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The effect of subsurface deformation on the wear behavior of steam generator tube materials
Wear 253 (2002) 438–447
The effect of subsurface deformation on the wear behavior of steam generator tube materials Young-Ho Lee, In-Sup Kim∗ Department of Nuclear Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea Received 29 October 2001; received in revised form 4 April 2002; accepted 27 May 2002
Abstract Fretting wear behavior of steam generator (SG) tube materials (Inconel 600MA and 690TT) against ferritic stainless steels was investigated in a room temperature water environment. The results indicated that the fretting wear rate and wear coefficient (K) in a work-rate model of Inconel 600MA were higher than those of Inconel 690TT with increasing normal loads and sliding amplitudes. From the results of scanning electron microscopy (SEM) observation, there was little evidence of particle agglomeration on the worn surfaces, while wear particles were released in the form of thin plates, which were generated from deformation substructures formed by severe plastic deformation during fretting wear. Therefore, the wear rates of SG tube materials in the room temperature water are closely related with plastic deformation behavior on contact surfaces. In subsurface layer, wear particle size seems to be determined by cell-structure thickness and closely related to the difference of stacking fault energy (SFE) of tube materials through chromium contents. Inconel 690TT showed lower wear rate because it should have relatively smaller cells due to lower SFE than Inconel 600MA and in turn may easily accommodate large strains. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Steam generator; Fretting wear; Work-rate model; Inconel alloys; Stacking fault energy
1. Introduction The formation of wear particle layers and the subsurface deformation are widely observed phenomena under sliding or fretting contact of metals. It was proposed that the wear rate is strongly influenced by those two phenomena in various test conditions [1–12]. If wear particles are easily oxidized and compacted on the rubbing surface, they act as a wear protective layer, which decreases the total wear rate. Otherwise, they may act as abrasives which accelerate wear damage [4]. Sometimes, the metal-to-metal contact is more dominant, especially in water or in air blowing condition [5]. Thus, it is very important that the wear rate at high flow rate condition such as the secondary side of nuclear power plant is controlled by the variation of mechanical properties in surface or in subsurface because wear particles are released after plastic deformation. The effect of wear particle layers on the wear rate of nickel-based alloy at temperatures of 20–250 ◦ C has been reported previously [13]. It was reported that wear transitions ∗ Corresponding author. Tel.: +82-42-869-3815; fax: +82-42-869-3810. E-mail addresses: [email protected] (Y.-H. Lee), [email protected] (I.-S. Kim).
from an initial high rate to a low rate were accompanied by the development of high-resistance load-bearing layers on the wear surface. With increasing temperature, wear particles were heavily oxidized, resulting in oxide-to-oxide contact between the rubbing surfaces. It was suggested that the mechanism of these transitions may have been due to the adhesion between the worn surface and wear particles, which prevents wear particles being released from the contact surfaces. On the other hand, in a water environment such as the secondary side of steam generator (SG) in a nuclear power plant, it is difficult to form the wear particle layer between tubes and their supports during fretting wear. This may be result of water lubrication or the easy removal of wear particles due to the high water flow rates. Therefore, it is expected that the wear rate of SG tube materials strongly depended on the deformation characteristics of the worn surface or in subsurface during fretting wear. Sauger et al. [14] proposed that a specific superficial layer is formed during the very first cycles of fretting loading, which is called the tribologically transformed structure (TTS). They emphasize that understanding the mechanisms of formation of the TTS is a key step in the modeling of wear induced by fretting. But this TTS will disappear after a large number of cycles.
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Y.-H. Lee, I.-S. Kim / Wear 253 (2002) 438–447
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With increasing number of fretting cycles, it is expected that the wear-determining factors are plastic deformation and fracture on wearing surface in the water environment. Recently, the fretting-related degradations in SG tube materials are reported from many nuclear power plants. The main causes are both flow-induced vibration (FIV) between tube and support materials and the loose parts which randomly impact against the tube. Previous studies [15,16] were focused on the evaluation of the wear coefficient in a work-rate model to calculate the exact degradation rate for the estimation of residual lifetime. In spite of the widely recognized importance of wear coefficients in SG fretting wear, there has been relatively little effort made to understand wear mechanisms. The authors [17] have performed the wear test of SG tube materials in the room temperature air environment, in which the wear coefficients and the role of wear particle layers in the SG tube materials such as Inconel 600MA and 690TT were analyzed. In the present work, fretting wear experiment was performed with SG tube materials against ferritic stainless steels in room temperature water. The objective was to examine the possible wear mechanisms before high temperature and pressure experiments. The discussion was focused on the relationship between the subsurface deformation and wear rate or wear coefficient K in the work-rate model.
2. Experimental procedure The alloys used in this study were Inconel 600MA and 690TT (abbreviated to 600MA and 690TT); these commercial nickel-based alloys have been used as SG tube materials in nuclear power plants. Their microstructures are shown in Fig. 1. The counterpart materials were selected as 405 and 409 ferritic stainless steel (abbreviated to 405SS and 409SS) because those are used as tube support materials in the operating power plants. Chemical composition and bulk hardness of tested materials are shown in Table 1. A reciprocating wear apparatus with tube-on-plate configuration was utilized and details of this test system is described in the previous studies [17]. In this test, a water tank is equipped so that the fretting experiment can be conducted in 25 ◦ C distilled water. The dimensions of the tube specimen were 19.05 mm in diameter and 14 mm in length and
Fig. 1. Microstructure of tested tubes: (a) Inconel 600MA; (b) Inconel 690TT.
