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New ECCC assessment of creep rupture strength for steel grade X10CrMoVNb9-1 (Grade 91)
International Journal of Pressure Vessels and Piping 87 (2010) 304e309
Contents lists available at ScienceDirect
International Journal of Pressure Vessels and Piping journal homepage: www.elsevier.com/locate/ijpvp
New ECCC assessment of creep rupture strength for steel grade X10CrMoVNb9-1 (Grade 91) W. Bendick a, *, L. Cipolla b, J. Gabrel c, J. Hald d a
Salzgitter Mannesmann Forschung GmbH, D-47259 Duisburg, Germany Centro Sviluppo Materiali, I-00128 Roma, Italy c Vallourec Research Center CEV, F-59620 Aulnoye-Aymeries, France d Technical University of Denmark, DK-2800 Lyngby, Denmark b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2008 Accepted 21 January 2010
A first assessment of creep rupture strength for steel grade X10CrMoVNb9-1 (Grade 91) was performed by ECCC in 1995. The results were included in the European standard EN 10216. Due to a significant increase of test data and test duration it was decided in 2005 to make a re-assessment of the extended database. Different procedures have been used independently by different assessors. The method with the best overall fit of the data set has found to be the ISO CRD method. This is characterized by a two steps procedure: in the first step the mean isotherms are evaluated from the test data, afterwards the evaluated isotherms are used for averaging by a MansoneHaferd master-curve. The results have been chosen as the basis to specify long term creep rupture strength values in a new ECCC data sheet for X10CrMoVNb9-1 (Grade 91). Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Creep rupture strength Data assessment X10CrMoVNb9-1 T/P91
1. Introduction The application of heat resistant steels in power plants requires reliable long term creep rupture strength values as a basis for design. Thus there is a need to evaluate such values for national and international standards or design codes. The unification of Europe demanded a harmonization of national standards and codes. That was one of the reasons to establish the European Creep Collaborative Committee (ECCC) in 1991 [1]. An important task of this association has been the development of a systematic European approach for analysing creep data. Guidance for the assessment of creep rupture data has been laid down in Volume 5 of the ECCC Recommendations [2]. Grade 91 (European designation: X10CrMoVNb9-1) was developed in the 1970s by Oak Ridge National Laboratory (ORNL) in the USA as a modified 9Cre1Mo steel. The modifications included additions of Vanadium, Niobium, Nitrogen and a low Carbon content. The chemistry of Grade 91 was especially designed to promote the formation of a microstructure which provides excellent long term high-temperature strength. The final desired microstructure, which can be achieved by a proper normalizing and tempering heat treatment, consists of tempered martensite, with a fine carbonitride precipitation (MX) inside the matrix, and an
* Corresponding author. E-mail address: [email protected] (W. Bendick). 0308-0161/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2010.03.010
extensive carbide precipitation (M23C6) along the grain boundaries [3e5]. The steel has been adopted in various forms in ASTM/ASME specifications: seamless tube T91 (ASTM A213), seamless pipe P91 (ASTM A335), forged pipe FP91 (ASTM A369), forging F91 (ASTM A182) and casting C12A (ASTM A217). This steel is also included in the European Standard EN 10216-2. The chemical composition is given in Table 1, together with Grade 9 (equivalent to European X12CrMo9-1) which has been the basis of development. The main application is tubing and piping for power plants. 2. Previous assessments on X10CrMoVNb9-1 (Grade 91) A first assessment of creep rupture strength values for Grade 91 steel was made in 1983 on the basis of an ORNL data package consisting of about 30 heats with longest test duration up to 30,000 h. The 105 h creep rupture strength at 600 C is used to provide a characteristic comparison parameter in the following paper. The value of this parameter determined in the first assessment of ORNL was 98 MPa [6]. This assessment was the basis for determination of allowable stress values in the ASME Code. A second assessment was made by ORNL in 1990 with a predicted 105 h value of 93 MPa for 600 C [7]. At that time single test results were available at 1000 F (538 C) and 1100 F (593 C) with durations of 80,000 h. Both assessments were performed by the same parametric method. In Europe an early assessment of X10CrMoVNb9-1 was made by Mannesmann in 1992 based on available European, American and
W. Bendick et al. / International Journal of Pressure Vessels and Piping 87 (2010) 304e309 2,8
Table 1 Chemical composition of T/P9 and T/P91 (X10CrMoVNb9-1) in weight-%.
