nd Preven.(2008)8: 41-47 DOI10.1007/sl1668-0079106-5 C·A·S·EH·I.s.T.0·R.Y-P·E.E·R.R·E.v·I·E.W·E·D Damage and residual life assessment of bends for X20CrMoV121 Main Steam Pipe after Long-Term Service Zheng-Fei Hu· Zheng-Guo Yang·Guo- Qiu He Cheng-Shu Chen Submitted: 30 June 2007 /in revised form: 5 December 2007/Published online: 31 January 2008 C ASM International 2008 165,000 h service at 550C under 13.73 MPa pressure. experience severe service conditions that lead to deg The residual life of the bend sections is determined by radation in the microstructure and properties. Because evaluation of the service stresses and testing to obtain creep of this degradation, it is necessary to assess the residual rupture data. Metallographic analysis and tensile, impact, life of exposed components. The creep damage of the nd hardness tests are also conducted. These combined materials from components in long-term service has tests show that the properties of the steel deteriorated been surveyed to evaluate the residual life. There are during service, displaying embrittlement tendencies: the few reports about residual life assessment of high corresponding microstructures exhibit grain boundary chromium( Cr) ferritic steels; however some weakening and creep damage characteristics. However, tions of creep damage have focused on considering no evidence of localized damage in the form of measurement, the recovery of lath structure, and creep cavitation or surface cracks was observed in the coarsening of precipitating carbides [1-3]. The hardness examined parts, considering the residual life of the bends at of high-Cr ferritic steels decreases during recovery of service condition, they are adequate for an additional the lath structure and coarsening precipitates during 44,000 h of operation. It is recommended that a health creep deformation. Additionally, the lath width assessment should be taken after 25,000 h service exposure precipitate size increase during accelerated creep for safety reasons the contrary, some of the investigations [1] of X20CrMoV12 1 steam pipes that have been in long Keywords Main steam pipe. Microstructure term service indicated that the properties degraded Finite element analysis. Residual life assessment severely but no softening occurred. It is believed that the precipitation hardening by fine carbide intralaths as well as stabilization of M23 C6 subgrain structure by Introduction carbides are decisive factors for the high creep strength of this steel [2]. Many power plants This paper describes the damage evaluation of the 20 years, and the components in these plants have been mechanical properties and microstructure as well as residual life assessment for bent pipe sections of a ZF.H(凶)·G.Q.HeC.S.Chen X20CrMoV12 1 main steam pipe in a power plant in School of Materials Science and Engineering, Tongji University, Shanghai. The steam pipe has been in service for Shanghai 200092. China 23 years, and it has been operating at isothermal and constant pressure condition (550C/13.73 MPa) Z.-G. Yang more than 165, 000 h. The chemical composition of Department of Materials Science, Fudan University the X20CrMoV121 steel investigated is given in Shanghai 200433, China Table 1 2 Spring
C Æ A Æ S Æ E H Æ I Æ S Æ T Æ O Æ R Æ Y—P Æ E Æ E Æ R-R Æ E Æ V Æ I Æ E Æ W Æ E Æ D Damage and Residual Life Assessment of Bends for X20CrMoV12.1 Main Steam Pipe after Long-Term Service Zheng-Fei Hu Æ Zheng-Guo Yang Æ Guo-Qiu He Æ Cheng-Shu Chen Submitted: 30 June 2007 / in revised form: 5 December 2007 / Published online: 31 January 2008 ASM International 2008 Abstract The bent sections from a main steam pipe in a thermal power plant in Shanghai were examined after 165,000 h service at 550 C under 13.73 MPa pressure. The residual life of the bend sections is determined by evaluation of the service stresses and testing to obtain creep rupture data. Metallographic analysis and tensile, impact, and hardness tests are also conducted. These combined tests show that the properties of the steel deteriorated during service, displaying embrittlement tendencies; the corresponding microstructures exhibit grain boundary weakening and creep damage characteristics. However, considering no evidence of localized damage in the form of creep cavitation or surface cracks was observed in the examined parts, considering the residual life of the bends at service condition, they are adequate for an additional 44,000 h of operation. It is recommended that a health assessment should be taken after 25,000 h service exposure for safety reasons. Keywords Main steam pipe Microstructure Finite element analysis Residual life assessment Introduction Many power plants have been operating for more than 20 years, and the components in these plants have been used at elevated temperatures up to or exceeding the original design life. High-temperature components experience severe service conditions that lead to degradation in the microstructure and properties. Because of this degradation, it is necessary to assess the residual life of exposed components. The creep damage of the materials from components in long-term