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Effect of carbide size and spacing on the fretting wear behavior of Inconel 690 SG tube mated with SUS 409
Wear 338-339 (2015) 252–257
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Effect of carbide size and spacing on the fretting wear behavior of Inconel 690 SG tube mated with SUS 409 Jae Yong Yun, Gyeong Su Shin, Dae Il Kim, Ho Sik Lee, Woong Soon Kang, Seon Jin Kim n Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 24 November 2014 Received in revised form 19 June 2015 Accepted 24 June 2015 Available online 6 July 2015
The effect of carbide size and spacing on the fretting wear resistance of Inconel 690 steam generator (SG) tube mated with SUS 409 was investigated in this study. Based on previous materials tribology research, we reasoned that increasing carbide size and spacing would increase the wear resistance of the Inconel 690. Experiments were performed to test this hypothesis. Commercialized Inconel 690 tube with mean carbide length, thickness and spacing of 0.29, 0.10, and 0.59 μm, respectively, was used for the wear test. Carbide length, thickness, and spacing increased to 0.36, 0.15, and 0.68 μm, respectively after heat treatment at 450 °C for 16 h without changes in carbide volume fraction, grain size or hardness. The results of the wear test showed that at the amplitude of 25 μm, wear coefficient did not change due to stick phenomenon. At the amplitude of 50 μm–150 μm, wear coefficient decreased about 50%. At the amplitude of 300 μm, the wear coefficient decreased from 2.7 × 10−13 m3/Nm to 2.0 × 10−13 m3/Nm due to increase in carbide size and spacing. & 2015 Elsevier B.V. All rights reserved.
Keywords: Inconel 690 Fretting wear Steam generator Nuclear power plant Carbide Coarsening
1. Introduction Inconel 690 is widely used for steam generator (SG) tubes in nuclear power plants [1]. Because the tubes experience flowinduced vibration, fretting wear, which is repeatedly rubbed between two materials under very low amplitude, can occur between SG tubes and anti-vibration structures. The amount of wear damage varies with column and row number of the tube; in other words, the position of the tube in the steam generator. For example, the central region in the OPR1000, which is a certain type of steam generator located in Korea, shows the highest wear damage due to large vibration amplitude [2]. Since the wear damage on the tube can cause leakage of radioactive substances in nuclear power plants [3], worn tubes are plugged before failure. Because heat exchange efficiency in nuclear power plants can be reduced by plugging, wear behavior of Inconel 690 SG tubes has been widely investigated. It has been reported that intergranular carbides in Inconel 690 tend to coarsen after they are totally precipitated at relatively high temperatures of 600–800 °C [4]. The coarsening of carbide in Inconel 690 has not been reported yet due to lack of long-term operation of nuclear power plants. However, at the operation temperature as low as 320 °C, carbide coarsening could possibly occur due to long operation times ranging from years to tens of n
Corresponding author. Tel.: þ 82 2 2220 0406; fax: þ 82 2293 7844. E-mail address: [email protected] (S.J. Kim).
http://dx.doi.org/10.1016/j.wear.2015.06.012 0043-1648/& 2015 Elsevier B.V. All rights reserved.
years. These changes in carbide size and spacing could affect fretting wear resistance [5] as well as stress corrosion cracking (SCC) [4,6]. Inconel 690 has good resistance to SCC because of its proper carbide size and spacing, which promote crack tip blunting and decrease the stress concentration at the crack tip [4–6], as well as its high concentration of Cr. However, despite the appropriate carbide size and spacing of Inconel 690, further coarsening of carbide decreases its resistance to SCC, because closely distributed small carbide is more beneficial for crack tip blunting [4]. Wear resistance of Inconel 690 increases with increasing carbide volume fraction [5]. However, the effect of size and spacing of intergranular carbide on the wear behavior of Inconel 690 has not yet been studied. The effect of carbide size and spacing on the wear resistance has been studied for steels, WC-coated alloys, Cobased PM alloys, and Al-based alloys [7–17]. Some researchers have reported that large carbides have a detrimental effect on wear resistance by making the grain boundary more susceptible to cracking [8–12]. Other researchers have reported that large carbides increase wear resistance because larger carbides are more effective in supporting the applied load [13–15] thereby more effectively preventing metal-to-metal contact [17]. The purpose of the current investigation is to investigate the effects of carbide size and spacing on the fretting wear behavior of Inconel 690 against type 409 stainless steel under varying amplitudes.
