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Microstructural effects on the fretting wear of Inconel 690 steam generator tube
Wear 259 (2005) 349–355
Microstructural effects on the fretting wear of Inconel 690 steam generator tube Jin-Ki Honga,∗ , In-Sup Kimb , Chi-Yong Parkc , Eung-Seon Kima b
a Korea Atomic Energy Reseach Institute, 150 Dukjin-dong, Yusung-gu, Daejeon, Republic of Korea Korea Advanced Institute of Science and Technology, 373-1 Gusung-dong, Yusung-gu, Daejeon, Republic of Korea c Korea Electric Power Research Institute, 103-16 Munji-dong, Yusung-gu, Daejeon, Republic of Korea
Received 28 July 2004; received in revised form 14 December 2004; accepted 16 December 2004 Available online 18 January 2005
Abstract Inconel 690 is used for steam generator tube in nuclear power plants. In this study, microstructural effects on fretting wear of Inconel 690 were investigated. To change the microstructure of test specimens, heat treatments were performed in two steps; solution annealing (at a high temperature) and thermal treatment (at a low temperature). Observations of microstructure indicated that the temperature and duration of solution annealing affect the grain size of Inconel 690. The carbide morphology along grain boundaries was affected mainly by duration and temperature of thermal treatment. The results of the wear test show that specimens with larger grain and coarse carbides along the grain boundaries have greater resistance to wear. Cracks were found in specimens with carbides along the grain boundary whereas few cracks were found in carbide-free specimens. The carbide along the grain boundary seemed to help form and propagate cracks in the specimens with carbides. On the other hand, the microhardness of specimens had no major role in fretting wear. © 2004 Elsevier B.V. All rights reserved. Keywords: Fretting wear; Inconel 690; Microstructure; Steam generator
1. Introduction Inconel 690 is used as a pressurized water reactor (PWR) steam generator tubing. With twice as much chromium as Inconel 690, the Inconel 690 has greater resistance to corrosion [1]. However, in view of mechanical properties, steam generator (S/G) with Inconel 690 would be inferior to S/G with Inconel 600 [2]. Especially, because of the thermal conductivity of the Inconel 690, the length of the tube is increased by 10% and the more severe operating condition was expected. Consequently, mechanical damage, such as fretting or fatigue, is likely to occur more than pure corrosion damage. Thermal energy is transferred in a steam generator, when one fluid passes through the tubes while another (or fluid gas mixture) passes along the outside of the tubes. Cross-flow ∗
Corresponding author. Tel.: +82 42 868 8387; fax: +82 42 869 3852. E-mail address: [email protected] (J.-K. Hong).
0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.12.007
over the tubes induces the tube to vibrate. If the amplitudes of vibration become too large, damage or even failure can occur from mechanical degradation such as fatigue or fretting wear [3,4]. Fretting wear, which occurs as a result of the lowamplitude motion between contacting components, is distinct from the general sliding wear. In particular, fretting wear is affected by the parameters of the fluid flow such as the type of tube motion, the vibration frequency and the impact force at the supports. Fretting wear is also governed by the mechanical design; for example, the clearance of the tubes and supports clearance, the tube-supports area, the combination of materials and the operating temperature of the system [5]. Using environmental parameters and an analytical approach, previous research has focused on geometrical parameters between the tube and the tube support plate. Few studies have adopted a microstructural point of view. As a result, we focused on the microstructural effects of In-
350
J.-K. Hong et al. / Wear 259 (2005) 349–355
conel 690 and how heat treatment can minimize fretting wear.