service has been surveyed to evaluate the residual life. There are few reports about residual life assessment of highchromium (Cr) ferritic steels; however some investigations of creep damage have focused on hardness measurement, the recovery of lath structure, and coarsening of precipitating carbides [1–3]. The hardness of high-Cr ferritic steels decreases during recovery of the lath structure and coarsening precipitates during creep deformation. Additionally, the lath width and precipitate size increase during accelerated creep. On the contrary, some of the investigations [1] of X20CrMoV12.1 steam pipes that have been in longterm service indicated that the properties degraded severely but no softening occurred. It is believed that the precipitation hardening by fine carbide intralaths as well as stabilization of M23C6 subgrain structure by carbides are decisive factors for the high creep strength of this steel [2]. This paper describes the damage evaluation of the mechanical properties and microstructure as well as residual life assessment for bent pipe sections of a X20CrMoV12.1 main steam pipe in a power plant in Shanghai. The steam pipe has been in service for 23 years, and it has been operating at isothermal and constant pressure condition (550 C/13.73 MPa) more than 165,000 h. The chemical composition of the X20CrMoV12.1 steel investigated is given in Table 1. Z.-F. Hu (&) G.-Q. He C.-S. Chen School of Materials Science and Engineering, Tongji University, Shanghai 200092, China e-mail: huzhengf@mail.tongji.edu.cn Z.-G. Yang Department of Materials Science, Fudan University, Shanghai 200433, China 123 J Fail. Anal. and Preven. (2008) 8:41–47 DOI 10.1007/s11668-007-9106-5
J Fail. Anal and Preven. (2008)8: 41-47 Table 1 The chemical composition of examined pipe from the pipe samples. The methods of preparing the TEM samples are described in many references [4] Stress analysis was carried out using finite element C Mn S analysis software and reflects the stresses under service 0.23 0.64 0.006 0.02 0.36 11.23 1.00 0.32 0.72 0.08 conditions. The residual life assessment of the main steam pipe is determined from the comprehensive results, including mechanical tests, microstructure investigation, Experiment and procedures and stress analysis. The test material was cut from three bends at 45 outboard position of a 273 mm outside diameter(OD) by 26 mm Results and Discussion thick main steam pipe of X20CrMo V12 1. A sample from a section of virgin pipe in the as-received state was also Mechanical Properties studied. The mechanical properties were investigat under room temperature and at the service temperature of The mechanical properties of the examined materials tested machine operated at constant strain rate. Full-size Charpy strength oo.2 and ultimate tensile strength ob of exposed V-notch impact specimens were also tested. Hardness samples decrease about 30%o, and the toughness is less than measurements were performed at a standardized machine 20% of that of the material in the as-received state: how- load of 375 kg on testpieces. Tensile tests were carried out ever, the hardness shows no significant changes. These on the base metal in both the longitudinal and transverse results show that the properties of the exposed materials directions of the main steam pipe. The accelerated stress degraded, and the spectacular change of the rupture tests were carried out at 550C at various stress mechanical property decrease of toughness after levels in the range of 150 to 260 MPa. The variation of the long-term service exposure. This means that the base metal yield strength (0.2% proof stress) and ultimate tensile of the exposed pipe is much more brittle at room temper strength (UTS)with temperature of testing are shown in ature. The same result has been obtained by others [5, 6 later tables The mechanical properties of the samples tested at high The microstructural investigations were performed temperature are given in Table 3. The strength as well as using analytical transmission electron microscopy (TEM); toughness of exposed samples decreases slightly, and there the transmission microscopy was carried out on both foil is a distinct change in toughness. Toughness of the exposed samples and carbon extraction replicas using a PhiLIPs materials at high temperature is about 30% less compared CM200(National Microanalysis Center, Materials Science with the as-received sample tested at the same condition. It Department,Fudan University, Shanghai) transmission is shown that there is an increasing trend of toughness and lectron microscope operating at 200 kV. Both the foil and brittleness transformation temperature with prolonged carbon extraction replicas for TEM study were prepared exposure time at service condition Table 2 Mechanical properties of virgin and service-ex aterials at room temperature Material SI No and direction Tensile stren Pa EA(A), RA(Z). Toughness(Akv), J Hardness, HB Bend 21 4465149 21 24 l19 ENI0216-2 >490 690-840 axial; H, hoop 2 Springer
