JMEPEG(2012)21:1313-1319 ◎ ASM International DOI:10.1007/s11665-011-0048-4 1059-9495/S1900 Heat strength evaluation and Microstructures Observation of the welded joints of one china-Made T91 Steel Yi Gong, Zhen-Guo Yang, and Fa-Yun Yang Submitted December 8, 2010: in revised form May 16, 2011) T91(9CrlMoVNb), the martensitic heat-resistant steel, is widely applied in industries like power genera- on,petrochemical, nuclear, etc, and a wealth of researches has been conducted on its properties so far. century. Hence, thorough assessments of the China-made T91 steels are always urgently required, especial However, actually for China, T91 was begun to be domestically manufactured only from the end of last for its welded joints. In this paper, the relationship between mechanical properties and microstructures of the welded joints of one China-made T91 steel was experimentally discussed. Moreover, aging test and creep rupture test were utilized for both analyzing the heat strength and predicting the service life of the joints. Results showed that welded joints of this China-made T91 steel could exhibit sufficient strength under the operating conditions of most nuclear reactors used nowadays Keywords Creep rupture, Microstructure, T91, Welded joints microstructures evolution(Ref 20-23) of this familiar material at elevated temperatures, even exposed to the nuclear environ- ments(Ref 24, 25). With respect to the similar and/or the dissimilar welded joints of it, Das and co-workers(Ref 26, 27) analyzed the relationship between mechanical properties and 1. Introduction microstructures, Thomas(Ref 28)investigated the residual stresses after welding, Spigarelli(Ref 29)studied the creep rate, ugh it has been mainly applied as the matrix material and Li and co-workers(Ref 30, 31, 32)evaluated the creep rupture properties and predicted the service lives. However, in for steam tubes of superheaters, reheaters, etc, in supercritical fact, T91 was not imported into China until the beginning of the (SC)and/or ultra-supercritical (USC) plants in power genera- 90s in last century, and was started to be domestically made tion industry for over 30 years, T91(9CrIMoVNb), the only from the end of last century. Thus, comprehensive frequently used structural materials in nuclear industry(Ref assessment of the China-made T91 steels and their welded The original aim of developing T9 1 by ORNAL and CE Joints plays a critical role in popularizing them in engineering in 1970s was just for liquid metal fast breeder reactors practice, even supporting the national industry of China In this paper, study mainly accumulated in the welded joints (LMFBR)(Ref 3, 4). Actually, owing to the increasing steam that were produced by a kind of China-made T9I steel. In order parameters in next generation USC plants(abov e625°0 30 MPa) for the purpose of higher fuels utilization and lower microstructures, tensile tests were carried out at both room and novel martensitic heat-resistant steels with even superior high increasing temperatures on the welded joints, while optical temperature properties, such as T92(9CrO 5Mol 8WVNb), were utilized to observe the metallographic microstructures and T911(9CrlMol WNb), T122(12Cr0 5Mo2WCuVNb), and so the carbides precipitation across the joints, particularly in on(Ref 7-11). However, as the steam parameters are not as heat-affected zone(Haz) and the weld seam. Furthermore, at severe as that in USC plants, also considering its mature service experiences and high performance versus price ratio, T91 will 625C, not only the aging test was conducted to evaluate the performance deterioration of the joints, but also the creep ertainly maintain its popularity in the foreseeable future in rupture test was employed nuclear power plants, which are always attracting the incentive drives from the governments all over the world Achievements of this paper supplemented relevant heat streng data of T91 welded joints for engineering practice, and could In the past two decades, a great deal of researches has been also provide solid foundation for popularization of this China- carried out on the mechanical properties(Ref 12, 13 ), corrosion made T91 steel in the nuclear indust resistance(Ref 14-16), creep performances(Ref 17-19), and Yi Gong, and Zhen-Guo Yang, Department of Materials 2. Experimenta Fudan University, Shanghai 200433, People's Republic Fa-Yun Yang, Power Station, Baosteel Group Co. Ltd. 201900, People's Republic of China. Contact e-mail: zgyang@fudan Tested materials were nominal t91 he asistant steels with edu. cn scale of 47.6OD x 7 mm thick. Chemical compositions and Journal of materials Engineering and Performance Volume21(7)Jul2012—1313
Heat Strength Evaluation and Microstructures Observation of the Welded Joints of One China-Made T91 Steel Yi Gong, Zhen-Guo Yang, and Fa-Yun Yang (Submitted December 8, 2010; in revised form May 16, 2011) T91 (9Cr1MoVNb), the martensitic heat-resistant steel, is widely applied in industries like power generation, petrochemical, nuclear, etc., and a wealth of researches has been conducted on its properties so far. However, actually for China, T91 was begun to be domestically manufactured only from the end of last century. Hence, thorough assessments of the China-made T91 steels are always urgently required, especially for its welded joints. In this paper, the relationship between mechanical properties and microstructures of the welded joints of one China-made T91 steel was experimentally discussed. Moreover, aging test and creep rupture test were utilized for both analyzing the heat strength and predicting the service life of the