the counter-specimens were prepared from the flat strip. The friction load and fretting amplitude between sliding surfaces were continuously monitored during the wear test. The wear losses were determined from the weight measurements before and after the experiments using an analytical balance with an accuracy of the order of 0.1 mg. Prior to each test and weighing measurement, the specimens were acoustically cleaned in acetone for 5 min and dried in compressed air. The tube specimen oscillates with a peak-to-peak amplitude
Table 1 Chemical composition and bulk hardness of SG tubes and their support materials Specimen
Cr
Fe
C
Si
Mn
Ti
P
S
Co
Ni
Hardness (HV)
600MA 690TT 405SS 409SS
16.81 29.5 11.5–14.5 10.5–11.75
9.1 10.4 Balance Balance
0.026 0.02 0.08 0.08
0.32 0.33 1.00 1.00
0.81 0.26 1.00 1.00
0.35 0.32 0.1 (Al) 0.5
0.008 0.004 0.04 0.045
0.002 0.001 0.03 0.045
0.012 0.012 – –
Balance Balance – –
194 193 215 229
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of 50–400 m at a frequency of 30 Hz. The applied normal load was 10–40 N. In the present study, distilled water was used for the environment and the wear experiments were performed at 25 ◦ C. To evaluate the wear mechanism in the water environment, the worn surfaces and cross-sections below the contact surface were examined using a scanning electron microscopy (SEM) after tests. Also, nano-indentation tests were performed to assess the resistance to plastic deformation in the subsurface of the tube materials. Recently, a work-rate model has been used for the wear evaluation of SG tubes [15]. The work-rate is used to normalize the wear rate and is defined as the rate of energy being dissipated at the contact [16]. It
gives the following expression for the lifetime evaluation of SG tubes: ˙ W = F ds (1) V˙ = K W˙
(2)
where V˙ is wear rate, W˙ the work-rate, F the normal load, s the sliding distance and K is the wear coefficient. Thus, from the test results, wear coefficients (K, defined as the total volume loss per normal load and per total sliding distance) were calculated and compared between the test conditions.
Fig. 2. Effect of normal load on the specific wear rate (Vs , mg/m) of SG tube materials: (a) Inconel 600MA; (b) Inconel 690TT.
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3. Experimental results and discussion 3.1. Wear rate and wear coefficients of work-rate model Fig. 2 show the relationship between applied normal load and specific wear rate (weight loss per sliding distance, mg/m) of tube materials in water environment. The sliding amplitude and number of cycles have been changed at each test condition. The specific wear rate slowly increased for both tube materials and showed much scattering of values with increasing normal load. Fig. 3 shows the effect of sliding distance on the mass loss of the tube materials at the
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same load. With increasing sliding distance, the wear rate, which is the slope of the tangent to the curve, was slowly decreased. Hence, the fretting wear rate does not linearly increase with sliding distance. Fig. 4 shows the calculated wear coefficient, K, in the work-rate model of SG tube materials. 600MA has similar values with two support materials. Average wear coefficients, K, are 56.1 × 10−15 Pa−1 for 405SS and 58.1 × 10−15 Pa−1 for 409SS. But, with 690TT, the wear coefficient, K, shows a much low value (K = 18.5 × 10−15 Pa−1 ) when the support material is selected as 409SS. However, when we compare the average values of wear coefficient (K) in the work-rate model, 600MA has
Fig. 3. The variation of mass loss of SG tube materials with sliding distance at the same load condition: (a) Inconel 600MA against 405 stainless steel; (b) Inconel 690TT against 409 stainless steel. Wear rate slowly decreases at both materials with increasing sliding distance.
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Fig. 4. Wear coefficient K in the work-rate model from test results: (a) Inconel 600MA; (b) Inconel 690TT. The more resistant condition in room temperature water is Inconel 690TT against 409 stainless steel.
about twice the value as with 690TT in room temperature water environment. Generally, the wear transition phenomena are closely related with the formation of wear particle layers in an air environment. But, Hiratsuka et al. [18] pointed out that the initial steady wear transition was also observed during rubbing in a vacuum. He emphasized that the oxide film is not a necessary condition for establishing steady-state wear. Therefore, there seems to be two possibilities for these lower wear rates with increasing sliding distance. One is due to wear particles remaining in the contact surface. Even if it is expected that the generated wear particles are easily removed between contact surfaces in water environment during sliding, wear particles could be adhered to the worn surface due
to other factors such as magnetization on the worn surface. The other is the change in hardness of surface or subsurface. From the previous study [19], when the subsurfaces were hardened to the same extent as the transfer particles and wear particles, the wear mode changed to the steady-state wear. To confirm the magnetization effect between wear particles and worn surface, we measured the magnetic field at the worn surface. As a result, the intensity of magnetic field is almost the same before and after wear test and these effects could be ignored in those experiments. So, in water environment, wear rate of tube materials are determined by the variation of mechanical properties, such as work hardening, abilities of strain accommodation, etc., between contact surfaces.
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Fig. 5. Variation of friction coefficient during wear test. After 10 000 cycles, friction coefficient of Inconel 690TT slowly decreased.
