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—1313Heat 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 genera￾tion, 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 genera￾tion 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 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 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 China￾made 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