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Residual lifetime assessment of cold-reheater pipe in coal-fired power plant through accelerated degradation test
Reliability Engineering and System Safety 188 (2019) 330–335
Contents lists available at ScienceDirect
Reliability Engineering and System Safety journal homepage: www.elsevier.com/locate/ress
Residual lifetime assessment of cold-reheater pipe in coal-fired power plant through accelerated degradation test Myung-Yeon Kima,b, Dong-Ju Chua,c, Young-Kook Leeb, Jae-Hyeok Shima, Woo-Sang Junga,
T
⁎
a
Center for Energy Materials Research, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic of Korea c Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: 1.25Cr-0.5Mo steel Cold-reheater pipe Creep LMP Residual lifetime
Non-destructive testing (NDT) is the process of evaluating and inspecting materials without damage. The purpose of present study is to evaluate the residual lifetime of materials by proposing a new concept of nondestructive testing. The residual lifetime of service-exposed 1.25Cr-0.5Mo (P11) steel cold-reheater pipes in a coal-fired power plant, which were used for approximately 23 years at 310 °C, was investigated with accelerated degradation tests of a fresh P11 steel. The hardness and the microstructure of the P11 steel aged at 550 °C for 1000 h were similar to those of the service-exposed cold-reheater pipe. The creep-rupture tests of the steel aged at 550 °C for 1000 h were performed under various temperature and stress conditions together with the fresh steel. The creep-rupture properties analyzed using a Larson–Miller parameter (LMP) approach suggested that the residual lifetime of the service-exposed pipes is approximately 53% at the current conditions. The proposed methodology is expected to serve as a useful tool to check the timing of materials replacement and to maintain safety in a coal-fired power plant.
1. Introduction Various Cr-Mo ferritic/martensitic steels have been widely used in coal-fired power plants as pressure vessels, super-heater boiler tubes and steam pipes due to their superior mechanical properties and lower thermal expansion coefficients at high temperature as well as relatively low cost [1–5]. However, these types of steel are exposed to high temperatures for a long time, making it difficult to maintain their original properties. In coal-fired power plants that have operated for more than 20 years, the lifetime of main steam lines such as boiler pipes decreases significantly. Premature failure of these main steam lines can cause serious problems. For this reason, much research has been done on microstructural changes and creep properties of the steels at high temperature [5–11]. Instead of obsolete coal-fired power plants that have operated for more than 20 years, the construction of new power plants is being discussed for safety and air pollution reasons. The construction of a new plant requires much cost and time. On the other hand, the plant performance can be improved only by changing core plant components, maintaining existing frames, which eventually contributes to improving energy efficiency and reducing CO2 emissions by increasing operating temperature and steam pressure [2,12]. Therefore, the main steam lines
⁎
need to be replaced by materials that are suitable for increased temperature and pressure. Before the renewal of the components in a power plant, it is important to assess the residual lifetime of the components in order to judge if the renewal is necessary. Non-destructive tests (NDT) can be used to characterize materials in use. Magnetic technique including the use of parameters related to the magnetic hysteresis loop (such as coercivity, retentivity, permeability) is most commonly used as tools for cracking and defect detection [13]. Magnetic Barkhausen Noise (MBN) technique is applicable to ferromagnetic materials, which are composed of small order magnetic regions called magnetic domains. Yelbay et al. measured residual stresses in the welded steel plates by MBN technique [14]. Ultrasonic technique (UT) uses high frequency sound waves to conduct examinations. For crack detection, UTs such as defect echo height method, decibel drop method and edge echo method are well known technique [13]. Advantages of UT are high scan speed, flaw detecting capabilities and convenience of field use [15]. Chatillon proposed a new concept of ultrasonic contact phased array transducer having flexible radiating surface able to fit the actual surface of the piece [16]. Furthermore, Radiographic testing (RT) for pipeline inspection is outstanding due to their efficiency and ease of operation [17].
Corresponding author. E-mail address: [email protected] (W.-S. Jung).
https://doi.org/10.1016/j.ress.2019.03.043 Received 16 July 2018; Received in revised form 21 March 2019; Accepted 23 March 2019 Available online 24 March 2019 0951-8320/ © 2019 Elsevier Ltd. All rights reserved.
Reliability Engineering and System Safety 188 (2019) 330–335
M.-Y. Kim, et al.
Non-destructive inspection methods such as magnetic test, MBN, UT and RT are generally used to evaluate the soundness of welds and cast parts by detecting flaw such as voids, inclusions, macro and micro cracks inside the materials. However, only structural changes such as carbide growth, recovery of dislocations and microstructures of matrix take place during the early stage of service period. These changes do not significantly affect the measured parameters obtained from NDT. So, it is difficult to determine the quantitative lifetime consumption of materials by non-destructive testing method although structural changes reduce the lifetime of materials. An effective way to quantitatively determine the residual lifetime of materials is to check the creep and stress-rupture properties [18–20]. KOrea MIdland POwer Co., LTD (KOMIPO) in South Korea is planning to retrofit the Unit 3 of the Boryeong 500 MW coal-fired power plant complex by replacing the main components of turbines and boilers with new components of advanced heat-resistant materials. The Unit 3 of the power plant complex had operated for approximately 23 years from the operation start in March 1993 to May 2016 at a main steam temperature of 538 °C. Among the various components, cold-reheater pipes made from Gr.11 steel is not being considered for the replacement because of their enough margin of initial design. The coldreheater had been exposed to 310 °C under an operating pressure of 5.3 MPa. The dimension and the operating conditions of the cold-reheater are summarized in Table 1. The purpose of present study is to estimate the residual life of the cold-reheater pipes by investigating their structural degradation using non-destructive material testing. Destructive sampling of the cold-reheater pipes from power plant was not possible, because the power plant was operating. The only data obtained from the cold-reheater pipes were microstructural replica images and hardness values during a maintenance period. Also, we tried to quantitatively determine the lifetime consumption rate of the cold-reheater from a creep point of view.