3.2. Variation of friction coefficients Fig. 5 shows the variation of friction coefficient during fretting wear test. In the case of 690TT against 405SS, the friction coefficient decreased rapidly at the beginning of the tests and decreased slowly after around 10000 cycles. If the difference of initial friction coefficient originates from the difference of roughness of the tube surfaces, these will be eliminated at the beginning of test. Therefore, the friction coefficient of 690TT decreases at a slower rate with increas-
ing cycles. In the case of 600MA, friction coefficients fluctuated in the regions from 0.5 to 0.55. However, the general trend of these friction curves show that there is little change of friction coefficient in 600MA, but show some decrease in 690TT with increasing cycles. If the contact surface became harder due to severe plastic deformation, the contact area under the same load may decrease and consequently the friction coefficient will decrease. From the results, we deduced that 690TT experiences more work-hardening on its worn surface. Also, if wear
Fig. 6. The effect of shear load on the specific wear rate of SG tube materials. Note the reduction of scattering as compared to Fig. 2 when friction coefficient was correlated to wear rate.
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particles were released from the worn surface after severe plastic deformation and fracture, the shear load, which is generated by friction force on worn surface, is dominantly related to deformation and fracture. Therefore, to identify the relationship between specific wear rate and shear load, we expressed wear rate against shear load, which was correlated with friction coefficient in Fig. 6. In this figure, much of scattering as shown in Fig. 2 disappeared and specific wear rate is linearly increased with shear load. So, we deduced that wear rate of tubes in water is determined by the abilities of strain accommodation on contact surfaces because this is closely related with the generation rate of wear particles. Fouvry et al. [20] proposed the wear energy approach correlating friction coefficient during fretting wear and showed a linear relationship between dissipated energy and wear volume. Therefore, the wear coefficient (K) of the work-rate model for the lifetime evaluation of SG tubes should reflect wear mechanism in water condition and this could be archived by the correlation of wear coefficient with friction coefficient. 3.3. Worn surface and subsurface observation In the previous test results in air condition [17], wear rate and wear coefficients were dependent on the tribological properties of wear particle layers that were agglomerated on the worn surface of SG tubes. However, Fig. 7 shows that almost all wear particles were removed and their layer was not observable any more in water condition. The worn surface was fractured in thin plate form and wear particles seem to be released from the edge of these layers. Therefore, they do not stay between contact surfaces for a long time and metal-to-metal contact is dominant. In the present experiment, the roles of water lubricant are to restrain the formation of wear particle layers as well as to remove easily the wear particles when they reach a critical size. In Fig. 7, it seems apparent that the worn surfaces consisted of fractured thin plates, which were generated from the severe plastic deformation during wear. In 600MA, the thickness of those thin plates was about 2–3 m and cracks appeared under the worn surface in a perpendicular direction. But, in case of 690TT, the thickness of these plates and generated wear particles are relatively small and it is difficult to observe the crack in the direction of depth on the worn surface. Fig. 8 shows cross-sections of worn surfaces in two tube materials. The plastic deformation layer apparently appeared and they have specific thickness which is very small compared with their grain sizes as shown in Fig. 1. Besides, microcracks before wear particles propagate in deformed layer and not in grain boundaries. This means that if wear particles are generated by the fracture of these thin plates due to the hardness differences between upper and lower plates after severe plastic deformation, wear rate difference of two tube materials is, therefore, closely related with resistance to plastic deformation near worn surface during wear.
Fig. 7. SEM micrographs of the worn surfaces of SG tube materials after wear test: (a) Inconel 600MA; (b) Inconel 690TT.
The hardness distribution in subsurface regions was measured. Since severe deformation was observed in these regions, nano-indentation tests were performed to depth direction from worn surface. On each tube specimen, hardness tests were performed at three different locations and the average values are plotted in Fig. 9. The standard deviations of the mean values were about 20% of the mean. Before the wear test, two tube materials have similar hardness values. But the hardness of 690TT near the worn surface rapidly increased with respect to that of 600MA and this hardness difference gradually reduced with increasing distance from worn surface. 600MA and 690TT both have a single phase with stable face-centered cubic (fcc) structured up to temperatures of 1000 ◦ C and they have chromium carbides in the grain boundaries, such as dominant types of Cr7 C3 for 600MA and Cr23 C6 for 690TT [21–23]. Unlike austenite stainless steels that could be transformed to martensitic structures during sliding wear, these tube materials seem to maintain the initial phase. So, it is expected that the dislocation structure changes to accommodate the large strains generated during fretting wear. From the study of Rigney and Glaeser [24],
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Fig. 8. The subsurface observation using SEM: (a) Inconel 600MA; (b) Inconel 690TT. Each tube material has plastic deformation layers with specific thickness formed during wear.
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the repeated ploughing of asperity contacts over a mating surface can produce high dislocation densities and eventually change the microstructure to a cell-type structure found in heavily deformed metals; the cell size depending on material characteristics such as stacking fault energy (SFE), the applied stress and the temperature. The most significant difference in the chemical composition between 600MA and 690TT is chromium contents: 690TT has twice the chromium content (∼30 w/o) for the benefit of corrosion resistance. From the result of Symons’ study [25] about the effect of chromium content on the SFE of Ni–XCr–Fe alloy, he proposed that SFE rapidly decreased with increasing chromium contents in nickel-base alloys. If the material has high SFE, then dislocation climb and cross slip are easier and the tangled cell walls are well formed. The cell structure may be entirely absent in low SFE materials. However, at high strains cells could develop even for low SFE materials, but the cell sizes slowly decreased to a very thin layer near worn surface. Also, they proposed that a cell structure can present many suitable pathways for subsurface crack generation and the release of thin wear flakes [24]. So, 690TT has relatively low SFE due to high chromium content and is not easy to develop cell structures. But at high strain region such as worn surface, small size cell could develop near worn surface. Therefore, when wear particles are separated due to microcracks which are dominantly propagated to cell boundaries, wear particle size is larger in 690MA than in 690TT at worn surface as shown in Fig. 7. Thompson [26] proposed that yield strength is weakly dependent on grain size below 1 m and this behavior was compared to yielding in a cell-type structure. So, cell
Fig. 9. Microhardness variation beneath the worn surfaces of SG tube materials.
