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Residual life prediction of service exposed main steam pipe of boilers in a thermal power plant
Engineering Failure Analysis 7 (2000) 359±376
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Residual life prediction of service exposed main steam pipe of boilers in a thermal power plant A.K. Ray*, Y.N. Tiwari, R.K. Sinha, S. Chaudhuri, R. Singh National Metallurgical Laboratory, Jamshedpur 831007, India Received 25 May 1999; accepted 3 July 1999
Abstract This paper deals with residual life prediction methodology for more than 12.5 years service exposed main steam pipes of various boilers in a thermal power plant. Health assessment was made using destructive accelerated stress rupture and tensile tests at dierent temperatures, and some nondestructive tests. There was no evidence of localised damage in the form of surface cracks, cavitation or dents in the service exposed main steam pipes of all the boilers. So far as the remaining life at 5508C is concerned, it is possible to obtain a life of greater than 100,000 h at the hoop stress level of the service exposed pipes, provided no localised damage in the form of cracks or dents have been developed. It is recommended that a health check may be carried out after 50,000 h of service exposure at 5508C. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Service exposed; Boiler failures; Stress rupture test; Tensile properties; Remaining life
1. Introduction Carbon and Cr±Mo steels are extensively used as high temperature components in power plants [1±7]. Even though most of these components have a speci®c design life of 20 years, many of these have been known to have survived much longer. In view of the increasing cost of setting up a new plant, there is now considerable interest in life extension of the existing units. In order to arrive at a quantitative estimate of the remaining life of such ageing components, it is necessary to have some creep and stress rupture data. Attention has been paid [1] to the most critical point of super heater (SH) and reheater (RH) tubes of power plants. The application of the short-term accelerated creep rupture test methodology for life expectancy determination of boiler tubes required a more accurate de®nition of the * Corresponding author. Tel.: +91-657-426091, extn 6; fax: +91-657-426527. E-mail address: [email protected] (A.K. Ray). 1350-6307/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 6 3 0 7 ( 9 9 ) 0 0 0 2 4 - 2
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sensitivity and accuracy of this method. For this reason, rupture tests were carried out on service exposed 2.25 Cr±1 Mo steel of a RH tube which burst during operation in a 320 MW(e) thermal power plant. The residual life obtained by extrapolating experimental isostress line at service temperature of dierent zones of the tube was analysed and various hypotheses suggested, taking into account the dierent initial metallurgical condition of the steel and the local stress and temperature interaction during operation. It was concluded that the test methodology, applied to SH and RH tubes, appears to be sensitive enough to discriminate between only very dierent creep damage levels. Determination of consumption of the service life of the steam line pipe in conditions of creep [2] was considered taking into account not only the total time of equipment operation but also actual operational parameters (temperature, pressure, start up and shut down). The method consisted of various stages, namely: production of data base, total history of the steam line pipe operation, determination of the strength characteristics of a steam line material and determination of real consumption of service life. The latest studies on the design methodology and life estimation for major components for steam turbines were reviewed in [3]. Relationships which enable the calculation of actual creep damage were clari®ed by many studies. Several new devices were introduced, which enable the detection of local creep and fatigue damage. Studies of life estimation of welded joints in the design stages were discussed. It was shown that the remaining life of such components can be analysed from creep and stress rupture properties by de®ning a reference stress such that the component life equals the life of a simple specimen tested at the reference stress. Keeping this in view, over the years attempts have been made to generate such data on similar components [4±7]. The aim of the present work is therefore to evaluate the remaining life of >12 years service exposed main steam pipe of boilers, based on experimentally determined tensile and stress rupture properties of service exposed materials. 2. Material and history of the service exposed main steam pipes The material speci®cation with service condition and history of operation of the service exposed main steam pipe of all the boilers are given in Tables 1 and 2 respectively. 2.1. Dimensional and visual examination of the pipe Dimensions of the outer diameter (OD) measured in two mutually perpendicular directions at intervals of 150 mm along the length of the pipe are shown in Table 3. There was no evidence of localised or general corrosion/oxidation on either external or internal surfaces of the pipes.
Table 1 Material speci®cation and service condition of the service exposed main steam pipes Material Design stress: Operating temperature: Allowable stress: Service Condition Steam pressure in the main steam pipes: Steam temperature: Boiler capacity:
1 Cr±0.5 Mo±0.25 V Ð Russian graded steel (12 1 M4) 45.80 MPa 5508C 65 MPa 9025 atm 5352138C 240 tonnes of steam/h
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Table 2 History of operation Sl no
Type of main steam pipe
Service life exposed (h)
1 2 3 4 5
Boiler Boiler Boiler Boiler Boiler
100,000 100,000 140,000 164,000 163,000
A B C D E
3. Experimental Chemical analysis shows that the materials under the present investigation are basically plain carbon steels as revealed in Table 4. They also conform to the Russian grade speci®ed. Optical metallographic examinations (Figs. 1±6) were carried out on the virgin steel, service exposed main steam pipe of all boilers (A, B, C, D and E), heat aected zone (HAZ) of service exposed main steam pipe of boiler B and the weldments of service exposed pipe of boilers B, D and E respectively. Specimens containing a welded joint at the centre were cut from the service exposed main steam line pipe of boilers B, D and E which were to be examined critically by optical microscopy. The hardness values in the base and weldment (for boilers B, D and E) are shown in Table 5. Tensile tests at room temperature and elevated temperature (300±6508C) of the service exposed base metal were performed using software on a digitally controlled 8562 Instron servo-electric testing system, equipped with a 3-zone split furnace with PID control. Standard tensile specimens were made from the service exposed materials as per ASTM E8-79 speci®cation. Tensile tests were carried out on the base metal only from the longitudinal direction of the main steam pipes of the service exposed boilers. During tensile tests, constant test temperature within 228C and constant displacement of 20.2 mm/min were maintained. The variation of the yield strength (0.2% proof stress) and ultimate tensile strength (UTS) with temperature of testing is shown in later ®gures. Also illustrated is the variation of % reduction in area (RA) and % elongation (EL) with temperature of testing respectively. Tensile test results are displayed in Table 6. Although the design stress and operating temperature were 45.80 MPa and 5508C respectively it was decided that the accelerated stress rupture tests should be carried out in the range of 525±6908C and the lives were evaluated at 65 MPa, since the allowable stress speci®ed in designing the boilers is 65 MPa. Accelerated stress rupture tests using Maye's creep testing machines were carried out as per ASTM 139/ 83 speci®cation with specimens made from the cord direction of the main steam pipe of each boilers. These tests were carried out at various stress levels in the range of 65±200 MPa for the virgin pipe and Table 3 Dimensions of the virgin and service exposed main steam pipes Sl no Type of main steam pipe Outer diameter (mm) 1 2 3 4 5 6
Virgin Boiler A Boiler B Boiler C Boiler D Boiler E
273 273 272.5 273 272.8 273
273 273 272.5 273 273 273
273 274 272 273 272.8 273
Average wall thickness (mm) Length of pipe for investigation (mm) 273 273 272.5 273 272.8 273
22 23 21.5 23 19 19
430 450 500 450 450 430
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Table 4 Chemical analysis and virgin and service exposed boilers Sl no
1 2 3 4 5 6 7 8
Type of main steam pipe
Virgin Boiler A Boiler B Boiler C Boiler D Boiler E Boiler D (weldment) Boiler E (weldment)
Wt % of elements present C
Mn
Si
S
P
N
Cr
Mo
V
Ni
Cu
Sb
0.08 0.05 0.072 0.11 0.10 0.12 0.12 0.11
0.40 0.55 0.301 0.59 0.50 0.56 0.57 0.56
0.17 0.33 0.58 0.35 0.25 0.28 0.30 0.28
0.025 0.015 0.019 0.007 0.011 0.011 0.015 0.015
0.025 0.01 0.01 0.003 0.007 0.007 0.01 0.01
± 0.012 ± ± ± ± ± ±
0.90 0.83 0.83 1.01 1.11 1.11 0.57 0.57
0.35 0.26 0.26 0.32 0.25 0.22 0.33 0.315
0.20 0.27 0.22 0.28 0.23 0.22 0.21 0.22
0.25 0.19 0.19 ± ± ± ± ±
0.20 ± ± 0.20 ± ± 0.21 0.20
± ± ± 0.013 ± ± ± ±
Fig. 1. (a) Optical micrograph of the virgin pipe at 100, revealing ferrite grains dispersed with carbides. (b) Optical micrograph of the virgin pipe at 600, revealing ferrite grains dispersed with carbides. There is no evidence of graphitization and creep damage in the form of cavities and decarburization.
