ultimate tensile strength (rb) and the yield strength (r0.2) are 1277 and 985 MPa, respectively. And the elongation (d) and the area reduction (W) are 25% and 43%, respectively. The strength properties, especially the plastic properties, decrease with the increase of solution temperature. When the solution treatment temperature reaches 1150 jC, elongation and area reduction decrease to 17% and 14%, respectively. Fig. 2 shows the metallographs of the alloy solution-treated at different temperatures. Austenitic
Fig. 1. The influence of solution treatment temperature on tensile properties of the Fe – Ni – Cr alloy.
grains in specimens coarsen with the increase of solution temperature, especially in those solution-treated at temperature above 1100 jC. EDS analysis indicates that the main retained particles in the microstructure are TiC. They increase in size and decrease in number with the increase of temperature. The amounts of TiC phase in different samples can be compared directly from the XRD results shown in Fig. 3, where the intensity of diffraction peaks was normalized. The intensity of diffraction peaks of the TiC phase after solution treatment at 1000 jC are higher than those at 1050, 1100 and 1150 jC. The diffraction peaks of TiC phase for the specimen solution-treated at 1150 jC are very weak. Due to the decrease of the amount of TiC particles, the pinning effect on the growth of austenitic grains during holding at high temperatures is lowered, which induces the coarsening of grains. Fractographs of the alloy after different solution treatments and aging are shown in Fig. 4. For specimen solution-treated at 1000 jC (Fig. 4a), fracture surface is characterized as fine and homogeneous dimples with TiC particles at their bottom. For specimen solutiontreated at 1050 jC (Fig. 4b), it shows both dimples and intergranular fracture patterns. While, for specimens solution-treated at 1100 and 1150 jC, the fracture surfaces are mainly intergranular and secondary cracks exist (Fig. 4c and d). Thus, the specimens solutiontreated at lower temperature are fractured with a microvoid coalescence mechanism and both its strength and ductility are relatively high. With the increase of solution temperature, fracture mechanism changes into part or full intergranular mechanism and the strength as well as the ductility decreases. Fig. 5 shows TEM micrographs of the grain boundary morphologies of the alloy after different solution treatments. In the specimens solution-treated at 1050, 1100 and 1150 jC, flake-like precipitates can be observed along the grain boundaries (Fig. 5a), while no precipitates can be detected in the specimen solution-treated at 1000 jC (Fig. 5c). The higher the solution temperature, the more the secondary phase can be found. During the following aging process, the
D. Cai et al. / Materials Letters 57 (2003) 3805–3809
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Fig. 2. Effects of solution treatment temperature on the microstructure of the Fe – Ni – Cr alloy, solution-treated at (a) 1000 jC, (b) 1050 jC, (c) 1100 jC and (d) 1150 jC.
Fig. 3. XRD spectrum of the Fe – Ni – Cr alloy after different solution treatments.
formed flake-like phase cannot be effectively eliminated (Fig. 5d). Result of selected area electron diffraction (SAD) reveals that the grain boundary phase is M3B2, an interstitial phase, shown in Fig. 5b. M3B2 phase is characterized as high hardness and high brittleness, and its occupation on the grain boundary with flake-like morphology can cause the weakness of grain boundary [5]. That is why the fracture of the specimens solution-treated at higher temperature appears as part or complete intergranular feature and the mechanical properties become worse. With regard to the effect of boron addition to some stainless steels and superalloys, there is substantial experimental evidence that boron significantly increases creep and tensile strength at high temperatures [6,7]. This effect is assumed to be either a direct consequence of segregation of boron or an indirect influence on the B-containing precipitates at grain boundary. But unfortunately, the segregation of trace elements to grain boundary has dramatic effects on an alloy’s ductile properties, such as temper embrittlement and intergranular embrittlement. The enrichment
3808
D. Cai et al. / Materials Letters 57 (2003) 3805–3809
Fig. 4. Influence of solution temperature on fracture mechanism of the Fe – Ni – Cr alloy, solution-treated at (a) 1000 jC, (b) 1050 jC, (c) 1100 jC and (d) 1150 jC.
Fig. 5. TEM micrographs of the grain boundary of the Fe – Ni – Cr alloy, (a) solution-treated at 1150 jC, (b) SAD pattern of grain boundary phase for (a), (c) solution-treated at 1000 jC, (d) solution-treated at 1150 jC followed by aging.