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structures were much stronger because the relatively short sources in cell walls caused higher yield strengths than the long sources in normal grain boundaries. This means that 690TT with relatively small cell size from Symons’ study shows high hardening behavior on its worn surface and low wear rate in room temperature water because cell structure could easily accommodate large strains. In addition to the measurements of hardness distributions below worn surface, plastic deformation energy was calculated to examine the relationship between hardened layer and wear properties. The plastic deformation energy is defined as the closed loop of load–displacement curve as shown in Fig. 10. The plastic deformation energies per unit volume
at the depth of 15 m from worn surface were calculated as 6.73 J mm−3 for 600MA and 7.58 J mm−3 for 690TT. The result indicates that about 15% more energy was needed to produce the same deformation volume in 690TT than in 600MA and could be connected to the low wear rate of materials. Also, the deformed layer should be propagated to the depth direction in order to maintain the specific thickness as shown in Fig. 8 as wear particles are continuously removed on the worn surface during wear. Therefore, the wear rate of SG tube materials in room temperature water is determined by the balance between the rate of wear particle removal on the worn surface and the formation rate of plastic deformation layers in the subsurface.
Fig. 10. Load–Indentation depth plots of SG tube materials in subsurface. The closed areas represent the plastic deformation energy during nano-indentation tests.
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4. Conclusions Fretting wear behaviors of SG tube materials against ferritic stainless steels were investigated in room temperature water. The following conclusions are drawn; 1. With increasing normal load and sliding distance, 690TT has low wear rate compared with 600MA in room temperature water. This result is mainly related to the changing mechanical properties of the tube materials during wear. 2. Wear particles are mostly removed from worn surface after severe plastic deformation and fracture. Thus, it is possible to relate the wear rate to shear load in room temperature water because stress component causing plastic deformation and fracture is mainly shear type. 3. The worn surface examination confirmed that wear particles are ejected in the form of thin plates on worn surface. Main cause is the difference of SFE with chromium contents between Inconel 600MA and 690TT and this is closely related to the formation of cell-type structure that could easily accommodate large strains. Also, plastic deformation layers appear in subsurface of both tube materials, which have specific thickness and are small compared with their grain sizes. 4. In room temperature water, the wear rate of SG tube materials is determined by the balance between the removal rates of wear particles from the worn surface and the formation rate of plastic deformation layers in the subsurface. Acknowledgements This research was partially supported by the Brain Korea 21 project and Korea Electric Power Research Institute of Korea Electric Power Corporation. References [1] F.H. Stott, The role of oxidation in the wear of alloys, Tribol. Int. 31 (1998) 61–71. [2] T.F.J. Quinn, Review of oxidational wear—Part 1: The origins of oxidational wear, Tribol. Int. 16 (1983) 257–271. [3] J. Jiang, F.H. Stott, M.M. Stack, The effect of partial pressure of oxygen on the tribological behaviour of a nickel-based alloy, N80A, at elevated temperatures, Wear 203–204 (1997) 615–625. [4] A. Iwabuchi, The role of oxide particles in the fretting wear of mild steel, Wear 151 (1991) 301–311. [5] E.R. Leheup, R.E. Pendlebury, Unlubricated reciprocating wear of stainless steel with an interfacial air flow, Wear 142 (1991) 351–372.
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[6] X.C. Lu, K. Shi, S.Z. Li, X.X. Jiang, Effects of surface deformation on corrosive wear of stainless steel in sulfuric acid solution, Wear 225–229 (1999) 537–543. [7] S.Y. Tarassov, A.V. Kolubaev, Effect of friction on subsurface layer microstructure in austenitic and martensitic steels, Wear 231 (1999) 228–234. [8] H. Mishina, Surface deformation and formation of original element of wear particles in sliding friction, Wear 215 (1998) 10–17. [9] A.A. Torrance, The influence of surface deformation on mechanical wear, Wear 200 (1996) 45–54. [10] D.A. Rigney, R. Divakar, S.M. Kuo, Deformation substructures associated with very large plastic strains, Scripta Metall. Mater. 27 (1992) 975–980. [11] S.L. Rice, Characteristics of metallic subsurface zones in sliding and impact wear, Wear 74 (1981–1982) 131–142. [12] M.A. Moore, R.M. Douthwaite, Plastic deformation below worn surfaces, Metall. Trans. A 7 (1976) 1833–1839. [13] J. Jiang, F.H. Stott, M.M. Stack, The role of triboparticulates in dry sliding wear, Tribol. Int. 31 (1998) 245–256. [14] E. Sauger, S. Fouvry, L. Ponsonnet, Ph. Kapsa, J.M. Martin, L. Vincent, Tribologically transformed structure in fretting, Wear 245 (2000) 39–52. [15] N.J. Fisher, A.B. Chow, M.K. Weckwerth, Experimental fretting-wear studies of steam generator materials, J. Pressure Vessel Technol. 