Table 3 Summary of heat treatment conditions Designation
2. Experimental procedure 2.1. Material preparation The materials we used, namely the tube-shaped, thermally-treated Inconel 690 alloy and the plate-shaped AISI 405 alloy, were supplied by the Doosan Heavy Industries and Construction Co., Ltd. Our specimen material had a carbon content of 0.02%, and the material of the plate that supported the tube was ferritic stainless steel. The other chemical compositions are listed in Tables 1 and 2. 2.2. Heat treatments The heat treatment consisted of two steps; solution annealing for carbide dissolution and thermal treatment for carbide precipitation. To obtain various grain sizes and carbide morphologies, the heat treatment was conducted at various temperatures and periods. The solution annealing treatment was performed at 1150 ◦ C or at 1070 ◦ C, whereas the aging treatment was performed at 700 ◦ C or at 800 ◦ C. Table 3 lists the details of the heat treatment. To prevent reactions involving oxygen, all specimens were thermally treated in vacuum-sealed quartz tubes. 2.3. Microstructural observation and hardness test Metallographic specimens were prepared and electrolytically etched in 6% nital solution at 4 V for 60 s to reveal the microstructural features. Grain size measurements were performed using a linear intercept method according to ASTM E 112 [6]. Phosphoric etching was used to determine the presTable 1 Chemical composition of Inconel 690 tube (wt.%) C Si Mn P S Cr Ni Mo
0.02 0.27 0.28 0.008 0.001 29.4 59.2 0.01
Co Ti Cu Al Nb B N Fe
0.011 0.28 0.01 0.027 0.01 0.004 0.012 10.5
AS SAH SAH701 SAH715 SAH801 3SAH701 SAL701 SAL715
SA temperature (◦ C)
SA time (h)
1150 1150 1150 1150 1150 1070 1070
1 1 1 1 3 1 1
TT temperature (◦ C)
TT time (h)
700 700 800 700 700 700
1 15 1 1 1 15
SA: solution annealing; TT: thermal treatment; AS: as received.
ence of carbides in grain boundary area. The etching was performed at 3 V for 20 s in the solution of eight parts phosphoric acid and one part water. Carbide morphology was examined through scanning electron microscopy (SEM, Philips SEM 515). Microhardness was measured on surface of matrix and grain boundary using micro-Vickers hardness tester (Tukon 300). In this measurement, 25 g load was applied for 15 s. 2.4. Wear test The test system consists of wear test machine, load cell, strain gauge and weighing balance. Load cell and strain gauge are connected to a power supply and a digital multi-meter. The load cell was used for measuring the normal load during the test and the strain gauge was used to measure the specimen displacement. Fig. 1 illustrates the wear test machine and specimen contact geometry. The wear tests were performed with various displacements and cycles under the same frequency. The testing displacement was 100 m, and the tests were performed for 54,000 cycles with a fixed frequency of 30 Hz. We performed the tests more than three times for each experimental condition. Before and after each test, we measured the weight of each specimen 10 times and calculated the average. Before weighing the specimens, we cleaned them in an ultrasonic cleaning bath.
3. Results and discussion 3.1. Microstructure observation and hardness
Table 2 Chemical composition of tube support plate (AISI 405) (wt.%) C Mn P S Si Cr Ni Fe
0.08 1.00 0.04 0.03 1.00 13.0 0.60 Val.
The grain sizes of the thermally treated specimens were larger than that of the as-received specimen. The grain size of the specimen that was solution annealed at 1150 ◦ C for 3 h was larger than the grain size of the specimen that was solution annealed at 1150 ◦ C for 1 h, while the smallest grain size was found in the specimen that was solution annealed at 1070 ◦ C for 1 h. Table 4 lists the grain sizes and ASTM numbers of specimens.
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351
Fig. 1. Wear testing machine (TSP: tube support plate, lever: used to change displacement). Table 4 Results of grain size measurements
Grain no. Grain size (m)
AS
SAL
SAH
3SAH
8.07 25
7.49 30
5.53 60
5.2 80
Fig. 2, which shows the carbide morphologies for the asreceived and thermally treated specimens, indicates that the temperature and duration of the thermal treatment influence the carbide morphology of the Inconel 690. From these results, it is clear that solution annealing alone affects the grain size of the material, whereas the low-
temperature heat treatment, which precipitates carbides in the region of the grain boundary, barely affects the grain size of the material. The size of the carbides was about 0.2 m for AS (as-received) specimen and about 0.5 m for the SAH801 (solution annealed at 1150 ◦ C for 1 h, thermally treated at 800 ◦ C for 1 h) specimen. The temperature and duration of the low-temperature heat treatment affected the carbide morphology of the material. The Vickers micro-hardness values are shown in Fig. 3. The hardness values in grain boundaries were higher than those in the matrices. The hardness of a grain boundary with few carbides was lower than that of a grain boundary with carbides. The difference between grain boundary and ma-
Fig. 2. Carbide morphology of (a) SAH (solution annealed at 1150 ◦ C for 1 h), (b) SAH701 (solution annealed at 1150 ◦ C for 1 h, thermally treated 700◦ 1 h), (c) AS (as-received) and (d) SAH801 (solution annealed at 1150 ◦ C for 1 h, thermally treated at 800 ◦ C, 1 h).
