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Evaluation on toughness degradation of Cr–Mo–V steel using miniaturized impact specimen technology
International Journal of Impact Engineering 25 (2001) 805–816
Evaluation on toughness degradation of Cr–Mo–V steel using miniaturized impact specimen technology Seung Hoon Nahma,*, Amkee Kimb, Jonghwa Parkc a
Strength Evaluation Group, Korea Research Institute of Standards and Science, P.O. Box 102, Yusong, Taejon 305-600, South Korea b Department of Mechanical Engineering, Kongju National University, 182 Shinkwan-Dong, Kongju, Chungnam 314-701, South Korea c School of Transport Vehicle Engineering, Gyeongsang National University, Chinju, Gyeongnam 660-701, South Korea Received 26 September 2000; received in revised form 31 January 2001
Abstract Miniaturized specimen technology is inevitable when the amount of available material for test is limited. In this study, miniaturized Charpy V-notched specimens of 1Cr–1Mo–0.25 V rotor steel with five different aging periods were artificially prepared by an isothermal aging heat treatment at 6308C and tested. For the miniaturized specimens, two different types of specimens with or without side groove were utilized. A correlation between the ductile brittle transition temperature (DBTT) obtained by the miniaturized specimen and that by the standard specimen was investigated. In addition, the relationship between fracture toughness and DBTT by the miniaturized specimen of degraded 1Cr–1Mo–0.25 V rotor steels was proposed. # 2001 Published by Elsevier Science Ltd. Keywords: Miniaturized impact specimen; Fracture toughness; DBTT; Degradation; Normalization
1. Introduction Miniaturized specimen technology is very effective for the characterization of material properties because the collection of material can be done without critical damage to structures or machinery components [1]. Charpy V-notched (CVN) impact test is one of the most adequate methods for that purpose because of simplicity and convenience. CVN impact test provides various informations such as impact energy, lateral expansion, ductile fracture area percentage, etc. It is also employed for clarifying factors affecting ductile *Corresponding author. Tel.: +82-42-868-5383; fax: +82-42-868-5027. E-mail address: [email protected] (S.H. Nahm). 0734-743X/01/$ - see front matter # 2001 Published by Elsevier Science Ltd. PII: S 0 7 3 4 - 7 4 3 X ( 0 1 ) 0 0 0 0 6 - 9
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brittle transition temperature (DBTT) [2–4]. In order to generate an impact energy curve for the measurement of upper self energy (USE) or DBTT, more than 10 standard specimens are required. Thus the miniaturized specimen technology could be necessary as one may find that the amount of material for sample with different operating histories is limited in variety and quantity. The miniaturized specimen technology may produce an impact energy curve even with amount of material for one standard specimen. However, despite such merits, the usage of miniaturized specimen has still been restricted by the size effect that USE and DBTT of miniaturized specimen appear lower than those of standard impact specimen. The decrease of USE is resulted from the reduction of fracture volume of specimen, and the decrease of DBTT is caused by the ductile fracture that is promoted by the plane stress state at the notch tip of miniaturized specimen [5]. Therefore, for the use of miniaturized specimen, one must utilize either a relationship between data of standard impact specimen and those of miniaturized specimen, or a specially designed miniaturized specimen like the side grooved specimen which gives rise to the same strain constraint of fracture plane as that in standard CVN specimen. In this paper, the evaluation technology on standard specimen DBTTstan of degraded turbine rotor steel was attempted using the 18 miniaturized specimens with or without side groove. An isothermal heat treatment was employed for the artificial degradation process of material. The correlation between the fracture toughness in terms of critical stress intensity factor, KIC , and the DBTTstan of degraded turbine rotor steel was also established. Moreover, its validity was verified by the experimental data. 2. Experimental 2.1. Material and specimen The test material is 1Cr–1Mo–0.25V steel that is widely used as a steam turbine rotor material