a
T/P9
T/P91 (X10CrMoVNb9-1)
Max. 0.15 0.25e1.00 0.30e0.60 Max. 0.025 Max. 0.025
0.08e0.12 0.20e0.50 0.30e0.60 Max. 0.020 Max. 0.010 Max. 0.040a 8.00e9.50 Max. 0.040 0.85e1.05 0.18e0.25 0.06e0.10 0.030e0.070
8.00e10.00 0.90e1.10
600°C Data Regression Curve
2,6 2,4 Log Stress (MPa)
C Si Mn P S Al Cr Ni Mo V Nb N
305
2,2 2 1,8 1,6 1,4 1
Value has recently been changed by ASTM/ASME to Max. 0.020.
1,5
2
2,5
3
3,5
4
4,5
5
5,5
Log Time (h)
Fig. 1. Regression analysis of test data at 600 C.
Japanese data with a cumulative testing time of about 8e10 million hours [8]. The assessment was made by using a graphical multiheat averaging and cross plotting method [9] revealing a 105 h value at 600 C of 90 MPa. The long term values of this assessment were included in the VdTÜV data sheet 511/2. In 1995 a first ECCC assessment was made with a database of approximately 14 million testing hours. According to the ECCC rules two separate assessments were made using different methods. The results were close to each other so that average values were published in an ECCC data sheet in 1995 with 94 MPa as 105 h value for 600 C [10]. The long term values of this assessment were also introduced in the European standard EN 10216-2.
Strength Steels [13]. The 105 h values at 600 C were 86 MPa for the MRM-, 94 MPa for the LM- and 84 MPa for the MC-assessment. The predicted long term values of the MRM- and MC-assessments were quite close to each other in the whole temperature range, whereas the assessed LM-values were distinctly higher, especially at 600 C. It was considered that the LM-assessment overestimated the creep rupture strength of Grade 91. On the other hand the ECCC post assessment test PAT 2.1 [14] gave the indication that the other two assessments might be too conservative, especially at temperatures <600 C. Therefore it was decided by ECCC to make an additional assessment in order to improve the prediction in this most important temperature range for the steel grade. The new assessment was performed at Salzgitter Mannesmann Forschung (SZMF) using the ISO CRDA method [15]. A detailed description is given below.
3. New assessments on X10CrMoVNb9-1 (Grade 91) More than ten years later a re-assessment of creep rupture strength of Grade 91 was initiated by ECCC. Available long term tests had passed 100,000 h in the range 575e625 C, which would allow the establishment of valid extrapolations to 200,000 h. Furthermore some deviations from previously extrapolated curves from 1995 gave indications that a re-assessment would be reasonable. For these reasons a new ECCC database was generated. The data were collated from Japan, USA and major European Countries (mainly from Germany, France, England and Italy). Centro Sviluppo Materiali (CSM) and Vallourec Research Center (CEV) were officially designated as ECCC assessors of the new creep rupture database. Two parametric assessments were performed by CSM, one according to PD6605 procedure using the MendelsoneRobertseManson (MRM) equation as timeetemperature parameter [11] and a second according to the Larson-Miller (LM) parameter method. CEV used the Minimum Commitment (MC) equation [12]. Details of the assessments were published in 2005 at the 1st Int. Conf. of Super High
3.1. Data base used for assessment The new assessment was based on practically the same data collation which has been used for the assessments by CSM and CEV. First the meta-data was checked to be in accordance with the European Standard EN 10216-2, in terms of chemical composition, heat treatment and mechanical properties. After this check a total of 2195 creep data (including running tests) remained with cumulative test duration of nearly 18 million hours. A further data reduction was necessary because of the assessment procedure which has been used (the reason is given below). An overview on the used test data is shown in Table 2. An overwhelming number of test results came from tube and pipe material. Less than 300 data points were from bars, plates or
Table 2 Overview on evaluated Grade 91 creep test data. Temp C
450 500 538 550 575 593 600 625 649 650 700
Number of tests with durations specified <10 kh
10e20 kh
20e30 kh
8 89 (6) 13 253 (5) 52 26 (1) 515 (6) 73 55 377 (5) 39
2 7 (3) 3 37 (9) 13 9 101 (13) 3 (1) 2 42 (5) 4
2 (1) 2 (11) (2) 21 (10) 1 2 38 (9) (1) 3 16 (5)
(): Unbroken tests, tR (max): longest rupture time.