Table 2 Chemical composition of the P11 steel (wt.%). Steel
Fe
C
Si
Mn
Cr
Mo
P
S
1.25Cr-0.5Mo
Bal.
0.11
0.56
0.45
1.13
0.46
0.008
0.004
15,000 h. The specimens for creep tests with a diameter of 6.35 mm and a gage length of 32 mm were machined from the aged samples according to ASTM E139. The creep-rupture tests were conducted in air at 450, 500 and 550 °C under various stress conditions using lever-arm creep testers. The furnaces were equipped with 3-zone temperature control units and temperature was carefully controlled in a range of less than ± 2 °C during the creep test. The creep strain was measured by high temperature extensometers. The Vickers hardness was measured with a test load of 500 g using a Mitutoyo HM-122 hardness tester. The microstructural observation was carried out using an FEI Inspect F50 scanning electron microscope (SEM). Precipitates were extracted from the as-tempered and aged samples by anode decomposition method using electrolyte solution (95% methanol + 5% HCl) for X-ray diffraction (XRD) measurement. The precipitates were identified using a Bruker D8 Advanced X-ray diffractometer with Cu Kα radiation. The XRD scan was performed under a step size of 0.02 o in 2-theta with a counting time of 10 s per step.
3. Results and discussion 3.1. Long-term service-exposed cold-reheater pipes The matrix and precipitate microstructures of materials change with service-exposure time at high temperatures. As a result, the creep rupture strength of materials tends to decrease with time. Studies on the creep damage by the microstructural changes of heat-resistant steels have been carried out by many researchers [21–24]. Neubauer et al. [21] classified the cavity evolution in heat-resistant steels at four stages such as isolated cavities, oriented cavities, linked cavities and macrocracks. They suggested the A parameter defined as the number fraction of cavitated grain boundaries, which is proportional to the creep damage. Kuroishi et al. [22] considered the microstructural changes on the creep damage as well as the morphology of cavities. They classified the microstructural degradation of heat-resistant steels into three stages. It is defined as the I-M stage (life consumption ratio = 0–20%) in case that the microstructures are the nearly same as those of fresh materials. In the II-M stage (life consumption ratio = 20–40%), precipitates form in grain boundaries. Also, it is classified as the III-M stage (life consumption ratio = 40–100%) in case that the spheroidization of precipitates takes place. They also categorized the degradation into two stages (I-D and II-D) depending on whether creep cavities form in heat-resistant steels. It is defined as the ID stage (life consumption ratio = 0–50%) in case that creep cavities do not form. The damage extent of the sevice-exposed cold reheater pipes was estimated by comparing with the damage accumulation stages proposed by Neubauer et al. [21] and Kuroishi et al. [22]. Fig. 1a and b show the locations for the hardness measurement and the extracted replica in the service-exposed cold-reheater pipes used for 23 years, which are located on the 8th and 14th floors, respectively, in the power plant. As shown in Table 3, hardness was measured at four points every 90 ° in the clockwise direction with respect to the upper
2. Experimental procedure 2.1. Long-term service-exposed cold-reheater pipes The hardness of the cold-reheater after the long-term service exposure for 23 years was on-site measured using an ultrasonic hardness tester (GE inspection MIC 10) in the power plant. To observe the microstructure of the cold-reheater, the surface of a cold reheater pipe, which was made of a 1.25Cr-0.5Mo (P11) steel, was polished and etched using 5% acidic Nital solution (mixture of 5% nitric acid and methanol). Replicas for the microstructural observation were prepared from the etched samples by placing an adhesive diacetate replicating tape (LADD No.12010) on their surface and detaching the tape from the samples. Extracted replicas were stored in glass slides. 2.2. Aged materials for degradation acceleration Since it was not possible to prepare specimens for creep-rupture tests with the service-exposed cold-reheater, fresh P11 steel samples provided by Doosan Heavy Industries & Construction were used. Table 2 shows the chemical composition of the P11 steel used in this study. The samples were normalized at 910 °C for 10 min and subsequently tempered at 740 °C for 45 min. For accelerated degradation, the samples were aged in air atmosphere at 450, 500 and 550 °C for up to Table 1 Dimension and operation conditions of the service-exposed cold-reheater pipe. Dimension Cold-reheater
Thickness (mm) 17.5
Operating conditions Pressure (kgf/cm2) 54
Outer diameter (mm) 711.2
331
Temperature (°C) 310
Hoop Stress (MPa) 105
Reliability Engineering and System Safety 188 (2019) 330–335
M.-Y. Kim, et al.