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Fig. 3. Hardness values of (a) matrix and (b) grain boundary.
trix microhardness was due to the strain energy of the grain boundary [7], which resulted from inclusions or precipitates in grain boundaries. Solution annealing treatments decreased the hardness. For the same low temperature heat treatment, the longer solution annealing lower the hardness. 3SAH 701 (solution annealed at 1150 ◦ C for 3 h, thermally treated at 700 ◦ C for 1 h) had lower hardness in the matrix. These results are in good agreement with other investigators [8].
This phenomenon was discussed by Ko and Suh [5,11]. The wear property depends so little on hardness because shear mechanisms are the primary cause of fretting wear; consequently, the hardness of the material has no significant effect on wear resistance [4,5]. According to our test results, hardness is not a major variable in determining the wear property of Inconel 690.
3.2. Effect of hardness
Table 4 shows that solution annealing increase the grain size of the Inconel 690. To determine the effect of the grain size, we conducted the thermal treatment under the same temperature and duration (thermally treated at 700 ◦ C for 15 h and thermally treated at 800 ◦ C for 1 h), and we found that the carbide morphology was similar for each specimen, even at different grain sizes. Fig. 5 shows the changes of wear loss according to the grain size. In Fig. 5a and b, the wear loss decreases with increasing grain size. As-received specimen wore more than larger grain size specimens with the same carbide morphology, and low temperature solution annealed specimen wore more than high temperature solution annealed specimen. The trend in Fig. 5b is similar to the test results shown in Fig. 5a, specimens with various grain sizes and finer carbide morphology. These results show that the larger grain size specimens have better wear resistance than the smaller grain size specimen with the same grain boundary morphology. In the SEM images of Fig. 6, similar patterns appear on the worn surface of each specimen with carbides in the grain boundary, regardless of the grain size or carbide morphology. From the SEM images, we can see that the worn surfaces are rough and that the worn of the surface have cracks. Fig. 7 shows that subsurface cracks have been propagated along the grain boundary. According to Suh [11,12], decohesion between the matrix and the inclusions or precipitates cause cracks. When the crack extends and reaches a critical length, the sheet is
In solution annealing and carbide precipitation, the mechanical properties are affected by both the high temperature and the low temperature [9,10]. To investigate the effect of hardness on wear property of Inconel 690, we correlated the data of the wear loss to hardness. Fig. 4 shows that the wear resistance of much harder specimens is poor. The softer, heat treated specimens showed less wear than the harder asreceived specimen.
Fig. 4. Weight loss related to hardness of specimens.
3.3. Effect of grain size
J.-K. Hong et al. / Wear 259 (2005) 349–355
353
Fig. 5. Weight loss related to grain sizes of (a) thermally treated at 700 ◦ C for 15 h and (b) thermally treated 700 ◦ C for 1 h.
Fig. 6. Image of worn surface related to grain size of (a) AS (as-received), (b) SAL715 (solution annealed at 1070 ◦ C for 1 h, thermally treated at 700 ◦ C for 15 h) and (c) SAH 715 (solution annealed 1050 ◦ C for 1 h, thermally treated at 700 ◦ C for 15 h).
Fig. 7. SEM image of surface section: (a) grain boundary and (b) cracks in grain boundary.
