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Towards the correlation of fracture toughness in an ex-service power generating rotor
International Journal of Pressure Vessels and Piping 77 (2000) 113±116
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Towards the correlation of fracture toughness in an ex-service power generating rotor A. Shekhter a,*, A.B.L. Croker b, A.K. Hellier b, C.J. Moss c, S.P. Ringer a a
b
Department of Materials Engineering, Monash University, Clayton, Victoria 3168, Australia Materials Division, Australian Nuclear Science and Technology Organisation, PMB 1 Menai, New South Wales 2234, Australia c Santos, P.O. Box 1010, Brisbane, Queensland, 7000 Australia Received 8 June 1999; accepted 19 November 1999
Abstract This work presents preliminary results from a project aimed at deriving fracture toughness correlations for an improved assessment of the structural integrity and remaining life of low alloy steel pressure equipment where susceptibility to temper embrittlement is a consideration. Room temperature KIc tests, Charpy tests and small punch tests were performed on samples extracted from three different locations of a 1Cr± 1Mo±0.25V wt% ex-service (136,000 h) turbine rotor in the ex-service condition. Proposed correlations for two mechanical methods are presented and discussed. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Fracture toughness; Ex-service power generating rotor; Temper embrittlement
1. Introduction 1.1. Background The fact that much of Australian power generation and petrochemical plants are more than 25 years old [1] requires increasing innovation in material testing and research, so as to provide improved remaining life analysis. The capacity, ef®ciency, availability and safety of plants depend critically on the integrity of the components and materials employed. A number of damage mechanisms such as temper embrittlement, creep, hydrogen attack and hydrogen embrittlement can impair plant integrity. The aim of the present work is to assess the validity of mechanical testing correlations (Fig. 1) of low alloy steel pressure equipment, susceptible to temper embrittlement, where impurity segregation to the prior austenite grain boundaries reduces the critical ¯aw size and changes the fracture mechanism from ductile to brittle cleavage, resulting in a deterioration of the fracture toughness. This damage mechanism is of signi®cance given the increasingly cyclical nature of turbine rotor operation. 1.2. Mechanical methods and correlation procedures Charpy V-Notch (CVN) testing is the most commonly used method to evaluate the ductile-to-brittle transition in * Corresponding author.
steels through the fracture appearance transition temperature (FATT) [2]. However, the most important material parameter in assessing rotor integrity is the fracture toughness, which is expressed in terms of plane strain fracture toughness KIc, or stress±strain fracture toughness JIc. Correlations between fracture toughness and small-scale test results are useful in assessing the structural integrity of pressure vessels due to cost, availability of material, ease of testing and because fracture toughness can be used directly in design analysis [3]. The small-scale test results may provide this information through correlation with the fracture toughness. Further, KIc measurements involve use of large specimens which are dif®cult to excise from operating components. A large body of CVN impact-test data is already available because the Charpy impact transition curve has been the most common basis for specifying toughness, and correlations between CVN testing and KIc have been extensively reviewed in the literature [3,4]. In this work only selective correlation approaches were examined with the emphasis of possible implication of such correlations in further miniaturisation such as small punch testing (SPT). A method outlined by Jones [4] was applied in this study to predict a minimum KIc from CVN testing FATT, which is given by an equation: KIc 6600=60 2 T 2 FATT
1
where T (testing temperature)and FATT are in 8C and KIc is in MPa m 1/2.
0308-0161/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0308-016 1(99)00091-5
114
A. Shekhter et al. / International Journal of Pressure Vessels and Piping 77 (2000) 113±116
Fig. 2. Small punch experimental setup.
Fig. 1. Mechanical testing correlations.