Fig. 1. Images of the long-term service-exposed P11 steel cold-reheater pipe in the power plant; (a), (b) locations of hardness measurement and replica extraction in the power plant; (c) SEM micrographs of the replica in (b).
estimated to range from 20 to 50%. As mentioned before, the only data obtained from the cold-reheater pipes were microstructural replica images and hardness values during a maintenance period. Generally, the hardness of Gr.11 steel decreased with increasing exposure time to high temperature due to the growth of carbides. The replica images of the cold-reheater pipes exhibit mixed microstructure of ferrite and pearlite. Also the measured hardness of the cold-reheater pipes ranges from 157 to 180 Hv values. The mechanical properties of the materials are determined by the matrix and precipitate microstructure. Accelerated degradation tests were carried out to verify the validity of the creep damage ranging from 20 to 50% estimated from the replica images. The accelerated degradation condition was selected according to the equivalent conditions representing the minimum hardness without any structural changes of ferrite + pearlite. It was considered that the concept would allow us to evaluate the minimum residual lifetime of the service-exposed cold-reheater pipes.
Table 3 Vickers hardness values of the service-exposed cold-reheater pipe. 0 Bending (8th floor) Near boiler (14th floor)
o
179.8 ± 6.9 –
90
o
168.3 ± 9.4 156.9 ± 5.5
180
o
175.5 ± 8.0 –
270
o
171.2 ± 9.4 –
side in the bending portion on the 8th floor and at one point in the vicinity of the boiler on the 14th floor. In the case of the boiler periphery on the 14th floor, hardness could be measured at only one point due to a safety problem. The hardness value was averaged over 10 measurements at every location. The lowest hardness value, which is 157 ± 5.5 Hv, is obtained at this point. SEM micrographs show that the microstructure of the replicas from the service-exposed cold-reheater pipes consists mainly of ferrite matrix containing spheroidized carbide particles (Fig. 1c). However, we failed to directly measure the volume fraction of carbide particles and the particle size distribution. Fig. 1c shows the microstructure of carbide spheroidization without creep cavities. By careful comparison with the degradation stages proposed by Kuroishi et al. [22], it corresponds to the II-M and I-D stages from the viewpoint of microstructure and creep cavity, respectively. Therefore, the overall creep damage ratio is
3.2. Aged materials for degradation acceleration The as-tempered P11 steel was aged at high temperatures to make the microstructure and the hardness value similar to those of the longterm service-exposed cold-reheater pipes shown in Fig. 1c and Table 3. The aging for degradation acceleration was performed for up to 15,000 h at 450, 500 and 550 °C, where the mixed microstructure of
Fig. 2. (a) Hardness values of the sample aged for up to 15,000 h at 450, 500 and 550 °C, and (b) the hardness values as a function of the LMP. 332
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ferrite and pearlite was maintained. Vickers hardness value variation of the sample aged for up to 15,000 h at 450, 500 and 550 °C is shown in Fig. 2a. The initial hardness value of the fresh (as-tempered) steel is 181 Hv. The hardness value tends to decrease with aging time. In addition, it is confirmed that the degradation tendency of the sample aged at 550 °C is higher than those aged at 450 and 500 °C. In Fig. 2b, the measured hardness values are plotted as a function of a Larson–Miller parameter (LMP), which is given by:
LMP = T (20 + logt )
(1)
where T and t are the aging temperature in kelvins and the aging time in hours, respectively. In this plot, the hardness values generally exhibit similar behavior with small scattering irrespective of the aging temperature and time. The results indicate that it takes 5861,400 h at 450 °C, 72,400 h at 500 °C and 1000 h at 550 °C to make the optimum condition for reaching 157 Hv, which is the lowest hardness directly measured at the service-exposed cold-reheater pipe. We selected the condition of 1000 h at 550 °C for accelerated degradation, which produces the hardness value similar to that of the service-exposed coldreheater pipe. Fig. 3a presents an SEM image of the microstructure of the fresh steel, showing polygonal ferrite grains together with pearlite structures with segmented carbide. Although the microstructure does not change significantly after the aging at 550 °C for 1000 h, more spheroidized carbide particles are observed (Fig. 3(b)). The overall microstructure of the aged sample does not seem to be significantly different from that of the service-exposed cold-heater pipe (Fig. 1c). Fig. 4 shows XRD patterns of extracted residues from the fresh sample and the sample aged at 550 °C for 1000 h. M3C, which is mainly Fe3C, is identified as a major precipitate in both samples. It is noted that other precipitates do not form during the aging, whereas the microstructure gradually changes. The results of the creep-rupture tests of the fresh and aged samples are shown in Fig. 5. A general trend that the creep-rupture time increases with decreasing creep-rupture stress and decreasing temperature is observed, although many tests at other temperature-stress conditions are still on-going. It should be noted that the creep-rupture time of the aged sample is shorter than that of the fresh sample irrespective of creep-rupture stress and temperature. Fig. 6 plots the creep-rupture stress of the fresh and aged samples as a function of a LMP, Eq. (1), in which t is the creep-rupture time. The data of both samples can be categorized into two different curves. It is obviously confirmed in the plots that the creep-rupture time of the aged samples becomes shorter compared with that of the as-tempered samples. This implies that the progressive microstructural change due to long-term exposure to high temperature leads to the degradation in
Fig. 4. XRD patterns of precipitates extracted from (a) the fresh P11 steel and (b) the sample aged for 1000 h at 550 °C.