Experiment and Procedures The test material was cut from three bends at 45 outboard position of a 273 mm outside diameter (OD) by 26 mm thick main steam pipe of X20CrMoV12.1. A sample from a section of virgin pipe in the as-received state was also studied. The mechanical properties were investigated under room temperature and at the service temperature of 550 C. Tensile tests were conducted using a 100 kN machine operated at constant strain rate. Full-size Charpy V-notch impact specimens were also tested. Hardness measurements were performed at a standardized machine load of 375 kg on testpieces. Tensile tests were carried out on the base metal in both the longitudinal and transverse directions of the main steam pipe. The accelerated stress rupture tests were carried out at 550 C at various stress levels in the range of 150 to 260 MPa. The variation of the yield strength (0.2% proof stress) and ultimate tensile strength (UTS) with temperature of testing are shown in later tables. The microstructural investigations were performed using analytical transmission electron microscopy (TEM); the transmission microscopy was carried out on both foil samples and carbon extraction replicas using a PHILIPS CM200 (National Microanalysis Center, Materials Science Department, Fudan University, Shanghai) transmission electron microscope operating at 200 kV. Both the foil and carbon extraction replicas for TEM study were prepared from the pipe samples. The methods of preparing the TEM samples are described in many references [4]. Stress analysis was carried out using finite element analysis software and reflects the stresses under service conditions. The residual life assessment of the main steam pipe is determined from the comprehensive results, including mechanical tests, microstructure investigation, and stress analysis. Results and Discussion Mechanical Properties The mechanical properties of the examined materials tested at room temperature are given in Table 2. The yield strength r0.2 and ultimate tensile strength rb of exposed samples decrease about 30%, and the toughness is less than 20% of that of the material in the as-received state; however, the hardness shows no significant changes. These results show that the properties of the exposed materials degraded, and the most spectacular change of the mechanical property is the decrease of toughness after long-term service exposure. This means that the base metal of the exposed pipe is much more brittle at room temperature. The same result has been obtained by others [5, 6]. The mechanical properties of the samples tested at high temperature are given in Table 3. The strength as well as toughness of exposed samples decreases slightly, and there is a distinct change in toughness. Toughness of the exposed materials at high temperature is about 30% less compared with the as-received sample tested at the same condition. It is shown that there is an increasing trend of toughness and brittleness transformation temperature with prolonged exposure time at service condition. Table 1 The chemical composition of examined pipe Composition, wt% C Mn S P Si Cr Mo V Ni Cu 0.23 0.64 0.006 0.02 0.36 11.23 1.00 0.32 0.72 0.08 Table 2 Mechanical properties of virgin and service-exposed materials at room temperature Material SI No. and direction Tensile strength, MPa EA (A), % RA (Z), % Toughness (Akv), J Hardness, HB Rp0.2 Rm Bend 1 A 575 778 20 44 20 220 H 556 766 21 44 20 227 2 A 566 775 20 46 23 230 H 568 776 20 45 20 232 3 A 561 774 19 47 21 230 H 567 777 19 41 21 224 Virgin 1 A 695 777 26 59 113 230 2 A 702 776 26 59 114 234 3 A 685 776 24 60 119 229 EN10216-2 [490 690–840 [14 [27 A, axial; H, hoop 42 J Fail. Anal. and Preven. (2008) 8:41–47 123
J Fail. Anal. and Preven. (2008)8: 41-47 Table 3 Short-time tensile properties of virgin and service-exposed materials at high temperature SI No and direction Tensile strength. MPa EA (A),% RA(2), %o Toughness(Akv), J 445 48 AHAHAHAAA I15 442 l13 101 136 465 ENI >250 A, axial; H, hoop Microstructure creep process. A significant reduction of dislocation density is observed and few dislocation-free regions can be seen Scanning electron microscopy(SEM) was used Extensive carbide precipitates can be seen at prior aus the fracture surfaces of the test samples. Figure 1 vs tenite and martensite lath boundaries with the finer the fracture surfaces of tensile test from an exposed sample precipitates in martensitic laths. Large coarsening carbides at room temperature. This sample displays almost complete in irregular spheroid formed along the boundaries. Com- intergranular fracture on a macroscopic level, although the pared with virgin material, the carbide morpholog fracture mode is ductile rupture. Subcracks propagate along coarsened distinctly grain boundaries, and wedge cracks appear at triple points. The observations indicate that the matrix of the tem- The fracture facets are quite flat, even though a process of pered martensite has undergone a deterioration durin tensile necking had taken place. Such fracture implies a long-term creep. The dislocations climbed or glided and weakness of the grain boundaries, which is caused by the terminated at boundaries. As the number of dislocations at segregation of