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Fig. 2. (a) Optical micrograph of the service exposed main steam pipe of boiler A at 300, showing ferrite grains dispersed with carbide. (b) Typical optical micrograph of the service exposed main steam pipe in the case of weldment of boiler A at 300, revealing tempered bainitic structure. There is evidence of weld defects like microporosity (indicated by arrow).
Table 5 Hardness values of the virgin and service exposed main steam pipes Sl no
Type of main steam pipe
Hardness value (VHN)
1 2 3 4 5 6 7 8 9
Virgin Boiler A Boiler B Boiler B Boiler C Boiler D Boiler D Boiler E Boiler E
153 149 139 214 146 143 209 149 219
for for for for for for for for
base metal base metal weldment base metal base metal weldment base metal weldment
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65±190 MPa for the service exposed steam pipes, to generate data with high extrapolation capability. The stress levels above the operating stress at each temperature were selected in such a way as to obtain rupture within a reasonable span of time. The hoop stress sh acting on the service exposed pipes was calculated using the following formula to predict the remaining life: sh PD=2t
1
where P is the operating pressure in MPa, D is the mean diameter in mm and t is the thickness of the vessel in mm. The service hoop stress evaluated is 066.70 MPa. The plan for generation of stress rupture data was carried out in two ways. 1. Increased temperature tests: at a constant stress of 65 MPa, acceleration was achieved by increasing the test temperature above the speci®ed service temperature which is about 5508C. The range of temperature selected for the tests for various service exposed main steam pipes varied from 525± 6908C. To generate more meaningful stress rupture data for life prediction methodology, stress levels between 65 MPa (allowable stress) and 190 MPa were selected.
Fig. 3. (a) Optical micrograph of the service exposed main steam pipe of boiler B at 300, away from the weldment (i.e. in the base metal) revealing ferrite grains dispersed with carbides. There is no evidence of graphitization, decarburization and creep damage in the form of cavities. (b) Typical optical micrograph of the service exposed main steam pipe in the case of weldment of boiler B at 300, showing the presence of tempered bainite. (c) Cross section of the weldment of service exposed steam pipe of boiler B examined at 10, when macroetched revealed presence of weld defects like porosity (indicated by arrow). (d) Optical micrograph of the service exposed main steam pipe of boiler B at 300, showing the heat aected zone (HAZ) on the left and the base metal on the right as indicated. The HAZ is found to reveal coarsening of grains with some amount of spheroidization of carbides, but is free from creep damage in the form of cavities.
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Fig. 4. (a) Optical micrograph of the service exposed main steam pipe of boiler C at 100, showing ferrite grains dispersed with carbides. (b) Optical micrograph of the service exposed main steam pipe of boiler C at 600, showing ferrite grains dispersed with carbides. There is no evidence of graphitization and creep damage in the form of cavities and decarburization. (c) Microstructure of the inner wall (controlled section) of the service exposed main steam pipe of boiler C at 100, in as received condition showing ferrite grains dispersed with carbides. There is no evidence of any appreciable decarburization. (d) Microstructure of the outer wall of the service exposed main steam pipe of boiler C at 250, in as received condition showing ferrite grains dispersed with carbides. There is no evidence of any appreciable decarburization. (e) Typical optical micrograph of the service exposed main steam pipe in the case of weldment in as received condition (taken at circular section of the pipe) of boiler C at 600, showing the presence of tempered bainite. (f) Typical optical micrograph of the service exposed main steam pipe in the HAZ of a welded portion in as received condition (taken at circular section of the pipe) of boiler C at 600, showing coarsening of grains.
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2. Increased stress test: acceleration was achieved by increasing the stress level to 66.70 MPa which is above the service stress level speci®ed. Keeping the temperature constant at 5508C, the stress levels selected for these tests were in the range 110±190 MPa. The stress rupture data under the above stipulated conditions of acceleration is given in Tables 7 and 8. The rupture data for the specimens have been plotted in terms of log (stress) vs Larson Miller parameter fLMP T 20 log trg,
2
where T is the absolute temperature in K and tr is the rupture time in hours. For the purpose of comparison, the best ®t curve using a third order polynomial for the virgin pipe has been superimposed on this plot. Regression analysis of stress rupture data for virgin as well as service exposed main steam
Fig. 5. (a) Optical micrograph of the service exposed main steam pipe of boiler D at 300, revealing ferrite grains dispersed with carbides. There is no evidence of graphitization and creep damage in the form of cavities and decarburization. (b) Typical optical micrograph of the service exposed main steam pipe in the case of weldment of boiler D at 300, showing tempered bainitic structure with discrete microporosity (indicated by arrow) and without any evidence of their coalesence.
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pipes has been carried out using a standard software package, in order to evaluate the rupture strength of the components at various rupture times and temperatures. The Larson Miller parameter (LMP) is: T C log tr ao a1 log S a2 log S 2 . . . am log Sm
3
where T is the temperature in (K), tr is the rupture time in (h), S is the rupture strength in (MPa), m is the order of polynomial, C = 20. The values of m, ao, a1, a2 and a3 are tabulated in Table 9. Table 10 shows the rupture strengths (S ) of the service exposed pipes for various rupture times and at dierent m values.
Fig. 6. (a) Optical micrograph of the service exposed main steam pipe of boiler E at 300, revealing ferrite grains dispersed with carbides. There is no evidence of graphitization and creep damage in the form of cavities and decarburization. (b) Typical optical micrograph of the service exposed main steam pipe in the case of weldment of boiler E at 300, showing tempered bainitic structure with discrete microporosity (indicated by arrow) and without any evidence of their coalesence. The % microposity present as weld defects in this case is greater than that present in Fig. 5(b).