joints. Results showed that welded joints of this China-made T91 steel could exhibit sufficient strength under the operating conditions of most nuclear reactors used nowadays. Keywords Creep rupture, Microstructure, T91, Welded joints 1. Introduction Although it has been mainly applied as the matrix material for steam tubes of superheaters, reheaters, etc., in supercritical (SC) and/or ultra-supercritical (USC) plants in power generation industry for over 30 years, T91 (9Cr1MoVNb), the martensitic heat-resistant steel, is still one of the most frequently used structural materials in nuclear industry (Ref 1, 2). The original aim of developing T91 by ORNAL and CE in 1970s was just for liquid metal fast breeder reactors (LMFBR) (Ref 3, 4). Actually, owing to the increasing steam parameters in next generation USC plants (above 625 C, 30 MPa) for the purpose of higher fuels utilization and lower CO2 emission (Ref 5, 6), T91 is now gradually substituted by novel martensitic heat-resistant steels with even superior high temperature properties, such as T92 (9Cr0.5Mo1.8WVNb), T911 (9Cr1Mo1WNb), T122 (12Cr0.5Mo2WCuVNb), and so on (Ref 7–11). However, as the steam parameters are not as severe as that in USC plants, also considering its mature service experiences and high performance versus price ratio, T91 will certainly maintain its popularity in the foreseeable future in nuclear power plants, which are always attracting the incentive drives from the governments all over the world. In the past two decades, a great deal of researches has been carried out on the mechanical properties (Ref 12, 13), corrosion resistance (Ref 14–16), creep performances (Ref 17–19), and microstructures evolution (Ref 20–23) of this familiar material at elevated temperatures, even exposed to the nuclear environments (Ref 24, 25). With respect to the similar and/or the dissimilar welded joints of it, Das and co-workers (Ref 26, 27) analyzed the relationship between mechanical properties and microstructures, Thomas (Ref 28) investigated the residual stresses after welding, Spigarelli (Ref 29) studied the creep rate, and Li and co-workers (Ref 30, 31, 32) evaluated the creep rupture properties and predicted the service lives. However, in fact, T91 was not imported into China until the beginning of the 90s in last century, and was started to be domestically made only from the end of last century. Thus, comprehensive assessment of the China-made T91 steels and their welded joints plays a critical role in popularizing them in engineering practice, even supporting the national industry of China. In this paper, study mainly accumulated in the welded joints that were produced by a kind of China-made T91 steel. In order to discuss the relationship between mechanical properties and microstructures, tensile tests were carried out at both room and increasing temperatures on the welded joints, while optical microscope (OM) and transmission electron microscope (TEM) were utilized to observe the metallographic microstructures and the carbides precipitation across the joints, particularly in the heat-affected zone (HAZ) and the weld seam. Furthermore, at 625 C, not only the aging test was conducted to evaluate the performance deterioration of the joints, but also the creep rupture test was employed to predict their service lives. Achievements of this paper supplemented relevant heat strength data of T91 welded joints for engineering practice, and could also provide solid foundation for popularization of this Chinamade T91 steel in the nuclear industry. 2. Experimental Tested materials were nominal T91 heat-resistant steels with scale of 47.6OD 9 7 mm thick. Chemical compositions and Yi Gong, and Zhen-Guo Yang, Department of Materials Science, Fudan University, Shanghai 200433, Peoples Republic of China; Fa-Yun Yang, Power Station, Baosteel Group Co. Ltd., Shanghai 201900, Peoples Republic of China. Contact e-mail: zgyang@fudan. edu.cn. JMEPEG (2012) 21:1313–1319 ASM International DOI: 10.1007/s11665-011-0048-4 1059-9495/$19.00 Journal of Materials Engineering and Performance Volume 21(7) July 2012—1313
leat treatment conditions of them are listed in Table l, which carbides evolution across the welded joints before and after are in accordance with the requirements of ASME SA-213 T91 welding, especially in the HAZ and the weld seam, were then pecifications(Ref 33). As is shown in Fig. 1(a), no obvious inspected, respectively, under LEICA DMLM optical micro- arse nonmetallic inclusions were present in the material(Ref scope and PHILIPS EM 430 TEM. Finally, in accordance with 34). Furthermore, etched in agent of picric acid (2, 4, 6- ASTM E139-06(Ref 39)standard, creep rupture test was trinitrophenol) 1.25 g, HCl 20 mL, ethanol 10 mL, and H20 conducted at 625C under load stresses from 65 to 150 MPawith 10 mL for 40 s, its metallographic microstructure is shown in increment of 5 or 10 MPa. Meanwhile, high temperature aging Fig. 1(b), which displays a typical tempered lath martensitic test was achieved on the welded joints at this temperature as well microstructure with average lath width of about I um. The T91 welded joints were welded by means of gas tungsten arc welding(GTAw) with pure argon gas(Ar)as the shielding gas and AWS ER90S-B9 as the welding wire(D 3. Results and Discussion 0 mm), whose chemical compositions are listed in Table 2 (Ref 35). The welding current and voltage were, respectively, 3.1 Mechanical Properties at Room and 230 A and 14 V, and the number of weld passes was three. Temperatures Then, the joints were subjected to the post-weld heat treatment PWHT)at 730-760C for I h to eliminate