tR (max) h 30e50 kh
50e70 kh
70e100 kh
(1) 11 (3)
1 (4) (5) 5
(1) 1 (1) 2 (1)
3 (1) 27 (5)
9 (5)
(1) 1 (6)
6
6
(2)
>100 kh
1 (1)
2 (1) 1 (2)
30,343 59,492 84,309 110,301 24,202 49,514 113,431 12,447 29,363 62,532 14,106
306
W. Bendick et al. / International Journal of Pressure Vessels and Piping 87 (2010) 304e309 2,8
Heat A Heat B Regression Heat A Regression Heat B
Log Stress (MPa)
2,6 2,4
600°C
2,2 2,0 100 h
1,8
300 h
1000 h
3000 h
10000
30000 h
20000 h 40000 h
1,6 1,4 1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
Log Time (h)
Fig. 2. Creep rupture strength of individual heats at 600 C.
Fig. 5. Iso stress series extracted from averaged isotherms given in Fig. 4.
2,8 600°C
Log Stress (MPa)
2,6 2,4 2,2 2,0
600°C-Data 600°C-Mean
1,8 1,6 1,4 1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
Log Time (h)
Fig. 3. Determination of mean 600 C isotherm by heat-wise averaging. Fig. 6. Optimised description of the iso stress series (log ta ¼ 17.5 and Ta ¼ 550 C).
forgings; 550 C, 600 C and 649/650 C can be identified as main testing temperatures covering 81% of the rupture points. 3.2. Assessment procedure The ISO CRD method is a creep data assessment that involves a two steps process. In the first step the experimental data is used to determine mean isotherms separately for each test temperature. This should be the most direct information from the test data on the creep rupture strength at specific temperatures. Although mathematical procedures are used, the material is not forced to behave according to specific mathematical relationships at this step. In the second step a timeetemperature parameter is used to describe the behaviour of the individual isotherms in the whole timee temperatureestress-range by a master-curve. An optimum assessment should be achieved, if the master-curve can describe the
Fig. 4. Mean isotherms determined by heat-wise averaging.
individual isotherms in a most accurate way. The main difference to other parametric assessment procedures is that the ISO CRD method applies the timeetemperature parameter to pre-assessed data (individual isotherms) and not to experimental raw data. The first assessment step shall be explained for test data at 600 C, serving as an example. A straight forward method of determining the mean 600 C isotherm is shown in Fig. 1. All the experimental test data at 600 C are plotted in a stressetimediagram with logarithmic axes. An optimum mean line is received by computerized regression analysis. In an ideal situation this mean line should represent the mean creep rupture strength of Grade 91 at 600 C. However, an inhomogeneous data set can cause severe distortions. For example this can happen, when a tested material is strongly over-represented and dominates the
Fig. 7. MansoneHaferd plot of the evaluated mean isotherms.