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4. Results and discussion 4.1. Visual observation and metallography Dimensional measurement revealed that there was no change in the outer diameter and thickness of the service exposed pipes either in the base metal or in the weldment. It seems that the pipes have not Table 6 Hot tensile properties of virgin and service exposed boiler pipes Sl no
Material
Test temperature (8C)
0.2% Proof stress (MPa)
Ultimate tensile strength (MPa)
% EL
% RA
1 2 3 4 5
Virgin Virgin Virgin Virgin Virgin
RT=25 350 450 550 650
360 240 230 210 180
480 400 440 310 190
25 30 30 39 53
75 77 79 79 94
1 2 3 4 5 6 7
Boiler Boiler Boiler Boiler Boiler Boiler Boiler
A A A A A A A
RT=25 400 450 500 550 600 650
270 210 200 180 160 140 120
460 380 340 300 240 220 170
31 24 29 37 37 24 38
72 68 68 71 74 77 84
1 2 3 4 5 6
Boiler Boiler Boiler Boiler Boiler Boiler
B B B B B B
RT=25 350 450 550 600 650
260 210 210 170 150 120
470 420 370 250 210 170
27 19 27 30 28 37
77 75 72 78 85 82
1 2 3 4 5 6
Boiler Boiler Boiler Boiler Boiler Boiler
C C C C C C
RT=25 300 400 500 550 600
270 210 220 210 180 160
470 390 390 310 250 220
25 22 24 34 36 48
75 75 68 71 76 83
1 2 3 4 5 6 7
Boiler Boiler Boiler Boiler Boiler Boiler Boiler
D D D D D D D
RT=25 400 450 500 550 600 650
270 230 200 200 180 160 130
470 400 350 310 260 220 180
32 28 21 32 15 41 40
75 69 60 76 78 85 92
1 2 3 4 5 6 7
Boiler Boiler Boiler Boiler Boiler Boiler Boiler
E E E E E E E
RT=25 400 450 500 550 600 650
270 230 200 200 180 160 140
470 400 360 320 270 240 190
30 26 30 34 34 50 36
75 70 72 76 78 80 89
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undergone any appreciable deformation during actual operating conditions. Any evidence of localised and general corrosion/oxidation was not observed on either external or internal surfaces. The microstructure of the virgin material mainly consisted of ferrite grains dispersed with carbides (Fig. 1(a) and (b)). Resolved pearlite is revealed at high magni®cation (Fig. 1(b)). Evidence of graphitization and creep damage in the form of cavities was not observed (Figs. 2(a), 3(a), 4(a)±(d), 5(a) Table 7 Stress rupture properties of virgin and service exposed boiler pipes Material
Sl no
Test temperature (8C)
Stress (MPa)
Rupture time (h)
% EL
% RA
Virgin (increased stress test)
1 2 3 4 5 6
550 550 550 550 550 550
200 180 150 140 130 120
46 205 960 1440 6048 11088
32 36 36 25 Interrupted Interrupted
Virgin (increased temperature test)
1 2 3 4 5
690 670 650 630 610
65 65 65 65 65
77 218 672 1673 5032
13 17 15 10 10
71 85 80 18 210
Boiler A (increased stress test)
1 2 3 4
525 525 525 525
180 170 160 140
106 116 168 912
55 54 51 60
79 79 81 80
1 2 3 4 5
550 550 550 550 550
160 150 140 130 110
60 130 130 504 3504
57 52 66 57 56
82 77 81 81 80
1 2 3 4
600 600 600 600
120 110 100 90
36 240 408 1819
62 57 54 22
82 81 81 91
Boiler A (increased temperature test)
1 2 3
670 650 630
65 70 75
336 588 2816
43 41 42
92 90 79
Boiler B (increased stress test)
1 2 3 4
550 550 550 550
150 140 130 120
48 146 1200 3312
44 44 64 50
79 75 64 47
1 2 3 4
600 600 600 600
120 110 100 90
72 120 528 4872
62 58 58 24
63 72 67 21
1
650
70
522
21
53
56 54 56 71 ± ±
370
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Table 8 Stress rupture properties of service exposed boiler pipes Material
Sl no
Test temperature (8C)
Stress (MPa)
Rupture time (h)
% EL
% RA
Boiler C (increased stress test)
1 2 3 4 5 6
550 550 550 550 550 550
190 170 150 140 130 120
48 330 504 576 1296 4969
38 29 38 30 22 Interrupted
73 67 79 82 74 ±
Boiler C (increased stress test)
7 8 9
550 550 550
110 100 90
3692 3700 3694
Interrupted Interrupted Interrupted
± ± ±
Boiler C (increased temperature test)
1 2 3 4 5
690 670 650 630 610
65 65 65 65 65
36 332 576 2255 5282
18 36 23 7 Interrupted
51 54 75 22 ±
Boiler D (increased stress test)
1 2 3 4
525 525 525 525
180 170 160 140
170 441 432 2688
53 47 54 53
80 79 80 72
1 2 3 4 5
550 550 550 550 550
160 150 140 130 110
70 242 730 1739 5352
53 52 51 54 24
77 77 75 77 47
1 2 3 4 5
600 600 600 600 600
120 110 100 90 80
121 912 1656 2544 5352
58 45 17 27 16
80 67 53 42 50
Boiler D (increased temperature test)
1 2 3
670 650 630
65 70 75
466 610 2696
43 38 31
51 44 51
Boiler E (increased stress test)
1 2 3 4
525 525 525 525
180 170 160 140
178 533 264 1512
54 55 53 55
82 82 77 82
1 2 3 4 5
550 550 550 550 550
160 150 140 130 110
103 270 462 4104 6216
56 48 38 35 45
80 77 83 65 76
1 2
600 600
120 110
216 556
51 27
71 77
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Table 8 (continued ) Material
Boiler E (increased temperature test)
Sl no
Test temperature (8C)
Stress (MPa)
Rupture time (h)
% EL
% RA
3 4 5
600 600 600
100 90 80
1288 3905 6672
27 42 Interrupted
67 71 ±
1 2 3
670 650 630
65 70 75
269 740 2940
49 52 44
88 88 84
and 6(a)) for all the service exposed main steam pipe of boilers A, B, C, D and E. The base metal of the service exposed main steam pipe of boiler A essentially consisted of carbides dispersed in a ferrite matrix (Fig. 2(a)). Typical microstructures of the weldment of the service exposed main steam pipe of boiler A (shown in Fig. 2(b)) consisted of tempered bainite. There was no evidence of creep damage or graphitization. The microstructure of the service exposed main steam pipe of boilers B, C, D and E also revealed carbides dispersed in a ferrite matrix (Figs. 3(a), 4(a)±(d), 5(a) and 6(a)). Creep cavities were not found. The weldment of service exposed main steam line pipe (for boilers B, C, D and E) showed a bainitic structure and presence of discrete microporosity (Figs. 3(c), 4(e), 5(b) and 6(b)) without any evidence of their coalescence. The HAZ (Fig. 3(d)) of service exposed pipe of boiler B revealed coarsening of grains with some amount of spheroidization of carbides, but free from creep cavitation.