J.Y. Yun et al. / Wear 338-339 (2015) 252–257
253
Table 1 Chemical compositions of Inconel 690TT and SUS 409SS. Specimen
Inconel 690TT SUS 409SS
Element (wt%) Ni
Fe
Cr
Co
C
Si
Mn
Ti
S
Bal. 0.310
10.810 Bal.
28.542 11.7
0.030 –
0.030 0.015
0.176 0.654
0.092 0.215
0.330 0.128
– 0.028
from SEM images. More than 1500 carbides per specimen were measured.
3. Results and discussion
Fig. 1. Schematic of custom-designed fretting wear test apparatus.
Table 2 Average grain diameter and hardness after heat treatment at 450 °C for various times. Heat treatment time (hours)
AR
5h
9h
12.5 h
16 h
Average grain diameter (μm) Hardness (HrB)
46.9 90.0
48.4 90.5
48.2 88.6
47.0 89.0
46.5 90.7
2. Experimental procedures Commercialized Inconel 690TT tube made by Sandvik was used for the wear test. SUS 409SS, which is generally used for antivibration structures in steam generators, was used as a mating material. The chemical compositions of Inconel 690TT and SUS 409SS were confirmed by optical emission spectroscopy and are presented in Table 1. Heat treatment was conducted at 450 °C for 5–16 h to obtain various carbide sizes without changing the carbide volume fraction or grain size [4,18,19]. To observe carbide morphology, specimens were electrolytically etched in 68% phosphoric acid at 4 V for 10 s. Grain size was measured for each heat treated specimens by Heyn's method in ASTM E112. Wear test was conducted using a modified fretting wear tester which covers reciprocating amplitudes from 5 to 300 μm, as shown in Fig. 1. The tube surface was machined into a rectangular shape (5 mm 20 mm) to make contact stress constant during the test. The wear test was performed with increasing amplitude from 25 to 300 μm at 30 Hz. Constant dead weight of 100 N was applied during the wear test. True amplitude was measured by a displacement meter (DM) during the wear test and amplitude was controlled by resetting the cam. Although the normal load of 100 N seems to be higher than the peak force between the tube and support plates [20], the stress in this research (1 Mpa) was less than the peak stress due to the large contact area. Weight losses were measured at 15, 30, 60 and 120 min. More than three replicate tests were performed to obtain reliable data. The relative deviation in the amount of wear loss between tests was about 78%. Scanning electron microscopy (SEM) was used to observe carbide morphology and the worn surfaces of specimens. Intergranular carbide length, thickness, and spacing were measured
The average grain diameter and hardness of Inconel 690 after heat treatment at 450 °C for various times up to 16 h are presented in Table 2. As shown in Table 2, hardness and average grain diameter did not change with increasing heat treatment time. Therefore, in the present work, the effects of hardness and grain size on the wear resistance of Inconel 690 were considered negligible. Fig. 2 shows SEM images of carbide morphology before and after heat treatment at 450 °C. All the carbides, as shown in Fig. 2, were precipitated along the grain boundaries. Carbide volume fraction, carbide size, and spacing were measured and the results are presented in Figs. 3–5. As shown in Fig. 3, carbide volume fractions at various heat treatment times were about the same (0.55 vol%). Therefore, the effect of carbide volume fraction was not considered. Only carbide size and spacing were considered to be parameters affecting the wear resistance of Inconel 690. As shown in Figs. 4 and 5, carbide length thickness and spacing increased with increasing heat treatment time. Carbide length and thickness increased from 0.29 and 0.1 μm to 0.36 and 0.15 μm, respectively, and carbide spacing also increased from 0.59 μm to 0.68 μm after 16 h of heat treatment at 450 °C. Weight loss as a function of test distance at various amplitudes and heat treatment times is shown in