the residual stresses. It can be learned from Table 3 that the T91 welded joints vanety of mechanical tests for the welded joints were also exhibited qualified tensile strength according to the t9I base uccessively carried out. Tensile test and bending test were material specification, only the elongation was a bit lower. rformed at room temperature according to the ASTM E8-04 Table 4 reveals that the welded joints also presented eligible (Ref 36)and E290-97a(2004)(Ref 37)standards. Also, tensile toughness, and no cracks were found on the bended surfaces properties of the joints were evaluated at increasing temperatures Table 5 and Fig. 2 show the tensile properties of the T91 from 50 to 650C with increment of 50oC based on ISO 783 welded joints at increasing temperatures. Compared with the 1999(Ref 38)standard. Metallographic microstructures and requirements of T91 base material in GB 5310 standard of Table 1 Chemical compositions and heat treatment conditions of the t91 sample (wt % Elements Mo 0.09 0.0110.0020.298.820.900.200.080.040 0.120.013 0.08-0.120.30-0.60≤0.020≤0.0100.20-0.508.00-9.500.85-1.050.18-1050.06-0.100.030-0.070≤040≤0.04 SA-213T91 Heat treatment conditions:1060°C×20min( normalizing)+780°C×60min( tempering) 20 Fig. 1 Metallographic microstructures of the T91 sample(a) polished state(b)etched state Table 2 Chemical compositions of the welding wire ER90S-B9(wt % ER90S-B9 Welding 0.112 0.570.30 0.0070.0020.6890 02000.0090.080.0570.036 ASME SFA-5.280.070.13≤1.250.15-0.30≤0.010≤0.010≤1.008.00-9500.80-1.100.15-0.25≤0.04≤0.200.02-0.100.03-0.0 (AWS) ER9OS-B 1314-Volume 21(7) July 2012 Journal of Materials Engineering and Performance
heat treatment conditions of them are listed in Table 1, which are in accordance with the requirements of ASME SA-213 T91 specifications (Ref 33). As is shown in Fig. 1(a), no obvious coarse nonmetallic inclusions were present in the material (Ref 34). Furthermore, etched in agent of picric acid (2,4,6- trinitrophenol) 1.25 g, HCl 20 mL, ethanol 10 mL, and H2O 10 mL for 40 s, its metallographic microstructure is shown in Fig. 1(b), which displays a typical tempered lath martensitic microstructure with average lath width of about 1 lm. The T91 welded joints were welded by means of gas tungsten arc welding (GTAW) with pure argon gas (Ar) as the shielding gas and AWS ER90S-B9 as the welding wire (U 1.0 mm), whose chemical compositions are listed in Table 2 (Ref 35). The welding current and voltage were, respectively, 230 A and 14 V, and the number of weld passes was three. Then, the joints were subjected to the post-weld heat treatment (PWHT) at 730-760 C for 1 h to eliminate the residual stresses. A variety of mechanical tests for the welded joints were also successively carried out. Tensile test and bending test were performed at room temperature according to the ASTM E8-04 (Ref 36) and E290-97a(2004) (Ref 37) standards. Also, tensile properties of the joints were evaluated at increasing temperatures from 50 to 650 C with increment of 50 C based on ISO 783- 1999 (Ref 38) standard. Metallographic microstructures and carbides evolution across the welded joints before and after welding, especially in the HAZ and the weld seam, were then inspected, respectively, under LEICA DMLM optical microscope and PHILIPS EM 430 TEM. Finally, in accordance with ASTM E139-06 (Ref 39) standard, creep rupture test was conducted at 625 C under load stresses from 65 to 150 MPa with increment of 5 or 10 MPa. Meanwhile, high temperature aging test was achieved on the welded joints at this temperature as well. 3. Results and Discussion 3.1 Mechanical Properties at Room and Increasing Temperatures It can be learned from Table 3 that the T91 welded joints exhibited qualified tensile strength according to the T91 base material specification, only the elongation was a bit lower. Table 4 reveals that the welded joints also presented eligible toughness, and no cracks were found on the bended surfaces. Table 5 and Fig. 2 show the tensile properties of the T91 welded joints at increasing temperatures. Compared with the requirements of T91 base material in GB 5310 standard of Table 1 Chemical compositions and heat treatment conditions of the T91 sample (wt.%) Elements C Mn P S Si Cr Mo V Nb N Ni Al T91 Sample 0.09 0.41 0.011 0.002 0.29 8.82 0.90 0.20 0.08 0.040 0.12 0.013 ASME SA-213 T91 0.08–0.12 0.30-0.60 £0.020 £0.010 0.20-0.50 8.00-9.50 0.85-1.05 0.18-1.05 0.06-0.10 0.030-0.070 £0.40 £0.04 Heat treatment conditions: 1060 C 9 20 min (normalizing) + 780 C 9 60 min (tempering) Fig. 1 Metallographic microstructures of the T91 sample (a) polished state (b) etched state Table 2 Chemical compositions of the welding wire ER90S-B9 (wt.%) Elements C Mn Si P S Ni Cr Mo V Al Cu Nb N ER90S-B9 Welding wire 0.112 0.57 0.30 0.007 0.002 0.68 9.00 0.93 0.200 0.009 0.08 0.057 0.036 ASME SFA-5.28 (AWS) ER90S-B9 0.07-0.13 £1.25 0.15-0.30 £0.010 £0.010 £1.00 8.00-9.50 0.80-1.10 0.15-0.25 £0.04 £0.20 0.02-0.10 0.03-0.07 1314—Volume 21(7) July 2012 Journal of Materials Engineering and Performance