Table 9 Polynomial constants from regression analysis Type of main steam pipe
Order of polynomial
Average sum square error
ao
a1
a2
a3
Virgin
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
0.46979 10ÿ1 0.891476 10ÿ2 0.1632459 100
0.2591062 105 0.1823921 105 0.3034407 106
0.5954646 104 0.9704682 104 0.8160072 106
± 0.7724902 104 0.7732164 106
± ± 0.2420369 106
Boiler A
m = 1, C = 20 m = 2, C = 20
0.3135 10ÿ1 0.3126 10ÿ1
30650.006 29892.383
ÿ9594.574 8408.811
± ±
± ±
Boiler B
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
0.7479 10ÿ1 0.3747 10ÿ1 0.3713 10ÿ1
28695.60828 14101.15669 308.6995885
ÿ8908.30182 20773.42164 62866.42592
± ÿ14888.1921 ÿ57362.7563
± ± 14176.40756
Boiler C
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
0.25497 10ÿ1 0.254967 10ÿ1 0.2018821 10ÿ1
0.2636881 105 0.263797 105 0.8908564 105
0.6567206 104 0.6589497 104 0.1883069 106
± 0.1099281 102 0.1721597 106
± ± 0.5351054 106
Boiler D
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
0.55 10ÿ1 0.44 10ÿ1 0.53 10ÿ1
27055.96 18297.96
ÿ8093.4 5115.41
± ÿ6108.46
± ÿ
Boiler E
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
0.60 10ÿ1 0.42 10ÿ1 0.62 10ÿ1
28057.23 16850.36
ÿ8286.96 8439.19
± ÿ7711.40
± ±
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Coarsening of grains with no creep cavitation is also revealed in the HAZ of service exposed main steam pipe of boiler C, taken at a circular section of the pipe (see Fig. 4(f)). The weldment of main steam pipes of boilers A, B, C, D and E revealed a bainitic structure with no evidence of creep cavitation or graphitization. The micropores in the weldment of boiler E (see Fig. 6(b) were more extensive compared to those in the weldment of boiler D (see Fig. 5(b)), but these pores did not show any tendency to coalesce during operation and service exposure. Hence, it is noteworthy that the virgin as well the service exposed main steam pipes of all the boilers have had hardly any appreciable degradation from a microstructural point of view as there was no evidence of creep damage, graphitization, cavitation or decarburization. 4.2. Mechanical properties Room temperature as well as high temperature tensile properties as obtained from experiments are reported in Table 6 and Fig. 7. It is evident from the results that 0.2% proof stress (yield strength) and the ultimate tensile strength (UTS) values for the service exposed pipes showed a decreasing trend with increasing temperature. However, % reduction in area (RA) and % elongation (EL) showed an increasing trend with temperature. Analysis of tensile data revealed that there is some deterioration in Table 10 Estimated rupture strength Type of main steam pipe
Order of polynomial
Virgin
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
Boiler A
Temperature (8C)
Rupture strength (MPa) tr = 3000 h
tr = 10,000 h
tr = 30,000 h
tr = 100,000 h
550 550 ±
127.80 135.70 ±
108.30 118.60 ±
93.00 103.00 ±
78.70 85.50 ±
m = 1, C = 20 and for m = 2, C = 20
500 525 550 575
± ± ± ±
136.60 116.90 100.70 86.30
125.00 107.10 91.60 78.20
113.50 96.80 82.40 70.15
Boiler B
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
500 525 550 575
± ± ± ±
135.20 120.70 105.90 90.40
126.30 111.40 95.90 78.80
116.50 100.90 84.20 62.70
Boiler C
m = 1, C = 20 and for m = 2, C = 20 and also for m = 3, C = 20
550 550 550
118.30 118.30 105.60
101.80 101.70 87.10
88.70 88.60 77.50
76.20 76.20 70.40
Boiler D
m = 1, C = 20 and for m = 2, C = 20 and also for m = 3, C = 20
500 525 550 575
± ± ± ±
141.90 123.20 105.60 88.70
128.40 110.20 92.80 76.20
114.50 96.50 78.30 62.40
Boiler E
m = 1, C = 20 and for m = 2, C = 20 and also for m = 3, C = 20
500 525 550 575
± ± ± ±
142.90 124.50 106.80 89.30
130.00 111.70 93.80 75.70
116.20 99.90 79.40 59.80
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yield stress (0.2 proof stress), UTS (see Figs. 7(a) and (b)), % RA and % EL (see Figs. 7(c) and (d)) of the service exposed main steam pipe of all the boilers compared to those of the virgin pipe. In the absence of discernible cavitation or ¯aws, stress rupture tests can be selectively used to assess the condition of components. One of the most widely used techniques for life assessment of components involves the removal of samples and conducting accelerated tests at temperatures above the service temperature [8]. An estimate of the remaining life is then made by extrapolation of the results to the service temperature. Several uncertainties relating to the validity and application of the technique have been resolved in recent research projects [8]. In the present investigation, long term rupture strengths were estimated with best ®tted curves up to third order polynomial as there was no signi®cant change in average sum square error, beyond second order polynomial. Analysis of rupture data clearly indicates that the properties of the present steel are closely comparable to those of the virgin pipe (Figs. 8 and 9) suggesting no appreciable creep damage. The data points of the Stress vs LMP plots (Figs. 8 and 9) for all the service exposed materials fall
Fig. 7. (a) Plot of yield strength (0.2% proof stress) vs temperature for virgin and service exposed boiler pipes. (b) Plot of ultimate tensile strength UTS vs temperature for virgin and service exposed boiler pipes. (c) Plot showing variation of % reduction in area (RA) with temperature for virgin and service exposed boiler pipes. (d) Plot showing variation of % elongation (EL) with temperature for virgin and service exposed boiler pipes.
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Fig. 8. Plot of stress vs Larson Miller parameter (LMP) for boilers A, B, C and virgin pipe.
Fig. 9. Plot of stress vs larson miller parameter (LMP) for boilers D, E and virgin pipe.
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375
within the 20% scatter band of the virgin material. The best ®t curve for the virgin material has been extrapolated to a lower stress value (65 MPa) which is the allowable stress level and slightly below the operating hoop stress (66.70 MPa) of the service exposed main steam pipes of all boilers, for the purpose of life prediction (Figs. 8 and 9). Previous investigators [9,10] have also estimated the remaining life of such ageing components by comparing the upper limit of the data scatter band in stress rupture tests with the ASTM and BSS mean data lines. The remaining lives of all the service exposed main steam pipes of all the boilers predicted at 65 MPa and at 5508C are shown in Figs. 8 and 9. At the operating hoop stress of 66.70 MPa for the service exposed pipes, the LMP value as read from the graph (Figs. 8 and 9) is about 20,850. At this value of LMP, one would expect a very long life. This is mainly because the creep phenomenon is operative only at a temperature above 4508C. As is customary, an inspection life of >100,000 h is recommended. So far as the remaining life at 5508C is concerned, it is possible to obtain a minimum life of >100,000 h for the service exposed main steam pipes provided there is no evidence of localised damage in the form of surface cracks, cavitation or dents. Another health check of the service exposed pipes in terms of residual life is recommended to be carried out after expiry of 50,000 h of service life from the view of economical and safety reasons. Also during shut down of the plant, NDT (nondestructive) tests viz. dimensional (thickness and diameter) measurement, hardness measurement and insitu metallography may be carried out to assess the condition of the materials for their future serviceability. NDT examination of the weld joints at regular intervals during operation is desirable as the microstructural examination of the weldment in some cases revealed the presence of defects like microporosities.
5. Conclusions It is concluded that so far as the residual life at 5508C is concerned, it is possible to obtain a minimum life of about 100,000 h for the service exposed main steam pipes provided there is no evidence of localised damage in the form of surface cracks, cavitation or dents. The service exposed main steam pipes of all the boilers appear to be in a reasonably good state of health. Another health check of the service exposed pipes in terms of residual life is recommended to be carried out after expiry of 50,000 h of service life from the view of economical and safety reasons. Also during shut down of the plant, NDT (nondestructive) tests viz. dimensional (thickness and diameter) measurement, hardness measurement and insitu metallography may be carried out to assess the condition of the materials for their future serviceability. NDT examination of the weld joints at regular intervals during operation is desirable as the microstructural examination of the weldment in some cases revealed the presence of defects like microporosities.
Acknowledgements The authors are grateful to Prof. P. Ramachandra Rao- Director, National Metallurgical Laboratory, Jamshedpur, India for his kind permission to publish this paper.
References [1] D'Angelo D, Percivate A. Proceedings of International Conference on Creep. Tokyo, Japan, 14±18 April, 1986. [2] Klevtsov IA, Tallermo KhA. Teploenergetika 1996;12:21. [3] Endo T. International Jr Pressure Vessel and Piping 1994;57(1):7.