Fig. 6. Weight loss increased linearly with increasing test distance in all test specimens. In other words, the wear coefficient did not change with increasing distance. The slope of weight loss increased with increasing amplitude, indicating that the wear mode changed with changing amplitude as reported in previous research [21]. And the slope decreased with increasing heat treatment time. Therefore, wear resistance may possibly change with increasing heat treatment time. To quantitatively analyze the wear behavior of the specimens, wear coefficients were calculated by using the Archard equation shown in Eq. (1):
V = KFS
(1)
where V is the wear volume, F is the applied normal load, K is the wear coefficient, and S is the sliding distance. The wear coefficient in Fig. 7 was calculated from weight loss data in Fig. 6. The distinction between wear modes for Inconel 690 at various amplitude was studied in the previous study [21]. At the amplitude of 25 μm, the wear coefficient did not change. In this region, the stick phenomenon occurred and so the effect of carbide was negligible. However, at the amplitude of 50 μm, the wear coefficient decreased from 4.6 × 10−14 m3/Nm to 2.2 × 10−14 m3/Nm in the mixed stick and slip region after 16 h heat treatment. In the gross slip region, which occurred at the amplitude of 75–150 μm, the wear coefficient decreased about 50% on average after 16 h heat treatment. At the amplitude of 300 μm, wear coefficient of
254
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Fig. 2. SEM images of carbide morphology before and after heat treatment; (a) as-received, (b) 5 h, (c) 9 h, (d) 12.5 h, (e) 16 h of heat treatment.
Carbide volume fraction(%)
1.0
0.8
0.6
0.4
0.2
0.0 -2
0
2
4
6
8
10
12
14
16
18
Heat treatment time(hours) Fig. 3. Changes of carbide volume fraction as a function of heat treatment time at 450 °C.
the
sliding
10−13
region
m3/Nm
decreased
from
2.7 × 10−13 m3/Nm
to
2.0 × after 16 h heat treatment. These results indicate that an increase in carbide size and spacing decreased the wear
Fig. 4. Changes of intergranular carbide length and thickness as a function of heat treatment time at 450 °C.
coefficient. Therefore, the fretting wear resistance of Inconel 690 may increase as the operation time of nuclear power plants increases.
J.Y. Yun et al. / Wear 338-339 (2015) 252–257
As mentioned above, at amplitude higher than 25 μm, the wear coefficient, K, decreased with increasing heat treatment time. The worn surface of the as-received specimen, shown in Fig. 8(a),
255
revealed more wear damage than the specimen heat treated for 16 h, shown in Fig. 8(b). Many disturbed layers were found on the as-received specimen due to adhesion of Inconel 690 to mating materials. However, in the 16 h heat-treated specimen, only
0.70 25µm 50µm 75µm 100µm 150µm 300µm
0.68
3.00E-013 0.66
Wear coefficient(m /Nm)
0.62 0.60
2.50E-013
3
Spacing(µm)
0.64
0.58 0.56 0.54 0.52
2.00E-013 1.00E-013
5.00E-014
0.50 -2
0
2
4
6
8
10
12
14
16
18
Heat treatment time(hours) Fig. 5. Changes of spacing between intergranular carbides as a function of heat treatment time at 450 °C.
0.00E+000 -2
0
2
4
6
8
10
12
14
16
18
Heat treatment time(hours) Fig. 7. Wear coefficient as a function of heat treatment time at various amplitudes.
Fig. 6. Weight loss behavior as a function of test distance at various amplitudes and heat treatment times; (a) amplitude range from 25 to 75 μm up to test distance of 75,000 mm; (b) amplitude range from 25 to 75 μm up to test distance of 25,000 mm; (c) amplitude range from 100 to 300 μm up to test distance of 300,000 mm (d) amplitude range from 100 to 300 μm up to test distance of 100,000 mm.
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Fig. 8. SEM images of worn surfaces after wear test at an amplitude of 300 μm: (a) as-received specimen and (b) specimen heat treated for 16 h.