This correlation was based on a large amount of CVN testing and KIc data collected on large alloy forgings similar to the component in the present study. The SPT utilises smaller amounts of material than the above techniques and has been also correlated with CVN impact test data [5]. The procedure involves determination of a transition temperature, from the small punch test (small punch transition temperature, SPTT), which is correlated with the Charpy FATT. The transition temperature from a series of small punch tests, TSP, is then de®ned as the temperature at which the fracture energy is equal to the mean of the upper and lower shelf energy. This is now a widely documented approach [6] and is based on similarities of the fracture transition behaviour in the small punch test and CVN impact test samples. Foulds and Viswanathan [5] have proposed that the following factors are important in relating the results of these approaches: ² while the transition temperature from a series of small punch tests is observed to be signi®cantly lower than the FATT, there is a material speci®c correlation between TSP and FATT that can be empirically established; ² experimental data indicate that fracture energy-based results are best obtained with the hemispherical punch head, as compared to earlier experiments with specimens having different geometries [6]; ² the correlation procedure is based largely on the size of the database used to derive such a correlation. Table 1 HP±IP rotor operating temperatures and FATTs Stage
Operating temperature (8C)
FATT (8C)
1 3 4 5 6 7 8 12
565 511 506 487 467 448 426 351
125 150 145 135 138 120 120 100
For Cr±Mo±V rotor steels, the linear regression applied to a database compiled from 17 rotors of grades C and D produced the following mean, or best-estimate FATT: FATT 8C 457:61 1 2:536TSP 8C
2
This correlation method has been successfully applied to components in service [5]. The present work examines the correlation for CVN, SPT and KIc fracture toughness and is part of a wider programme focused on specimen miniaturisation. 2. Experimental procedures An ex-service 1Cr±1Mo±0.25V (wt%) rotor steel was available for the present study, following 136,000 h of elapsed operation. The high pressure-intermediate pressure rotor had 12 stages with service temperatures ranging from 5658C at stage 1 to 3518C at stage 12. Table 1 summarises the operating temperatures and corresponding FATTs of the rotor determined in the earlier work [7]. Blocks were cut out of the core of stages 1, 3 and 12 to provide the fracture toughness and CVN specimens notched in the radial orientation [8]. Small punch tests were performed on the as-received samples of the core of stages 1, 3 and 12 in accordance with the method described by Foulds and Viswanathan [5]. For the SPT (the small punch experimental setup is shown in Fig. 2) a polished specimen of 0.5 mm thickness and 10 mm in diameter, is placed between the punch guide and die, and a hemispherical punch of 2.2 mm, in diameter then advanced upward through the guide to deform the specimen against the receiving die in an INSTRON 8505 servoelectric machine. The area under the punch displacement curve indicates the energy absorbed during the test. By conducting tests at different temperatures in a controlled environmental chamber, a curve of absorbed energy vs. temperature is obtained, similar to that of a Charpy energy vs. temperature curve. The midpoint of the energy curve can be used to de®ne a ductile±brittle transition temperature or SPTT. The KIc tests were performed in accordance with the ASTM standard E399-90 [9] for the three-point bend single
A. Shekhter et al. / International Journal of Pressure Vessels and Piping 77 (2000) 113±116
115
Fig. 3. CVN testing and SPT data for stage 1 (a and b), stage 3 (c and d) and stage 12 (e and f).
specimen technique. Specimens were notched, 20 £ 100 £ 10 mm3 : Specimens were fatigue-precracked according to the fatigue-precracking procedures outlined in ASTM E399-90. 3. Results and discussion Fig. 3(a), (c) and (e) are plots of the CVN and SPT data from samples taken near the core of stages 1, 3 and 12 in the ex-service condition and indicates FATT of 125, 150 and 1008C, respectively. Fig. 3(b), (d) and (f) provides the corresponding small punch energy plots vs. temperature. By constructing a line of best ®t and then ®nding a midpoint in these graphs, it was concluded that the corresponding SPTT was 2133, 2120 and 21458C, respectively. The ªCrMoV Steels Correlationº method [5] was used and Table 2 gives details about the derived relationship between the SPTT and FATT. The assessment of CrMoV rotor steel using the ªCrMoV Steels Correlationº [5] yields values for FATT that are within the con®dence limits of a best-®t approach.
Table 3 presents the experimental results of KIc fracture toughness measured at room temperature for ex-service samples of stages 1, 3 and 12 and includes values derived from empirical relationships (Eqs. (1) and (2)). The experimental KIc values are compared to values predicted by Jones' method (Eq. (1)), utilising the Foulds and Viswanathan SPTT±FATT relationship (Eq. (2)). These correlations appear to be satisfactory. Further work is in progress to examine this triangular CVN, SPT and KIc correlation in the de-embrittled condition and isothermally re-embrittled conditions. 4. Conclusion Following the CVN, SPT and KIc testing of an ex-service (136,000 h) 1Cr±1Mo±0.25V (wt%) rotor steel it was concluded that the SPT and CVN impact testing for stages 1, 3 and 12 produced transition temperature values that were within the limits of the con®dence interval approach. The results provide an empirical estimate of KIc which was comparable to the experimentally measured values. Table 3 Fracture toughness correlations
Table 2 Correlations between SPTT and FATT Stage
SPTT (8C)
FATT using correlation method (8C; Eq. (2))
CVN FATT (8C)
1 3 12
2133 2120 2145
120.3 153 90
125 150 100
Stage
Condition aa
Predicted KIc value, (MPa m 1/2; Eq. (1))
Predicted KIc value, (MPa m 1/2; derivation of Eq. (2))
Experimental KIc value (MPa m 1/2)
1 3 12
As-received As-received As-received
41.2 35.6 48.8
42.4 35 52.8
35.8 30 30.4
116
A. Shekhter et al. / International Journal of Pressure Vessels and Piping 77 (2000) 113±116
Acknowledgements The technical assistance and advice of David Carr, Paul Stathers, Dr David Mitchell, Graham Smith (ANSTO) and Dr Mike Drew (Paci®c Power Intl.) are gratefully appreciated. Financial support from Gladstone Power Station, BP (Bulwer Is.), Paci®c Power Intl., the Electricity Corporation of New Zealand, HRL Technology and the Australian Research Council is gratefully acknowledged.