of fossil power plant. The chemical compositions and mechanical properties are contained in Tables 1 and 2, respectively. Since it was difficult to obtain the degraded rotor steels from the turbine rotor, they were simulated by an isothermal heat treatment. Virgin (as received) materials were artificially aged at 6308C for 0, 455, 910, 1365, 1820, 3640, 5460 h, which is the accelerating degradation process for simulating the microstructures of materials which served at the turbine steam temperature of 5388C for about 0, 3, 5, 8, 11, 22, 33 years, respectively (see Table 3 for the details). The periods of time for the heat treatment were determined by the self-diffusion theory for iron element [6]. The geometry of tested ASTM E-23 standard specimen [7] and miniaturized specimen is depicted in Fig. 1, which follows other researchers’ proposition [4]. The size of
Table 1 Chemical compositions of tested material (wt%) Element
C
Si
Mn
P
S
Sb
N
Cu
Ni
Cr
Mo
V
Sn
As
Composition
0.31
0.29
0.80
0.015
0.022
0.015
0.013
0.21
0.39
1.19
1.42
0.25
0.004
0.015
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Table 2 Mechanical properties Temp. (8C)
Yield stress (MPa)
Tensile strength (MPa)
Elongation (%)
Reduction of area (%)
24 538
665.2 533.5
823.1 580.6
18.8 22.7
59.4 55.5
Table 3 Isothermal heat treatment times to simulate the microstructure of the turbine rotor steel aged at the steam temperature of 5388C Aging time at 5388C (h)
25,000
50,000
75,000
100,000
200,000
300,000
Heat treatment time at 6308C (h)
455
910
1365
1820
3640
5460
Fig. 1. Dimensions of standard and miniaturized CVN specimens. (all dimensions in mm).
miniaturized impact specimen is about 18 of standard specimen size. Two different kinds of miniaturized specimens with or without side groove (see Fig. 1(c)) were machined. The direction of all specimens is T-L oriented. 2.2. Test equipment and method The SATEC Charpy impact tester (maximum capacity 33.9 J) was utilized, which was specially designed to adjust the anvil and the hammer weight for the specimens with various dimensions.
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Table 4 describes the difference of basic experimental parameters [1] between standard and miniaturized specimens. The load variation during impact with time was measured and analyzed by the instrumented impact test software developed by Dynatup Co. Fig. 2 represents a typical load history that was obtained from the test. Pm and Pgy in the figure represent the maximum load
Table 4 Comparison of specimen dimensions and experimental parameters Standard Charpy specimen (mm)
Miniaturized Charpy specimen (mm)
Specimen Thickness Depth Width Notch depth Notch-root radius
10.00 10.00 55.00 2.00 0.25
4.50 4.50 24.00 0.90 0.25
Experimental Striker radius Striker tip width Anvil radius Anvil span
8.00 4.00 1.00 40.00
8.00 4.00 1.00 19.96
Fig. 2. A typical impact load history for 50,000 h aged miniaturized specimen tested at 758C. Pm and Pgy represent the maximum load and the general yield load.
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809
and the general yielding load, respectively, during impact. The absorbed impact energy was measured by the loss of initial potential energy of hammer after the breakage of specimen. Prior to the test, each specimen was kept in a thermostat in which the test temperature was maintained within 28C error for 10 min, and tested in 3 s after it was taken out of the thermostat for the test.
3. Results and discussion 3.1. Impact energy characteristics of standard specimen Fig. 3 represents the variation of absorbed impact energy for standard specimen with test temperature. The absorbed impact energy variation curves fitted with hyperbolic tangent Eq. (1) depend on the aging time. Here, E1 ; E2 ; T0 and T1 are fitting constants. The unit of E1 and E2 is J, and that of T0 and T1 is 8C. The difference in DBTT and upper shelf energy (USE) due to different aging time is manifest while the lower shelf energy (LSE) represents less difference. T T0 impact energyCVN ¼ E1 þ E2 tanh : ð1Þ T1 Fig. 4 represents the variation of standard specimen DBTTstan defined as the temperature at which the impact energy on fitted curve is equal to 12 the sum of USE and LSE with aging time. The DBTTstan increases rapidly at the initial stage of aging and approaches to the maximum
Fig. 3. Dependence of impact energy curve on temperature for standard specimen.