Table 3 Mechanical properties of T91 welded joints Table 5 Mechanical properties of T91 welded joints at room temperature at increasing temperatures Tensile strength, ob, MPa Yield strength, oo.2, MPa Tensile Sample no·1ⅡAwg6 Test Test GB 5310 Data fr temperature,C results specification(a) Metre MPa 708.5 1234 693.5 450 712 741.0 525 728.0 T9I specification ≥585 20 200 056000 Table 4 Bending test results of T91 welded joints 300 376 Bending style Test condition Sample no Results 371 Face bending 358 D=3T,a=50° 123451234 Qualified 3050805 60 D, denotes the bending diameter; T, denotes the material thickness; a denotes the bending angle 5350 China(Ref 40), yield strengths(oo.2)of the joints were all qualified with increase of temperatures. Also, corresponding data of the T91 welded joints with same welding wire and process from Metrode Products Ltd. (Ref 41) were listed in the table and the figure. Meanwhile, the tensile strengths of our (a) This GB 5310 specification is just for T91 base material joints at these increasing temperatures were displayed as well tion Figure 3(a) displays the metallographic microstructure of the haz after welding, which consisted of sorbite with finer t data from metrod laths than that of t91 base material. Further detailedly, by means of TEM, it could be obviously observed that coarsened rod-like carbides of M2C, had ripened and precipitated on the 2 5004 grain boundaries in the HAZ, marked with arrows in Fig. 4(a) Especially in some triangular grain boundaries, the carbides were even larger, marked in Fig. 4(b). In contrast, the weld as also martensite just like the t91 base material but der laths of about 3 um, seen in Fig 3(b). Moreover subgrains existed within the dislocation martensite ig 4c), and nearly no coarsened M23C6 carbides had boundaries(Fig. 4d) According to the metallographic microstructures and the 500600700 TEM micrographs, compared with the weld seam, HAZ of the Ter joints displayed fine sorbitic microstructure with coarsened M23C6 carbides on the grain boundaries. This could be ascribed Fig 2 Mechanical properties of T91 welded joints at increasing to the heat effect from the weld seam during the welding process, thus the initial martensites decomposed into finer sorbite. Meanwhile, since carbide as M23 C6 on the grai boundaries for boundaries precipitates prior to MC within the grains(Ref 42 ), turned to be coarsened under he the M23C6 carbides that originally existed on the grain the haZ decreased and made it Journal of Materials Engineering and Performance Volume 21(7) July 2012--1315
China (Ref 40), yield strengths (r0.2) of the joints were all qualified with increase of temperatures. Also, corresponding data of the T91 welded joints with same welding wire and process from Metrode Products Ltd. (Ref 41) were listed in the table and the figure. Meanwhile, the tensile strengths of our joints at these increasing temperatures were displayed as well. 3.2 Metallographic Microstructures Inspection Figure 3(a) displays the metallographic microstructure of the HAZ after welding, which consisted of sorbites with finer laths than that of T91 base material. Further detailedly, by means of TEM, it could be obviously observed that coarsened rod-like carbides of M23C6 had ripened and precipitated on the grain boundaries in the HAZ, marked with arrows in Fig. 4(a). Especially in some triangular grain boundaries, the carbides were even larger, marked in Fig. 4(b). In contrast, the weld seam was also martensite just like the T91 base material but with wider laths of about 3 lm, seen in Fig. 3(b). Moreover, tangling subgrains existed within the dislocation martensite laths (Fig. 4c), and nearly no coarsened M23C6 carbides had precipitated on the grain boundaries (Fig. 4d). According to the metallographic microstructures and the TEM micrographs, compared with the weld seam, HAZ of the joints displayed fine sorbitic microstructure with coarsened M23C6 carbides on the grain boundaries. This could be ascribed to the heat effect from the weld seam during the welding process, thus the initial martensites decomposed into finer sorbites. Meanwhile, since carbide as M23C6 on the grain boundaries precipitates prior to MC within the grains (Ref 42), the M23C6 carbides that originally existed on the grain boundaries for purpose of pinning strengthening ripened and turned to be coarsened under heat too. As a result, strength of the HAZ decreased and made it the weak region of the whole Table 3 Mechanical properties of T91 welded joints at room temperature Sample no. Tensile strength, rb, MPa Elongation, I II Avg. d5, % 1 706 711 708.5 18 2 693 694 693.5 3 707 712 709.5 4 730 752 741.0 5 735 721 728.0 T91 specification ‡585 ‡20 Table 4 Bending test results of T91 welded joints Bending style Test condition Sample no. Results Face bending 1 2 3 4 D = 3T, a = 50 5 Qualified Back bending 1 2 3 4 5 D, denotes the bending diameter; T, denotes the material thickness; a, denotes the bending angle Table 5 Mechanical properties of T91 welded joints at increasing temperatures Test temperature, C Yield strength, r0.2, MPa Tensile strength, rb, MPa Test results GB 5310 specification(a) Data from Metrode 50 450 660 100 470 384 580 525 650 150 475 378 590 500 595 200 377 565 580 250 377 560 560 300 376 535 530 350 371 535 535 400 358 510 515 450 337 480 485 500 380 306 460 385 455 515 490 540 550 350 260 395 365 410 575 350 375 330 375 600 280 198 335 290 345 625 275 310 290 315 650 250 265 245 280 (a) This GB 5310 specification is just for T91 base material Fig. 2 Mechanical properties of T91 welded joints at increasing temperatures Journal of Materials Engineering and Performance Volume 21(7) July 2012—1315
(a) Fig 3 Metallographic microstructures of the different regions across the t91 welded joints(a)HAZ (b)weld seam (b) M23C6 (c)、 a Fig 4 TEM micrographs of the different regions across the T91 welded joints (a) HAZ, 28,000;(b)HAZ, x35,000;(c)weld seam, 22,000;(d) weld seam,x35,000 1316-Volume 21(7) July 2012 Journal of Materials Engineering and Performance