phosphorus and the presence of coarsened the boundaries increased, networks formed and substruc carbides in the grain boundaries during long-term exposed tures developed. The carbides morphology in boundaries service at high temperature [7]. Figure 1(b) shows the coarsened distinctly, and most of the strengthening phase fracture surfaces of tensile test from the same exposed have dissolved or coarsened. However, the tiny V-rich sample tested at 550C. It shows that the fracture was precipitates that formed during service increased the transgranular ductile rupture; however, a few subinter- hardness and are responsible for the hardness maintenance granular cracks can be seen in the fracture facets. These in the material [9] observations are consistent with the trend toward weak ening of the grain boundaries with exposed time The general TEM microstructure of the sample is shown Stress Analysis in Fig. 1(c). Obviously, the main structure is a typical tempered martensite containing laths and many coarsened In the absence of discernible cavitation and flaws, stress carbide precipitates along grain boundaries. The ferrite rupture tests were selected to assess the condition of the regions appear very clean in the TEM image, and most main pipe. One of the most widely used techniques for life regions contain some finer precipitates. No form of inter- assessment of components involves the removal of samples granular cavities was observed in the foil sample, but some and conducting accelerated tests at service temperature. An evidence of creep damage can be seen in TEM. The shapes estimate of the residual life is then made by extrapolatio of the laths are changed; in particular, the lath boundaries of the results to the service conditions look like bamboo knots called cell structure. which is a Table 4 list the results of accelerated test at 550 oC typical microstructure morphology caused by creep [8]. Stress rupture data have been plotted in term of log stress Many low dislocation density regions appeared in the lath with rupture time in Fig. 2. The curves show that stresses structure, and some typical substructures can be seen in are linear with rupture times; thus, the threshold strength of Fig. I(d). The substructure seems to develop as subgrains the main steam pipe at 550C was determined by extrap- boundaries are formed by dislocation movement during the olation method to be 2 Spring
Microstructure Scanning electron microscopy (SEM) was used to inspect the fracture surfaces of the test samples. Figure 1 (a) shows the fracture surfaces of tensile test from an exposed sample at room temperature. This sample displays almost complete intergranular fracture on a macroscopic level, although the fracture mode is ductile rupture. Subcracks propagate along grain boundaries, and wedge cracks appear at triple points. The fracture facets are quite flat, even though a process of tensile necking had taken place. Such fracture implies a weakness of the grain boundaries, which is caused by the segregation of phosphorus and the presence of coarsened carbides in the grain boundaries during long-term exposed service at high temperature [7]. Figure 1(b) shows the fracture surfaces of tensile test from the same exposed sample tested at 550 C. It shows that the fracture was transgranular ductile rupture; however, a few subintergranular cracks can be seen in the fracture facets. These observations are consistent with the trend toward weakening of the grain boundaries with exposed time. The general TEM microstructure of the sample is shown in Fig. 1(c). Obviously, the main structure is a typical tempered martensite containing laths and many coarsened carbide precipitates along grain boundaries. The ferrite regions appear very clean in the TEM image, and most regions contain some finer precipitates. No form of intergranular cavities was observed in the foil sample, but some evidence of creep damage can be seen in TEM. The shapes of the laths are changed; in particular, the lath boundaries look like bamboo knots, called cell structure, which is a typical microstructure morphology caused by creep [8]. Many low dislocation density regions appeared in the lath structure, and some typical substructures can be seen in Fig. 1(d). The substructure seems to develop as subgrains boundaries are formed by dislocation movement during the creep process. A significant reduction of dislocation density is observed, and few dislocation-free regions can be seen. Extensive carbide precipitates can be seen at prior austenite and martensite lath boundaries, with the finer precipitates in martensitic laths. Large coarsening carbides in irregular spheroid formed along the boundaries. Compared with virgin material, the carbide morphology coarsened distinctly. The observations indicate that the matrix of the tempered martensite has undergone a deterioration during long-term creep. The dislocations climbed or glided and terminated at boundaries. As the number of dislocations at the boundaries increased, networks formed and substructures developed. The carbides morphology in boundaries coarsened distinctly, and most of the