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Fig. 4. Dependence of DBTTstan obtained by standard specimen on aging time.
around 50,000 h. After that, no further increment is observed even though the aging proceeds. The similar result for the turbine rotor steel in terms of fracture toughness has been observed by other researchers [8]. 3.2. Impact energy characteristics of miniaturized specimen Figs. 5 and 6 represent the variations of absorbed impact energy with test temperature for the miniaturized specimens with or without side groove. Comparing with those for standard specimens in Fig. 3, USEs of miniaturized specimens appear to be only 10–15% of those of standard specimens, which would be mainly due to the difference of fracture volume between standard and miniaturized specimens as previously mentioned. In deed, the volume of miniaturized specimen in this study is about 12.5% ð18Þ of that of standard specimen. DBTTminis by miniaturized specimens without side groove appear lower than those by standard specimens while DBTTminiSGs by miniaturized specimens with side groove represent almost same values as those by the standard specimens (see Table 5), which implies that the side groove of the 18 miniaturized specimen should provide almost the same strain constraint of fracture plane as that in standard specimen. On the other hand, Kumar et al. [5] proposed a method to predict the size effect on DBTT by normalizing DBTTs obtained from different size specimens by the maximum elastic tensile stress, s0 , at the notch tip. It is assumed that the fracture is controlled by the maximum elastic tensile stress, s0 , at the notch tip. When the stress at the notch tip reaches the critical value, snf , also called the fracture stress, the cleavage crack propagates. The maximum elastic tensile stress is calculated by following Eq. (2). Where Kt is a stress concentration factor at notch tip. Pnm in Eq. (2) is the maximum load at a temperature that the specimen fails by cleavage crack initiation at the point of
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Fig. 5. Dependence of impact energy curve on temperature for miniaturized specimen without side groove.
Fig. 6. Dependence of impact energy curve on temperature for miniaturized specimen with side groove.
general yield; i.e., Pnm ¼ Pngy (see Fig. 7). L, B and b are the span of anvil, the specimen width and the size of ligament, respectively. s0 ¼
3Kt Pnm L : 2Bb2
ð2Þ
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Table 5 Comparison between DBTTstans by standard specimens and DBTTminiSGs by miniaturized specimens with side groove Specimen Aging time (h)
DBTTstan by standard specimen (8C)
DBTTminiSG by miniaturized specimen with side groove (8C)
Error (%)
0 25,000 50,000 75,000 100,000
42 54 88 81 87
40 53 78 75 90
1.64 +2.16 6.75 2.73 +8.82
Fig. 7. Maximum load and general yield load vs. temperature diagrams for (a) 25,000 h aged miniaturized specimen and (b) 100,000 h aged miniaturized specimen.
The stress concentration factor, Kt , can be calculated by Eq. (3) [9]. Here, R is the radius of notch root. Kt ¼
f ¼
g ¼
2ðb=R þ 1Þ f ðb=R þ 1Þ1=2 ; 4ðb=R þ 1Þ=g 3f 2ðb=R þ 1Þ ðb=RÞ1=2
ðb=R þ 1Þarctanðb=RÞ1=2 þ ðb=RÞ1=2 4ðb=RÞ3=2 3½ðb=RÞ1=2 ðb=R 1Þarctanðb=RÞ1=2
;
:
ð3Þ
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Kt calculated by Eq. (3) with respect to the standard and miniaturized specimens in this study are 4.81 and 3.46, respectively. Fig. 7 represents typical examples of determination of Pnm using the Pm vs. temperature plot. According to Lucas et al. [10], maximum load Pnm in Eq. (2) can be also evaluated by Eq. (4) which includes the fracture stress, snf . Here, snf is known as a material constant independent of the size of specimen. Pnm ¼
snf Bb ; 6:52
ð4Þ
which leads to the equation snf ¼
6:52Pnm : Bb
ð5Þ
Fig. 8 represents the variation of snf with the aging time. Substituting Eq. (4) for Eq. (2), Eq. (2) reduces to s0 ¼
0:23Kt Lsnf : b
ð6Þ
Fig. 9 represents a linear relationship between normalized DBTTstan of standard specimen, DBTTstan =s0stan , and that of miniaturized specimen, DBTTmini =s0mini . The slope of correlation line appears one. It implies that the normalization by s0 is well applicable to the experimental results of this study. The correlation between DBTTstan =s0stan and DBTTmini =s0mini for the degraded turbine rotor steels can be expressed by Eq. (7). Thus Eq. (7) make it possible to predict the
Fig. 8. Variation of the fracture stress, s f , with the aging time.