Fig. 9. SEM images of cross-section of the worn surface of specimens after the wear test at an amplitude of 300 μm: (a) as-received specimen and (b) specimen heat-treated for 16 h.
smooth scratch marks and mild wear morphology were found on the worn surfaces. Wear morphology of the matrix and grain boundary area was observed from cross sections of the worn surfaces. As shown in Fig. 9(a), in the as-received specimen, wear damage was severe in both the matrix and grain boundary areas. In the 16 h heat treated specimen, shown in Fig. 9(b), wear damage in the matrix was more severe and deeper than near the grain boundary. Compared to the carbides in the as-received specimen, coarsened carbides with large spacing are considered more effective in bearing the load [13–15] and therefore more effectively prevent metal-tometal contact [17].
4. Summary 1. Wear tests were conducted on Inconel 690 tubes with various carbide sizes and spacings and wear behavior was investigated. Carbide length and thickness increased from 0.29 and 0.1 μm to 0.36 and 0.15 μm, and spacing also increased from 0.59 μm to 0.68 μm after 16 h of heat treatment at 450 °C. 2. Wear coefficients were calculated by using the Archard equation to determine the wear resistance of Inconel 690. In the stick region, the wear coefficient did not differ significantly among specimens. In the mixed stick and slip region, the wear coefficient decreased by about 50%. In the gross slip region, the wear coefficient also decreased by about 50%. In the sliding region, the wear coefficient decreased by about 35%. Compared to carbides in the as-received specimen, the coarsened carbides with larger spacing due to heat treatment were more effective in bearing the load and thereby more effectively prevented metal-to-metal contact.
3. The decrease in wear coefficient of Inconel 690 due to coarsening of carbide suggests that the fretting wear resistance of Inconel 690 may increase as the operation time of nuclear power plants increases.
Acknowledgments This work was supported by the Human Resource Development Program (No. 20114010203020) of a Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government Ministry of Trade, Industry and Energy. This work was also supported by the Nuclear Research & Development (No. 20111510100060) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government Ministry of Trade, Industry and Energy.
References [1] S.-H. Jeong, C.-W. Cho, Y.-Z. Lee, Friction and wear of Inconel 690 for steam generator tube in elevated temperature water under fretting condition, Tribol. Int. 38 (3) (2005) 283–288. [2] C.-Y. Park, K.-W. Ryu, H. Rhee, Development of a computer program to predict structural integrity against fretting wear of steam generator tubes: PIAT (program for integrity assessment of steam generator tubes), Nucl. Eng. Des. 257 (0) (2013) 146–154. [3] G.S. Park, G.G. Kim, S.J. Kim, Sliding wear behaviors of steam generator tube materials in high temperature water environment, J. Nucl. Mater. 352 (1–3) (2006) 80–84. [4] S.Y. Jiao, M.C. Zhang, L. Zheng, J.X. Dong, Investigation of carbide precipitation process and chromium depletion during thermal treatment of alloy 690, Metall. Mater. Trans. A 41 (1) (2010) 26–42.
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[5] Jin-Ki Hong, In-Sup Kim, Chi-Yong park, Eung-Seon Kim, Microstructural effects on the fretting wear of Inconel 690 steam generator tube, Wear 259 (1–6) (2005) 349–355. [6] S.M. Bruemmer, C.H. Henager Jr, High voltage electron microscopy observations of microdeformation in alloy 600 tubing, Scr. Metall. 20 (6) (1986) 909–914. [7] D.N. Hanlon, W.M. Rainforth, C.M. Sellars, The effect of processing route, composition and hardness on the wear response of chromium bearing steels in a rolling-sliding configuration, Wear 203–204 (0) (1997) 220–229. [8] A. Kasak, T.A. Neumeyer, Observations on wear of high-hardness steels, Wear 14 (6) (1969) 445–454. [9] K. Holmberg, A. Laukkanen, A. Ghabchi, M. Rombouts, E. Turunen, R. Waudby, T. Suhonen, K. Valtonen, E. Sarlin, Computational modelling based wear resistance analysis of thick composite coatings, Tribol. Int. 72 (0) (2014) 13–30. [10] T.Z. Kattamis, T. Suganuma, Solidification processing and tribological behavior of particulate TiC-Ferrous Matrix composites, Mater. Sci. Eng.: A 128 (2) (1990) 241–252. [11] Yoshio Ikawa, Tetsu Itami, Ken Kumagai, Yuji Kawashima, A.G. Leatham, J.S. Coombs, R.G. Brooks, Spray deposition method and its application to the production of mill rolls, ISIJ Int. 30 (9) (1990) 756–763. [12] M. Aksoy, O. Yilmaz, M.H. Korkut, The effect of strong carbide-forming elements on the adhesive wear resistance of ferritic stainless steel, Wear 249 (8) (2001) 639–646. [13] C. Perrin, W.M. Rainforth, The effect of alumina fibre reinforcement on the wear of an Al-4.3% Cu alloy, Wear 181–183 (0) (1995) 312–324, Part 1.