Fig. 3 Metallographic microstructures of the different regions across the T91 welded joints (a) HAZ (b) weld seam Fig. 4 TEM micrographs of the different regions across the T91 welded joints (a) HAZ, 928,000; (b) HAZ, 935,000; (c) weld seam, 922,000; (d) weld seam, 935,000 1316—Volume 21(7) July 2012 Journal of Materials Engineering and Performance
joints. However, based on the classic Ostwald ripening Creep rupture test was then conducted on tw mechanism(Ref 43 ), under heat effect the precipitated carbides round bar specimen of the joints at 625"C, respectively, named are first in form of rod and gradually change toward sphere with Test I and Test IL. Table 7 lists the rupture times under different low free energy. In the present case, the rod-like M23C6 load stresses. a carbides on the grain boundaries may be regarded only in the plotted in Fig. 6. Meanwhile, Fig. 6 also involves relevant data early stage of ripening, seen in Fig 4(a). In other words, by of the T91 welded joints at 600C(Ref 42), and at 550, 600 eans of this welding and PWHT process, carbides in the HAZ and 650C (Ref 30). Moreover, for purpose of comparison, the id not ripen seriously and could still ensure a relatively acceptable strength for the whole welded joint. In terms of the weld seam, it underwent a complete melt-to-recrystallize According to the classic equation finally form coarser martensite laths than that of the t91 base Igt=lg4-Blga (Eq1) aterial. What's more, the adequate welding process inhibited the relationships between load stresses a and rupture times t the M2 C6 carbides coarsened, and consequently ensured of the two groups of tests were both linearly fitted in Fig. 6 qualified strength and toughness for the weld seam. and mathematically expressed in Table 7. It is clearly dis- played in Fig. 6 that results of the two tests well conformed 3.3 Aging Test In order to investigate the performance deterioration of the Table 7 Creep rupture test(625C)results of the t91 T91 welded joints, aging test was carried out at 625C. Table 6 welded joints and Fig. 5 display the mechanical properties variation with the aging times. It can be concluded from these results that e tim Elongation. mechanical properties of the joints did not deteriorate signif- antly with increase of the aging time. In other words, such 91 welded joints could exhibit good structural stability at Load stress. o MPa Test I Test II Test I Test II elevated temperatures 13.1 10.84 3.4 Creep Rupture Test 570 l1.9 l1.05 13.1 Table 6 Mechanical properties of the T91 welded joints 438.5 aging test l142 l18 Aging Yield strength, Tensile strength, Elongation, 3.51 time Oo.2, MPa Ob, MPa 500506 0 450 660 1084 14.75 Fitted line equation 3000 20 8068 Test I: Igo=2.272-0.120lgt 125C=47MPa =40 MPa l0000 Test II: Ig o=2.454-0.170lgt 1000 —oa2xl0(MPa) 、%,×10(MPa) 550 4000 8000 10000 Time(h) Time(h) Fig 6 Double logarithmic plot of load stress versus rupture time Fig 5 Mechanical properties of the t9I welded joints in aging test for the T91 welded joints at 625C Journal of Materials Engineering and Performance Volume 21(7) July 2012--1317
joints. However, based on the classic Ostwald ripening mechanism (Ref 43), under heat effect the precipitated carbides are first in form of rod and gradually change toward sphere with low free energy. In the present case, the rod-like M23C6 carbides on the grain boundaries may be regarded only in the early stage of ripening, seen in Fig. 4(a). In other words, by means of this welding and PWHT process, carbides in the HAZ did not ripen seriously and could still ensure a relatively acceptable strength for the whole welded joint. In terms of the weld seam, it underwent a complete melt-to-recrystallize procedure, therefore its grains could sufficiently grow and finally form coarser martensite laths than that of the T91 base material. Whats more, the adequate welding process inhibited the M23C6 carbides coarsened, and consequently ensured qualified strength and toughness for the weld seam. 3.3 Aging Test In order to investigate the performance deterioration of the T91 welded joints, aging test was carried out at 625 C. Table 6 and Fig. 5 display the mechanical properties variation with the aging times. It can be concluded from these results that mechanical properties of the joints did not deteriorate significantly with increase of the aging time. In other words, such T91 welded joints could exhibit good structural stability at elevated temperatures. 3.4 Creep Rupture Test Creep rupture test was then conducted on two groups of round bar specimen of the joints at 625 C, respectively, named Test I and Test II. Table 7 lists the rupture times under different load stresses, and their double logarithmic relationship is plotted in Fig. 6. Meanwhile, Fig. 6 also involves relevant data of the T91 welded joints at 600 C (Ref 42), and at 550, 600, and 650 C (Ref 30). Moreover, for purpose of comparison, the creep rupture data of the T91 base material at 625 C (Ref 42) and our past research were both presented in Fig. 6 as well. According to the classic equation: lg t ¼ lg A B lg r ðEq 1Þ the relationships between load stresses r and rupture times t of the two groups of tests were both linearly fitted in Fig. 6 and mathematically expressed in Table 7. It is clearly displayed in Fig. 6 that results of the two tests well conformed Table 6 Mechanical properties of the T91 welded joints in aging test Aging time Yield strength, r0.2, MPa Tensile strength, rb, MPa Elongation, d5, % 0 450 660 18 1000 325 655 20 3000 450 680 16 5000 520 685 18 10000 495 660 15 Fig. 5 Mechanical properties of the T91 welded joints in aging test Table 7 Creep rupture test (625 C) results of the T91 welded joints Load stress, r, MPa Rupture time, T, h Elongation, d5, % Test I Test II Test I Test II 150 33 13.1 140 6 10.84 130 9 95 11.16 11.9 120 266 157 11.05 10 110 113 290 13.66 6.4 105 62 13.1 100 438.5 6 95 490 700 11.42 5.6 90 500 11.81 80 1197 1602 3.51 6.3 75 3337 10.11 70 1315 65 1084 14.75 Fitted line equation Test I : lg r ¼ 2:272 0:120 lg t r625 C 105 ¼ 47 MPa Test II : lg r ¼ 2:454 0:170 lg t r625 C 105 ¼ 40 MPa Fig. 6 Double logarithmic plot of load stress versus rupture time for the T91 welded joints at 625 C Journal of Materials Engineering and Performance Volume 21(7) July 2012—1317