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[4] de Witte M. International Jr Pressure Vessel and Piping 1996;39(12):41. [5] Li YD. International Jr Pressure Vessel and Piping 1996;69(2):161. [6] Klevtsov I, Tallermo H, Crane RA. Proceedings of International Conference of Plant Conditon & Life Measurement, vol I, Helsinki, Finland (6±8 June 1995). Pub-Technical Research Centre of Finland, 1995. [7] Wilshire B. International Jr Pressure Vessel and Piping 1989;39(12):73. [8] Viswanathan R, Dooley R, Saxena A. Proceedings of International Conference on Life Assessment and Extension, vol II, Congress Centre, The Hague/The Netherlands (13±15 June 1988), 1988. p. 175. [9] Singh AK, Bagdasarian AJ, Malhotra VK. Houston, TX: NACE International, 1994. p. 21. [10] Goodwall TW, Ainsworth RA. CEGB Res 1984;16:34.
www.elsevier.com/locate/engfailanal
Residual life prediction of service exposed main steam pipe of boilers in a thermal power plant A.K. Ray*, Y.N. Tiwari, R.K. Sinha, S. Chaudhuri, R. Singh National Metallurgical Laboratory, Jamshedpur 831007, India Received 25 May 1999; accepted 3 July 1999
Abstract This paper deals with residual life prediction methodology for more than 12.5 years service exposed main steam pipes of various boilers in a thermal power plant. Health assessment was made using destructive accelerated stress rupture and tensile tests at dierent temperatures, and some nondestructive tests. There was no evidence of localised damage in the form of surface cracks, cavitation or dents in the service exposed main steam pipes of all the boilers. So far as the remaining life at 5508C is concerned, it is possible to obtain a life of greater than 100,000 h at the hoop stress level of the service exposed pipes, provided no localised damage in the form of cracks or dents have been developed. It is recommended that a health check may be carried out after 50,000 h of service exposure at 5508C. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Service exposed; Boiler failures; Stress rupture test; Tensile properties; Remaining life
1. Introduction Carbon and Cr±Mo steels are extensively used as high temperature components in power plants [1±7]. Even though most of these components have a speci®c design life of 20 years, many of these have been known to have survived much longer. In view of the increasing cost of setting up a new plant, there is now considerable interest in life extension of the existing units. In order to arrive at a quantitative estimate of the remaining life of such ageing components, it is necessary to have some creep and stress rupture data. Attention has been paid [1] to the most critical point of super heater (SH) and reheater (RH) tubes of power plants. The application of the short-term accelerated creep rupture test methodology for life expectancy determination of boiler tubes required a more accurate de®nition of the * Corresponding author. Tel.: +91-657-426091, extn 6; fax: +91-657-426527. E-mail address: [email protected] (A.K. Ray). 1350-6307/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 6 3 0 7 ( 9 9 ) 0 0 0 2 4 - 2
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sensitivity and accuracy of this method. For this reason, rupture tests were carried out on service exposed 2.25 Cr±1 Mo steel of a RH tube which burst during operation in a 320 MW(e) thermal power plant. The residual life obtained by extrapolating experimental isostress line at service temperature of dierent zones of the tube was analysed and various hypotheses suggested, taking into account the dierent initial metallurgical condition of the steel and the local stress and temperature interaction during operation. It was concluded that the test methodology, applied to SH and RH tubes, appears to be sensitive enough to discriminate between only very dierent creep damage levels. Determination of consumption of the service life of the steam line pipe in conditions of creep [2] was considered taking into account not only the total time of equipment operation but also actual operational parameters (temperature, pressure, start up and shut down). The method consisted of various stages, namely: production of data base, total history of the steam line pipe operation, determination of the strength characteristics of a steam line material and determination of real consumption of service life. The latest studies on the design methodology and life estimation for major components for steam turbines were reviewed in [3]. Relationships which enable the calculation of actual creep damage were clari®ed by many studies. Several new devices were introduced, which enable the detection of local creep and fatigue damage. Studies of life estimation of welded joints in the design stages were discussed. It was shown that the remaining life of such components can be analysed from creep and stress rupture properties by de®ning a reference stress such that the component life equals the life of a simple specimen tested at the reference stress. Keeping this in view, over the years attempts have been made to generate such data on similar components [4±7]. The aim of the present work is therefore to evaluate the remaining life of >12 years service exposed main steam pipe of boilers, based on experimentally determined tensile and stress rupture properties of service exposed materials. 2. Material and history of the service exposed main steam pipes The material speci®cation with service condition and history of operation of the service exposed main steam pipe of all the boilers are given in Tables 1 and 2 respectively. 2.1. Dimensional and visual examination of the pipe Dimensions of the outer diameter (OD) measured in two mutually perpendicular directions at intervals of 150 mm along the length of the pipe are shown in Table 3. There was no evidence of localised or general corrosion/oxidation on either external or internal surfaces of the pipes.
Table 1 Material speci®cation and service condition of the service exposed main steam pipes Material Design stress: Operating temperature: Allowable stress: Service Condition Steam pressure in the main steam pipes: Steam temperature: Boiler capacity:
1 Cr±0.5 Mo±0.25 V Ð Russian graded steel (12 1 M4) 45.80 MPa 5508C 65 MPa 9025 atm 5352138C 240 tonnes of steam/h
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Table 2 History of operation Sl no
Type of main steam pipe
Service life exposed (h)
1 2 3 4 5
Boiler Boiler Boiler Boiler Boiler
100,000 100,000 140,000 164,000 163,000
A B C D E
3. Experimental Chemical analysis shows that the materials under the present investigation are basically plain carbon steels as revealed in Table 4. They also conform to the Russian grade speci®ed. Optical metallographic examinations (Figs. 1±6) were carried out on the virgin steel, service exposed main steam pipe of all boilers (A, B, C, D and E), heat aected zone (HAZ) of service exposed main steam pipe of boiler B and the weldments of service exposed pipe of boilers B, D and E respectively. Specimens containing a welded joint at the centre were cut from the service exposed main steam line pipe of boilers B, D and E which were to be examined critically by optical microscopy. The hardness values in the base and weldment (for boilers B, D and E) are shown in Table 5. Tensile tests at room temperature and elevated temperature (300±6508C) of the service exposed base metal were performed using software on a digitally controlled 8562 Instron servo-electric testing system, equipped with a 3-zone split furnace with PID control. Standard tensile specimens were made from the service exposed materials as per ASTM E8-79 speci®cation. Tensile tests were carried out on the base metal only from the longitudinal direction of the main steam pipes of the service exposed boilers. During tensile tests, constant test temperature within 228C and constant displacement of 20.2 mm/min were maintained. The variation of the yield strength (0.2% proof stress) and ultimate tensile strength (UTS) with temperature of testing is shown in later ®gures. Also illustrated is the variation of % reduction in area (RA) and % elongation (EL) with temperature of testing respectively. Tensile test results are displayed in Table 6. Although the design stress and operating temperature were 45.80 MPa and 5508C respectively it was decided that the accelerated stress rupture tests should be carried out in the range of 525±6908C and the lives were evaluated at 65 MPa, since the allowable stress speci®ed in designing the boilers is 65 MPa. Accelerated stress rupture tests using Maye's creep testing machines were carried out as per ASTM 139/ 83 speci®cation with specimens made from the cord direction of the main steam pipe of each boilers. These tests were carried out at various stress levels in the range of 65±200 MPa for the virgin pipe and Table 3 Dimensions of the virgin and service exposed main steam pipes Sl no Type of main steam pipe Outer diameter (mm) 1 2 3 4 5 6
Virgin Boiler A Boiler B Boiler C Boiler D Boiler E
273 273 272.5 273 272.8 273
273 273 272.5 273 273 273
273 274 272 273 272.8 273
Average wall thickness (mm) Length of pipe for investigation (mm) 273 273 272.5 273 272.8 273
22 23 21.5 23 19 19
430 450 500 450 450 430
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Table 4 Chemical analysis and virgin and service exposed boilers Sl no
1 2 3 4 5 6 7 8
Type of main steam pipe
Virgin Boiler A Boiler B Boiler C Boiler D Boiler E Boiler D (weldment) Boiler E (weldment)
Wt % of elements present C
Mn
Si
S
P
N
Cr
Mo
V
Ni
Cu
Sb
0.08 0.05 0.072 0.11 0.10 0.12 0.12 0.11
0.40 0.55 0.301 0.59 0.50 0.56 0.57 0.56
0.17 0.33 0.58 0.35 0.25 0.28 0.30 0.28
0.025 0.015 0.019 0.007 0.011 0.011 0.015 0.015
0.025 0.01 0.01 0.003 0.007 0.007 0.01 0.01
± 0.012 ± ± ± ± ± ±
0.90 0.83 0.83 1.01 1.11 1.11 0.57 0.57
0.35 0.26 0.26 0.32 0.25 0.22 0.33 0.315
0.20 0.27 0.22 0.28 0.23 0.22 0.21 0.22
0.25 0.19 0.19 ± ± ± ± ±
0.20 ± ± 0.20 ± ± 0.21 0.20
± ± ± 0.013 ± ± ± ±
Fig. 1. (a) Optical micrograph of the virgin pipe at 100, revealing ferrite grains dispersed with carbides. (b) Optical micrograph of the virgin pipe at 600, revealing ferrite grains dispersed with carbides. There is no evidence of graphitization and creep damage in the form of cavities and decarburization.