W. Bendick et al. / International Journal of Pressure Vessels and Piping 87 (2010) 304e309
307
Table 3 Coefficients of the evaluated MansoneHaferd master-curve. log ta
Ta
a0
a1
a2
a3
a4
R2
17.5
550
8.6363 E 01
1.8227 E þ 00
1.4795 E þ 00
5.3004 E 01
7.1486 E 02
0.9957
whole data set. Therefore a better choice is to make the isothermal averaging on a heat by heat basis which is described in Figs. 2 and 3. Fig. 2 shows the creep test results of two different heats. The creep rupture strength of Heat B is higher than that of Heat A. In this case Heat B is only represented by 4 data points, whereas Heat A has 25 data points. An averaging over both heats would shift the mean line towards the creep rupture strength of Heat A. The heat by heat averaging tries to diminish this distortion by describing the each heat by data points taken from an averaged line through the experimental points. The points are taken at fixed time intervals of 100, 300, 1000, 3000 and 10,000 h, followed by constant steps of 10,000 h. In the example shown in Fig. 2 both heat will be represented by almost the same number of data points. The final averaging overall heats is performed in Fig. 3. Heat-wise averaging implies that only heats could be considered with at least three data points at three different stress levels per test temperature. This was only fulfilled by the temperatures given in Table 2. Isolated test results from other temperatures could not be used. Fig. 4 gives an overview on all isotherms that have been determined separately by heat-wise averaging. The first step of the ISO CRD procedure does not involve any extrapolation. That is why the curves in Fig. 4 have different lengths corresponding to the respective longest testing times. As
Fig. 8. Evaluated mean creep isotherm and PAT 2.1 evaluation for 550 C.
a second step the ISO CRD procedure uses the extracted mean data points from the averaged isotherms to perform an analysis with a timeetemperature parameter, in most cases the MansoneHaferd parameter [15]. The MansoneHaferd description of creep rupture values refers to the following equation:
MH Parameter ¼
log t log ta ¼ f ðsÞ T Ta
(1)
with designations: t ¼ rupture time in h, T ¼ temperature in K and ta, Ta ¼ constants. The stress function is usually given by fourth order polynomial in log s:
f ðsÞ ¼ a0 þ a1 log s þ a2 ðlog sÞ2 þa3 ðlog sÞ3 þa4 ðlog sÞ4
(2)
Since the MH-Parameter should be a constant for a constant stress, iso stress plots received from the mean isotherm in Fig. 4 should principally be able to determine the constants log ta and Ta. Such plots are represented in Fig. 5. The ideal situation would be that all extrapolated lines intersect at one point with the coordinates log ta and Ta. In reality four intersection points were found. Starting with Fig. 5 a systematic survey of possible intersection points has been made in order to find the most optimum set of constants. Guidance was given by the determined overall
Fig. 9. Evaluated mean creep isotherm and PAT 2.1 evaluation for 600 C.
308
W. Bendick et al. / International Journal of Pressure Vessels and Piping 87 (2010) 304e309 1000
Table 4 Newly assessed long term creep rupture strength values for X10CrMoVNb9-1 (Grade 91).
650°C
Temperature C
Stress (MPa)
104 h 100
Data 650°C-Mean +/- 20%
Open Symbol: Running Test
10 10
100
1000
10000
100000
1000000
Time (h)
1,E+06 650°C Data Mean Line
tR (calculated), h
1,E+05
1,E+04
1,E+03
289 270 251 233 216 200 183 167 152 137 122 109 97 86 76 68 61 54
105 h 255 236 217 199 182 164 148 132 117 103 90 79 70 62 55 48 42 36
Values according EN 10216-2 (MPa) 2 105 h a
245 225a 206a 188 170 153 136 121 106 93 81 71 63 56a 49a 43a 36a
104 h 289 271 254 234 216 199 182 166 151 136 123 110 99 89 79 70 62 55
105 h a
258 239a 220a 201a 183a 166 150 134 120 106 94 83 73 65 56 49 42 36
2 105 h 246a 227a 208a 189a 171a 154a 139a 124a 110a 97a 86a 75a 65a 57a 49a 42a 35a
Extended time extrapolation.
3.3. Discussion of results Slope: 0,93 Outlyers: 1,4%
1,E+01 1,E+01
1,E+02
1,E+03
1,E+04
1,E+05
1,E+06
tR (observed), h
Fig. 10. Evaluated mean creep isotherm and PAT 2.1 evaluation for 650 C.
regression coefficient R2. Optimum constants were found to be log ta ¼ 17.5 and Ta ¼ 550 C (Fig. 6). The optimised constants were used for parameterisation of the evaluated mean isotherms according to equation (1). The results are plotted in Fig. 7. A master-curve according to Equation (2) was obtained by regression analysis. There is a good overlap of results from different temperatures and the scattering is rather low. The coefficients of the master-curve are listed in Table 3; the calculated 105 h value at 600 C is 90 MPa.