1000 However, HAZ of the joint changed into fine sorbite 口 Test I with coarsened M23 C6 carbides precipitated on the grain leading it the weak creep rupture test, such T91 welded joints could ensure sufficient heat strength under the operating conditions of most currently used nuclear reactors. And life was estimated to be over 10?h under 25 MPa and550°C. 10 The work was supported by both National Natural Science LMP=T(25+lgt)x10 Foundation of China ( Grant 50871076) and Shanghai Leading Academic Discipline Project(Project Number: B113). Meanwhile Fig. 7 Plot of stresses with LMP for the T91 welded joints part of the tests was cooperated by Shanghai Institute of Special Equipment Inspection Technical Research and Shanghai Boiler to and also supplemented the data(Ref 30)at other tempera- works Ltd. Finally, gratitude must also be given to Shanghai tures, but they were a bit inferior to the performance of the esearch Institute of Materials for providing various experimental T91 base material at the same temperature. Fu based on the two fitted lines, the threshold stresses 0623 C of the joints at 625C, after 10 h were approximately estimated: 47 and 40 MPa, already higher than the steam References pressures of USC condition. If considering the safety factor, these threshold stresses should also be divided by a safety 1. RL Klueh and A.T. Nelson. Ferritic/Martensitic Steels for Next- Generation Reactors, J. Nucl. Mater, 2007, 371, p 37-52 Defficient(Ref 44, 45)that ranges from 1. 2 to 1.6 to obtain 2. J. Van den Bosch and A. Almazouzi, Compatibility of Martensitic/ the permitted stresses in application. However, this is also Austenitic Steel Welds with Lead Bismuth Eutectic Environ- higher than the operating pressures of most nuclear reactors ment,J. Nucl. Mater, 2009, 385, P 504-50 presently used. 3. F Masuyama, History of Power Plants and Progress in Heat Resistant It is a common sense that Larson-Miller equation is always Steels, IS/ Int, 2001, 41, p 612-625 applied to predict service lives of components on basis of creep 4. R. Viswanathan and w. Bakker, Materials for Ultrasupercritical Coal Power Plants-Boiler Materials: Part 1, J. Mater. Eng. Perform, 2001 rupture data(Ref 46). As for T91 and its welded joints, the 10,p8l-95 Larson-Miller equation is defined 5. J. Hansen, M. Sato, R. Ruedy, K. Lo, D W. Lea, and M. Medina Elizade. Global Te re Change. Proc. Natl. Acad. Sci. USA LMP= T(25+Igt) 2006,103,p14288-1 6. Y. Gong and Z.G. Yang, Corrosion Evaluation of One Dry Desulfur- where LMP is the dimensionless Larson-Miller parameter, T 2011,32,p671-681 hour(Ref 47). Then, the creep rupture data can be plotted in 7. J. Cao. Y. Gong, K. Zhu, Z.G. Yang et al., Microstructure and form of lg o versus LMP, where LMP can be calculated by Mechanical Properties of Dissimilar Materials Joints Between Eq 2, seen in Fig. 7. After polynomial fitting, service lives of Martensitic and S304H Austenitic Steels. Mater: Des.. 2011 p2763-2770 the T91 welded joints can be easily predicted. For example, 8.K H. Lo, C H. Shek, and J K L. Lai, Recent Developments in Stainless under 30 MPa and 625C, the service life is nearly Steels, Mater. Sci. Eng. R, 2009, 60,000 h; under 25 MPa and 550C, the upper limit operat J. Cao, Y. Gong, Z.G. Yang et al., Creep Fracture Behavior of ing conditions of most current nuclear reactors(Ref 48-51) Dissimilar Weld Joint Between T92 Martensitic and HR3C Austenitic the service life could be over 10 h. Thus it can be con- Steels, Int J. Pres. Ves. Pip, 2011, 88, p 94-98 cluded that this kind of China-made T91 welded joints were 10. J.C. An, H.Y. Jing, G.C. Xiao, L. Zhao, and L.Y. Xu, Analysis of the Creep Behavior of P92 Steel Welded Joint, J. Mater: Eng. Perform. comprehensively qualified to be applied in nuclear power 010,doi:10.1007/s11665-010-9779- dustry 11. Y. Gong, J. Cao, L N. Ji, Z.G. Yang et al., Assessment of Cree 12. A. Roy, P. Kumar, and D. Maitra, The Effect of Silicon Content on 4. Conclusions pact Toughness of T91 Grade Steels, J. Mater. Eng. Perform, 2009 1. Mechanical properties of this China-made T91 welded 13. C. Keller. M.M es, Z. Hadjem-Hamouche, and L. Guillot Influence of the are on the tensile behaviour of a modified joints were qualified at both room and increasing temper 9Cr-IMo T91 Steel. Mater. Sci. Eng. 4, 2010, 527 atures according to relevant standards. What's more, aged p67586764 at 625C for 10,000 h, tensile properties of the joints did not exhibit obvious deterioration as well Oxidation of T91 Ferritic Steel Under Steam. Corros. Sci. 2004 p613-631 2. After welding, weld seam consisted of wider mar- 15. L Nieto Hierro, V. Rohr, P.J. Ennis, M Schutze, and w.J. Quadakkers, te laths than that of t91 base material and no coars- Effects on Creep Strength of Power ened carbides were observed on the grain boundaries. Station Materials. Mater. 1318--Volume 21(7) July 2012 Journal of Materials Engineering and Performance