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Fig. 9. Correlation between normalized DBTTstan and DBTTmini.
DBTTstans through the DBTTminis regardless of the extent of degradation. Here constant A is 0.58C/GPa for the turbine rotor steel. DBTTstan DBTTmini ¼ þ A: s0stan s0mini
ð7Þ
3.3. Fracture toughness evaluation by miniaturized specimen Since relationships between fracture toughness, KIC , and DBTTstan of standard specimen are available at the moment [8, 11, 12], Eq. (7) enables one to evaluate the fracture toughness of degraded turbine rotor steel by using the DBTTmini of miniaturized specimen. In this study, Eq. (8) was utilized in order to evaluate the fracture toughness, KIC [8], where the unit of KIC is MPa m1/2, and unit of test temperature T and DBTTstan of standard specimen is 8C. Moreover, constants B and T2 are 177058C MPa m1/2 and 1948C, respectively. B : ð8Þ KIC ¼ T2 ðT DBTTstan Þ Fig. 10 represents the fracture toughness, KIC , calculated by the excess temperature, TDBTTstan . Open marks in Fig. 10 indicate the KIC values of degraded Cr–Mo–V rotor steels evaluated by Eqs. (7) and (8). All the values are located within the scatter band that refers to Ref. [11]. Therefore the miniaturized specimen technology seems valid as an alternative for the fracture toughness evaluation when the available material for the standard specimen technology is not sufficient.
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Fig. 10. Excess temperature versus fracture toughness.
4. Conclusion The evaluation technology on the fracture toughness of degraded turbine rotor steel using miniaturized Charpy V-notched impact specimens with or without side groove was attempted in this study. Obtained results are as follows: (1) DBTTstan of degraded turbine rotor by standard specimen was obtained directly by using the 18 miniaturized specimen with side groove. (2) The conversion between DBTTminis of degraded turbine rotor steel by miniaturized specimens without side groove and those by standard specimens was achieved through Eq. (7) normalized by the maximum elastic tensile stress at the notch tip. (3) An empirical Eq. (8) for the fracture toughness evaluation of degraded turbine rotor steel was proposed. And its validity was supported by the previously reported experimental data.
References [1] Manahan MP. Determination of Charpy transition temperature of ferritic steels using miniaturized specimens. J Mater Sci 1990;25:3429–38. [2] Rolfe ST, Barsom JM. Fracture and fatigue control in structures. Englewood Cliffs, NJ: Prentice Hall, 1977. [3] Firrao D, Begley JA, Silva G, Roberti R, De Benedetti B. The influence of notch root radius and austenitizing temperature on fracture appearance of as-quenched Charpy-V type AISI 4340 steel specimens. Metall Trans A 1982;13A:1003–13. [4] Corwin WR and Houghland AM. Effect of specimen size and material condition on the Charpy impact properties of 9Cr-1Mo-V-Nb steel. ASTM STP 888, American Society for Testing and Materials, Philadelphia, 1986. p. 337.