Fig. 5. Creep-rupture time of the fresh P11 steel and sample aged for 1000 h at 550 °C under various stress and temperature conditions. The open and closed symbols represent the fresh P11 steel (as-tempered) and sample aged for 1000 h at 550 °C, respectively.
Fig. 3. SEM micrographs of (a) the fresh P11 steel and (b) the sample aged for 1000 h at 550 °C. 333
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microstructure. We decide that it will not be a problem to continue to use the service-exposed cold-reheater pipes after the power plant renewal, because the residual lifetime ratio is 0.53 at 310 °C under the current service conditions of use. However, it should be noted that the P11 steel used in this study experienced the accelerated degradation at 550 °C for up to 1000 h, not actual operation in the power plant. When the power plant was shut down, the degradation of the P11 steel pipe could be evaluated only by measuring the hardness, which is the non-destructive test, on the surface of the service-exposed cold-reheater pipes. We assessed the residual lifetime from a point of view of only creep properties by conducting a creep test of accelerated degraded P11 steel. The accelerated degraded P11 steel in this study was not exactly same microstructures and mechanical properties as the service-exposed coldreheater pipes. In order to accurately evaluate the residual lifetime of the serviceexposed cold-reheater pipes, it is necessary to analyze the creep properties obtained from destructive creep testing. Zieliński et al. [19] and M. Dziuba-Kaluza et al. [25] reported the residual lifetime of low-alloy Cr-Mo-V cast steel and 13CrMo4-5 steel by destructive creep testing of materials used for approximately 100,000 h and 200,000 h at high temperatures respectively. It is desirable to predict the more accurate residual lifetime of the cold-reheater pipe through the careful analysis of destructive tests such as tensile tests at room and high temperatures, impact tests and accelerated creep tests. In the future, destructive testing of the service-exposed cold-reheater pipe will be carried out, as the retrofit of the power plant progresses. Despite uncertainty due to the factors that were not considered in this study, the result of the residual lifetime assessment might be a great help in judging if the service-exposed cold-reheater pipes should be replaced or keep being used.
Fig. 6. Creep-rupture stress of fresh P11 steel and sample aged for 1000 h at 550 °C as a function of the LMP.
creep-rupture life of the steel. Assuming that the aged sample can represent the service-exposed cold-reheater pipe, it might be possible to estimate the residual lifetime of the service-exposed pipe using these plots. The blue star symbol corresponding to the hoop stress of the service-exposed pipe indicates the residual lifetime of the aged sample. Therefore, we estimate the ratio of the residual lifetime to the total lifetime using the two LMP plots as follows:
t − tR ⎞ Residual lifetime ratio = ⎛1 − T tT ⎠ ⎝ ⎜
⎟
(2)
4. Conclusions
where tT and tR are the rupture time of the fresh steel (total lifetime) and the aged steel (residual lifetime) representing the service-exposed pipe, respectively. Fig. 7 presents an enlarged view of Fig. 6 at the current service condition of the cold-reheater pipe. The residual lifetime ratio at 310 °C under a stress of 105 MPa derived from LMP values is also indicated on the top x-axis. At the current service condition, the LMP values of the fresh and aged samples are 19.68 and 19.52, respectively. Eventually, the residual lifetime ratio is estimated to be 0.53 at 310 °C by calculating tT and tR using Eq. (2). The result well agrees with the creep damage ranging from 20 to 50% estimated from the replicated
The assessment of the residual lifetime of the service-exposed P11 steel cold-reheater pipe used for approximately 23 years at 310 °C in the power plant was conducted by analyzing the creep properties of accelerated degraded P11 steel. Major conclusions based on the results and their analysis are as follows: (1) The microstructure of the cold-reheater pipe exposed for 23 years at 310 °C consist mainly of ferrite and pearlite area. The carbides in pearlite area of service-exposed pipe were spheroidized and coarsened as compared to carbides of fresh P11 steel. (2) The lowest hardness of the service-exposed pipe was 157 Hv and the aged condition for accelerated degradation in order to have similar hardness values with the service-exposed cold-reheater pipe was 1000 h at 550 °C (3) The creep-rupture strength of aged steel for accelerated degradation was decreased compared with that of fresh steel. Based on the LMP plots, the residual lifetime of the service-exposed pipe was estimated to be 53% at 310 °C with respect to the total life time under the current service conditions of use. (4) Only non-destructive properties such as hardness values and replica images of the cold-reheater pipes from power plant was possible in this research because the power plant was operating. (5) In the future, destructive creep testing to predict the more accurate residual lifetime of the service-exposed cold-reheater pipe will be carried out, as the retrofit of the power plant progresses. Acknowledgments The authors would like to acknowledge the financial support from the R&D Convergence Program of National Research Councilof Science & Technology (grant no. CAP 16-08-KITECH) and Energy Technology Evaluation and Planning (grant no. 20151110100040) of Ministry of Trade, Industry and Energy of Republic of Korea. The authors also
Fig. 7. Creep-rupture stress of the fresh and degraded samples as a function of the LMP and the residual lifetime ratio (enlarged view of Fig. 6). 334
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thank Doosan Heavy Industries & Construction Co., Ltd. for providing the materials.