1000 650°C
100 Tube/Pipe Plate Bar/Forging 650°C-Mean +/- 20% 10 10
500 510 520 530 540 550 560 570 580 590 600 610 620 630 640 650 660 670 a
1,E+02
Stress (MPa)
Values of new assessment (MPa)
100
1000
10000
100000
1000000
Time (h)
Fig. 11. Test results from different product forms at 650 C (only rupture data).
The master-curve is able to predict average creep rupture strength values within the range of evaluated temperatures and time scales. As a first step a comparison with the original test data has to be performed in order to show whether the assessment gives a good description of the data set. In addition post assessment tests (PATs) are recommended by ECCC. The PAT 2.1, described in the ECCC recommendations, compares the predicted rupture times from calculation with the actually observed rupture times. Figs. 8 to 10 show combined plots of creep isotherm and the corresponding PAT 2.1 evaluations for the main testing temperatures 550 C, 600 C and 650 C. The test results of the regarded isotherms are well represented by the calculated average line and the vast majority of data points are situated within the normal scatter-band of 20% around the average line. Concerning the PAT 2.1 evaluation, the ideal behaviour would be that all points are situated within a small scatter-band around the center line with a slope of 1. According to the description of PAT 2.1, the allowed width of the scatter-band is specified as 2.5 times the standard deviation (outer broken lines). Not more than 1.5% of the data points should fall outside this scatter-band. The mean line for the data set should lie within the inner broken lines having a slope between 0.78 and 1.22. As shown in the figures, the present assessment fulfils the requirements of PAT 2.1 very well. The mean line comes very close to the ideal line at 105 h for 550 C and 600 C, which covers the normal application range for Grade 91. For 650 C the assessment gives results for 105 h which are somewhat lower than the observed ones. As has been mentioned before, an overwhelming number of data points comes from tubes and pipes. The question of whether product form has an influence on creep rupture strength could best be investigated for 650 C, since many data points from plates, bars and forgings were available at this temperature (Fig. 11). The figure shows that a distinct difference in creep rupture strength for different product forms cannot be observed. As final result of the new assessment on X10CrMoVNb9-1 (Grade 91), creep rupture strength values for 104 h, 105 h and 2 105 h are tabulated in Table 4. These long term values are published in a new ECCC data sheet for X10CrMoVNb9-1 (Grade 91). For comparison the X10CrMoVNb9-1 values included in the European standard EN 10216-2 are also
W. Bendick et al. / International Journal of Pressure Vessels and Piping 87 (2010) 304e309
presented in Table 4. The number of values with extended time extrapolation decreased due to the achieved longer testing times. The 104 h-values did not differ from the older EN values, whereas a slight decrease up to 4 and 5 MPa was obtained for the 105 h and 2 105 h-values, respectively. This is equivalent to a decrease of 4e6%. The new ECCC values shall also be introduced in EN 10216-2. 4. Summary and conclusions More than ten years after the first ECCC assessment of creep rupture strength for Grade 91 a re-assessment was made due to available long term tests values >100,000 h in the range 575e625 C. Also some deviations from previously extrapolated curves gave indications that a re-assessment would be reasonable. With respect to the former values a slight decrease up to 4 and 5 MPa was obtained for the 105 h and 2 105 h-values, respectively. The ISO CRD method was used for creep data assessment which involves a two steps process. In the first step the experimental data is used to determine mean isotherms separately for each test temperature. Then a timeetemperature parameter is used to describe the behaviour of the individual isotherms in the whole timeetemperatureestress-range by a mastercurve. The main difference to other parametric assessment procedures is that the ISO CRD method applies the timeetemperature parameter to pre-assessed data (individual isotherms) and not to experimental raw data. This procedure has proved to be an excellent method for creep data assessment for other 9% Cr-steels, too [16].