117 (1995) 312–320. [16] F.M. Guerout, N.J. Fisher, D.A. Grandison, M.K. Weckwerth, Effect of temperature on steam generator fretting-wear, ASME PVP V.328 Flow-Induced Vibration (1996) 233–246. [17] Y.H. Lee, I.S. Kim, S.S. Kang, H.D. Chung, A study on wear coefficients and mechanisms of steam generator tube materials, Wear 250 (2001) 719–726. [18] K. Hiratsuka, Y. Ando, A. Sugahara, Effect of atmospheric oxygen on the enhancement of a growth process of transfer particles in adhesive wear, J. JSLE Int. Ed. 10 (1989) 33–38. [19] K. Hiratsuka, M. Goto, The role of changes in hardness of subsurfaces, transfer particles and wear particles in initial-steady wear transition, Wear 238 (2000) 70–77. [20] S. Fouvry, P. Kapsa, H. Zahouani, L. Vincent, Wear analysis in fretting of hard coatings through a dissipated energy concept, Wear 203–204 (1997) 393–403. [21] R.A. Page, A. Mcminn, Relative stress corrosion susceptibilities of alloy 690 and 600 in simulated boiling water reactor environments, Metall. Trans. A 17 (1986) 877–887. [22] K. Stiller, J.O. Nilsson, K. Norring, Structure, chemistry and stress corrosion cracking of grain boundaries in alloys 600 and 690, Metall. Mater. Trans. A 27 (1996) 327–341. [23] 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 on Inconel alloy 690, Metall. Trans. A 20 (1989) 2057–2067. [24] D.A. Rigney, W.A. Glaeser, The significance of near surface microstructure in the wear process, Wear 46 (1978) 241–250. [25] D.M. Symons, Hydrogen embrittlement of Ni–Cr–Fe alloys, Metall. Mater. Trans. A 28 (1997) 655–663. [26] A.A.W. Thompson, Yielding in nickel as a function of grain or cell size, Acta Metall. 23 (1975) 1337–1342.
The effect of subsurface deformation on the wear behavior of steam generator tube materials Young-Ho Lee, In-Sup Kim∗ Department of Nuclear Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea Received 29 October 2001; received in revised form 4 April 2002; accepted 27 May 2002
Abstract Fretting wear behavior of steam generator (SG) tube materials (Inconel 600MA and 690TT) against ferritic stainless steels was investigated in a room temperature water environment. The results indicated that the fretting wear rate and wear coefficient (K) in a work-rate model of Inconel 600MA were higher than those of Inconel 690TT with increasing normal loads and sliding amplitudes. From the results of scanning electron microscopy (SEM) observation, there was little evidence of particle agglomeration on the worn surfaces, while wear particles were released in the form of thin plates, which were generated from deformation substructures formed by severe plastic deformation during fretting wear. Therefore, the wear rates of SG tube materials in the room temperature water are closely related with plastic deformation behavior on contact surfaces. In subsurface layer, wear particle size seems to be determined by cell-structure thickness and closely related to the difference of stacking fault energy (SFE) of tube materials through chromium contents. Inconel 690TT showed lower wear rate because it should have relatively smaller cells due to lower SFE than Inconel 600MA and in turn may easily accommodate large strains. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Steam generator; Fretting wear; Work-rate model; Inconel alloys; Stacking fault energy
1. Introduction The formation of wear particle layers and the subsurface deformation are widely observed phenomena under sliding or fretting contact of metals. It was proposed that the wear rate is strongly influenced by those two phenomena in various test conditions [1–12]. If wear particles are easily oxidized and compacted on the rubbing surface, they act as a wear protective layer, which decreases the total wear rate. Otherwise, they may act as abrasives which accelerate wear damage [4]. Sometimes, the metal-to-metal contact is more dominant, especially in water or in air blowing condition [5]. Thus, it is very important that the wear rate at high flow rate condition such as the secondary side of nuclear power plant is controlled by the variation of mechanical properties in surface or in subsurface because wear particles are released after plastic deformation. The effect of wear particle layers on the wear rate of nickel-based alloy at temperatures of 20–250 ◦ C has been reported previously [13]. It was reported that wear transitions ∗ Corresponding author. Tel.: +82-42-869-3815; fax: +82-42-869-3810. E-mail addresses: [email protected] (Y.-H. Lee), [email protected] (I.-S. Kim).
from an initial high rate to a low rate were accompanied by the development of high-resistance load-bearing layers on the wear surface. With increasing temperature, wear particles were heavily oxidized, resulting in oxide-to-oxide contact between the rubbing surfaces. It was suggested that the mechanism of these transitions may have been due to the adhesion between the worn surface and wear particles, which prevents wear particles being released from the contact surfaces. On the other hand, in a water environment such as the secondary side of steam generator (SG) in a nuclear power plant, it is difficult to form the wear particle layer between tubes and their supports during fretting wear. This may be result of water lubrication or the easy removal of wear particles due to the high water flow rates. Therefore, it is expected that the wear rate of SG tube materials strongly depended on the deformation characteristics of the worn surface or in subsurface during fretting wear. Sauger et al. [14] proposed that a specific superficial layer is formed during the very first cycles of fretting loading, which is called the tribologically transformed structure (TTS). They emphasize that understanding the mechanisms of formation of the TTS is a key step in the modeling of wear induced by fretting. But this TTS will disappear after a large number of cycles.