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[5] Kumar AS, Louden BS, Garner FA, Hamilton ML. Recent improvements in size effects correlations for DBTT and upper shelf energy of ferritic steels. ASTM STP 1204, American Society for Testing and Materials. Philadelphia, 1993. p. 47–61. [6] Abdel-Latif AM, Corbett JM, Sidey D and Taplin DMR. Effects of microstructural degradation on creep life prediction of 2.25Cr–1Mo Steel. Proceedings of Fifth International Conference on Fracture (ICF5), Cannes (France), Vol. 4, 1981. 4: p. 1613–20. [7] ASTM Standard E23. Notched Bar impact testing of metallic materials. Philadelphia: ASTM, 1997. [8] Viswanathan R, Gehl S. A method for estimation of the fracture toughness of Cr–Mo–V rotor steels based on composition. J Eng Mater Technol 1991;113:263–70. [9] Neuber H. Theory of notch stresses. 2nd ed. Berlin: Springer, 1958. p. 71. [10] Lucas GE, Odette GR, Sheckherd JW, McConnell P, Perrin J. Subsized bend and Charpy V-notch specimens for irradiated testing. ASTM STP 888. American Society for Testing and Materials, Philadelphia, 1986. p. 305–24. [11] Schwant RC and Timo DP. Life assessment and improvement of turbogenerator rotors for fossil plants. New York: Pergamon Press, 1985. p. 325–40. [12] Rolfe ST and Novak SR. Slow bend KIC testing of medium-strength high toughness steels. ASTM STP 463, American Society for Testing and Materials, Philadelphia, 1970. p. 124–59.
Evaluation on toughness degradation of Cr–Mo–V steel using miniaturized impact specimen technology Seung Hoon Nahma,*, Amkee Kimb, Jonghwa Parkc a
Strength Evaluation Group, Korea Research Institute of Standards and Science, P.O. Box 102, Yusong, Taejon 305-600, South Korea b Department of Mechanical Engineering, Kongju National University, 182 Shinkwan-Dong, Kongju, Chungnam 314-701, South Korea c School of Transport Vehicle Engineering, Gyeongsang National University, Chinju, Gyeongnam 660-701, South Korea Received 26 September 2000; received in revised form 31 January 2001
Abstract Miniaturized specimen technology is inevitable when the amount of available material for test is limited. In this study, miniaturized Charpy V-notched specimens of 1Cr–1Mo–0.25 V rotor steel with five different aging periods were artificially prepared by an isothermal aging heat treatment at 6308C and tested. For the miniaturized specimens, two different types of specimens with or without side groove were utilized. A correlation between the ductile brittle transition temperature (DBTT) obtained by the miniaturized specimen and that by the standard specimen was investigated. In addition, the relationship between fracture toughness and DBTT by the miniaturized specimen of degraded 1Cr–1Mo–0.25 V rotor steels was proposed. # 2001 Published by Elsevier Science Ltd. Keywords: Miniaturized impact specimen; Fracture toughness; DBTT; Degradation; Normalization
1. Introduction Miniaturized specimen technology is very effective for the characterization of material properties because the collection of material can be done without critical damage to structures or machinery components [1]. Charpy V-notched (CVN) impact test is one of the most adequate methods for that purpose because of simplicity and convenience. CVN impact test provides various informations such as impact energy, lateral expansion, ductile fracture area percentage, etc. It is also employed for clarifying factors affecting ductile *Corresponding author. Tel.: +82-42-868-5383; fax: +82-42-868-5027. E-mail address: [email protected] (S.H. Nahm). 0734-743X/01/$ - see front matter # 2001 Published by Elsevier Science Ltd. PII: S 0 7 3 4 - 7 4 3 X ( 0 1 ) 0 0 0 0 6 - 9
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brittle transition temperature (DBTT) [2–4]. In order to generate an impact energy curve for the measurement of upper self energy (USE) or DBTT, more than 10 standard specimens are required. Thus the miniaturized specimen technology could be necessary as one may find that the amount of material for sample with different operating histories is limited in variety and quantity. The miniaturized specimen technology may produce an impact energy curve even with amount of material for one standard specimen. However, despite such merits, the usage of miniaturized specimen has still been restricted by the size effect that USE and DBTT of miniaturized specimen appear lower than those of standard impact specimen. The decrease of USE is resulted from the reduction of fracture volume of specimen, and the decrease of DBTT is caused by the