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J.-K. Hong et al. / Wear 259 (2005) 349–355
Fig. 8. Weight loss related to carbide morphology.
delaminated. The chromium carbides in the grain boundary seemed to enhance the formation and propagation of surface and subsurface cracks. From these results, we deduce that the grain boundary area may be an important variable of the wear property. The grain boundary area decreases as the specimen’s grain size increases. Furthermore, when delamination is the mechanism of wear, the reduced grain boundary area with carbides decreases the sites where the cracks are formed and propagated. Consequently, the wear resistance of Inconel 690 can be improved by enlarging the grain size.
for the grain boundaries. The specimen with few carbides in the grain boundary has the largest weight loss. Furthermore, the specimen with the relatively fine carbide morphology has a slightly larger weight loss than the specimen with the relatively coarse carbide morphology. We deduce from these results that the carbides in the grain boundaries influence the weight loss of wear. In Fig. 9, the worn surface of the specimen with few carbides differs from the other specimens that have carbides in the grain boundary. The worn surface of the specimen with few carbides has deformation or abrasion marks that is similar to surfaces worn by an abrasion [13]. Moreover, the SEM images of the other specimens with carbides show a rough worn surface and cracks, which are similar to the rough surface and cracks surfaces worn by a delamination [12]. According to these results, the wear mechanism of the specimen with few carbides differs from that of the specimen with carbides in the grain boundary. In a carbide-free specimen, the surface is easily deformed and a third body can be easily formed [14]. In the specimen with carbides, the carbides in the grain boundary may enable a delamination to form and propagate cracks [12]. Because the average length of the crack propagation was short in the grain boundaries with continuous carbide, many cracks were formed. The continuous carbides morphologies produce more crack formation and propagation than the morphology of discontinuous carbides. Hence, the discontinuous carbides in the grain boundaries improve the wear resistance of the Inconel 690. 4. Conclusions
3.4. Effect of carbide morphology To observe the effect of carbide morphology, we performed thermal treatments under various conditions, after first conducting solution anneailng at the same temperature (1150 ◦ C) for the same duration (1 h) to get the same grain size. The results of the heat treatments, which are shown in Fig. 2, indicate that the carbide morphology varies with the thermal treatment condition. Fig. 8 shows the wear resistance of specimens with the same grain size, along with various carbide morphologies
1. Microstructure of Inconel 690 was affected by temperature and duration of the heat treatment. Grain size increased with increasing the temperature and the duration of solution annealing, whereas the hardness decreased. The sizes and discontinuousness of carbides increased with temperature and duration of the thermal treatment. 2. The hardness of Inconel 690 was not a major affecting factor in comparison with other microstructural variables. 3. The wear loss decreased with increasing grain size of specimens. The mass loss of small grain size specimen (AS
Fig. 9. SEM images of worn surface related to carbide morphology: (a) SAH (solution annealed at 1050 ◦ C for 1 h) and (b) SAH 715 (solution annealed at 1050 ◦ C for 1 h, thermally treated at 700 ◦ C for 15 h).
J.-K. Hong et al. / Wear 259 (2005) 349–355
grain size: 25 m) was three times as much as that of large grain size specimen (SAH715 grain size: 60 m). 4. In the worn surface of the carbide-free specimen, few cracks were found and the deformation or abrasion marks were observed. The cracks were observed in the worn surface of the specimens with grain boundary carbides. 5. Specimens with discontinuous grain boundary carbides have more wear resistance than those with continuous grain boundary carbides. Wear loss of the specimen with coarse carbide (SAH801 carbide size 0.5 m) is smaller than that with fine carbides (SAH701 carbide size 0.2 m) by 30%. References [1] Vikram N. Shah, Phlip E. Macdonald, Aging and Life Extension of Major Light Water Reactor, Elsevier, 1993.