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[14] V.M. Desai, C.M. Rao, T.H. Kosel, N.F. Fiore, Effect of carbide size on the abrasion of cobalt-base powder metallurgy alloys, Wear 94 (1) (1984) 89–101. [15] M. Vardavoulias, C. Jouanny-Tresy, M. Jeandin, Sliding-wear behaviour of ceramic particle-reinforced high-speed steel obtained by powder metallurgy, Wear 165 (2) (1993) 141–149. [16] J.-K. Lee, On the dry wear behaviour of ASTM 4032H steel against the titanium carbide metal matrix composite in the early stage, Wear 265 (11–12) (2008) 1584–1591. [17] A.T. Alpas, J. Zhang, Effect of microstructure (particulate size and volume fraction) and counterface material on the sliding wear resistance of particulate-reinforced aluminum matrix composites, Metall. Mater. Trans. A 25 (5) (1994) 969–983. [18] Y.F. Yin, R.G. Faulkner, Model predictions of grain boundary chromium depletion in Inconel 690, Corros. Sci. 49 (5) (2007) 2177–2197. [19] J.J. Kai, G.P. Yu, C.H. Tsai, M.N. Liu, S.C. Yao, The effects of heat treatment on the chromium depletion, precipitate evolution, and corrosion resistance of INCONEL alloy 690, Metall. Trans. A 20 (10) (1989) 2057–2067. [20] I. Chung, M. Lee, An experimental study on fretting wear behavior of crosscontacting Inconel 690 tubes, Nucl. Eng. Des. 241 (10) (2011) 4103–4110. [21] J.Y. Yun, M.C. Park, G.S. Shin, J.H. Heo, D.I. Kim, S.J. Kim, Effects of amplitude and frequency on the wear mode change of Inconel 690 SG tube mated with SUS 409, Wear 313 (1–2) (2014) 83–88.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Effect of carbide size and spacing on the fretting wear behavior of Inconel 690 SG tube mated with SUS 409 Jae Yong Yun, Gyeong Su Shin, Dae Il Kim, Ho Sik Lee, Woong Soon Kang, Seon Jin Kim n Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 24 November 2014 Received in revised form 19 June 2015 Accepted 24 June 2015 Available online 6 July 2015
The effect of carbide size and spacing on the fretting wear resistance of Inconel 690 steam generator (SG) tube mated with SUS 409 was investigated in this study. Based on previous materials tribology research, we reasoned that increasing carbide size and spacing would increase the wear resistance of the Inconel 690. Experiments were performed to test this hypothesis. Commercialized Inconel 690 tube with mean carbide length, thickness and spacing of 0.29, 0.10, and 0.59 μm, respectively, was used for the wear test. Carbide length, thickness, and spacing increased to 0.36, 0.15, and 0.68 μm, respectively after heat treatment at 450 °C for 16 h without changes in carbide volume fraction, grain size or hardness. The results of the wear test showed that at the amplitude of 25 μm, wear coefficient did not change due to stick phenomenon. At the amplitude of 50 μm–150 μm, wear coefficient decreased about 50%. At the amplitude of 300 μm, the wear coefficient decreased from 2.7 × 10−13 m3/Nm to 2.0 × 10−13 m3/Nm due to increase in carbide size and spacing. & 2015 Elsevier B.V. All rights reserved.