D. Cai et al. / Materials Letters 57 (2003) 3805–3809
of trace elements, such as boron, at grain boundary can be induced by either equilibrium segregation or non-equilibrium segregation. Equilibrium segregation refers to the movement of solution atoms from the bulk alloy to loosely packed sites, such as different boundaries, free surface and stacking faults. In this case, the segregated structure forms a stable equilibrium condition. The driving force for equilibrium segregation is a reduction in the free energy of grain boundary. As the temperature of isothermal treatments decreases, the level of segregation should rise. While in the present work, the grain boundary M3B2 phase only exists in the specimens solution-treated at higher temperatures. Thus, the formation of the M3B2 phase cannot be interpreted by the equilibrium segregation of boron. A non-equilibrium segregation mechanism was proposed based on the preferential solution vacancy interaction and non-ideal thermodynamic behaviour of the binary system [8]. During the high temperature holding, an equilibrium concentration of vacancies is generated and distributed throughout the lattice. Additionally, there can be some vacancy-solute binding at this temperature, provided a positive attraction occurs between the solute and vacancy and the diffusion coefficient of the solute-vacancy complex exceeds that of the vacancy alone. During cooling to a lower temperature, the concentration of vacancies becomes supersaturated and the grain boundary can act as a strong sink for vacancies. Consequently, a concentration gradient of vacancy will develop near such interface and vacancies within diffusion range of the grain tend to migrate to the interface where they can be annihilated. As vacancies diffuse toward the grain boundary, they tend to drag certain solution atoms towards the boundary. The effective uphill diffusion of solute atoms, producing the solute-rich boundary region, is thermodynamically driven by the decrease in free energy associated with the annihilation of excess vacancies at the boundary sinks. The non-equilibrium segregation of boron is largely dependent on the solution temperature, which
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increases with the increase of heating temperature. Additionally, large grain size formed at higher temperatures also increases the segregation. Thus, the formation of the M3B2 phase observed in this study should be attributed to the non-equilibrium segregation of boron, owning to the high solution treatment temperature and large grain size.
4. Conclusion For the Fe – Ni – Cr alloy studied in this work, optimum mechanical properties can be obtained by solution-treated at 1000 jC followed by conventional aging. When the solution temperature is above 1050 jC, the flake-like M3B2 precipitates appear along grain boundary and its formation impairs the strength and ductility.
Acknowledgements The authors gratefully thank the Chinese National Natural Science Foundation for financial support (no. 59971039).
References [1] K. Kusabiraki, E. Amada, T. Ooka, J. Iron Steel Inst. Jpn. 82 (1996) 208. [2] K. Kusabiraki, H. Toda, H. Komatsu, S. Saji, J. Iron Steel Inst. Jpn. 84 (1998) 664. [3] A. Wang, K. Yang, C. Fan, Y. Li, Acta Metall. Sin. Series A Phys. Metall. Mater. Sci. 9 (1996) 32. [4] A.Q. He, H.Q. Ye, T. Van, G. Kuo, Philos. Mag. A 63 (1991) 1327. [5] R.M. Wang, Y.F. Han, C.Z. Li, S.W. Zhang, J. Mater. Sci. 33 (1998) 5069. [6] T. Takasugi, C.L. Ma, S. Hanada, Mater. Sci. Eng. A 192 – 193 (1995) 407. [7] X.X. Yao, Mater. Sci. Eng. A 271 (1999) 353. [8] X. Huang, M.C. Chaturvedi, N.L. Richards, J. Jackman, Acta Mater. 45 (1997) 3095.
Influence of solution treatment temperature on mechanical properties of a Fe–Ni–Cr alloy Dayong Cai *, Mei Yao, Pulin Nie, Wenchang Liu College of Materials Science and Engineering, Yanshan University, Qinhuangdao 066004, China Received 11 December 2002; received in revised form 18 February 2003; accepted 26 February 2003
Abstract Influence of solution treatment temperature on mechanical properties of a Fe – Ni – Cr alloy was studied in this work. The results indicate that the strength and the ductile properties are optimum after solution treatment at 1000 jC followed by conventional two-step aging, but decrease markedly with the increase of solution temperature. Microstructure analyses show that TiC phase particles in the microstructure partly dissolves into the matrix when the solution treatment temperature is higher than 1100 jC, resulting in the coarsening of austenitic grain. Flake-like M3B2 phase precipitates at the grain boundary in the specimens solution-treated at temperatures higher than 1050 jC and its formation induces the mechanical properties to be worse. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Fe – Ni – Cr alloy; Interstitial phase; Mechanical property; Microstructure; Heat treatment; Grain boundary
1. Introduction The material studied in this work is an iron-base superalloy used in manufacturing fastener and spring, working at elevated temperatures. This alloy is microalloyed with small amount of boron to strengthen the grain boundary. Boron is a common additive to many superalloys since it can significantly increase the high temperature properties, such as creep rupture life and tensile strength. But it is found that the mechanical properties, especially ductility, are quite sensitive to heat treatment process [1 –4]. In this work, influences of solution treatment temperature on the mechanical * Corresponding author. Tel.: +86-335-8074729; fax: +86-3358051148. E-mail address: [email protected] (D. Cai).
properties and the microstructures of Fe –Ni – Cr alloy were studied.