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Contents lists available at ScienceDirect
Reliability Engineering and System Safety journal homepage: www.elsevier.com/locate/ress
Residual lifetime assessment of cold-reheater pipe in coal-fired power plant through accelerated degradation test Myung-Yeon Kima,b, Dong-Ju Chua,c, Young-Kook Leeb, Jae-Hyeok Shima, Woo-Sang Junga,
T
⁎
a
Center for Energy Materials Research, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic of Korea c Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: 1.25Cr-0.5Mo steel Cold-reheater pipe Creep LMP Residual lifetime
Non-destructive testing (NDT) is the process of evaluating and inspecting materials without damage. The purpose of present study is to evaluate the residual lifetime of materials by proposing a new concept of nondestructive testing. The residual lifetime of service-exposed 1.25Cr-0.5Mo (P11) steel cold-reheater pipes in a coal-fired power plant, which were used for approximately 23 years at 310 °C, was investigated with accelerated degradation tests of a fresh P11 steel. The hardness and the microstructure of the P11 steel aged at 550 °C for 1000 h were similar to those of the service-exposed cold-reheater pipe. The creep-rupture tests of the steel aged at 550 °C for 1000 h were performed under various temperature and stress conditions together with the fresh steel. The creep-rupture properties analyzed using a Larson–Miller parameter (LMP) approach suggested that the residual lifetime of the service-exposed pipes is approximately 53% at the current conditions. The proposed methodology is expected to serve as a useful tool to check the timing of materials replacement and to maintain safety in a coal-fired power plant.
1. Introduction Various Cr-Mo ferritic/martensitic steels have been widely used in coal-fired power plants as pressure vessels, super-heater boiler tubes and steam pipes due to their superior mechanical properties and lower thermal expansion coefficients at high temperature as well as relatively low cost [1–5]. However, these types of steel are exposed to high temperatures for a long time, making it difficult to maintain their original properties. In coal-fired power plants that have operated for more than 20 years, the lifetime of main steam lines such as boiler pipes decreases significantly. Premature failure of these main steam lines can cause serious problems. For this reason, much research has been done on microstructural changes and creep properties of the steels at high temperature [5–11]. Instead of obsolete coal-fired power plants that have operated for more than 20 years, the construction of new power plants is being discussed for safety and air pollution reasons. The construction of a new plant requires much cost and time. On the other hand, the plant performance can be improved only by changing core plant components, maintaining existing frames, which eventually contributes to improving energy efficiency and reducing CO2 emissions by increasing operating temperature and steam pressure [2,12]. Therefore, the main steam lines
⁎
need to be replaced by materials that are suitable for increased temperature and pressure. Before the renewal of the components in a power plant, it is important to assess the residual lifetime of the components in order to judge if the renewal is necessary. Non-destructive tests (NDT) can be used to characterize materials in use. Magnetic technique including the use of parameters related to the magnetic hysteresis loop (such as coercivity, retentivity, permeability) is most commonly used as tools for cracking and defect detection [13]. Magnetic Barkhausen Noise (MBN) technique is applicable to ferromagnetic materials, which are composed of small order magnetic regions called magnetic domains. Yelbay et al. measured residual stresses in the welded steel plates by MBN technique [14]. Ultrasonic technique (UT) uses high frequency sound waves to conduct examinations. For crack detection, UTs such as defect echo height method, decibel drop method and edge echo method are well known technique [13]. Advantages of UT are high scan speed, flaw detecting capabilities and convenience of field use [15]. Chatillon proposed a new concept of ultrasonic contact phased array transducer having flexible radiating surface able to fit the actual surface of the piece [16]. Furthermore, Radiographic testing (RT) for pipeline inspection is outstanding due to their efficiency and ease of operation [17].
Corresponding author. E-mail address: [email protected] (W.-S. Jung).
https://doi.org/10.1016/j.ress.2019.03.043 Received 16 July 2018; Received in revised form 21 March 2019; Accepted 23 March 2019 Available online 24 March 2019 0951-8320/ © 2019 Elsevier Ltd. All rights reserved.
Reliability Engineering and System Safety 188 (2019) 330–335
M.-Y. Kim, et al.
Non-destructive inspection methods such as magnetic test, MBN, UT and RT are generally used to evaluate the soundness of welds and cast parts by detecting flaw such as voids, inclusions, macro and micro cracks inside the materials. However, only structural changes such as carbide growth, recovery of dislocations and microstructures of matrix take place during the early stage of service period. These changes do not significantly affect the measured parameters obtained from NDT. So, it is difficult to determine the quantitative lifetime consumption of materials by non-destructive testing method although structural changes reduce the lifetime of materials. An effective way to quantitatively determine the residual lifetime of materials is to check the creep and stress-rupture properties [18–20]. KOrea MIdland POwer Co., LTD (KOMIPO) in South Korea is planning to retrofit the Unit 3 of the Boryeong 500 MW coal-fired power plant complex by replacing the main components of turbines and boilers with new components of advanced heat-resistant materials. The Unit 3 of the power plant complex had operated for approximately 23 years from the operation start in March 1993 to May 2016 at a main steam temperature of 538 °C. Among the various components, cold-reheater pipes made from Gr.11 steel is not being considered for the replacement because of their enough margin of initial design. The coldreheater had been exposed to 310 °C under an operating pressure of 5.3 MPa. The dimension and the operating conditions of the cold-reheater are summarized in Table 1. The purpose of present study is to estimate the residual life of the cold-reheater pipes by investigating their structural degradation using non-destructive material testing. Destructive sampling of the cold-reheater pipes from power plant was not possible, because the power plant was operating. The only data obtained from the cold-reheater pipes were microstructural replica images and hardness values during a maintenance period. Also, we tried to quantitatively determine the lifetime consumption rate of the cold-reheater from a creep point of view.