Keywords: Inconel 690 Fretting wear Steam generator Nuclear power plant Carbide Coarsening
1. Introduction Inconel 690 is widely used for steam generator (SG) tubes in nuclear power plants [1]. Because the tubes experience flowinduced vibration, fretting wear, which is repeatedly rubbed between two materials under very low amplitude, can occur between SG tubes and anti-vibration structures. The amount of wear damage varies with column and row number of the tube; in other words, the position of the tube in the steam generator. For example, the central region in the OPR1000, which is a certain type of steam generator located in Korea, shows the highest wear damage due to large vibration amplitude [2]. Since the wear damage on the tube can cause leakage of radioactive substances in nuclear power plants [3], worn tubes are plugged before failure. Because heat exchange efficiency in nuclear power plants can be reduced by plugging, wear behavior of Inconel 690 SG tubes has been widely investigated. It has been reported that intergranular carbides in Inconel 690 tend to coarsen after they are totally precipitated at relatively high temperatures of 600–800 °C [4]. The coarsening of carbide in Inconel 690 has not been reported yet due to lack of long-term operation of nuclear power plants. However, at the operation temperature as low as 320 °C, carbide coarsening could possibly occur due to long operation times ranging from years to tens of n
Corresponding author. Tel.: þ 82 2 2220 0406; fax: þ 82 2293 7844. E-mail address: [email protected] (S.J. Kim).
http://dx.doi.org/10.1016/j.wear.2015.06.012 0043-1648/& 2015 Elsevier B.V. All rights reserved.
years. These changes in carbide size and spacing could affect fretting wear resistance [5] as well as stress corrosion cracking (SCC) [4,6]. Inconel 690 has good resistance to SCC because of its proper carbide size and spacing, which promote crack tip blunting and decrease the stress concentration at the crack tip [4–6], as well as its high concentration of Cr. However, despite the appropriate carbide size and spacing of Inconel 690, further coarsening of carbide decreases its resistance to SCC, because closely distributed small carbide is more beneficial for crack tip blunting [4]. Wear resistance of Inconel 690 increases with increasing carbide volume fraction [5]. However, the effect of size and spacing of intergranular carbide on the wear behavior of Inconel 690 has not yet been studied. The effect of carbide size and spacing on the wear resistance has been studied for steels, WC-coated alloys, Cobased PM alloys, and Al-based alloys [7–17]. Some researchers have reported that large carbides have a detrimental effect on wear resistance by making the grain boundary more susceptible to cracking [8–12]. Other researchers have reported that large carbides increase wear resistance because larger carbides are more effective in supporting the applied load [13–15] thereby more effectively preventing metal-to-metal contact [17]. The purpose of the current investigation is to investigate the effects of carbide size and spacing on the fretting wear behavior of Inconel 690 against type 409 stainless steel under varying amplitudes.
J.Y. Yun et al. / Wear 338-339 (2015) 252–257
253
Table 1 Chemical compositions of Inconel 690TT and SUS 409SS. Specimen
Inconel 690TT SUS 409SS
Element (wt%) Ni
Fe
Cr
Co
C
Si
Mn
Ti
S
Bal. 0.310
10.810 Bal.
28.542 11.7
0.030 –
0.030 0.015
0.176 0.654
0.092 0.215
0.330 0.128
– 0.028
from SEM images. More than 1500 carbides per specimen were measured.
3. Results and discussion
Fig. 1. Schematic of custom-designed fretting wear test apparatus.