[3] [4] [5] [6] [7]
References [1] Moss CJ, Croker ABL, Harrison RP. A perspective of pressure plant life management in Australia. Advanced Material Program, ANSTO, 1995. [2] Moscovic R, Flewitt PEJ. An overview of the principles of modelling
[8] [9]
Charpy impact energy data using statistical analysis, 1997;28A:2609± 23. Roberts R, Newton C. Interpretive report on small-scale test correlations with KIc data. Welding Research Council Bulletin 1981;265:1± 18. Jones GT. Discussion on N. Calderon and J.L. Gray. Proceedings of Institution of Mechanical Engineers 1972;186(31/32):D121±3. Foulds J, Viswanathan R. Journal of Engineering Materials and Technology 1994;116:457±64. Mao X, Shoji H, Takahashi H. Scripta Metallurgica et Materialia 1991;25:2481±5. Shekhter A, Hellier AK, Moss CJ, Ringer SP. Lifetime management and evaluation of plant structures and components. Proceedings of the Fourth International Conference on Engineering Structural Integrity Assessment. Cambridge: EMAS Publishing, 1998. p. 87±94. Annual Book of ASTM standards, 03.01, ASTM E23Ð96. p. 136±45. Annual Book of ASTM standards, 03.01, ASTM, E 399Ð90. p. 633± 47.
www.elsevier.com/locate/ijpvp
Towards the correlation of fracture toughness in an ex-service power generating rotor A. Shekhter a,*, A.B.L. Croker b, A.K. Hellier b, C.J. Moss c, S.P. Ringer a a
b
Department of Materials Engineering, Monash University, Clayton, Victoria 3168, Australia Materials Division, Australian Nuclear Science and Technology Organisation, PMB 1 Menai, New South Wales 2234, Australia c Santos, P.O. Box 1010, Brisbane, Queensland, 7000 Australia Received 8 June 1999; accepted 19 November 1999
Abstract This work presents preliminary results from a project aimed at deriving fracture toughness correlations for an improved assessment of the structural integrity and remaining life of low alloy steel pressure equipment where susceptibility to temper embrittlement is a consideration. Room temperature KIc tests, Charpy tests and small punch tests were performed on samples extracted from three different locations of a 1Cr± 1Mo±0.25V wt% ex-service (136,000 h) turbine rotor in the ex-service condition. Proposed correlations for two mechanical methods are presented and discussed. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Fracture toughness; Ex-service power generating rotor; Temper embrittlement
1. Introduction 1.1. Background The fact that much of Australian power generation and petrochemical plants are more than 25 years old [1] requires increasing innovation in material testing and research, so as to provide improved remaining life analysis. The capacity, ef®ciency, availability and safety of plants depend critically on the integrity of the components and materials employed. A number of damage mechanisms such as temper embrittlement, creep, hydrogen attack and hydrogen embrittlement can impair plant integrity. The aim of the present work is to assess the validity of mechanical testing correlations (Fig. 1) of low alloy steel pressure equipment, susceptible to temper embrittlement, where impurity segregation to the prior austenite grain boundaries reduces the critical ¯aw size and changes the fracture mechanism from ductile to brittle cleavage, resulting in a deterioration of the fracture toughness. This damage mechanism is of signi®cance given the increasingly cyclical nature of turbine rotor operation. 1.2. Mechanical methods and correlation procedures Charpy V-Notch (CVN) testing is the most commonly used method to evaluate the ductile-to-brittle transition in * Corresponding author.