2. Experimental Hot-rolled bars of Fe –Ni – Cr alloy with chemical composition presented in Table 1 were used in this work. Specimens cut from the rolled bars were subjected to the following heat treatments: solution treated at 1000, 1050, 1100 and 1150 jC for 3 h, respectively, followed by water cooling, then aged at 750 jC for 16 h followed by furnace cooling (50 jC/h) to 650 jC, and held at this temperature for 16 h, then air-cooled). Tensile properties of the as heat-treated alloy were measured at room temperature. Microstructure and
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00182-4
3806
D. Cai et al. / Materials Letters 57 (2003) 3805–3809
Table 1 Chemical composition of the Fe – Ni – Cr alloy (wt.%) C
Cr
Ni
Mo
Al
Ti
B
Mn
Si
P
S
Fe
0.05
11.6
23.4
1.14
0.55
3.06
< 0.01
< 0.09
< 0.60
< 0.02
< 0.01
Bal.
fracture surface were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Composition as well as amount of precipitates was analyzed with energy dispersive spectrum (EDS) and X-ray diffraction (XRD) methods.
3. Results and discussion The influence of solution treatment temperature on the mechanical properties of the alloy is shown in Fig. 1. It can be seen that the specimen solutiontreated at 1000 jC shows optimum mechanical properties. The ultimate tensile strength (rb) and the yield strength (r0.2) are 1277 and 985 MPa, respectively. And the elongation (d) and the area reduction (W) are 25% and 43%, respectively. The strength properties, especially the plastic properties, decrease with the increase of solution temperature. When the solution treatment temperature reaches 1150 jC, elongation and area reduction decrease to 17% and 14%, respectively. Fig. 2 shows the metallographs of the alloy solution-treated at different temperatures. Austenitic
Fig. 1. The influence of solution treatment temperature on tensile properties of the Fe – Ni – Cr alloy.
grains in specimens coarsen with the increase of solution temperature, especially in those solution-treated at temperature above 1100 jC. EDS analysis indicates that the main retained particles in the microstructure are TiC. They increase in size and decrease in number with the increase of temperature. The amounts of TiC phase in different samples can be compared directly from the XRD results shown in Fig. 3, where the intensity of diffraction peaks was normalized. The intensity of diffraction peaks of the TiC phase after solution treatment at 1000 jC are higher than those at 1050, 1100 and 1150 jC. The diffraction peaks of TiC phase for the specimen solution-treated at 1150 jC are very weak. Due to the decrease of the amount of TiC particles, the pinning effect on the growth of austenitic grains during holding at high temperatures is lowered, which induces the coarsening of grains. Fractographs of the alloy after different solution treatments and aging are shown in Fig. 4. For specimen solution-treated at 1000 jC (Fig. 4a), fracture surface is characterized as fine and homogeneous dimples with TiC particles at their bottom. For specimen solutiontreated at 1050 jC (Fig. 4b), it shows both dimples and intergranular fracture patterns. While, for specimens solution-treated at 1100 and 1150 jC, the fracture surfaces are mainly intergranular and secondary cracks exist (Fig. 4c and d). Thus, the specimens solutiontreated at lower temperature are fractured with a microvoid coalescence mechanism and both its strength and ductility are relatively high. With the increase of solution temperature, fracture mechanism changes into part or full intergranular mechanism and the strength as well as the ductility decreases. Fig. 5 shows TEM micrographs of the grain boundary morphologies of the alloy after different solution treatments. In the specimens solution-treated at 1050, 1100 and 1150 jC, flake-like precipitates can be observed along the grain boundaries (Fig. 5a), while no precipitates can be detected in the specimen solution-treated at 1000 jC (Fig. 5c). The higher the solution temperature, the more the secondary phase can be found. During the following aging process, the
D. Cai et al. / Materials Letters 57 (2003) 3805–3809
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Fig. 2. Effects of solution treatment temperature on the microstructure of the Fe – Ni – Cr alloy, solution-treated at (a) 1000 jC, (b) 1050 jC, (c) 1100 jC and (d) 1150 jC.