to and also supplemented the data (Ref 30) at other temperatures, but they were a bit inferior to the performance of the T91 base material at the same temperature. Furthermore, based on the two fitted lines, the threshold stresses r625 C 105 of the joints at 625 C, after 105 h were approximately estimated: 47 and 40 MPa, already higher than the steam pressures of USC condition. If considering the safety factor, these threshold stresses should also be divided by a safety coefficient (Ref 44, 45) that ranges from 1.2 to 1.6 to obtain the permitted stresses in application. However, this is also higher than the operating pressures of most nuclear reactors presently used. It is a common sense that Larson–Miller equation is always applied to predict service lives of components on basis of creep rupture data (Ref 46). As for T91 and its welded joints, the Larson–Miller equation is defined as: LMP ¼ Tð25 þ lg tÞ ðEq 2Þ where LMP is the dimensionless Larson–Miller parameter, T is the absolute temperature in K, and t is the rupture time in hour (Ref 47). Then, the creep rupture data can be plotted in form of lg r versus LMP, where LMP can be calculated by Eq 2, seen in Fig. 7. After polynomial fitting, service lives of the T91 welded joints can be easily predicted. For example, under 30 MPa and 625 C, the service life is nearly 60,000 h; under 25 MPa and 550 C, the upper limit operating conditions of most current nuclear reactors (Ref 48–51), the service life could be over 107 h. Thus, it can be concluded that this kind of China-made T91 welded joints were comprehensively qualified to be applied in nuclear power industry. 4. Conclusions 1. Mechanical properties of this China-made T91 welded joints were qualified at both room and increasing temperatures according to relevant standards. Whats more, aged at 625 C for 10,000 h, tensile properties of the joints did not exhibit obvious deterioration as well. 2. After welding, weld seam consisted of wider martensite laths than that of T91 base material, and no coarsened carbides were observed on the grain boundaries. However, HAZ of the joint changed into fine sorbites with coarsened M23C6 carbides precipitated on the grain boundaries, leading it the weak region of the whole joints. 3. Based on the creep rupture test, such T91 welded joints could ensure sufficient heat strength under the operating conditions of most currently used nuclear reactors. And its service life was estimated to be over 107 h under 25 MPa and 550 C. Acknowledgments The work was supported by both National Natural Science Foundation of China (Grant 50871076) and Shanghai Leading Academic Discipline Project (Project Number: B113). Meanwhile, part of the tests was cooperated by Shanghai Institute of Special Equipment Inspection & Technical Research and Shanghai Boiler works Ltd. Finally, gratitude must also be given to Shanghai Research Institute of Materials for providing various experimental conditions. References 1. R.L. Klueh and A.T. Nelson, Ferritic/Martensitic Steels for NextGeneration Reactors, J. Nucl. Mater., 2007, 371, p 37–52 2. J. Van den Bosch and A. Almazouzi, Compatibility of Martensitic/ Austenitic Steel Welds with Liquid Lead Bismuth Eutectic Environment, J. Nucl. Mater., 2009, 385, p 504–509 3. F. Masuyama, History of Power Plants and Progress in Heat Resistant Steels, ISIJ Int., 2001, 41, p 612–625 4. R. Viswanathan and W. Bakker, Materials for Ultrasupercritical Coal Power Plants–Boiler Materials: Part 1, J. Mater. Eng. Perform., 2001, 10, p 81–95 5. J. Hansen, M. Sato, R. Ruedy, K. Lo, D.W. Lea, and M. MedinaElizade, Global Temperature Change, Proc. Natl. Acad. Sci. USA, 2006, 103, p 14288–14293 6. Y. Gong and Z.G. Yang, Corrosion Evaluation of One Dry Desulfurization Equipment—Circulating Fluidized Bed Boiler, Mater. Des., 2011, 32, p 671–681 7. J. Cao, Y. Gong, K. Zhu, Z.G. Yang et al., Microstructure and Mechanical Properties of Dissimilar Materials Joints Between T92 Martensitic and S304H Austenitic Steels, Mater. Des., 2011, 32, p 2763–2770 8. K.H. Lo, C.H. Shek, and J.K.L. Lai, Recent Developments in Stainless Steels, Mater. Sci. Eng. R, 2009, 65, p 39–104 9. J. Cao, Y. Gong, Z.G. Yang et al., Creep Fracture Behavior of Dissimilar Weld Joint Between T92 Martensitic and HR3C Austenitic Steels, Int. J. Pres. Ves. Pip., 2011, 88, p 94–98 10. J.C. An, H.Y. Jing, G.C. Xiao, L. Zhao, and L.Y. Xu, Analysis of the Creep Behavior of P92 Steel Welded Joint, J. Mater. Eng. Perform., 2010, doi:10.1007/s11665-010-9779-x 11. Y. Gong, J. Cao, L.N. Ji, Z.G. Yang et al., Assessment of Creep Rupture Properties for Dissimilar Steels Welded Joints Between T92 and HR3C, Fatigue Fract. Eng. M, 2011, 34, p 83–96 12. A. Roy, P. Kumar, and D. Maitra, The Effect of Silicon Content on Impact Toughness of T91 Grade Steels, J. Mater. Eng. Perform., 2009, 18, p 205–210 13. C. Keller, M.M. Margulies, Z. Hadjem-Hamouche, and I. Guillot, Influence of the Temperature on the Tensile Behaviour of a Modified 9Cr–1Mo T91 Martensitic Steel, Mater. Sci. Eng. A, 2010, 527, p 6758–6764 14. D. Laverde, T. Go´mez-Acebo, and F. Castro, Continuous and Cyclic Oxidation of T91 Ferritic Steel Under Steam, Corros. Sci., 2004, 46, p 613–631 15. L. Nieto Hierro, V. Rohr, P.J. Ennis, M. Schu¨tze, and W.J. Quadakkers, Steam Oxidation and Its Potential Effects on Creep Strength of Power Station Materials, Mater. Corros., 2005, 56, p 890–896 Fig. 7 Plot of stresses with LMP for the T91 welded joints 1318—Volume 21(7) July 2012 Journal of Materials Engineering and Performance