0043-1648/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 2 ) 0 0 1 5 7 - 6
Y.-H. Lee, I.-S. Kim / Wear 253 (2002) 438–447
439
With increasing number of fretting cycles, it is expected that the wear-determining factors are plastic deformation and fracture on wearing surface in the water environment. Recently, the fretting-related degradations in SG tube materials are reported from many nuclear power plants. The main causes are both flow-induced vibration (FIV) between tube and support materials and the loose parts which randomly impact against the tube. Previous studies [15,16] were focused on the evaluation of the wear coefficient in a work-rate model to calculate the exact degradation rate for the estimation of residual lifetime. In spite of the widely recognized importance of wear coefficients in SG fretting wear, there has been relatively little effort made to understand wear mechanisms. The authors [17] have performed the wear test of SG tube materials in the room temperature air environment, in which the wear coefficients and the role of wear particle layers in the SG tube materials such as Inconel 600MA and 690TT were analyzed. In the present work, fretting wear experiment was performed with SG tube materials against ferritic stainless steels in room temperature water. The objective was to examine the possible wear mechanisms before high temperature and pressure experiments. The discussion was focused on the relationship between the subsurface deformation and wear rate or wear coefficient K in the work-rate model.
2. Experimental procedure The alloys used in this study were Inconel 600MA and 690TT (abbreviated to 600MA and 690TT); these commercial nickel-based alloys have been used as SG tube materials in nuclear power plants. Their microstructures are shown in Fig. 1. The counterpart materials were selected as 405 and 409 ferritic stainless steel (abbreviated to 405SS and 409SS) because those are used as tube support materials in the operating power plants. Chemical composition and bulk hardness of tested materials are shown in Table 1. A reciprocating wear apparatus with tube-on-plate configuration was utilized and details of this test system is described in the previous studies [17]. In this test, a water tank is equipped so that the fretting experiment can be conducted in 25 ◦ C distilled water. The dimensions of the tube specimen were 19.05 mm in diameter and 14 mm in length and
Fig. 1. Microstructure of tested tubes: (a) Inconel 600MA; (b) Inconel 690TT.
the counter-specimens were prepared from the flat strip. The friction load and fretting amplitude between sliding surfaces were continuously monitored during the wear test. The wear losses were determined from the weight measurements before and after the experiments using an analytical balance with an accuracy of the order of 0.1 mg. Prior to each test and weighing measurement, the specimens were acoustically cleaned in acetone for 5 min and dried in compressed air. The tube specimen oscillates with a peak-to-peak amplitude
Table 1 Chemical composition and bulk hardness of SG tubes and their support materials Specimen
Cr
Fe
C
Si
Mn
Ti
P
S
Co
Ni
Hardness (HV)
600MA 690TT 405SS 409SS
16.81 29.5 11.5–14.5 10.5–11.75
9.1 10.4 Balance Balance
0.026 0.02 0.08 0.08
0.32 0.33 1.00 1.00
0.81 0.26 1.00 1.00
0.35 0.32 0.1 (Al) 0.5
0.008 0.004 0.04 0.045
0.002 0.001 0.03 0.045
0.012 0.012 – –
Balance Balance – –
194 193 215 229
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of 50–400 m at a frequency of 30 Hz. The applied normal load was 10–40 N. In the present study, distilled water was used for the environment and the wear experiments were performed at 25 ◦ C. To evaluate the wear mechanism in the water environment, the worn surfaces and cross-sections below the contact surface were examined using a scanning electron microscopy (SEM) after tests. Also, nano-indentation tests were performed to assess the resistance to plastic deformation in the subsurface of the tube materials. Recently, a work-rate model has been used for the wear evaluation of SG tubes [15]. The work-rate is used to normalize the wear rate and is defined as the rate of energy being dissipated at the contact [16]. It
gives the following expression for the lifetime evaluation of SG tubes: ˙ W = F ds (1) V˙ = K W˙
(2)
where V˙ is wear rate, W˙ the work-rate, F the normal load, s the sliding distance and K is the wear coefficient. Thus, from the test results, wear coefficients (K, defined as the total volume loss per normal load and per total sliding distance) were calculated and compared between the test conditions.
Fig. 2. Effect of normal load on the specific wear rate (Vs , mg/m) of SG tube materials: (a) Inconel 600MA; (b) Inconel 690TT.
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3. Experimental results and discussion 3.1. Wear rate and wear coefficients of work-rate model Fig. 2 show the relationship between applied normal load and specific wear rate (weight loss per sliding distance, mg/m) of tube materials in water environment. The sliding amplitude and number of cycles have been changed at each test condition. The specific wear rate slowly increased for both tube materials and showed much scattering of values with increasing normal load. Fig. 3 shows the effect of sliding distance on the mass loss of the tube materials at the
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same load. With increasing sliding distance, the wear rate, which is the slope of the tangent to the curve, was slowly decreased. Hence, the fretting wear rate does not linearly increase with sliding distance. Fig. 4 shows the calculated wear coefficient, K, in the work-rate model of SG tube materials. 600MA has similar values with two support materials. Average wear coefficients, K, are 56.1 × 10−15 Pa−1 for 405SS and 58.1 × 10−15 Pa−1 for 409SS. But, with 690TT, the wear coefficient, K, shows a much low value (K = 18.5 × 10−15 Pa−1 ) when the support material is selected as 409SS. However, when we compare the average values of wear coefficient (K) in the work-rate model, 600MA has
Fig. 3. The variation of mass loss of SG tube materials with sliding distance at the same load condition: (a) Inconel 600MA against 405 stainless steel; (b) Inconel 690TT against 409 stainless steel. Wear rate slowly decreases at both materials with increasing sliding distance.