steels through the fracture appearance transition temperature (FATT) [2]. However, the most important material parameter in assessing rotor integrity is the fracture toughness, which is expressed in terms of plane strain fracture toughness KIc, or stress±strain fracture toughness JIc. Correlations between fracture toughness and small-scale test results are useful in assessing the structural integrity of pressure vessels due to cost, availability of material, ease of testing and because fracture toughness can be used directly in design analysis [3]. The small-scale test results may provide this information through correlation with the fracture toughness. Further, KIc measurements involve use of large specimens which are dif®cult to excise from operating components. A large body of CVN impact-test data is already available because the Charpy impact transition curve has been the most common basis for specifying toughness, and correlations between CVN testing and KIc have been extensively reviewed in the literature [3,4]. In this work only selective correlation approaches were examined with the emphasis of possible implication of such correlations in further miniaturisation such as small punch testing (SPT). A method outlined by Jones [4] was applied in this study to predict a minimum KIc from CVN testing FATT, which is given by an equation: KIc 6600=60 2 T 2 FATT
1
where T (testing temperature)and FATT are in 8C and KIc is in MPa m 1/2.
0308-0161/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0308-016 1(99)00091-5
114
A. Shekhter et al. / International Journal of Pressure Vessels and Piping 77 (2000) 113±116
Fig. 2. Small punch experimental setup.
Fig. 1. Mechanical testing correlations.
This correlation was based on a large amount of CVN testing and KIc data collected on large alloy forgings similar to the component in the present study. The SPT utilises smaller amounts of material than the above techniques and has been also correlated with CVN impact test data [5]. The procedure involves determination of a transition temperature, from the small punch test (small punch transition temperature, SPTT), which is correlated with the Charpy FATT. The transition temperature from a series of small punch tests, TSP, is then de®ned as the temperature at which the fracture energy is equal to the mean of the upper and lower shelf energy. This is now a widely documented approach [6] and is based on similarities of the fracture transition behaviour in the small punch test and CVN impact test samples. Foulds and Viswanathan [5] have proposed that the following factors are important in relating the results of these approaches: ² while the transition temperature from a series of small punch tests is observed to be signi®cantly lower than the FATT, there is a material speci®c correlation between TSP and FATT that can be empirically established; ² experimental data indicate that fracture energy-based results are best obtained with the hemispherical punch head, as compared to earlier experiments with specimens having different geometries [6]; ² the correlation procedure is based largely on the size of the database used to derive such a correlation. Table 1 HP±IP rotor operating temperatures and FATTs Stage
Operating temperature (8C)
FATT (8C)
1 3 4 5 6 7 8 12
565 511 506 487 467 448 426 351
125 150 145 135 138 120 120 100
For Cr±Mo±V rotor steels, the linear regression applied to a database compiled from 17 rotors of grades C and D produced the following mean, or best-estimate FATT: FATT 8C 457:61 1 2:536TSP 8C
2
This correlation method has been successfully applied to components in service [5]. The present work examines the correlation for CVN, SPT and KIc fracture toughness and is part of a wider programme focused on specimen miniaturisation. 2. Experimental procedures An ex-service 1Cr±1Mo±0.25V (wt%) rotor steel was available for the present study, following 136,000 h of elapsed operation. The high pressure-intermediate pressure rotor had 12 stages with service temperatures ranging from 5658C at stage 1 to 3518C at stage 12. Table 1 summarises the operating temperatures and corresponding FATTs of the rotor determined in the earlier work [7]. Blocks were cut out of the core of stages 1, 3 and 12 to provide the fracture toughness and CVN specimens notched in the radial orientation [8]. Small punch tests were performed on the as-received samples of the core of stages 1, 3 and 12 in accordance with the method described by Foulds and Viswanathan [5]. For the SPT (the small punch experimental setup is shown in Fig. 2) a polished specimen of 0.5 mm thickness and 10 mm in diameter, is placed between the punch guide and die, and a hemispherical punch of 2.2 mm, in diameter then advanced upward through the guide to deform the specimen against the receiving die in an INSTRON 8505 servoelectric machine. The area under the punch displacement curve indicates the energy absorbed during the test. By conducting tests at different temperatures in a controlled environmental chamber, a curve of absorbed energy vs. temperature is obtained, similar to that of a Charpy energy vs. temperature curve. The midpoint of the energy curve can be used to de®ne a ductile±brittle transition temperature or SPTT. The KIc tests were performed in accordance with the ASTM standard E399-90 [9] for the three-point bend single
A. Shekhter et al. / International Journal of Pressure Vessels and Piping 77 (2000) 113±116
115
Fig. 3. CVN testing and SPT data for stage 1 (a and b), stage 3 (c and d) and stage 12 (e and f).