309
References [1] Thornton DV, Ewald J. In: Proc. international colloquium “high temperature materials and design e objectives and achievements of joint research activities. Düsseldorf: German Iron and Steel Institute; 1999. p. 117e26. [2] Creep data validation and assessment procedures, ECCC recommendations. ETD; 2005. [3] Hald J. Proc. ECCC creep conference. London, UK; 2005. p. 20e30. [4] Korcakova L, Hald J. Microstructure evolution in high strength steels for power plant application: microscopy and modelling. Ph.D. Thesis. TU Denmark; 2002. [5] Henry JF. first quarter. Combined Cycle Journal; 2005:24. [6] Sikka VK, Ward CT, Thomas KC. Technical program and data package for use of modified 9%Cre1%Mo steel in ASME Sect. I and VIII; April 7, 1982. [7] Brinkman CR, Alexander DJ, Maziasz PJ. In: Proc. Jt. ASME/IEEE power generation conference. Boston, MA/USA; Oct. 21e25, 1990. [8] Bendick W, Haarmann K, Zschau M. In: Proc. Conf. “Hard coal fired power stations in the years 2000/2015” (Technische Akademie Mannheim e.V.). Dresden, Germany; March 30e31, 1995. [9] Granacher J, Schwienheer M. ECCC procedure document for graphical multiheat averaging and cross plotting method. ECCC Recommendations 2005;5 [Appendix D4]. [10] ECCC data sheets. ETD; 2005. [11] Holdsworth SR, Bullough CK, Orr J. BS PD6605 creep rupture data assessment procedure. ECCC Recommendations 2005;5 [Appendix D3]. [12] Bullough CK. Review of creep rupture assessment methods. ECCC Recommendations 2005;5 [Appendix B1]. [13] Cipolla L, Gabrel J. In: 1st Int. conf. super high strength steels. Rome, Italy; Nov. 2e4, 2005. [14] Holdsworth SR. Review of WG1 evaluation of data assessment methods e recommendation validation. ECCC Recommendations 2005;5 [Appendix C1]. [15] Orr J. ECCC ISO CRDA procedure document. ECCC Recommendations 2005;5 [Appendix D1a]. [16] Bendick W, Gabrel J. ECCC creep conference. London, UK; Sept. 12e14, 2005.
Contents lists available at ScienceDirect
International Journal of Pressure Vessels and Piping journal homepage: www.elsevier.com/locate/ijpvp
New ECCC assessment of creep rupture strength for steel grade X10CrMoVNb9-1 (Grade 91) W. Bendick a, *, L. Cipolla b, J. Gabrel c, J. Hald d a
Salzgitter Mannesmann Forschung GmbH, D-47259 Duisburg, Germany Centro Sviluppo Materiali, I-00128 Roma, Italy c Vallourec Research Center CEV, F-59620 Aulnoye-Aymeries, France d Technical University of Denmark, DK-2800 Lyngby, Denmark b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 November 2008 Accepted 21 January 2010
A first assessment of creep rupture strength for steel grade X10CrMoVNb9-1 (Grade 91) was performed by ECCC in 1995. The results were included in the European standard EN 10216. Due to a significant increase of test data and test duration it was decided in 2005 to make a re-assessment of the extended database. Different procedures have been used independently by different assessors. The method with the best overall fit of the data set has found to be the ISO CRD method. This is characterized by a two steps procedure: in the first step the mean isotherms are evaluated from the test data, afterwards the evaluated isotherms are used for averaging by a MansoneHaferd master-curve. The results have been chosen as the basis to specify long term creep rupture strength values in a new ECCC data sheet for X10CrMoVNb9-1 (Grade 91). Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Creep rupture strength Data assessment X10CrMoVNb9-1 T/P91