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Fig. 4. Wear coefficient K in the work-rate model from test results: (a) Inconel 600MA; (b) Inconel 690TT. The more resistant condition in room temperature water is Inconel 690TT against 409 stainless steel.
about twice the value as with 690TT in room temperature water environment. Generally, the wear transition phenomena are closely related with the formation of wear particle layers in an air environment. But, Hiratsuka et al. [18] pointed out that the initial steady wear transition was also observed during rubbing in a vacuum. He emphasized that the oxide film is not a necessary condition for establishing steady-state wear. Therefore, there seems to be two possibilities for these lower wear rates with increasing sliding distance. One is due to wear particles remaining in the contact surface. Even if it is expected that the generated wear particles are easily removed between contact surfaces in water environment during sliding, wear particles could be adhered to the worn surface due
to other factors such as magnetization on the worn surface. The other is the change in hardness of surface or subsurface. From the previous study [19], when the subsurfaces were hardened to the same extent as the transfer particles and wear particles, the wear mode changed to the steady-state wear. To confirm the magnetization effect between wear particles and worn surface, we measured the magnetic field at the worn surface. As a result, the intensity of magnetic field is almost the same before and after wear test and these effects could be ignored in those experiments. So, in water environment, wear rate of tube materials are determined by the variation of mechanical properties, such as work hardening, abilities of strain accommodation, etc., between contact surfaces.
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Fig. 5. Variation of friction coefficient during wear test. After 10 000 cycles, friction coefficient of Inconel 690TT slowly decreased.
3.2. Variation of friction coefficients Fig. 5 shows the variation of friction coefficient during fretting wear test. In the case of 690TT against 405SS, the friction coefficient decreased rapidly at the beginning of the tests and decreased slowly after around 10000 cycles. If the difference of initial friction coefficient originates from the difference of roughness of the tube surfaces, these will be eliminated at the beginning of test. Therefore, the friction coefficient of 690TT decreases at a slower rate with increas-
ing cycles. In the case of 600MA, friction coefficients fluctuated in the regions from 0.5 to 0.55. However, the general trend of these friction curves show that there is little change of friction coefficient in 600MA, but show some decrease in 690TT with increasing cycles. If the contact surface became harder due to severe plastic deformation, the contact area under the same load may decrease and consequently the friction coefficient will decrease. From the results, we deduced that 690TT experiences more work-hardening on its worn surface. Also, if wear
Fig. 6. The effect of shear load on the specific wear rate of SG tube materials. Note the reduction of scattering as compared to Fig. 2 when friction coefficient was correlated to wear rate.
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particles were released from the worn surface after severe plastic deformation and fracture, the shear load, which is generated by friction force on worn surface, is dominantly related to deformation and fracture. Therefore, to identify the relationship between specific wear rate and shear load, we expressed wear rate against shear load, which was correlated with friction coefficient in Fig. 6. In this figure, much of scattering as shown in Fig. 2 disappeared and specific wear rate is linearly increased with shear load. So, we deduced that wear rate of tubes in water is determined by the abilities of strain accommodation on contact surfaces because this is closely related with the generation rate of wear particles. Fouvry et al. [20] proposed the wear energy approach correlating friction coefficient during fretting wear and showed a linear relationship between dissipated energy and wear volume. Therefore, the wear coefficient (K) of the work-rate model for the lifetime evaluation of SG tubes should reflect wear mechanism in water condition and this could be archived by the correlation of wear coefficient with friction coefficient. 3.3. Worn surface and subsurface observation In the previous test results in air condition [17], wear rate and wear coefficients were dependent on the tribological properties of wear particle layers that were agglomerated on the worn surface of SG tubes. However, Fig. 7 shows that almost all wear particles were removed and their layer was not observable any more in water condition. The worn surface was fractured in thin plate form and wear particles seem to be released from the edge of these layers. Therefore, they do not stay between contact surfaces for a long time and metal-to-metal contact is dominant. In the present experiment, the roles of water lubricant are to restrain the formation of wear particle layers as well as to remove easily the wear particles when they reach a critical size. In Fig. 7, it seems apparent that the worn surfaces consisted of fractured thin plates, which were generated from the severe plastic deformation during wear. In 600MA, the thickness of those thin plates was about 2–3 m and cracks appeared under the worn surface in a perpendicular direction. But, in case of 690TT, the thickness of these plates and generated wear particles are relatively small and it is difficult to observe the crack in the direction of depth on the worn surface. Fig. 8 shows cross-sections of worn surfaces in two tube materials. The plastic deformation layer apparently appeared and they have specific thickness which is very small compared with their grain sizes as shown in Fig. 1. Besides, microcracks before wear particles propagate in deformed layer and not in grain boundaries. This means that if wear particles are generated by the fracture of these thin plates due to the hardness differences between upper and lower plates after severe plastic deformation, wear rate difference of two tube materials is, therefore, closely related with resistance to plastic deformation near worn surface during wear.
Fig. 7. SEM micrographs of the worn surfaces of SG tube materials after wear test: (a) Inconel 600MA; (b) Inconel 690TT.