ductile fracture that is promoted by the plane stress state at the notch tip of miniaturized specimen [5]. Therefore, for the use of miniaturized specimen, one must utilize either a relationship between data of standard impact specimen and those of miniaturized specimen, or a specially designed miniaturized specimen like the side grooved specimen which gives rise to the same strain constraint of fracture plane as that in standard CVN specimen. In this paper, the evaluation technology on standard specimen DBTTstan of degraded turbine rotor steel was attempted using the 18 miniaturized specimens with or without side groove. An isothermal heat treatment was employed for the artificial degradation process of material. The correlation between the fracture toughness in terms of critical stress intensity factor, KIC , and the DBTTstan of degraded turbine rotor steel was also established. Moreover, its validity was verified by the experimental data. 2. Experimental 2.1. Material and specimen The test material is 1Cr–1Mo–0.25V steel that is widely used as a steam turbine rotor material of fossil power plant. The chemical compositions and mechanical properties are contained in Tables 1 and 2, respectively. Since it was difficult to obtain the degraded rotor steels from the turbine rotor, they were simulated by an isothermal heat treatment. Virgin (as received) materials were artificially aged at 6308C for 0, 455, 910, 1365, 1820, 3640, 5460 h, which is the accelerating degradation process for simulating the microstructures of materials which served at the turbine steam temperature of 5388C for about 0, 3, 5, 8, 11, 22, 33 years, respectively (see Table 3 for the details). The periods of time for the heat treatment were determined by the self-diffusion theory for iron element [6]. The geometry of tested ASTM E-23 standard specimen [7] and miniaturized specimen is depicted in Fig. 1, which follows other researchers’ proposition [4]. The size of
Table 1 Chemical compositions of tested material (wt%) Element
C
Si
Mn
P
S
Sb
N
Cu
Ni
Cr
Mo
V
Sn
As
Composition
0.31
0.29
0.80
0.015
0.022
0.015
0.013
0.21
0.39
1.19
1.42
0.25
0.004
0.015
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Table 2 Mechanical properties Temp. (8C)
Yield stress (MPa)
Tensile strength (MPa)
Elongation (%)
Reduction of area (%)
24 538
665.2 533.5
823.1 580.6
18.8 22.7
59.4 55.5
Table 3 Isothermal heat treatment times to simulate the microstructure of the turbine rotor steel aged at the steam temperature of 5388C Aging time at 5388C (h)
25,000
50,000
75,000
100,000
200,000
300,000
Heat treatment time at 6308C (h)
455
910
1365
1820
3640
5460
Fig. 1. Dimensions of standard and miniaturized CVN specimens. (all dimensions in mm).
miniaturized impact specimen is about 18 of standard specimen size. Two different kinds of miniaturized specimens with or without side groove (see Fig. 1(c)) were machined. The direction of all specimens is T-L oriented. 2.2. Test equipment and method The SATEC Charpy impact tester (maximum capacity 33.9 J) was utilized, which was specially designed to adjust the anvil and the hammer weight for the specimens with various dimensions.
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Table 4 describes the difference of basic experimental parameters [1] between standard and miniaturized specimens. The load variation during impact with time was measured and analyzed by the instrumented impact test software developed by Dynatup Co. Fig. 2 represents a typical load history that was obtained from the test. Pm and Pgy in the figure represent the maximum load
Table 4 Comparison of specimen dimensions and experimental parameters Standard Charpy specimen (mm)
Miniaturized Charpy specimen (mm)
Specimen Thickness Depth Width Notch depth Notch-root radius
10.00 10.00 55.00 2.00 0.25
4.50 4.50 24.00 0.90 0.25
Experimental Striker radius Striker tip width Anvil radius Anvil span
8.00 4.00 1.00 40.00
8.00 4.00 1.00 19.96
Fig. 2. A typical impact load history for 50,000 h aged miniaturized specimen tested at 758C. Pm and Pgy represent the maximum load and the general yield load.
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809
and the general yielding load, respectively, during impact. The absorbed impact energy was measured by the loss of initial potential energy of hammer after the breakage of specimen. Prior to the test, each specimen was kept in a thermostat in which the test temperature was maintained within 28C error for 10 min, and tested in 3 s after it was taken out of the thermostat for the test.