specimen technique. Specimens were notched, 20 £ 100 £ 10 mm3 : Specimens were fatigue-precracked according to the fatigue-precracking procedures outlined in ASTM E399-90. 3. Results and discussion Fig. 3(a), (c) and (e) are plots of the CVN and SPT data from samples taken near the core of stages 1, 3 and 12 in the ex-service condition and indicates FATT of 125, 150 and 1008C, respectively. Fig. 3(b), (d) and (f) provides the corresponding small punch energy plots vs. temperature. By constructing a line of best ®t and then ®nding a midpoint in these graphs, it was concluded that the corresponding SPTT was 2133, 2120 and 21458C, respectively. The ªCrMoV Steels Correlationº method [5] was used and Table 2 gives details about the derived relationship between the SPTT and FATT. The assessment of CrMoV rotor steel using the ªCrMoV Steels Correlationº [5] yields values for FATT that are within the con®dence limits of a best-®t approach.
Table 3 presents the experimental results of KIc fracture toughness measured at room temperature for ex-service samples of stages 1, 3 and 12 and includes values derived from empirical relationships (Eqs. (1) and (2)). The experimental KIc values are compared to values predicted by Jones' method (Eq. (1)), utilising the Foulds and Viswanathan SPTT±FATT relationship (Eq. (2)). These correlations appear to be satisfactory. Further work is in progress to examine this triangular CVN, SPT and KIc correlation in the de-embrittled condition and isothermally re-embrittled conditions. 4. Conclusion Following the CVN, SPT and KIc testing of an ex-service (136,000 h) 1Cr±1Mo±0.25V (wt%) rotor steel it was concluded that the SPT and CVN impact testing for stages 1, 3 and 12 produced transition temperature values that were within the limits of the con®dence interval approach. The results provide an empirical estimate of KIc which was comparable to the experimentally measured values. Table 3 Fracture toughness correlations
Table 2 Correlations between SPTT and FATT Stage
SPTT (8C)
FATT using correlation method (8C; Eq. (2))
CVN FATT (8C)
1 3 12
2133 2120 2145
120.3 153 90
125 150 100
Stage
Condition aa
Predicted KIc value, (MPa m 1/2; Eq. (1))
Predicted KIc value, (MPa m 1/2; derivation of Eq. (2))
Experimental KIc value (MPa m 1/2)
1 3 12
As-received As-received As-received
41.2 35.6 48.8
42.4 35 52.8
35.8 30 30.4
116
A. Shekhter et al. / International Journal of Pressure Vessels and Piping 77 (2000) 113±116
Acknowledgements The technical assistance and advice of David Carr, Paul Stathers, Dr David Mitchell, Graham Smith (ANSTO) and Dr Mike Drew (Paci®c Power Intl.) are gratefully appreciated. Financial support from Gladstone Power Station, BP (Bulwer Is.), Paci®c Power Intl., the Electricity Corporation of New Zealand, HRL Technology and the Australian Research Council is gratefully acknowledged.
[3] [4] [5] [6] [7]
References [1] Moss CJ, Croker ABL, Harrison RP. A perspective of pressure plant life management in Australia. Advanced Material Program, ANSTO, 1995. [2] Moscovic R, Flewitt PEJ. An overview of the principles of modelling
[8] [9]
Charpy impact energy data using statistical analysis, 1997;28A:2609± 23. Roberts R, Newton C. Interpretive report on small-scale test correlations with KIc data. Welding Research Council Bulletin 1981;265:1± 18. Jones GT. Discussion on N. Calderon and J.L. Gray. Proceedings of Institution of Mechanical Engineers 1972;186(31/32):D121±3. Foulds J, Viswanathan R. Journal of Engineering Materials and Technology 1994;116:457±64. Mao X, Shoji H, Takahashi H. Scripta Metallurgica et Materialia 1991;25:2481±5. Shekhter A, Hellier AK, Moss CJ, Ringer SP. Lifetime management and evaluation of plant structures and components. Proceedings of the Fourth International Conference on Engineering Structural Integrity Assessment. Cambridge: EMAS Publishing, 1998. p. 87±94. Annual Book of ASTM standards, 03.01, ASTM E23Ð96. p. 136±45. Annual Book of ASTM standards, 03.01, ASTM, E 399Ð90. p. 633± 47.