The hardness distribution in subsurface regions was measured. Since severe deformation was observed in these regions, nano-indentation tests were performed to depth direction from worn surface. On each tube specimen, hardness tests were performed at three different locations and the average values are plotted in Fig. 9. The standard deviations of the mean values were about 20% of the mean. Before the wear test, two tube materials have similar hardness values. But the hardness of 690TT near the worn surface rapidly increased with respect to that of 600MA and this hardness difference gradually reduced with increasing distance from worn surface. 600MA and 690TT both have a single phase with stable face-centered cubic (fcc) structured up to temperatures of 1000 ◦ C and they have chromium carbides in the grain boundaries, such as dominant types of Cr7 C3 for 600MA and Cr23 C6 for 690TT [21–23]. Unlike austenite stainless steels that could be transformed to martensitic structures during sliding wear, these tube materials seem to maintain the initial phase. So, it is expected that the dislocation structure changes to accommodate the large strains generated during fretting wear. From the study of Rigney and Glaeser [24],
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Fig. 8. The subsurface observation using SEM: (a) Inconel 600MA; (b) Inconel 690TT. Each tube material has plastic deformation layers with specific thickness formed during wear.
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the repeated ploughing of asperity contacts over a mating surface can produce high dislocation densities and eventually change the microstructure to a cell-type structure found in heavily deformed metals; the cell size depending on material characteristics such as stacking fault energy (SFE), the applied stress and the temperature. The most significant difference in the chemical composition between 600MA and 690TT is chromium contents: 690TT has twice the chromium content (∼30 w/o) for the benefit of corrosion resistance. From the result of Symons’ study [25] about the effect of chromium content on the SFE of Ni–XCr–Fe alloy, he proposed that SFE rapidly decreased with increasing chromium contents in nickel-base alloys. If the material has high SFE, then dislocation climb and cross slip are easier and the tangled cell walls are well formed. The cell structure may be entirely absent in low SFE materials. However, at high strains cells could develop even for low SFE materials, but the cell sizes slowly decreased to a very thin layer near worn surface. Also, they proposed that a cell structure can present many suitable pathways for subsurface crack generation and the release of thin wear flakes [24]. So, 690TT has relatively low SFE due to high chromium content and is not easy to develop cell structures. But at high strain region such as worn surface, small size cell could develop near worn surface. Therefore, when wear particles are separated due to microcracks which are dominantly propagated to cell boundaries, wear particle size is larger in 690MA than in 690TT at worn surface as shown in Fig. 7. Thompson [26] proposed that yield strength is weakly dependent on grain size below 1 m and this behavior was compared to yielding in a cell-type structure. So, cell
Fig. 9. Microhardness variation beneath the worn surfaces of SG tube materials.
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structures were much stronger because the relatively short sources in cell walls caused higher yield strengths than the long sources in normal grain boundaries. This means that 690TT with relatively small cell size from Symons’ study shows high hardening behavior on its worn surface and low wear rate in room temperature water because cell structure could easily accommodate large strains. In addition to the measurements of hardness distributions below worn surface, plastic deformation energy was calculated to examine the relationship between hardened layer and wear properties. The plastic deformation energy is defined as the closed loop of load–displacement curve as shown in Fig. 10. The plastic deformation energies per unit volume
at the depth of 15 m from worn surface were calculated as 6.73 J mm−3 for 600MA and 7.58 J mm−3 for 690TT. The result indicates that about 15% more energy was needed to produce the same deformation volume in 690TT than in 600MA and could be connected to the low wear rate of materials. Also, the deformed layer should be propagated to the depth direction in order to maintain the specific thickness as shown in Fig. 8 as wear particles are continuously removed on the worn surface during wear. Therefore, the wear rate of SG tube materials in room temperature water is determined by the balance between the rate of wear particle removal on the worn surface and the formation rate of plastic deformation layers in the subsurface.
Fig. 10. Load–Indentation depth plots of SG tube materials in subsurface. The closed areas represent the plastic deformation energy during nano-indentation tests.
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4. Conclusions Fretting wear behaviors of SG tube materials against ferritic stainless steels were investigated in room temperature water. The following conclusions are drawn; 1. With increasing normal load and sliding distance, 690TT has low wear rate compared with 600MA in room temperature water. This result is mainly related to the changing mechanical properties of the tube materials during wear. 2. Wear particles are mostly removed from worn surface after severe plastic deformation and fracture. Thus, it is possible to relate the wear rate to shear load in room temperature water because stress component causing plastic deformation and fracture is mainly shear type. 3. The worn surface examination confirmed that wear particles are ejected in the form of thin plates on worn surface. Main cause is the difference of SFE with chromium contents between Inconel 600MA and 690TT and this is closely related to the formation of cell-type structure that could easily accommodate large strains. Also, plastic deformation layers appear in subsurface of both tube materials, which have specific thickness and are small compared with their grain sizes. 4. In room temperature water, the wear rate of SG tube materials is determined by the balance between the removal rates of wear particles from the worn surface and the formation rate of plastic deformation layers in the subsurface. Acknowledgements This research was partially supported by the Brain Korea 21 project and Korea Electric Power Research Institute of Korea Electric Power Corporation. References [1] F.H. Stott, The role of oxidation in the wear of alloys, Tribol. Int. 31 (1998) 61–71. [2] T.F.J. Quinn, Review of oxidational wear—Part 1: The origins of oxidational wear, Tribol. Int. 16 (1983) 257–271. [3] J. Jiang, F.H. Stott, M.M. Stack, The effect of partial pressure of oxygen on the tribological behaviour of a nickel-based alloy, N80A, at elevated temperatures, Wear 203–204 (1997) 615–625. [4] A. Iwabuchi, The role of oxide particles in the fretting wear of mild steel, Wear 151 (1991) 301–311. [5] E.R. Leheup, R.E. Pendlebury, Unlubricated reciprocating wear of stainless steel with an interfacial air flow, Wear 142 (1991) 351–372.
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