3. Results and discussion 3.1. Impact energy characteristics of standard specimen Fig. 3 represents the variation of absorbed impact energy for standard specimen with test temperature. The absorbed impact energy variation curves fitted with hyperbolic tangent Eq. (1) depend on the aging time. Here, E1 ; E2 ; T0 and T1 are fitting constants. The unit of E1 and E2 is J, and that of T0 and T1 is 8C. The difference in DBTT and upper shelf energy (USE) due to different aging time is manifest while the lower shelf energy (LSE) represents less difference. T T0 impact energyCVN ¼ E1 þ E2 tanh : ð1Þ T1 Fig. 4 represents the variation of standard specimen DBTTstan defined as the temperature at which the impact energy on fitted curve is equal to 12 the sum of USE and LSE with aging time. The DBTTstan increases rapidly at the initial stage of aging and approaches to the maximum
Fig. 3. Dependence of impact energy curve on temperature for standard specimen.
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Fig. 4. Dependence of DBTTstan obtained by standard specimen on aging time.
around 50,000 h. After that, no further increment is observed even though the aging proceeds. The similar result for the turbine rotor steel in terms of fracture toughness has been observed by other researchers [8]. 3.2. Impact energy characteristics of miniaturized specimen Figs. 5 and 6 represent the variations of absorbed impact energy with test temperature for the miniaturized specimens with or without side groove. Comparing with those for standard specimens in Fig. 3, USEs of miniaturized specimens appear to be only 10–15% of those of standard specimens, which would be mainly due to the difference of fracture volume between standard and miniaturized specimens as previously mentioned. In deed, the volume of miniaturized specimen in this study is about 12.5% ð18Þ of that of standard specimen. DBTTminis by miniaturized specimens without side groove appear lower than those by standard specimens while DBTTminiSGs by miniaturized specimens with side groove represent almost same values as those by the standard specimens (see Table 5), which implies that the side groove of the 18 miniaturized specimen should provide almost the same strain constraint of fracture plane as that in standard specimen. On the other hand, Kumar et al. [5] proposed a method to predict the size effect on DBTT by normalizing DBTTs obtained from different size specimens by the maximum elastic tensile stress, s0 , at the notch tip. It is assumed that the fracture is controlled by the maximum elastic tensile stress, s0 , at the notch tip. When the stress at the notch tip reaches the critical value, snf , also called the fracture stress, the cleavage crack propagates. The maximum elastic tensile stress is calculated by following Eq. (2). Where Kt is a stress concentration factor at notch tip. Pnm in Eq. (2) is the maximum load at a temperature that the specimen fails by cleavage crack initiation at the point of
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Fig. 5. Dependence of impact energy curve on temperature for miniaturized specimen without side groove.
Fig. 6. Dependence of impact energy curve on temperature for miniaturized specimen with side groove.
general yield; i.e., Pnm ¼ Pngy (see Fig. 7). L, B and b are the span of anvil, the specimen width and the size of ligament, respectively. s0 ¼
3Kt Pnm L : 2Bb2
ð2Þ
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Table 5 Comparison between DBTTstans by standard specimens and DBTTminiSGs by miniaturized specimens with side groove Specimen Aging time (h)
DBTTstan by standard specimen (8C)
DBTTminiSG by miniaturized specimen with side groove (8C)
Error (%)
0 25,000 50,000 75,000 100,000
42 54 88 81 87
40 53 78 75 90
1.64 +2.16 6.75 2.73 +8.82
Fig. 7. Maximum load and general yield load vs. temperature diagrams for (a) 25,000 h aged miniaturized specimen and (b) 100,000 h aged miniaturized specimen.