strengthening phase have dissolved or coarsened. However, the tiny V-rich precipitates that formed during service increased the hardness and are responsible for the hardness maintenance in the material [9]. Stress Analysis In the absence of discernible cavitation and flaws, stress rupture tests were selected to assess the condition of the main pipe. One of the most widely used techniques for life assessment of components involves the removal of samples and conducting accelerated tests at service temperature. An estimate of the residual life is then made by extrapolation of the results to the service conditions. Table 4 list the results of accelerated test at 550 C. Stress rupture data have been plotted in term of log stress with rupture time in Fig. 2. The curves show that stresses are linear with rupture times; thus, the threshold strength of the main steam pipe at 550 C was determined by extrapolation method to be: Table 3 Short-time tensile properties of virgin and service-exposed materials at high temperature Material SI No. and direction Tensile strength, MPa EA (A), % RA (Z), % Toughness (Akv), J Rp0.2 Rm Bend 1 A 357 444 18 50 109 H 360 445 20 48 100 2 A 365 447 18 51 115 H 358 442 19 54 102 3 A 378 466 21 73 113 H 361 441 19 50 101 Virgin 1 A 379 472 24 71 136 2 A 369 465 22 72 142 3 A 378 466 21 73 135 EN10216-2 [250 A, axial; H, hoop J Fail. Anal. and Preven. (2008) 8:41–47 43 123
J Fail. Anal and Preven. (2008)8: 41-47 Fig 1 (a)Scanning electron fractographs of exposed sample from tensile test at room Temperature.(b) Scanning lectron fractographs of of thin foil from thin fo exposed sample. (d) TEM micrograph of thin foil from in foil from exposed sample Table 4 Stress rupture properties of service exposed bend pipe at 1000 550°C 菲丰 Sample No. Stress(os), Rupture EL(A),% EA(2), %o MPa time. h 000 23456789 824 35.6 82.8 l190.5 35.1 10 82.1 100000 394 Time(hours) 10 Fig 2 Plot of stress with rupture time for service exposed bend at Sample is not ruptured 550° 131 MPa This threshold strength of virgin material is less than the d bent tube sectio properties of determined by extrapolation from accelerated testing., The mechanical tests and rupture data as well as the strengt the exposed material, 0559 C=131 M Finite-element analysis is used to calculate the stress damage compared with those of the virgin pipe distribution in a pipe section. The model comprises a pipe bend(90%) to which straight pipes were attached to both ends. Thus numerical influences from boundary conditions Residual Life assessment on the calculated stresses and strains in the pipe bend co be minimized. The stress analysis takes into account only Standard EN10216-2 [10] gives the creep rupture stre the internal pressure and ignores all other loads. Further, it of X20CrMoV121 under 550C GR.100000= 128 was decided to fix one straight pipe end and load an axial 2 Springer
r550 C 105 ¼ 131 MPa ð1Þ The mechanical tests and rupture data as well as the corresponding microstructure indicate that the properties of the parent metal of exposed bent tube sections show creep damage compared with those of the virgin pipe. Residual Life Assessment Standard EN10216-2 [10] gives the creep rupture strength of X20CrMoV12.1 under 550 C rR,100,000 = 128 MPa. This threshold strength of virgin material is less than the strength of the exposed material, r550 C 105 ¼ 131 MPa, determined by extrapolation from accelerated testing. Finite-element analysis is used to calculate the stress distribution in a pipe section. The model comprises a pipe bend (90) to which straight pipes were attached to both ends. Thus numerical influences from boundary conditions on the calculated stresses and strains in the pipe bend could be minimized. The stress analysis takes into account only the internal pressure and ignores all other loads. Further, it was decided to fix one straight pipe end and load an axial Table 4 Stress rupture properties of service exposed bend pipe at 550 C Sample No. Stress (rs), MPa Rupture time, h EL (A), % EA (Z), % 1 260 17.0 33.1 79.6 2 250 24.5 34.0 79.8 3 220 171.5 35.8 82.0 4 210 366.0 39.6 82.4 5 200 605.0 35.6 82.8 6 190 1190.5 46.5 83.2 7 180 1695.0 35.1 83.4 8 170 3334.0 30.4 82.1 9 160 5346.0 39.4 82.5 10 150a –– – a Sample is not ruptured Fig. 2 Plot of stress with rupture time for service exposed bend at 550 C Fig. 1 (a) Scanning electron fractographs of exposed sample from tensile test at room Temperature. (b) Scanning electron fractographs of exposed sample from tensile test at 550 C. (c) TEM micrograph of thin foil from thin foil from exposed sample. (d) TEM micrograph of thin foil from thin foil from exposed sample (magnified) 44 J Fail. Anal. and Preven. (2008) 8:41–47 123