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Fig. 2. (a) Optical micrograph of the service exposed main steam pipe of boiler A at 300, showing ferrite grains dispersed with carbide. (b) Typical optical micrograph of the service exposed main steam pipe in the case of weldment of boiler A at 300, revealing tempered bainitic structure. There is evidence of weld defects like microporosity (indicated by arrow).
Table 5 Hardness values of the virgin and service exposed main steam pipes Sl no
Type of main steam pipe
Hardness value (VHN)
1 2 3 4 5 6 7 8 9
Virgin Boiler A Boiler B Boiler B Boiler C Boiler D Boiler D Boiler E Boiler E
153 149 139 214 146 143 209 149 219
for for for for for for for for
base metal base metal weldment base metal base metal weldment base metal weldment
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65±190 MPa for the service exposed steam pipes, to generate data with high extrapolation capability. The stress levels above the operating stress at each temperature were selected in such a way as to obtain rupture within a reasonable span of time. The hoop stress sh acting on the service exposed pipes was calculated using the following formula to predict the remaining life: sh PD=2t
1
where P is the operating pressure in MPa, D is the mean diameter in mm and t is the thickness of the vessel in mm. The service hoop stress evaluated is 066.70 MPa. The plan for generation of stress rupture data was carried out in two ways. 1. Increased temperature tests: at a constant stress of 65 MPa, acceleration was achieved by increasing the test temperature above the speci®ed service temperature which is about 5508C. The range of temperature selected for the tests for various service exposed main steam pipes varied from 525± 6908C. To generate more meaningful stress rupture data for life prediction methodology, stress levels between 65 MPa (allowable stress) and 190 MPa were selected.
Fig. 3. (a) Optical micrograph of the service exposed main steam pipe of boiler B at 300, away from the weldment (i.e. in the base metal) revealing ferrite grains dispersed with carbides. There is no evidence of graphitization, decarburization and creep damage in the form of cavities. (b) Typical optical micrograph of the service exposed main steam pipe in the case of weldment of boiler B at 300, showing the presence of tempered bainite. (c) Cross section of the weldment of service exposed steam pipe of boiler B examined at 10, when macroetched revealed presence of weld defects like porosity (indicated by arrow). (d) Optical micrograph of the service exposed main steam pipe of boiler B at 300, showing the heat aected zone (HAZ) on the left and the base metal on the right as indicated. The HAZ is found to reveal coarsening of grains with some amount of spheroidization of carbides, but is free from creep damage in the form of cavities.
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Fig. 4. (a) Optical micrograph of the service exposed main steam pipe of boiler C at 100, showing ferrite grains dispersed with carbides. (b) Optical micrograph of the service exposed main steam pipe of boiler C at 600, showing ferrite grains dispersed with carbides. There is no evidence of graphitization and creep damage in the form of cavities and decarburization. (c) Microstructure of the inner wall (controlled section) of the service exposed main steam pipe of boiler C at 100, in as received condition showing ferrite grains dispersed with carbides. There is no evidence of any appreciable decarburization. (d) Microstructure of the outer wall of the service exposed main steam pipe of boiler C at 250, in as received condition showing ferrite grains dispersed with carbides. There is no evidence of any appreciable decarburization. (e) Typical optical micrograph of the service exposed main steam pipe in the case of weldment in as received condition (taken at circular section of the pipe) of boiler C at 600, showing the presence of tempered bainite. (f) Typical optical micrograph of the service exposed main steam pipe in the HAZ of a welded portion in as received condition (taken at circular section of the pipe) of boiler C at 600, showing coarsening of grains.
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2. Increased stress test: acceleration was achieved by increasing the stress level to 66.70 MPa which is above the service stress level speci®ed. Keeping the temperature constant at 5508C, the stress levels selected for these tests were in the range 110±190 MPa. The stress rupture data under the above stipulated conditions of acceleration is given in Tables 7 and 8. The rupture data for the specimens have been plotted in terms of log (stress) vs Larson Miller parameter fLMP T 20 log trg,
2
where T is the absolute temperature in K and tr is the rupture time in hours. For the purpose of comparison, the best ®t curve using a third order polynomial for the virgin pipe has been superimposed on this plot. Regression analysis of stress rupture data for virgin as well as service exposed main steam
Fig. 5. (a) Optical micrograph of the service exposed main steam pipe of boiler D at 300, revealing ferrite grains dispersed with carbides. There is no evidence of graphitization and creep damage in the form of cavities and decarburization. (b) Typical optical micrograph of the service exposed main steam pipe in the case of weldment of boiler D at 300, showing tempered bainitic structure with discrete microporosity (indicated by arrow) and without any evidence of their coalesence.
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367
pipes has been carried out using a standard software package, in order to evaluate the rupture strength of the components at various rupture times and temperatures. The Larson Miller parameter (LMP) is: T C log tr ao a1 log S a2 log S 2 . . . am log Sm
3
where T is the temperature in (K), tr is the rupture time in (h), S is the rupture strength in (MPa), m is the order of polynomial, C = 20. The values of m, ao, a1, a2 and a3 are tabulated in Table 9. Table 10 shows the rupture strengths (S ) of the service exposed pipes for various rupture times and at dierent m values.
Fig. 6. (a) Optical micrograph of the service exposed main steam pipe of boiler E at 300, revealing ferrite grains dispersed with carbides. There is no evidence of graphitization and creep damage in the form of cavities and decarburization. (b) Typical optical micrograph of the service exposed main steam pipe in the case of weldment of boiler E at 300, showing tempered bainitic structure with discrete microporosity (indicated by arrow) and without any evidence of their coalesence. The % microposity present as weld defects in this case is greater than that present in Fig. 5(b).