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[2] P. Berge, J.R. Donati, Nuclear Technol. 55 (1981) 88. [3] C.E. Taylor, M.J. Pettigrew, T.J. Dickinson, I.G. Currie, P. Vidalou, J. Press. Vessel Technol. 120 (1998) 283. [4] P.L. Ko, Transaction of ASME, J. Press. Vessel Technol. 101 (1979) 125. [5] P.L. Ko, J. Tribol. 107 (1985) 149. [6] Annual book of ASTM Standard sec. 3, ASTM E 112, (1994). [7] D.A. Porter, K.E. Easterling, Phase Transformations in Metals and Alloys, second ed., Van Norstrand Reinhold, 1981, p. 156. [8] H.-B. Park, Y.-H. Kim, B.-W. Lee, K.-S. Lim, J. Nuclear Mater. 231 (1996) 204. [9] N.P. Suh, Tribophysics, Prentice-Hall, 1986, p. 200. [10] N.J. Fisher, M.J. Olesen, R.J. Rogers, P.L. Ko, Transaction of the ASME, J. Press. Vessel Technol. 111 (1989) 378. [11] N. Saka, Proceeding of the International Conference on the Fundamentals of Tribology, 1978, p. 135. [12] N.P. Suh, S. Jahanmir, E.P. Abrahamson, A.P.L. Turner, Transaction of ASME, J. Lubricat. Technol. 96 (1975) 631. [13] K. Kato, Wear 153 (1992) 277. [14] T. Sada, M. Oike, N. Emory, Wear 97 (1980) 291.
Microstructural effects on the fretting wear of Inconel 690 steam generator tube Jin-Ki Honga,∗ , In-Sup Kimb , Chi-Yong Parkc , Eung-Seon Kima b
a Korea Atomic Energy Reseach Institute, 150 Dukjin-dong, Yusung-gu, Daejeon, Republic of Korea Korea Advanced Institute of Science and Technology, 373-1 Gusung-dong, Yusung-gu, Daejeon, Republic of Korea c Korea Electric Power Research Institute, 103-16 Munji-dong, Yusung-gu, Daejeon, Republic of Korea
Received 28 July 2004; received in revised form 14 December 2004; accepted 16 December 2004 Available online 18 January 2005
Abstract Inconel 690 is used for steam generator tube in nuclear power plants. In this study, microstructural effects on fretting wear of Inconel 690 were investigated. To change the microstructure of test specimens, heat treatments were performed in two steps; solution annealing (at a high temperature) and thermal treatment (at a low temperature). Observations of microstructure indicated that the temperature and duration of solution annealing affect the grain size of Inconel 690. The carbide morphology along grain boundaries was affected mainly by duration and temperature of thermal treatment. The results of the wear test show that specimens with larger grain and coarse carbides along the grain boundaries have greater resistance to wear. Cracks were found in specimens with carbides along the grain boundary whereas few cracks were found in carbide-free specimens. The carbide along the grain boundary seemed to help form and propagate cracks in the specimens with carbides. On the other hand, the microhardness of specimens had no major role in fretting wear. © 2004 Elsevier B.V. All rights reserved. Keywords: Fretting wear; Inconel 690; Microstructure; Steam generator
1. Introduction Inconel 690 is used as a pressurized water reactor (PWR) steam generator tubing. With twice as much chromium as Inconel 690, the Inconel 690 has greater resistance to corrosion [1]. However, in view of mechanical properties, steam generator (S/G) with Inconel 690 would be inferior to S/G with Inconel 600 [2]. Especially, because of the thermal conductivity of the Inconel 690, the length of the tube is increased by 10% and the more severe operating condition was expected. Consequently, mechanical damage, such as fretting or fatigue, is likely to occur more than pure corrosion damage. Thermal energy is transferred in a steam generator, when one fluid passes through the tubes while another (or fluid gas mixture) passes along the outside of the tubes. Cross-flow ∗
Corresponding author. Tel.: +82 42 868 8387; fax: +82 42 869 3852. E-mail address: [email protected] (J.-K. Hong).
0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.12.007
over the tubes induces the tube to vibrate. If the amplitudes of vibration become too large, damage or even failure can occur from mechanical degradation such as fatigue or fretting wear [3,4]. Fretting wear, which occurs as a result of the lowamplitude motion between contacting components, is distinct from the general sliding wear. In particular, fretting wear is affected by the parameters of the fluid flow such as the type of tube motion, the vibration frequency and the impact force at the supports. Fretting wear is also governed by the mechanical design; for example, the clearance of the tubes and supports clearance, the tube-supports area, the combination of materials and the operating temperature of the system [5]. Using environmental parameters and an analytical approach, previous research has focused on geometrical parameters between the tube and the tube support plate. Few studies have adopted a microstructural point of view. As a result, we focused on the microstructural effects of In-
350
J.-K. Hong et al. / Wear 259 (2005) 349–355
conel 690 and how heat treatment can minimize fretting wear.