J Fail. Anal. and Preven. (2008)8: 41-47 2.18=107 .80 141·10 1,01·107 8.21*105 8.21=105 1.58*1 6.28106 Fig. 3(a) Distribution of the principal stress (internal of bend).(b) Distribution of the principal stress(extemal of bend).(c) Distribution of the on Misesl stress (internal of bend). (d) Distribution of the von Misesl stress(external of bend) pressure of (PD/4t)=36.4 MPa on the other end; D and t value induced by loading artificially, the principal stress is are the diameter and thickness of the surveyed pipe, 101 MPa. The von Mises stress is given in Fig 3(c)and(d) respectively. Youngs modulus for the material is given as and displays the peak stress of about 125 MPa. From above 80,000 MPa (N/mm) from tensile test, and Poisson's results, under actual operating conditions, the maximum ratio is assumed to be 0.3. In order to obtain reasonable and resultant stress loading on the pipe is a1= 125 MPa. This accurate results, the analysis of the thick-wall bend stress was used to evaluate residual life of the bent attached to straight ends is defined by three-dimensional sections (3D)cells. To incorporate the actual sizes and space with the results of accelerated test and stress analysi structure of the exposed pipe, the model was set using a the permitted stress [a] expressed is geometry model tool in NASTRAN software. The opti- mized finite element mesh method was employed after the d model was established Generally, the stress to cause failure may be consic Here n is safety factor (generally, 1. 2-1.65), and we take to be either yielding or rupture stress, depending the value of n=1.5. Therefore the bent section of the criteria used. The corresponding failure theories also main pipe has permitted stress, from redesign viewpoint sider two types of failure: reaching the ultimate tensile of: stress for rupture and reaching the Mises yield criterion for [o]=131/1.5=87.3 MPa yield failure. The calculated results were presented in form of principal stress(rupture)and Von Mises Compared with the maximum resultant stress derived from (yielding)simultaneously. The distributions of principal finite element analysi stress are given in Fig 3(a) and(b). Except for the peak d=87. 3 MPa <OT= 125 MPa 2 Spring
pressure of (PD/4t) = 36.4 MPa on the other end; D and t are the diameter and thickness of the surveyed pipe, respectively. Young’s modulus for the material is given as 180,000 MPa (N/mm2 ) from tensile test, and Poisson’s ratio is assumed to be 0.3. In order to obtain reasonable and accurate results, the analysis of the thick-wall bend attached to straight ends is defined by three-dimensional (3D) cells. To incorporate the actual sizes and space structure of the exposed pipe, the model was set using a geometry model tool in NASTRAN software. The optimized finite element mesh method was employed after the model was established. Generally, the stress to cause failure may be considered to be either yielding or rupture stress, depending on the criteria used. The corresponding failure theories also consider two types of failure: reaching the ultimate tensile stress for rupture and reaching the Mises yield criterion for yield failure. The calculated results were presented in the form of principal stress (rupture) and Von Mises stress (yielding) simultaneously. The distributions of principal stress are given in Fig. 3(a) and (b). Except for the peak value induced by loading artificially, the principal stress is 101 MPa. The von Mises stress is given in Fig. 3(c) and (d) and displays the peak stress of about 125 MPa. From above results, under actual operating conditions, the maximum resultant stress loading on the pipe is r1 = 125 MPa. This stress was used to evaluate residual life of the bent sections. With the results of accelerated test and stress analysis, the permitted stress [r] expressed is: ½r ¼ r550 C 105 n ð2Þ Here n is safety factor (generally, 1.2–1.65), and we take the value of n = 1.5. Therefore, the bent section of the main pipe has permitted stress, from redesign viewpoint of: [r] = 131=1.5 = 87.3 MPa ð3Þ Compared with the maximum resultant stress derived from finite element analysis: ½r ¼ 87:3 MPa\rT ¼ 125 MPa ð4Þ Fig. 3 (a) Distribution of the principal stress (internal of bend). (b) Distribution of the principal stress (external of bend). (c) Distribution of the von Misesl stress (internal of bend). (d) Distribution of the von Misesl stress (external of bend) J Fail. Anal. and Preven. (2008) 8:41–47 45 123
J Fail. Anal and Preven. (2008)8: 41-47 est data and extrapolated curve surface cracks or cavitation. Additionally, it is recom Master curve from ref 9 mended that another health check of the expected service of the exposed bent sections be carried out after 22,000 h of service. During shutdown of the plant, nondestructive tests, namely, dimensional(thickness and diameter)mea- surement, and in situ metallography may be carried out to assess the condition of the materials for their future serviceabil 10 202122232425262728 Larson Miller Parameter T(25+log(tr))/1000 An increasing trend of brittleness of the parent metal of X20CrMoV12. 