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4. Results and discussion 4.1. Visual observation and metallography Dimensional measurement revealed that there was no change in the outer diameter and thickness of the service exposed pipes either in the base metal or in the weldment. It seems that the pipes have not Table 6 Hot tensile properties of virgin and service exposed boiler pipes Sl no
Material
Test temperature (8C)
0.2% Proof stress (MPa)
Ultimate tensile strength (MPa)
% EL
% RA
1 2 3 4 5
Virgin Virgin Virgin Virgin Virgin
RT=25 350 450 550 650
360 240 230 210 180
480 400 440 310 190
25 30 30 39 53
75 77 79 79 94
1 2 3 4 5 6 7
Boiler Boiler Boiler Boiler Boiler Boiler Boiler
A A A A A A A
RT=25 400 450 500 550 600 650
270 210 200 180 160 140 120
460 380 340 300 240 220 170
31 24 29 37 37 24 38
72 68 68 71 74 77 84
1 2 3 4 5 6
Boiler Boiler Boiler Boiler Boiler Boiler
B B B B B B
RT=25 350 450 550 600 650
260 210 210 170 150 120
470 420 370 250 210 170
27 19 27 30 28 37
77 75 72 78 85 82
1 2 3 4 5 6
Boiler Boiler Boiler Boiler Boiler Boiler
C C C C C C
RT=25 300 400 500 550 600
270 210 220 210 180 160
470 390 390 310 250 220
25 22 24 34 36 48
75 75 68 71 76 83
1 2 3 4 5 6 7
Boiler Boiler Boiler Boiler Boiler Boiler Boiler
D D D D D D D
RT=25 400 450 500 550 600 650
270 230 200 200 180 160 130
470 400 350 310 260 220 180
32 28 21 32 15 41 40
75 69 60 76 78 85 92
1 2 3 4 5 6 7
Boiler Boiler Boiler Boiler Boiler Boiler Boiler
E E E E E E E
RT=25 400 450 500 550 600 650
270 230 200 200 180 160 140
470 400 360 320 270 240 190
30 26 30 34 34 50 36
75 70 72 76 78 80 89
A.K. Ray et al. / Engineering Failure Analysis 7 (2000) 359±376
369
undergone any appreciable deformation during actual operating conditions. Any evidence of localised and general corrosion/oxidation was not observed on either external or internal surfaces. The microstructure of the virgin material mainly consisted of ferrite grains dispersed with carbides (Fig. 1(a) and (b)). Resolved pearlite is revealed at high magni®cation (Fig. 1(b)). Evidence of graphitization and creep damage in the form of cavities was not observed (Figs. 2(a), 3(a), 4(a)±(d), 5(a) Table 7 Stress rupture properties of virgin and service exposed boiler pipes Material
Sl no
Test temperature (8C)
Stress (MPa)
Rupture time (h)
% EL
% RA
Virgin (increased stress test)
1 2 3 4 5 6
550 550 550 550 550 550
200 180 150 140 130 120
46 205 960 1440 6048 11088
32 36 36 25 Interrupted Interrupted
Virgin (increased temperature test)
1 2 3 4 5
690 670 650 630 610
65 65 65 65 65
77 218 672 1673 5032
13 17 15 10 10
71 85 80 18 210
Boiler A (increased stress test)
1 2 3 4
525 525 525 525
180 170 160 140
106 116 168 912
55 54 51 60
79 79 81 80
1 2 3 4 5
550 550 550 550 550
160 150 140 130 110
60 130 130 504 3504
57 52 66 57 56
82 77 81 81 80
1 2 3 4
600 600 600 600
120 110 100 90
36 240 408 1819
62 57 54 22
82 81 81 91
Boiler A (increased temperature test)
1 2 3
670 650 630
65 70 75
336 588 2816
43 41 42
92 90 79
Boiler B (increased stress test)
1 2 3 4
550 550 550 550
150 140 130 120
48 146 1200 3312
44 44 64 50
79 75 64 47
1 2 3 4
600 600 600 600
120 110 100 90
72 120 528 4872
62 58 58 24
63 72 67 21
1
650
70
522
21
53
56 54 56 71 ± ±
370
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Table 8 Stress rupture properties of service exposed boiler pipes Material
Sl no
Test temperature (8C)
Stress (MPa)
Rupture time (h)
% EL
% RA
Boiler C (increased stress test)
1 2 3 4 5 6
550 550 550 550 550 550
190 170 150 140 130 120
48 330 504 576 1296 4969
38 29 38 30 22 Interrupted
73 67 79 82 74 ±
Boiler C (increased stress test)
7 8 9
550 550 550
110 100 90
3692 3700 3694
Interrupted Interrupted Interrupted
± ± ±
Boiler C (increased temperature test)
1 2 3 4 5
690 670 650 630 610
65 65 65 65 65
36 332 576 2255 5282
18 36 23 7 Interrupted
51 54 75 22 ±
Boiler D (increased stress test)
1 2 3 4
525 525 525 525
180 170 160 140
170 441 432 2688
53 47 54 53
80 79 80 72
1 2 3 4 5
550 550 550 550 550
160 150 140 130 110
70 242 730 1739 5352
53 52 51 54 24
77 77 75 77 47
1 2 3 4 5
600 600 600 600 600
120 110 100 90 80
121 912 1656 2544 5352
58 45 17 27 16
80 67 53 42 50
Boiler D (increased temperature test)
1 2 3
670 650 630
65 70 75
466 610 2696
43 38 31
51 44 51
Boiler E (increased stress test)
1 2 3 4
525 525 525 525
180 170 160 140
178 533 264 1512
54 55 53 55
82 82 77 82
1 2 3 4 5
550 550 550 550 550
160 150 140 130 110
103 270 462 4104 6216
56 48 38 35 45
80 77 83 65 76
1 2
600 600
120 110
216 556
51 27
71 77
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371
Table 8 (continued ) Material
Boiler E (increased temperature test)
Sl no
Test temperature (8C)
Stress (MPa)
Rupture time (h)
% EL
% RA
3 4 5
600 600 600
100 90 80
1288 3905 6672
27 42 Interrupted
67 71 ±
1 2 3
670 650 630
65 70 75
269 740 2940
49 52 44
88 88 84
and 6(a)) for all the service exposed main steam pipe of boilers A, B, C, D and E. The base metal of the service exposed main steam pipe of boiler A essentially consisted of carbides dispersed in a ferrite matrix (Fig. 2(a)). Typical microstructures of the weldment of the service exposed main steam pipe of boiler A (shown in Fig. 2(b)) consisted of tempered bainite. There was no evidence of creep damage or graphitization. The microstructure of the service exposed main steam pipe of boilers B, C, D and E also revealed carbides dispersed in a ferrite matrix (Figs. 3(a), 4(a)±(d), 5(a) and 6(a)). Creep cavities were not found. The weldment of service exposed main steam line pipe (for boilers B, C, D and E) showed a bainitic structure and presence of discrete microporosity (Figs. 3(c), 4(e), 5(b) and 6(b)) without any evidence of their coalescence. The HAZ (Fig. 3(d)) of service exposed pipe of boiler B revealed coarsening of grains with some amount of spheroidization of carbides, but free from creep cavitation.
