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 Performanceheat 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 micro￾scope 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