Table 3 Summary of heat treatment conditions Designation
2. Experimental procedure 2.1. Material preparation The materials we used, namely the tube-shaped, thermally-treated Inconel 690 alloy and the plate-shaped AISI 405 alloy, were supplied by the Doosan Heavy Industries and Construction Co., Ltd. Our specimen material had a carbon content of 0.02%, and the material of the plate that supported the tube was ferritic stainless steel. The other chemical compositions are listed in Tables 1 and 2. 2.2. Heat treatments The heat treatment consisted of two steps; solution annealing for carbide dissolution and thermal treatment for carbide precipitation. To obtain various grain sizes and carbide morphologies, the heat treatment was conducted at various temperatures and periods. The solution annealing treatment was performed at 1150 ◦ C or at 1070 ◦ C, whereas the aging treatment was performed at 700 ◦ C or at 800 ◦ C. Table 3 lists the details of the heat treatment. To prevent reactions involving oxygen, all specimens were thermally treated in vacuum-sealed quartz tubes. 2.3. Microstructural observation and hardness test Metallographic specimens were prepared and electrolytically etched in 6% nital solution at 4 V for 60 s to reveal the microstructural features. Grain size measurements were performed using a linear intercept method according to ASTM E 112 [6]. Phosphoric etching was used to determine the presTable 1 Chemical composition of Inconel 690 tube (wt.%) C Si Mn P S Cr Ni Mo
0.02 0.27 0.28 0.008 0.001 29.4 59.2 0.01
Co Ti Cu Al Nb B N Fe
0.011 0.28 0.01 0.027 0.01 0.004 0.012 10.5
AS SAH SAH701 SAH715 SAH801 3SAH701 SAL701 SAL715
SA temperature (◦ C)
SA time (h)
1150 1150 1150 1150 1150 1070 1070
1 1 1 1 3 1 1
TT temperature (◦ C)
TT time (h)
700 700 800 700 700 700
1 15 1 1 1 15
SA: solution annealing; TT: thermal treatment; AS: as received.
ence of carbides in grain boundary area. The etching was performed at 3 V for 20 s in the solution of eight parts phosphoric acid and one part water. Carbide morphology was examined through scanning electron microscopy (SEM, Philips SEM 515). Microhardness was measured on surface of matrix and grain boundary using micro-Vickers hardness tester (Tukon 300). In this measurement, 25 g load was applied for 15 s. 2.4. Wear test The test system consists of wear test machine, load cell, strain gauge and weighing balance. Load cell and strain gauge are connected to a power supply and a digital multi-meter. The load cell was used for measuring the normal load during the test and the strain gauge was used to measure the specimen displacement. Fig. 1 illustrates the wear test machine and specimen contact geometry. The wear tests were performed with various displacements and cycles under the same frequency. The testing displacement was 100 m, and the tests were performed for 54,000 cycles with a fixed frequency of 30 Hz. We performed the tests more than three times for each experimental condition. Before and after each test, we measured the weight of each specimen 10 times and calculated the average. Before weighing the specimens, we cleaned them in an ultrasonic cleaning bath.
3. Results and discussion 3.1. Microstructure observation and hardness
Table 2 Chemical composition of tube support plate (AISI 405) (wt.%) C Mn P S Si Cr Ni Fe
0.08 1.00 0.04 0.03 1.00 13.0 0.60 Val.
The grain sizes of the thermally treated specimens were larger than that of the as-received specimen. The grain size of the specimen that was solution annealed at 1150 ◦ C for 3 h was larger than the grain size of the specimen that was solution annealed at 1150 ◦ C for 1 h, while the smallest grain size was found in the specimen that was solution annealed at 1070 ◦ C for 1 h. Table 4 lists the grain sizes and ASTM numbers of specimens.