The stress concentration factor, Kt , can be calculated by Eq. (3) [9]. Here, R is the radius of notch root. Kt ¼
f ¼
g ¼
2ðb=R þ 1Þ f ðb=R þ 1Þ1=2 ; 4ðb=R þ 1Þ=g 3f 2ðb=R þ 1Þ ðb=RÞ1=2
ðb=R þ 1Þarctanðb=RÞ1=2 þ ðb=RÞ1=2 4ðb=RÞ3=2 3½ðb=RÞ1=2 ðb=R 1Þarctanðb=RÞ1=2
;
:
ð3Þ
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Kt calculated by Eq. (3) with respect to the standard and miniaturized specimens in this study are 4.81 and 3.46, respectively. Fig. 7 represents typical examples of determination of Pnm using the Pm vs. temperature plot. According to Lucas et al. [10], maximum load Pnm in Eq. (2) can be also evaluated by Eq. (4) which includes the fracture stress, snf . Here, snf is known as a material constant independent of the size of specimen. Pnm ¼
snf Bb ; 6:52
ð4Þ
which leads to the equation snf ¼
6:52Pnm : Bb
ð5Þ
Fig. 8 represents the variation of snf with the aging time. Substituting Eq. (4) for Eq. (2), Eq. (2) reduces to s0 ¼
0:23Kt Lsnf : b
ð6Þ
Fig. 9 represents a linear relationship between normalized DBTTstan of standard specimen, DBTTstan =s0stan , and that of miniaturized specimen, DBTTmini =s0mini . The slope of correlation line appears one. It implies that the normalization by s0 is well applicable to the experimental results of this study. The correlation between DBTTstan =s0stan and DBTTmini =s0mini for the degraded turbine rotor steels can be expressed by Eq. (7). Thus Eq. (7) make it possible to predict the
Fig. 8. Variation of the fracture stress, s f , with the aging time.
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Fig. 9. Correlation between normalized DBTTstan and DBTTmini.
DBTTstans through the DBTTminis regardless of the extent of degradation. Here constant A is 0.58C/GPa for the turbine rotor steel. DBTTstan DBTTmini ¼ þ A: s0stan s0mini
ð7Þ
3.3. Fracture toughness evaluation by miniaturized specimen Since relationships between fracture toughness, KIC , and DBTTstan of standard specimen are available at the moment [8, 11, 12], Eq. (7) enables one to evaluate the fracture toughness of degraded turbine rotor steel by using the DBTTmini of miniaturized specimen. In this study, Eq. (8) was utilized in order to evaluate the fracture toughness, KIC [8], where the unit of KIC is MPa m1/2, and unit of test temperature T and DBTTstan of standard specimen is 8C. Moreover, constants B and T2 are 177058C MPa m1/2 and 1948C, respectively. B : ð8Þ KIC ¼ T2 ðT DBTTstan Þ Fig. 10 represents the fracture toughness, KIC , calculated by the excess temperature, TDBTTstan . Open marks in Fig. 10 indicate the KIC values of degraded Cr–Mo–V rotor steels evaluated by Eqs. (7) and (8). All the values are located within the scatter band that refers to Ref. [11]. Therefore the miniaturized specimen technology seems valid as an alternative for the fracture toughness evaluation when the available material for the standard specimen technology is not sufficient.
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Fig. 10. Excess temperature versus fracture toughness.
4. Conclusion The evaluation technology on the fracture toughness of degraded turbine rotor steel using miniaturized Charpy V-notched impact specimens with or without side groove was attempted in this study. Obtained results are as follows: (1) DBTTstan of degraded turbine rotor by standard specimen was obtained directly by using the 18 miniaturized specimen with side groove. (2) The conversion between DBTTminis of degraded turbine rotor steel by miniaturized specimens without side groove and those by standard specimens was achieved through Eq. (7) normalized by the maximum elastic tensile stress at the notch tip. (3) An empirical Eq. (8) for the fracture toughness evaluation of degraded turbine rotor steel was proposed. And its validity was supported by the previously reported experimental data.
References [1] Manahan MP. Determination of Charpy transition temperature of ferritic steels using miniaturized specimens. J Mater Sci 1990;25:3429–38. [2] Rolfe ST, Barsom JM. Fracture and fatigue control in structures. Englewood Cliffs, NJ: Prentice Hall, 1977. [3] Firrao D, Begley JA, Silva G, Roberti R, De Benedetti B. The influence of notch root radius and austenitizing temperature on fracture appearance of as-quenched Charpy-V type AISI 4340 steel specimens. Metall Trans A 1982;13A:1003–13. [4] Corwin WR and Houghland AM. Effect of specimen size and material condition on the Charpy impact properties of 9Cr-1Mo-V-Nb steel. ASTM STP 888, American Society for Testing and Materials, Philadelphia, 1986. p. 337.
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