Table 2 Chemical composition of the P11 steel (wt.%). Steel
Fe
C
Si
Mn
Cr
Mo
P
S
1.25Cr-0.5Mo
Bal.
0.11
0.56
0.45
1.13
0.46
0.008
0.004
15,000 h. The specimens for creep tests with a diameter of 6.35 mm and a gage length of 32 mm were machined from the aged samples according to ASTM E139. The creep-rupture tests were conducted in air at 450, 500 and 550 °C under various stress conditions using lever-arm creep testers. The furnaces were equipped with 3-zone temperature control units and temperature was carefully controlled in a range of less than ± 2 °C during the creep test. The creep strain was measured by high temperature extensometers. The Vickers hardness was measured with a test load of 500 g using a Mitutoyo HM-122 hardness tester. The microstructural observation was carried out using an FEI Inspect F50 scanning electron microscope (SEM). Precipitates were extracted from the as-tempered and aged samples by anode decomposition method using electrolyte solution (95% methanol + 5% HCl) for X-ray diffraction (XRD) measurement. The precipitates were identified using a Bruker D8 Advanced X-ray diffractometer with Cu Kα radiation. The XRD scan was performed under a step size of 0.02 o in 2-theta with a counting time of 10 s per step.
3. Results and discussion 3.1. Long-term service-exposed cold-reheater pipes The matrix and precipitate microstructures of materials change with service-exposure time at high temperatures. As a result, the creep rupture strength of materials tends to decrease with time. Studies on the creep damage by the microstructural changes of heat-resistant steels have been carried out by many researchers [21–24]. Neubauer et al. [21] classified the cavity evolution in heat-resistant steels at four stages such as isolated cavities, oriented cavities, linked cavities and macrocracks. They suggested the A parameter defined as the number fraction of cavitated grain boundaries, which is proportional to the creep damage. Kuroishi et al. [22] considered the microstructural changes on the creep damage as well as the morphology of cavities. They classified the microstructural degradation of heat-resistant steels into three stages. It is defined as the I-M stage (life consumption ratio = 0–20%) in case that the microstructures are the nearly same as those of fresh materials. In the II-M stage (life consumption ratio = 20–40%), precipitates form in grain boundaries. Also, it is classified as the III-M stage (life consumption ratio = 40–100%) in case that the spheroidization of precipitates takes place. They also categorized the degradation into two stages (I-D and II-D) depending on whether creep cavities form in heat-resistant steels. It is defined as the ID stage (life consumption ratio = 0–50%) in case that creep cavities do not form. The damage extent of the sevice-exposed cold reheater pipes was estimated by comparing with the damage accumulation stages proposed by Neubauer et al. [21] and Kuroishi et al. [22]. Fig. 1a and b show the locations for the hardness measurement and the extracted replica in the service-exposed cold-reheater pipes used for 23 years, which are located on the 8th and 14th floors, respectively, in the power plant. As shown in Table 3, hardness was measured at four points every 90 ° in the clockwise direction with respect to the upper
2. Experimental procedure 2.1. Long-term service-exposed cold-reheater pipes The hardness of the cold-reheater after the long-term service exposure for 23 years was on-site measured using an ultrasonic hardness tester (GE inspection MIC 10) in the power plant. To observe the microstructure of the cold-reheater, the surface of a cold reheater pipe, which was made of a 1.25Cr-0.5Mo (P11) steel, was polished and etched using 5% acidic Nital solution (mixture of 5% nitric acid and methanol). Replicas for the microstructural observation were prepared from the etched samples by placing an adhesive diacetate replicating tape (LADD No.12010) on their surface and detaching the tape from the samples. Extracted replicas were stored in glass slides. 2.2. Aged materials for degradation acceleration Since it was not possible to prepare specimens for creep-rupture tests with the service-exposed cold-reheater, fresh P11 steel samples provided by Doosan Heavy Industries & Construction were used. Table 2 shows the chemical composition of the P11 steel used in this study. The samples were normalized at 910 °C for 10 min and subsequently tempered at 740 °C for 45 min. For accelerated degradation, the samples were aged in air atmosphere at 450, 500 and 550 °C for up to Table 1 Dimension and operation conditions of the service-exposed cold-reheater pipe. Dimension Cold-reheater
Thickness (mm) 17.5
Operating conditions Pressure (kgf/cm2) 54
Outer diameter (mm) 711.2
331
Temperature (°C) 310
Hoop Stress (MPa) 105
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Fig. 1. Images of the long-term service-exposed P11 steel cold-reheater pipe in the power plant; (a), (b) locations of hardness measurement and replica extraction in the power plant; (c) SEM micrographs of the replica in (b).