Fig. 3. XRD spectrum of the Fe – Ni – Cr alloy after different solution treatments.
formed flake-like phase cannot be effectively eliminated (Fig. 5d). Result of selected area electron diffraction (SAD) reveals that the grain boundary phase is M3B2, an interstitial phase, shown in Fig. 5b. M3B2 phase is characterized as high hardness and high brittleness, and its occupation on the grain boundary with flake-like morphology can cause the weakness of grain boundary [5]. That is why the fracture of the specimens solution-treated at higher temperature appears as part or complete intergranular feature and the mechanical properties become worse. With regard to the effect of boron addition to some stainless steels and superalloys, there is substantial experimental evidence that boron significantly increases creep and tensile strength at high temperatures [6,7]. This effect is assumed to be either a direct consequence of segregation of boron or an indirect influence on the B-containing precipitates at grain boundary. But unfortunately, the segregation of trace elements to grain boundary has dramatic effects on an alloy’s ductile properties, such as temper embrittlement and intergranular embrittlement. The enrichment
3808
D. Cai et al. / Materials Letters 57 (2003) 3805–3809
Fig. 4. Influence of solution temperature on fracture mechanism of the Fe – Ni – Cr alloy, solution-treated at (a) 1000 jC, (b) 1050 jC, (c) 1100 jC and (d) 1150 jC.
Fig. 5. TEM micrographs of the grain boundary of the Fe – Ni – Cr alloy, (a) solution-treated at 1150 jC, (b) SAD pattern of grain boundary phase for (a), (c) solution-treated at 1000 jC, (d) solution-treated at 1150 jC followed by aging.
D. Cai et al. / Materials Letters 57 (2003) 3805–3809
of trace elements, such as boron, at grain boundary can be induced by either equilibrium segregation or non-equilibrium segregation. Equilibrium segregation refers to the movement of solution atoms from the bulk alloy to loosely packed sites, such as different boundaries, free surface and stacking faults. In this case, the segregated structure forms a stable equilibrium condition. The driving force for equilibrium segregation is a reduction in the free energy of grain boundary. As the temperature of isothermal treatments decreases, the level of segregation should rise. While in the present work, the grain boundary M3B2 phase only exists in the specimens solution-treated at higher temperatures. Thus, the formation of the M3B2 phase cannot be interpreted by the equilibrium segregation of boron. A non-equilibrium segregation mechanism was proposed based on the preferential solution vacancy interaction and non-ideal thermodynamic behaviour of the binary system [8]. During the high temperature holding, an equilibrium concentration of vacancies is generated and distributed throughout the lattice. Additionally, there can be some vacancy-solute binding at this temperature, provided a positive attraction occurs between the solute and vacancy and the diffusion coefficient of the solute-vacancy complex exceeds that of the vacancy alone. During cooling to a lower temperature, the concentration of vacancies becomes supersaturated and the grain boundary can act as a strong sink for vacancies. Consequently, a concentration gradient of vacancy will develop near such interface and vacancies within diffusion range of the grain tend to migrate to the interface where they can be annihilated. As vacancies diffuse toward the grain boundary, they tend to drag certain solution atoms towards the boundary. The effective uphill diffusion of solute atoms, producing the solute-rich boundary region, is thermodynamically driven by the decrease in free energy associated with the annihilation of excess vacancies at the boundary sinks. The non-equilibrium segregation of boron is largely dependent on the solution temperature, which
3809
increases with the increase of heating temperature. Additionally, large grain size formed at higher temperatures also increases the segregation. Thus, the formation of the M3B2 phase observed in this study should be attributed to the non-equilibrium segregation of boron, owning to the high solution treatment temperature and large grain size.
4. Conclusion For the Fe – Ni – Cr alloy studied in this work, optimum mechanical properties can be obtained by solution-treated at 1000 jC followed by conventional aging. When the solution temperature is above 1050 jC, the flake-like M3B2 precipitates appear along grain boundary and its formation impairs the strength and ductility.
Acknowledgements The authors gratefully thank the Chinese National Natural Science Foundation for financial support (no. 59971039).
References [1] K. Kusabiraki, E. Amada, T. Ooka, J. Iron Steel Inst. Jpn. 82 (1996) 208. [2] K. Kusabiraki, H. Toda, H. Komatsu, S. Saji, J. Iron Steel Inst. Jpn. 84 (1998) 664. [3] A. Wang, K. Yang, C. Fan, Y. Li, Acta Metall. Sin. Series A Phys. Metall. Mater. Sci. 9 (1996) 32. [4] A.Q. He, H.Q. Ye, T. Van, G. Kuo, Philos. Mag. A 63 (1991) 1327. [5] R.M. Wang, Y.F. Han, C.Z. Li, S.W. Zhang, J. Mater. Sci. 33 (1998) 5069. [6] T. Takasugi, C.L. Ma, S. Hanada, Mater. Sci. Eng. A 192 – 193 (1995) 407. [7] X.X. Yao, Mater. Sci. Eng. A 271 (1999) 353. [8] X. Huang, M.C. Chaturvedi, N.L. Richards, J. Jackman, Acta Mater. 45 (1997) 3095.