16. R. Viswanathan and J M. Tanzosh, boiler Mater 32. F. Vivier, A.F. Gourgues-Lorenzon, and J. Besson, Creep Rupture of er Plants-Steamside Oxidation, J. 9CrlMoNbV Steel at 500@C: Base Metal and Welded Joint, Nucl. Eng Des,2010.240,p2704270 17.J.Cadek,V. Sustek, and M. Pahutova, An Analysis of a Set of Creep 33. ASME SA-213M-2001, Seamless Stainless Steel Tubes for Boiler and Data for a 9Cr-IMo-0 2V(P91 type)Steel, Mater. Sci. Eng. 4, 199 Heat Exchanger, ASME, Washington, DC, 2001 225,p22-28 18. V. Sklenicka, K. Kucharova, M. Svoboda, L. Kloc, J. Bursik, and Inclusions-Micrographic Method Using Standard Diagrams. Iso, Behavior of 9-12%Cr Power Plant Geneve. Switze 998 Steels, Mater. Charact., 2003, 51, p 35-48 35. ASME SFA-528M-2007, Low-Alloy Steel Electrodes and Rods for 19. B Fournier, M. Salvi, F. Dalle, Y De Carlan, C. Caes et al., Lifetime Gas Shielded Are Welding, ASME, Washington, DC, 2007 Prediction of 9-12%Cr Martensitic Ste 36. ASTM E8-04, Standard Test Methods for Tension Testing of Metallic at High Temperature, Int. J. Fatigue, 2010, 32, p 971-9 20. A Kumar, K. Laha, T Jayakumar, K. Bhanu Sankara Rao, and B Raj, 37. ASTM E290-97a(2004), Standard Test Methods for Bend Testing of Material for Ductility, ASTM, West Conshohocken, 2004 Ferritic Steel by Ultrasonic Measurements, Metall. Mater. Trans. A, 38. ISO 783-1999, Metallic Materials-Tensile Testing at Elevated Tem- 2002,33A,p1617- perature, ISO, Geneve, Switzerland, 1999 21. V. Homolova, J. Janovec, P. Zahumensky, and A. Vyrostkova, 39. ASTM E139-06, Standard Test Methods for Conducting Creep, Creep- Phases in P91 Steel, Mater: Sci. Eng. 4, 2003, 349, p Ipture, and Stress-Rupture Tests of Metallic Materials, ASTM, West Conshohocken. 2006 2. D.R.G. Mitchell and S Sulaiman, Advanced TEM Specimen Prepa 40. GB 5310-2008, Seamless Steel Tubes and Pipes for High Pressure ation Methods for Replication of P91 Steel, Mater Charact, 2006, 56 Boiler, SAC, Beijing, 2008 Welding Consumables for p91 Steels for the Power Generation 23. A K. Roy, D. Maitra, and P. Kumar, The Role of Silicon Content on Industry, Metrode Products Ltd Environmental Degradations of T91 Steels, J. Mater. Eng. Perform. 42. G.G. Shu, J N. Liu, C.Z. Shi, Z P. Wang, and Y F. Zhao, Microstruc- tural Pro and Engineering Applications of T/P9l Steel used in and Proton-Irradiated T91 Ferritic/Martensitic Steel, J. Nucl. Mater Supercritical Boilers, Shaanxi Science Technology Press, Xi'an, Shaanxi province. 2006 2007,367-370,p440-445 43. W. Ostwald, Lehrbuch der allgemeinen Chemie, vol. 2, part 1, Leipzig, 25. D.C. Foley, K.T. Hartwig, S.A. Maloy, P Hosemann, and x. Zhang, Germany, 1896 Grain Refinement of T91 Alloy by Equal Channel Angular Pressing, JMcl. Mater,2009,389,p221-224 44. Z.F. Hu and Z.G. Yang, An Investigation of the Embrittlement in X20CrMoV121 Power Plant Steel after Long-Term Service Exposure 6. C.R. Das, S.K. Albert, A.K. Bhaduri, G. Srinivasan, and B.S. Mur Effect of Prior microstructure on microstructure and mechanical at Elevated Temperature, Mater: Sci. Eng. A, 2004, 383, P 224-228 roperties of Modified 9Cr-lMo Steel Weld Joints, Mater: Sci. Eng. A, 45. Z.F. Hu and Z.G. Yang, Identification of the Precipitates by TEM and 008,477,p185-192 EDS in X20CrMoV121 after Long- Term Service at Elevated Temperature, J. Mater. Eng. Perform., 2003, 12, P 106-1l1 27. M. Sireesha, K Shaju Albert, and S Sundaresan, Microstructure and 46. F.R. Larson and J. Miller, A Time-Temperature Relationship for Mechanical Properties of Weld Fusion Zones in Modified 9Cr-lMo Steel, J. Mater: Eng. Perform, 2001, 10, p 320-33 Rupture and Creep Stresses, Trans. ASME, 1952, 74, p 765-775 28. A. Thomas, B. Pathiraj, and P. Veron, Feature Tests on Welded D. Jandova, J. Kasl, and V. Kanta, Creep Resistance of Similar and Dissimilar Weld Joints of P91 Steel, Mater: High. Temp., 2006, 23, Residual Stress Evaluation, Eng. Fract. Mech., 2007, 74, p 969-979 29. S. Spigarelli and E. Quadrini, Analysis of the Creep Behaviour of 48. M.M. Abu-Khader. Recent Advances in Nuclear Power: A Review Pog.Mc. Energ,2009,51,p225-235 Modified P91(9Cr-IMo-NbV) Welds, Mater. Des., 2002, 23, p 54 552 49. D.T. Ingersoll, Deliberately Small Reactors and the Second Nuclear 30. Y.K. Li, H. Hongo, M. Tabuch Era, Prog. Nucl. Energ., 2009, 51, P 589-603 Evaluation of Creep Damage in Heat Affected Zone of Thick Welded 50. M. Lenzen, Life Cycle Energy and Greenhouse Gas Joint for Mod.9Cr-lMo Steel, Int. J. Pres. Ves. Pip, 2009, 86, p 58 Nuclear Energy: A Review, Energ. Convers. Manag p2178-2199 31. T. Watanabe, M. Tabuchi, M. Yamazaki, H. Hongo, and T. Tanat 51. M. Piera, A. Lafuente, A. Abanades, and J M. Martinez-Val, Hybrid Creep Damage Evaluation of 9Cr-1Mo-V-Nb Steel Welded Joints Reactors: Nuclear Breeding or Energy Production?, Energ. Convers. Showing Type IV Fracture, Int J. Pres Ves. Pip, 2006, 83, p 63-7 manage,2010,51,p1758-1763 Journal of Materials Engineering and Performance Volume 21(7)July 2012--1319
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Mater. Eng. Perform., 2003, 12, p 106–111 46. F.R. Larson and J. Miller, A Time-Temperature Relationship for Rupture and Creep Stresses, Trans. ASME, 1952, 74, p 765–775 47. D. Jandova´, J. Kasl, and V. Kanta, Creep Resistance of Similar and Dissimilar Weld Joints of P91 Steel, Mater. High. Temp., 2006, 23, p 165–170 48. M.M. Abu-Khader, Recent Advances in Nuclear Power: A Review, Prog. Nucl. Energ., 2009, 51, p 225–235 49. D.T. Ingersoll, Deliberately Small Reactors and the Second Nuclear Era, Prog. Nucl. Energ., 2009, 51, p 589–603 50. M. Lenzen, Life Cycle Energy and Greenhouse Gas Emissions of Nuclear Energy: A Review, Energ. Convers. Manage., 2008, 49, p 2178–2199 51. M. Piera, A. Lafuente, A. Aba´nades, and J.M. Martinez-Val, Hybrid Reactors: Nuclear Breeding or Energy Production?, Energ. Convers. Manage., 2010, 51, p 1758–1763 Journal of Materials Engineering and Performance Volume 21(7) July 2012—1319