Table 2 Average grain diameter and hardness after heat treatment at 450 °C for various times. Heat treatment time (hours)
AR
5h
9h
12.5 h
16 h
Average grain diameter (μm) Hardness (HrB)
46.9 90.0
48.4 90.5
48.2 88.6
47.0 89.0
46.5 90.7
2. Experimental procedures Commercialized Inconel 690TT tube made by Sandvik was used for the wear test. SUS 409SS, which is generally used for antivibration structures in steam generators, was used as a mating material. The chemical compositions of Inconel 690TT and SUS 409SS were confirmed by optical emission spectroscopy and are presented in Table 1. Heat treatment was conducted at 450 °C for 5–16 h to obtain various carbide sizes without changing the carbide volume fraction or grain size [4,18,19]. To observe carbide morphology, specimens were electrolytically etched in 68% phosphoric acid at 4 V for 10 s. Grain size was measured for each heat treated specimens by Heyn's method in ASTM E112. Wear test was conducted using a modified fretting wear tester which covers reciprocating amplitudes from 5 to 300 μm, as shown in Fig. 1. The tube surface was machined into a rectangular shape (5 mm 20 mm) to make contact stress constant during the test. The wear test was performed with increasing amplitude from 25 to 300 μm at 30 Hz. Constant dead weight of 100 N was applied during the wear test. True amplitude was measured by a displacement meter (DM) during the wear test and amplitude was controlled by resetting the cam. Although the normal load of 100 N seems to be higher than the peak force between the tube and support plates [20], the stress in this research (1 Mpa) was less than the peak stress due to the large contact area. Weight losses were measured at 15, 30, 60 and 120 min. More than three replicate tests were performed to obtain reliable data. The relative deviation in the amount of wear loss between tests was about 78%. Scanning electron microscopy (SEM) was used to observe carbide morphology and the worn surfaces of specimens. Intergranular carbide length, thickness, and spacing were measured
The average grain diameter and hardness of Inconel 690 after heat treatment at 450 °C for various times up to 16 h are presented in Table 2. As shown in Table 2, hardness and average grain diameter did not change with increasing heat treatment time. Therefore, in the present work, the effects of hardness and grain size on the wear resistance of Inconel 690 were considered negligible. Fig. 2 shows SEM images of carbide morphology before and after heat treatment at 450 °C. All the carbides, as shown in Fig. 2, were precipitated along the grain boundaries. Carbide volume fraction, carbide size, and spacing were measured and the results are presented in Figs. 3–5. As shown in Fig. 3, carbide volume fractions at various heat treatment times were about the same (0.55 vol%). Therefore, the effect of carbide volume fraction was not considered. Only carbide size and spacing were considered to be parameters affecting the wear resistance of Inconel 690. As shown in Figs. 4 and 5, carbide length thickness and spacing increased with increasing heat treatment time. Carbide length and thickness increased from 0.29 and 0.1 μm to 0.36 and 0.15 μm, respectively, and carbide spacing also increased from 0.59 μm to 0.68 μm after 16 h of heat treatment at 450 °C. Weight loss as a function of test distance at various amplitudes and heat treatment times is shown in Fig. 6. Weight loss increased linearly with increasing test distance in all test specimens. In other words, the wear coefficient did not change with increasing distance. The slope of weight loss increased with increasing amplitude, indicating that the wear mode changed with changing amplitude as reported in previous research [21]. And the slope decreased with increasing heat treatment time. Therefore, wear resistance may possibly change with increasing heat treatment time. To quantitatively analyze the wear behavior of the specimens, wear coefficients were calculated by using the Archard equation shown in Eq. (1):
V = KFS
(1)
where V is the wear volume, F is the applied normal load, K is the wear coefficient, and S is the sliding distance. The wear coefficient in Fig. 7 was calculated from weight loss data in Fig. 6. The distinction between wear modes for Inconel 690 at various amplitude was studied in the previous study [21]. At the amplitude of 25 μm, the wear coefficient did not change. In this region, the stick phenomenon occurred and so the effect of carbide was negligible. However, at the amplitude of 50 μm, the wear coefficient decreased from 4.6 × 10−14 m3/Nm to 2.2 × 10−14 m3/Nm in the mixed stick and slip region after 16 h heat treatment. In the gross slip region, which occurred at the amplitude of 75–150 μm, the wear coefficient decreased about 50% on average after 16 h heat treatment. At the amplitude of 300 μm, wear coefficient of
254
J.Y. Yun et al. / Wear 338-339 (2015) 252–257
Fig. 2. SEM images of carbide morphology before and after heat treatment; (a) as-received, (b) 5 h, (c) 9 h, (d) 12.5 h, (e) 16 h of heat treatment.
Carbide volume fraction(%)
1.0
0.8
0.6
0.4
0.2
0.0 -2
0
2
4
6
8
10
12
14
16
18
Heat treatment time(hours) Fig. 3. Changes of carbide volume fraction as a function of heat treatment time at 450 °C.
the
sliding
10−13
region
m3/Nm
decreased
from
2.7 × 10−13 m3/Nm
to
2.0 × after 16 h heat treatment. These results indicate that an increase in carbide size and spacing decreased the wear
Fig. 4. Changes of intergranular carbide length and thickness as a function of heat treatment time at 450 °C.
coefficient. Therefore, the fretting wear resistance of Inconel 690 may increase as the operation time of nuclear power plants increases.