Table 9 Polynomial constants from regression analysis Type of main steam pipe
Order of polynomial
Average sum square error
ao
a1
a2
a3
Virgin
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
0.46979 10ÿ1 0.891476 10ÿ2 0.1632459 100
0.2591062 105 0.1823921 105 0.3034407 106
0.5954646 104 0.9704682 104 0.8160072 106
± 0.7724902 104 0.7732164 106
± ± 0.2420369 106
Boiler A
m = 1, C = 20 m = 2, C = 20
0.3135 10ÿ1 0.3126 10ÿ1
30650.006 29892.383
ÿ9594.574 8408.811
± ±
± ±
Boiler B
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
0.7479 10ÿ1 0.3747 10ÿ1 0.3713 10ÿ1
28695.60828 14101.15669 308.6995885
ÿ8908.30182 20773.42164 62866.42592
± ÿ14888.1921 ÿ57362.7563
± ± 14176.40756
Boiler C
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
0.25497 10ÿ1 0.254967 10ÿ1 0.2018821 10ÿ1
0.2636881 105 0.263797 105 0.8908564 105
0.6567206 104 0.6589497 104 0.1883069 106
± 0.1099281 102 0.1721597 106
± ± 0.5351054 106
Boiler D
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
0.55 10ÿ1 0.44 10ÿ1 0.53 10ÿ1
27055.96 18297.96
ÿ8093.4 5115.41
± ÿ6108.46
± ÿ
Boiler E
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
0.60 10ÿ1 0.42 10ÿ1 0.62 10ÿ1
28057.23 16850.36
ÿ8286.96 8439.19
± ÿ7711.40
± ±
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Coarsening of grains with no creep cavitation is also revealed in the HAZ of service exposed main steam pipe of boiler C, taken at a circular section of the pipe (see Fig. 4(f)). The weldment of main steam pipes of boilers A, B, C, D and E revealed a bainitic structure with no evidence of creep cavitation or graphitization. The micropores in the weldment of boiler E (see Fig. 6(b) were more extensive compared to those in the weldment of boiler D (see Fig. 5(b)), but these pores did not show any tendency to coalesce during operation and service exposure. Hence, it is noteworthy that the virgin as well the service exposed main steam pipes of all the boilers have had hardly any appreciable degradation from a microstructural point of view as there was no evidence of creep damage, graphitization, cavitation or decarburization. 4.2. Mechanical properties Room temperature as well as high temperature tensile properties as obtained from experiments are reported in Table 6 and Fig. 7. It is evident from the results that 0.2% proof stress (yield strength) and the ultimate tensile strength (UTS) values for the service exposed pipes showed a decreasing trend with increasing temperature. However, % reduction in area (RA) and % elongation (EL) showed an increasing trend with temperature. Analysis of tensile data revealed that there is some deterioration in Table 10 Estimated rupture strength Type of main steam pipe
Order of polynomial
Virgin
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
Boiler A
Temperature (8C)
Rupture strength (MPa) tr = 3000 h
tr = 10,000 h
tr = 30,000 h
tr = 100,000 h
550 550 ±
127.80 135.70 ±
108.30 118.60 ±
93.00 103.00 ±
78.70 85.50 ±
m = 1, C = 20 and for m = 2, C = 20
500 525 550 575
± ± ± ±
136.60 116.90 100.70 86.30
125.00 107.10 91.60 78.20
113.50 96.80 82.40 70.15
Boiler B
m = 1, C = 20 m = 2, C = 20 m = 3, C = 20
500 525 550 575
± ± ± ±
135.20 120.70 105.90 90.40
126.30 111.40 95.90 78.80
116.50 100.90 84.20 62.70
Boiler C
m = 1, C = 20 and for m = 2, C = 20 and also for m = 3, C = 20
550 550 550
118.30 118.30 105.60
101.80 101.70 87.10
88.70 88.60 77.50
76.20 76.20 70.40
Boiler D
m = 1, C = 20 and for m = 2, C = 20 and also for m = 3, C = 20
500 525 550 575
± ± ± ±
141.90 123.20 105.60 88.70
128.40 110.20 92.80 76.20
114.50 96.50 78.30 62.40
Boiler E
m = 1, C = 20 and for m = 2, C = 20 and also for m = 3, C = 20
500 525 550 575
± ± ± ±
142.90 124.50 106.80 89.30
130.00 111.70 93.80 75.70
116.20 99.90 79.40 59.80
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yield stress (0.2 proof stress), UTS (see Figs. 7(a) and (b)), % RA and % EL (see Figs. 7(c) and (d)) of the service exposed main steam pipe of all the boilers compared to those of the virgin pipe. In the absence of discernible cavitation or ¯aws, stress rupture tests can be selectively used to assess the condition of components. One of the most widely used techniques for life assessment of components involves the removal of samples and conducting accelerated tests at temperatures above the service temperature [8]. An estimate of the remaining life is then made by extrapolation of the results to the service temperature. Several uncertainties relating to the validity and application of the technique have been resolved in recent research projects [8]. In the present investigation, long term rupture strengths were estimated with best ®tted curves up to third order polynomial as there was no signi®cant change in average sum square error, beyond second order polynomial. Analysis of rupture data clearly indicates that the properties of the present steel are closely comparable to those of the virgin pipe (Figs. 8 and 9) suggesting no appreciable creep damage. The data points of the Stress vs LMP plots (Figs. 8 and 9) for all the service exposed materials fall
Fig. 7. (a) Plot of yield strength (0.2% proof stress) vs temperature for virgin and service exposed boiler pipes. (b) Plot of ultimate tensile strength UTS vs temperature for virgin and service exposed boiler pipes. (c) Plot showing variation of % reduction in area (RA) with temperature for virgin and service exposed boiler pipes. (d) Plot showing variation of % elongation (EL) with temperature for virgin and service exposed boiler pipes.
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Fig. 8. Plot of stress vs Larson Miller parameter (LMP) for boilers A, B, C and virgin pipe.
Fig. 9. Plot of stress vs larson miller parameter (LMP) for boilers D, E and virgin pipe.
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within the 20% scatter band of the virgin material. The best ®t curve for the virgin material has been extrapolated to a lower stress value (65 MPa) which is the allowable stress level and slightly below the operating hoop stress (66.70 MPa) of the service exposed main steam pipes of all boilers, for the purpose of life prediction (Figs. 8 and 9). Previous investigators [9,10] have also estimated the remaining life of such ageing components by comparing the upper limit of the data scatter band in stress rupture tests with the ASTM and BSS mean data lines. The remaining lives of all the service exposed main steam pipes of all the boilers predicted at 65 MPa and at 5508C are shown in Figs. 8 and 9. At the operating hoop stress of 66.70 MPa for the service exposed pipes, the LMP value as read from the graph (Figs. 8 and 9) is about 20,850. At this value of LMP, one would expect a very long life. This is mainly because the creep phenomenon is operative only at a temperature above 4508C. As is customary, an inspection life of >100,000 h is recommended. So far as the remaining life at 5508C is concerned, it is possible to obtain a minimum life of >100,000 h for the service exposed main steam pipes provided there is no evidence of localised damage in the form of surface cracks, cavitation or dents. Another health check of the service exposed pipes in terms of residual life is recommended to be carried out after expiry of 50,000 h of service life from the view of economical and safety reasons. Also during shut down of the plant, NDT (nondestructive) tests viz. dimensional (thickness and diameter) measurement, hardness measurement and insitu metallography may be carried out to assess the condition of the materials for their future serviceability. NDT examination of the weld joints at regular intervals during operation is desirable as the microstructural examination of the weldment in some cases revealed the presence of defects like microporosities.
5. Conclusions It is concluded that so far as the residual life at 5508C is concerned, it is possible to obtain a minimum life of about 100,000 h for the service exposed main steam pipes provided there is no evidence of localised damage in the form of surface cracks, cavitation or dents. The service exposed main steam pipes of all the boilers appear to be in a reasonably good state of health. Another health check of the service exposed pipes in terms of residual life is recommended to be carried out after expiry of 50,000 h of service life from the view of economical and safety reasons. Also during shut down of the plant, NDT (nondestructive) tests viz. dimensional (thickness and diameter) measurement, hardness measurement and insitu metallography may be carried out to assess the condition of the materials for their future serviceability. NDT examination of the weld joints at regular intervals during operation is desirable as the microstructural examination of the weldment in some cases revealed the presence of defects like microporosities.
Acknowledgements The authors are grateful to Prof. P. Ramachandra Rao- Director, National Metallurgical Laboratory, Jamshedpur, India for his kind permission to publish this paper.
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[4] de Witte M. International Jr Pressure Vessel and Piping 1996;39(12):41. [5] Li YD. International Jr Pressure Vessel and Piping 1996;69(2):161. [6] Klevtsov I, Tallermo H, Crane RA. Proceedings of International Conference of Plant Conditon & Life Measurement, vol I, Helsinki, Finland (6±8 June 1995). Pub-Technical Research Centre of Finland, 1995. [7] Wilshire B. International Jr Pressure Vessel and Piping 1989;39(12):73. [8] Viswanathan R, Dooley R, Saxena A. Proceedings of International Conference on Life Assessment and Extension, vol II, Congress Centre, The Hague/The Netherlands (13±15 June 1988), 1988. p. 175. [9] Singh AK, Bagdasarian AJ, Malhotra VK. Houston, TX: NACE International, 1994. p. 21. [10] Goodwall TW, Ainsworth RA. CEGB Res 1984;16:34.