estimated to range from 20 to 50%. As mentioned before, the only data obtained from the cold-reheater pipes were microstructural replica images and hardness values during a maintenance period. Generally, the hardness of Gr.11 steel decreased with increasing exposure time to high temperature due to the growth of carbides. The replica images of the cold-reheater pipes exhibit mixed microstructure of ferrite and pearlite. Also the measured hardness of the cold-reheater pipes ranges from 157 to 180 Hv values. The mechanical properties of the materials are determined by the matrix and precipitate microstructure. Accelerated degradation tests were carried out to verify the validity of the creep damage ranging from 20 to 50% estimated from the replica images. The accelerated degradation condition was selected according to the equivalent conditions representing the minimum hardness without any structural changes of ferrite + pearlite. It was considered that the concept would allow us to evaluate the minimum residual lifetime of the service-exposed cold-reheater pipes.
Table 3 Vickers hardness values of the service-exposed cold-reheater pipe. 0 Bending (8th floor) Near boiler (14th floor)
o
179.8 ± 6.9 –
90
o
168.3 ± 9.4 156.9 ± 5.5
180
o
175.5 ± 8.0 –
270
o
171.2 ± 9.4 –
side in the bending portion on the 8th floor and at one point in the vicinity of the boiler on the 14th floor. In the case of the boiler periphery on the 14th floor, hardness could be measured at only one point due to a safety problem. The hardness value was averaged over 10 measurements at every location. The lowest hardness value, which is 157 ± 5.5 Hv, is obtained at this point. SEM micrographs show that the microstructure of the replicas from the service-exposed cold-reheater pipes consists mainly of ferrite matrix containing spheroidized carbide particles (Fig. 1c). However, we failed to directly measure the volume fraction of carbide particles and the particle size distribution. Fig. 1c shows the microstructure of carbide spheroidization without creep cavities. By careful comparison with the degradation stages proposed by Kuroishi et al. [22], it corresponds to the II-M and I-D stages from the viewpoint of microstructure and creep cavity, respectively. Therefore, the overall creep damage ratio is
3.2. Aged materials for degradation acceleration The as-tempered P11 steel was aged at high temperatures to make the microstructure and the hardness value similar to those of the longterm service-exposed cold-reheater pipes shown in Fig. 1c and Table 3. The aging for degradation acceleration was performed for up to 15,000 h at 450, 500 and 550 °C, where the mixed microstructure of
Fig. 2. (a) Hardness values of the sample aged for up to 15,000 h at 450, 500 and 550 °C, and (b) the hardness values as a function of the LMP. 332
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ferrite and pearlite was maintained. Vickers hardness value variation of the sample aged for up to 15,000 h at 450, 500 and 550 °C is shown in Fig. 2a. The initial hardness value of the fresh (as-tempered) steel is 181 Hv. The hardness value tends to decrease with aging time. In addition, it is confirmed that the degradation tendency of the sample aged at 550 °C is higher than those aged at 450 and 500 °C. In Fig. 2b, the measured hardness values are plotted as a function of a Larson–Miller parameter (LMP), which is given by:
LMP = T (20 + logt )
(1)
where T and t are the aging temperature in kelvins and the aging time in hours, respectively. In this plot, the hardness values generally exhibit similar behavior with small scattering irrespective of the aging temperature and time. The results indicate that it takes 5861,400 h at 450 °C, 72,400 h at 500 °C and 1000 h at 550 °C to make the optimum condition for reaching 157 Hv, which is the lowest hardness directly measured at the service-exposed cold-reheater pipe. We selected the condition of 1000 h at 550 °C for accelerated degradation, which produces the hardness value similar to that of the service-exposed coldreheater pipe. Fig. 3a presents an SEM image of the microstructure of the fresh steel, showing polygonal ferrite grains together with pearlite structures with segmented carbide. Although the microstructure does not change significantly after the aging at 550 °C for 1000 h, more spheroidized carbide particles are observed (Fig. 3(b)). The overall microstructure of the aged sample does not seem to be significantly different from that of the service-exposed cold-heater pipe (Fig. 1c). Fig. 4 shows XRD patterns of extracted residues from the fresh sample and the sample aged at 550 °C for 1000 h. M3C, which is mainly Fe3C, is identified as a major precipitate in both samples. It is noted that other precipitates do not form during the aging, whereas the microstructure gradually changes. The results of the creep-rupture tests of the fresh and aged samples are shown in Fig. 5. A general trend that the creep-rupture time increases with decreasing creep-rupture stress and decreasing temperature is observed, although many tests at other temperature-stress conditions are still on-going. It should be noted that the creep-rupture time of the aged sample is shorter than that of the fresh sample irrespective of creep-rupture stress and temperature. Fig. 6 plots the creep-rupture stress of the fresh and aged samples as a function of a LMP, Eq. (1), in which t is the creep-rupture time. The data of both samples can be categorized into two different curves. It is obviously confirmed in the plots that the creep-rupture time of the aged samples becomes shorter compared with that of the as-tempered samples. This implies that the progressive microstructural change due to long-term exposure to high temperature leads to the degradation in
Fig. 4. XRD patterns of precipitates extracted from (a) the fresh P11 steel and (b) the sample aged for 1000 h at 550 °C.