1 is seen with prolonged exposure at service Fig4 Plot of stress vs Larson Miller parameter(LMP)for parent conditions. Corresponding microstructural investigations metal of bend show that carbide coarsening and martensite structure deterioration also occur during long-term exposure service Equation 8 means, from the redesign viewpoint, that the at 550C. The residual life of bent sections of main pipe of pipe could not meet the safety criteria for opera- X20CrMoV12I is about 44,000 h, provided there is tion for 100,000 h after 23 years service evidence of localized damage in the form of surface cracks The rupture data for the specimens have been plotted in or cavitation. Even though the straight sections of the main terms of log(stress) versus Larson-Miller parameter steam pipe appear to be in a reasonably good state of Fig 4, where LMP is expressed health. it is recommended that another health check be LMP= T(25+log tr) (5) carried out after 22,000 h of additional service. Addition- ally, during shutdown of the plant, nondestructive tests where T is the absolute temperature in K, and tr is the (NDT), namely, dimensional(thickness and diameter) rupture time in hours measurement. hardness measurement. and in situ metal- Analysis of rupture data indicates that the properties of lography should be carried out to assess the condition of the exposed steel are comparable to those of the master curve the materials for their future serviceability of virgin pipe [11], suggesting no appreciable creep damage The test data points of the stress versus LMP plot for the Acknow service-exposed material are fairly close to the master curve ural Science Foundation of China under contract no. 50771073 and for virgin material when the stress is less than 200 MPa. The 2007CB714705 esearch Program of China under contract no master curve for the virgin material has been extrapolated to a lower stress value that is below the operating hoop stress (on= PD/2t=72. 1 MPa)of the service-exposed main References steam pipe. The remaining life of the service-exposed steam pipe predicted at 72.1 MPa and at 550C is shown in I. Masuyama, F, Nishimura, N, Sasada, A: CAMP-ISD, 11. 614 Fig. 4. At the operating hoop stress of 72.1 MPa for the engthening mechanisms in high chromium service-exposed pipes, the LMP value as read from the graph steels. In: Bakker, W.T., Parker, J D(editors ). Proceedings of the is about 25,730. At this value of LMP, one would expect a Third Conference on Advances in Materials. Technology for very long life. As is customary, an inspection life of straight Fossil Power Plants. London (UK): The Institute of Materials, pp. 187-19402001). pipe after >100,000 h is recommended. However, consid 3. Sawada, K, Maruyama, K Hasegawa, Y, Muraki, T. Creep life ering that the maximum resultant stress on the bent section is assessment of high chromium ferritic steels by recovery of 01=125 MPa, the LMP value as read from the graph is martensitic lath structure. Proceedings of Eighth International about 24, 398 and the expected residual life is about Conf. on Creep and Fracture of Engineering Materials and Structures, Tsukuba, Japan, Nov. 1-5, 1999. 44,200 h. The results for the parent metal in the bent section 4. Hu, Z F, Wu, X F, Wang, C.X. HRTEM study on precipitates in predict a remaining life of about 44, 200 h at most highl high CoNi steel. J. Mater. Sci. Tech 20. 425/528 stressed locations, although failure of the straight sectior 5. Vodarek, V, Strang. A: Effect of nickel on the should not occur in 100,000 h of operation rocesses in 12CrMoV steels during creep at 550C. 38,101(1998) This analysis thus suggests that the remaining life at 6.Ennis, P.., Zielinska-Lipiec, A, Filemonowicz, A Quantitative 550C is a minimum of 44, 200 h for the service, provided omparison of the microstructure of high chromium steels for 2 Springer
Equation 8 means, from the redesign viewpoint, that the exposed pipe could not meet the safety criteria for operation for 100,000 h after 23 years service. The rupture data for the specimens have been plotted in terms of log (stress) versus Larson-Miller parameter in Fig. 4, where LMP is expressed: LMP ¼ Tð25 þ log trÞ ð5Þ where T is the absolute temperature in K, and tr is the rupture time in hours. Analysis of rupture data indicates that the properties of the exposed steel are comparable to those of the master curve of virgin pipe [11], suggesting no appreciable creep damage. The test data points of the stress versus LMP plot for the service-exposed material are fairly close to the master curve for virgin material when the stress is less than 200 MPa. The master curve for the virgin material has been extrapolated to a lower stress value that is below the operating hoop stress (rh = PD/2t = 72.1 MPa) of the service-exposed main steam pipe. The remaining life of the service-exposed main steam pipe predicted at 72.1 MPa and at 550 C is shown in Fig. 4. At the operating hoop stress of 72.1 MPa for the service-exposed pipes, the LMP value as read from the graph is about 25,730. At this value of LMP, one would expect a very long life. As is customary, an inspection life of straight pipe after [100,000 h is recommended. However, considering that the maximum resultant stress on the bent section is r1 = 125 MPa, the LMP value as read from the graph is about 24,398 and the expected residual life is about 44,200 h. The results for the parent metal in the bent section predict a remaining life of about 44,200 h at most highly stressed locations, although failure of the straight sections should not occur