J.Y. Yun et al. / Wear 338-339 (2015) 252–257
As mentioned above, at amplitude higher than 25 μm, the wear coefficient, K, decreased with increasing heat treatment time. The worn surface of the as-received specimen, shown in Fig. 8(a),
255
revealed more wear damage than the specimen heat treated for 16 h, shown in Fig. 8(b). Many disturbed layers were found on the as-received specimen due to adhesion of Inconel 690 to mating materials. However, in the 16 h heat-treated specimen, only
0.70 25µm 50µm 75µm 100µm 150µm 300µm
0.68
3.00E-013 0.66
Wear coefficient(m /Nm)
0.62 0.60
2.50E-013
3
Spacing(µm)
0.64
0.58 0.56 0.54 0.52
2.00E-013 1.00E-013
5.00E-014
0.50 -2
0
2
4
6
8
10
12
14
16
18
Heat treatment time(hours) Fig. 5. Changes of spacing between intergranular carbides as a function of heat treatment time at 450 °C.
0.00E+000 -2
0
2
4
6
8
10
12
14
16
18
Heat treatment time(hours) Fig. 7. Wear coefficient as a function of heat treatment time at various amplitudes.
Fig. 6. Weight loss behavior as a function of test distance at various amplitudes and heat treatment times; (a) amplitude range from 25 to 75 μm up to test distance of 75,000 mm; (b) amplitude range from 25 to 75 μm up to test distance of 25,000 mm; (c) amplitude range from 100 to 300 μm up to test distance of 300,000 mm (d) amplitude range from 100 to 300 μm up to test distance of 100,000 mm.
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Fig. 8. SEM images of worn surfaces after wear test at an amplitude of 300 μm: (a) as-received specimen and (b) specimen heat treated for 16 h.
Fig. 9. SEM images of cross-section of the worn surface of specimens after the wear test at an amplitude of 300 μm: (a) as-received specimen and (b) specimen heat-treated for 16 h.
smooth scratch marks and mild wear morphology were found on the worn surfaces. Wear morphology of the matrix and grain boundary area was observed from cross sections of the worn surfaces. As shown in Fig. 9(a), in the as-received specimen, wear damage was severe in both the matrix and grain boundary areas. In the 16 h heat treated specimen, shown in Fig. 9(b), wear damage in the matrix was more severe and deeper than near the grain boundary. Compared to the carbides in the as-received specimen, coarsened carbides with large spacing are considered more effective in bearing the load [13–15] and therefore more effectively prevent metal-tometal contact [17].
4. Summary 1. Wear tests were conducted on Inconel 690 tubes with various carbide sizes and spacings and wear behavior was investigated. Carbide length and thickness increased from 0.29 and 0.1 μm to 0.36 and 0.15 μm, and spacing also increased from 0.59 μm to 0.68 μm after 16 h of heat treatment at 450 °C. 2. Wear coefficients were calculated by using the Archard equation to determine the wear resistance of Inconel 690. In the stick region, the wear coefficient did not differ significantly among specimens. In the mixed stick and slip region, the wear coefficient decreased by about 50%. In the gross slip region, the wear coefficient also decreased by about 50%. In the sliding region, the wear coefficient decreased by about 35%. Compared to carbides in the as-received specimen, the coarsened carbides with larger spacing due to heat treatment were more effective in bearing the load and thereby more effectively prevented metal-to-metal contact.
3. The decrease in wear coefficient of Inconel 690 due to coarsening of carbide suggests that the fretting wear resistance of Inconel 690 may increase as the operation time of nuclear power plants increases.
Acknowledgments This work was supported by the Human Resource Development Program (No. 20114010203020) of a Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government Ministry of Trade, Industry and Energy. This work was also supported by the Nuclear Research & Development (No. 20111510100060) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government Ministry of Trade, Industry and Energy.
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