J.-K. Hong et al. / Wear 259 (2005) 349–355
351
Fig. 1. Wear testing machine (TSP: tube support plate, lever: used to change displacement). Table 4 Results of grain size measurements
Grain no. Grain size (m)
AS
SAL
SAH
3SAH
8.07 25
7.49 30
5.53 60
5.2 80
Fig. 2, which shows the carbide morphologies for the asreceived and thermally treated specimens, indicates that the temperature and duration of the thermal treatment influence the carbide morphology of the Inconel 690. From these results, it is clear that solution annealing alone affects the grain size of the material, whereas the low-
temperature heat treatment, which precipitates carbides in the region of the grain boundary, barely affects the grain size of the material. The size of the carbides was about 0.2 m for AS (as-received) specimen and about 0.5 m for the SAH801 (solution annealed at 1150 ◦ C for 1 h, thermally treated at 800 ◦ C for 1 h) specimen. The temperature and duration of the low-temperature heat treatment affected the carbide morphology of the material. The Vickers micro-hardness values are shown in Fig. 3. The hardness values in grain boundaries were higher than those in the matrices. The hardness of a grain boundary with few carbides was lower than that of a grain boundary with carbides. The difference between grain boundary and ma-
Fig. 2. Carbide morphology of (a) SAH (solution annealed at 1150 ◦ C for 1 h), (b) SAH701 (solution annealed at 1150 ◦ C for 1 h, thermally treated 700◦ 1 h), (c) AS (as-received) and (d) SAH801 (solution annealed at 1150 ◦ C for 1 h, thermally treated at 800 ◦ C, 1 h).
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J.-K. Hong et al. / Wear 259 (2005) 349–355
Fig. 3. Hardness values of (a) matrix and (b) grain boundary.
trix microhardness was due to the strain energy of the grain boundary [7], which resulted from inclusions or precipitates in grain boundaries. Solution annealing treatments decreased the hardness. For the same low temperature heat treatment, the longer solution annealing lower the hardness. 3SAH 701 (solution annealed at 1150 ◦ C for 3 h, thermally treated at 700 ◦ C for 1 h) had lower hardness in the matrix. These results are in good agreement with other investigators [8].
This phenomenon was discussed by Ko and Suh [5,11]. The wear property depends so little on hardness because shear mechanisms are the primary cause of fretting wear; consequently, the hardness of the material has no significant effect on wear resistance [4,5]. According to our test results, hardness is not a major variable in determining the wear property of Inconel 690.
3.2. Effect of hardness
Table 4 shows that solution annealing increase the grain size of the Inconel 690. To determine the effect of the grain size, we conducted the thermal treatment under the same temperature and duration (thermally treated at 700 ◦ C for 15 h and thermally treated at 800 ◦ C for 1 h), and we found that the carbide morphology was similar for each specimen, even at different grain sizes. Fig. 5 shows the changes of wear loss according to the grain size. In Fig. 5a and b, the wear loss decreases with increasing grain size. As-received specimen wore more than larger grain size specimens with the same carbide morphology, and low temperature solution annealed specimen wore more than high temperature solution annealed specimen. The trend in Fig. 5b is similar to the test results shown in Fig. 5a, specimens with various grain sizes and finer carbide morphology. These results show that the larger grain size specimens have better wear resistance than the smaller grain size specimen with the same grain boundary morphology. In the SEM images of Fig. 6, similar patterns appear on the worn surface of each specimen with carbides in the grain boundary, regardless of the grain size or carbide morphology. From the SEM images, we can see that the worn surfaces are rough and that the worn of the surface have cracks. Fig. 7 shows that subsurface cracks have been propagated along the grain boundary. According to Suh [11,12], decohesion between the matrix and the inclusions or precipitates cause cracks. When the crack extends and reaches a critical length, the sheet is
In solution annealing and carbide precipitation, the mechanical properties are affected by both the high temperature and the low temperature [9,10]. To investigate the effect of hardness on wear property of Inconel 690, we correlated the data of the wear loss to hardness. Fig. 4 shows that the wear resistance of much harder specimens is poor. The softer, heat treated specimens showed less wear than the harder asreceived specimen.
Fig. 4. Weight loss related to hardness of specimens.
3.3. Effect of grain size
J.-K. Hong et al. / Wear 259 (2005) 349–355
353
Fig. 5. Weight loss related to grain sizes of (a) thermally treated at 700 ◦ C for 15 h and (b) thermally treated 700 ◦ C for 1 h.
