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--1317joints. 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 signif￾icantly 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 dis￾played 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