Fig. 5. Creep-rupture time of the fresh P11 steel and sample aged for 1000 h at 550 °C under various stress and temperature conditions. The open and closed symbols represent the fresh P11 steel (as-tempered) and sample aged for 1000 h at 550 °C, respectively.
Fig. 3. SEM micrographs of (a) the fresh P11 steel and (b) the sample aged for 1000 h at 550 °C. 333
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microstructure. We decide that it will not be a problem to continue to use the service-exposed cold-reheater pipes after the power plant renewal, because the residual lifetime ratio is 0.53 at 310 °C under the current service conditions of use. However, it should be noted that the P11 steel used in this study experienced the accelerated degradation at 550 °C for up to 1000 h, not actual operation in the power plant. When the power plant was shut down, the degradation of the P11 steel pipe could be evaluated only by measuring the hardness, which is the non-destructive test, on the surface of the service-exposed cold-reheater pipes. We assessed the residual lifetime from a point of view of only creep properties by conducting a creep test of accelerated degraded P11 steel. The accelerated degraded P11 steel in this study was not exactly same microstructures and mechanical properties as the service-exposed coldreheater pipes. In order to accurately evaluate the residual lifetime of the serviceexposed cold-reheater pipes, it is necessary to analyze the creep properties obtained from destructive creep testing. Zieliński et al. [19] and M. Dziuba-Kaluza et al. [25] reported the residual lifetime of low-alloy Cr-Mo-V cast steel and 13CrMo4-5 steel by destructive creep testing of materials used for approximately 100,000 h and 200,000 h at high temperatures respectively. It is desirable to predict the more accurate residual lifetime of the cold-reheater pipe through the careful analysis of destructive tests such as tensile tests at room and high temperatures, impact tests and accelerated creep tests. In the future, destructive testing of the service-exposed cold-reheater pipe will be carried out, as the retrofit of the power plant progresses. Despite uncertainty due to the factors that were not considered in this study, the result of the residual lifetime assessment might be a great help in judging if the service-exposed cold-reheater pipes should be replaced or keep being used.
Fig. 6. Creep-rupture stress of fresh P11 steel and sample aged for 1000 h at 550 °C as a function of the LMP.
creep-rupture life of the steel. Assuming that the aged sample can represent the service-exposed cold-reheater pipe, it might be possible to estimate the residual lifetime of the service-exposed pipe using these plots. The blue star symbol corresponding to the hoop stress of the service-exposed pipe indicates the residual lifetime of the aged sample. Therefore, we estimate the ratio of the residual lifetime to the total lifetime using the two LMP plots as follows:
t − tR ⎞ Residual lifetime ratio = ⎛1 − T tT ⎠ ⎝ ⎜
⎟
(2)
4. Conclusions
where tT and tR are the rupture time of the fresh steel (total lifetime) and the aged steel (residual lifetime) representing the service-exposed pipe, respectively. Fig. 7 presents an enlarged view of Fig. 6 at the current service condition of the cold-reheater pipe. The residual lifetime ratio at 310 °C under a stress of 105 MPa derived from LMP values is also indicated on the top x-axis. At the current service condition, the LMP values of the fresh and aged samples are 19.68 and 19.52, respectively. Eventually, the residual lifetime ratio is estimated to be 0.53 at 310 °C by calculating tT and tR using Eq. (2). The result well agrees with the creep damage ranging from 20 to 50% estimated from the replicated
The assessment of the residual lifetime of the service-exposed P11 steel cold-reheater pipe used for approximately 23 years at 310 °C in the power plant was conducted by analyzing the creep properties of accelerated degraded P11 steel. Major conclusions based on the results and their analysis are as follows: (1) The microstructure of the cold-reheater pipe exposed for 23 years at 310 °C consist mainly of ferrite and pearlite area. The carbides in pearlite area of service-exposed pipe were spheroidized and coarsened as compared to carbides of fresh P11 steel. (2) The lowest hardness of the service-exposed pipe was 157 Hv and the aged condition for accelerated degradation in order to have similar hardness values with the service-exposed cold-reheater pipe was 1000 h at 550 °C (3) The creep-rupture strength of aged steel for accelerated degradation was decreased compared with that of fresh steel. Based on the LMP plots, the residual lifetime of the service-exposed pipe was estimated to be 53% at 310 °C with respect to the total life time under the current service conditions of use. (4) Only non-destructive properties such as hardness values and replica images of the cold-reheater pipes from power plant was possible in this research because the power plant was operating. (5) In the future, destructive creep testing to predict the more accurate residual lifetime of the service-exposed cold-reheater pipe will be carried out, as the retrofit of the power plant progresses. Acknowledgments The authors would like to acknowledge the financial support from the R&D Convergence Program of National Research Councilof Science & Technology (grant no. CAP 16-08-KITECH) and Energy Technology Evaluation and Planning (grant no. 20151110100040) of Ministry of Trade, Industry and Energy of Republic of Korea. The authors also
Fig. 7. Creep-rupture stress of the fresh and degraded samples as a function of the LMP and the residual lifetime ratio (enlarged view of Fig. 6). 334
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thank Doosan Heavy Industries & Construction Co., Ltd. for providing the materials.
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