Fig. 6. Image of worn surface related to grain size of (a) AS (as-received), (b) SAL715 (solution annealed at 1070 ◦ C for 1 h, thermally treated at 700 ◦ C for 15 h) and (c) SAH 715 (solution annealed 1050 ◦ C for 1 h, thermally treated at 700 ◦ C for 15 h).
Fig. 7. SEM image of surface section: (a) grain boundary and (b) cracks in grain boundary.
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Fig. 8. Weight loss related to carbide morphology.
delaminated. The chromium carbides in the grain boundary seemed to enhance the formation and propagation of surface and subsurface cracks. From these results, we deduce that the grain boundary area may be an important variable of the wear property. The grain boundary area decreases as the specimen’s grain size increases. Furthermore, when delamination is the mechanism of wear, the reduced grain boundary area with carbides decreases the sites where the cracks are formed and propagated. Consequently, the wear resistance of Inconel 690 can be improved by enlarging the grain size.
for the grain boundaries. The specimen with few carbides in the grain boundary has the largest weight loss. Furthermore, the specimen with the relatively fine carbide morphology has a slightly larger weight loss than the specimen with the relatively coarse carbide morphology. We deduce from these results that the carbides in the grain boundaries influence the weight loss of wear. In Fig. 9, the worn surface of the specimen with few carbides differs from the other specimens that have carbides in the grain boundary. The worn surface of the specimen with few carbides has deformation or abrasion marks that is similar to surfaces worn by an abrasion [13]. Moreover, the SEM images of the other specimens with carbides show a rough worn surface and cracks, which are similar to the rough surface and cracks surfaces worn by a delamination [12]. According to these results, the wear mechanism of the specimen with few carbides differs from that of the specimen with carbides in the grain boundary. In a carbide-free specimen, the surface is easily deformed and a third body can be easily formed [14]. In the specimen with carbides, the carbides in the grain boundary may enable a delamination to form and propagate cracks [12]. Because the average length of the crack propagation was short in the grain boundaries with continuous carbide, many cracks were formed. The continuous carbides morphologies produce more crack formation and propagation than the morphology of discontinuous carbides. Hence, the discontinuous carbides in the grain boundaries improve the wear resistance of the Inconel 690. 4. Conclusions
3.4. Effect of carbide morphology To observe the effect of carbide morphology, we performed thermal treatments under various conditions, after first conducting solution anneailng at the same temperature (1150 ◦ C) for the same duration (1 h) to get the same grain size. The results of the heat treatments, which are shown in Fig. 2, indicate that the carbide morphology varies with the thermal treatment condition. Fig. 8 shows the wear resistance of specimens with the same grain size, along with various carbide morphologies
1. Microstructure of Inconel 690 was affected by temperature and duration of the heat treatment. Grain size increased with increasing the temperature and the duration of solution annealing, whereas the hardness decreased. The sizes and discontinuousness of carbides increased with temperature and duration of the thermal treatment. 2. The hardness of Inconel 690 was not a major affecting factor in comparison with other microstructural variables. 3. The wear loss decreased with increasing grain size of specimens. The mass loss of small grain size specimen (AS
Fig. 9. SEM images of worn surface related to carbide morphology: (a) SAH (solution annealed at 1050 ◦ C for 1 h) and (b) SAH 715 (solution annealed at 1050 ◦ C for 1 h, thermally treated at 700 ◦ C for 15 h).
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grain size: 25 m) was three times as much as that of large grain size specimen (SAH715 grain size: 60 m). 4. In the worn surface of the carbide-free specimen, few cracks were found and the deformation or abrasion marks were observed. The cracks were observed in the worn surface of the specimens with grain boundary carbides. 5. Specimens with discontinuous grain boundary carbides have more wear resistance than those with continuous grain boundary carbides. Wear loss of the specimen with coarse carbide (SAH801 carbide size 0.5 m) is smaller than that with fine carbides (SAH701 carbide size 0.2 m) by 30%. References [1] Vikram N. Shah, Phlip E. Macdonald, Aging and Life Extension of Major Light Water Reactor, Elsevier, 1993.
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