in 100,000 h of operation. This analysis thus suggests that the remaining life at 550 C is a minimum of 44,200 h for the service, provided there is no evidence of localized damage in the form of surface cracks or cavitation. Additionally, it is recommended that another health check of the expected service of the exposed bent sections be carried out after 22,000 h of service. During shutdown of the plant, nondestructive tests, namely, dimensional (thickness and diameter) measurement, and in situ metallography may be carried out to assess the condition of the materials for their future serviceability. Conclusions An increasing trend of brittleness of the parent metal of X20CrMoV12.1 is seen with prolonged exposure at service conditions. Corresponding microstructural investigations show that carbide coarsening and martensite structure deterioration also occur during long-term exposure service at 550 C. The residual life of bent sections of main pipe of X20CrMoV12.1 is about 44,000 h, provided there is no evidence of localized damage in the form of surface cracks or cavitation. Even though the straight sections of the main steam pipe appear to be in a reasonably good state of health, it is recommended that another health check be carried out after 22,000 h of additional service. Additionally, during shutdown of the plant, nondestructive tests (NDT), namely, dimensional (thickness and diameter) measurement, hardness measurement, and in situ metallography should be carried out to assess the condition of the materials for their future serviceability. Acknowledgments This work was supported by the National Natural Science Foundation of China under contract no. 50771073 and National Basic Research Program of China under contract no. 2007CB714705. References 1. Masuyama, F., Nishimura, N., Sasada, A.: CAMP-ISIJ, 11, 614 (1998). 2. Ennis, J.P.: Creep strengthening mechanisms in high chromium steels. In: Bakker, W.T., Parker, J.D. (editors). Proceedings of the Third Conference on Advances in Materials. Technology for Fossil Power Plants. London (UK): The Institute of Materials, pp. 187–194 (2001). 3. Sawada, K., Maruyama, K., Hasegawa, Y., Muraki, T.: Creep life assessment of high chromium ferritic steels by recovery of martensitic lath structure. Proceedings of Eighth International Conf. on Creep and Fracture of Engineering Materials and Structures, Tsukuba, Japan, Nov. 1–5, 1999. 4. Hu, Z.F., Wu, X.F., Wang, C.X.: HRTEM study on precipitates in high CoNi steel. J. Mater. Sci. & Tech. 20, 425/528 (2004). 5. Vodarek, V., Strang, A.: Effect of nickel on the precipitation processes in 12CrMoV steels during creep at 550 C. Scr. Mater. 38, 101 (1998). 6. Ennis, P.J., Zielinska-Lipiec, A., Filemonowicz, A.: Quantitative comparison of the microstructure of high chromium steels for Fig. 4 Plot of stress vs Larson Miller parameter (LMP) for parent metal of bend 46 J Fail. Anal. and Preven. (2008) 8:41–47 123
J Fail. Anal. and Preven. (2008)8: 41-47 advanced power stations. In: Strang, A, Cawley, J, Greenwoof, 9. Hu, Z.F., Yang, Z.G.: Identification of the precipitates by TEM Gw.(editors): Microstructure of High Temperature Materials and eds in X20CrMov12. for long-term service at elevated No. 2, The Institute of Materials, London, U. K., Pp. 135-143 emperature. J Mater. Eng. Perform. 12(1), 106-11l(2003) (1998) 10. European Standard: EN10216-2, Seamless steel tubes for pres- 7. Hu, Z.F.. Yang, Z.G.: An investigation of the embrittlement in ure purpose-Technical delivery conditions, Part 2: Non alloy <20CrMoV121 power plant steel after long-term service expo- loy steel tubes with specified elevated temperature sure at elevated temperature. Mater. Sci Eng. A, 383, 224-228 2004) I1. Weber. J, Klenk, A. Rieke, M.A.: new method of strength cal 8. Chan, R.W. Haasen, P: Physical Metallurgy, part 2 M], North- culation and lifetime prediction of pipe bends operating in the Holland Physics Publishing, Netherlands (1983) creep range. Int J. Pressure Vessels Piping 82, 77-84(2005) 2 Spring
advanced power stations. In: Strang, A., Cawley, J., Greenwoof, G.W. (editors): Microstructure of High Temperature Materials, No. 2, The Institute of Materials, London, U.K., pp. 135–143 (1998). 7. Hu, Z.F., Yang, Z.G.: An investigation of the embrittlement in X20CrMoV12.1 power plant steel after long-term service exposure at elevated temperature. Mater. Sci. Eng. A, 383, 224–228 (2004). 8. Chan, R.W., Haasen, P.: Physical Metallurgy, part 2 [M], NorthHolland Physics Publishing, Netherlands (1983). 9. Hu, Z.F., Yang, Z.G.: Identification of the precipitates by TEM and EDS in X20CrMoV12.1 for long-term service at elevated temperature. J. Mater. Eng. Perform. 12(1), 106–111 (2003). 10. European Standard: EN10216-2, Seamless steel tubes for pressure purpose—Technical delivery conditions, Part 2: Non alloy and alloy steel tubes with specified elevated temperature properties. 11. Weber, J., Klenk, A., Rieke, M.A.: new method of strength calculation and lifetime prediction of pipe bends operating in the creep range. Int. J. Pressure Vessels Piping 82, 77–84 (2005). J Fail. Anal. and Preven. (2008) 8:41–47 47 123