1. Introduction The application of heat resistant steels in power plants requires reliable long term creep rupture strength values as a basis for design. Thus there is a need to evaluate such values for national and international standards or design codes. The unification of Europe demanded a harmonization of national standards and codes. That was one of the reasons to establish the European Creep Collaborative Committee (ECCC) in 1991 [1]. An important task of this association has been the development of a systematic European approach for analysing creep data. Guidance for the assessment of creep rupture data has been laid down in Volume 5 of the ECCC Recommendations [2]. Grade 91 (European designation: X10CrMoVNb9-1) was developed in the 1970s by Oak Ridge National Laboratory (ORNL) in the USA as a modified 9Cre1Mo steel. The modifications included additions of Vanadium, Niobium, Nitrogen and a low Carbon content. The chemistry of Grade 91 was especially designed to promote the formation of a microstructure which provides excellent long term high-temperature strength. The final desired microstructure, which can be achieved by a proper normalizing and tempering heat treatment, consists of tempered martensite, with a fine carbonitride precipitation (MX) inside the matrix, and an
* Corresponding author. E-mail address: [email protected] (W. Bendick). 0308-0161/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2010.03.010
extensive carbide precipitation (M23C6) along the grain boundaries [3e5]. The steel has been adopted in various forms in ASTM/ASME specifications: seamless tube T91 (ASTM A213), seamless pipe P91 (ASTM A335), forged pipe FP91 (ASTM A369), forging F91 (ASTM A182) and casting C12A (ASTM A217). This steel is also included in the European Standard EN 10216-2. The chemical composition is given in Table 1, together with Grade 9 (equivalent to European X12CrMo9-1) which has been the basis of development. The main application is tubing and piping for power plants. 2. Previous assessments on X10CrMoVNb9-1 (Grade 91) A first assessment of creep rupture strength values for Grade 91 steel was made in 1983 on the basis of an ORNL data package consisting of about 30 heats with longest test duration up to 30,000 h. The 105 h creep rupture strength at 600 C is used to provide a characteristic comparison parameter in the following paper. The value of this parameter determined in the first assessment of ORNL was 98 MPa [6]. This assessment was the basis for determination of allowable stress values in the ASME Code. A second assessment was made by ORNL in 1990 with a predicted 105 h value of 93 MPa for 600 C [7]. At that time single test results were available at 1000 F (538 C) and 1100 F (593 C) with durations of 80,000 h. Both assessments were performed by the same parametric method. In Europe an early assessment of X10CrMoVNb9-1 was made by Mannesmann in 1992 based on available European, American and
W. Bendick et al. / International Journal of Pressure Vessels and Piping 87 (2010) 304e309 2,8
Table 1 Chemical composition of T/P9 and T/P91 (X10CrMoVNb9-1) in weight-%.
a
T/P9
T/P91 (X10CrMoVNb9-1)
Max. 0.15 0.25e1.00 0.30e0.60 Max. 0.025 Max. 0.025
0.08e0.12 0.20e0.50 0.30e0.60 Max. 0.020 Max. 0.010 Max. 0.040a 8.00e9.50 Max. 0.040 0.85e1.05 0.18e0.25 0.06e0.10 0.030e0.070
8.00e10.00 0.90e1.10
600°C Data Regression Curve
2,6 2,4 Log Stress (MPa)
C Si Mn P S Al Cr Ni Mo V Nb N
305
2,2 2 1,8 1,6 1,4 1
Value has recently been changed by ASTM/ASME to Max. 0.020.
1,5
2
2,5
3
3,5
4
4,5
5
5,5
Log Time (h)
Fig. 1. Regression analysis of test data at 600 C.
Japanese data with a cumulative testing time of about 8e10 million hours [8]. The assessment was made by using a graphical multiheat averaging and cross plotting method [9] revealing a 105 h value at 600 C of 90 MPa. The long term values of this assessment were included in the VdTÜV data sheet 511/2. In 1995 a first ECCC assessment was made with a database of approximately 14 million testing hours. According to the ECCC rules two separate assessments were made using different methods. The results were close to each other so that average values were published in an ECCC data sheet in 1995 with 94 MPa as 105 h value for 600 C [10]. The long term values of this assessment were also introduced in the European standard EN 10216-2.
Strength Steels [13]. The 105 h values at 600 C were 86 MPa for the MRM-, 94 MPa for the LM- and 84 MPa for the MC-assessment. The predicted long term values of the MRM- and MC-assessments were quite close to each other in the whole temperature range, whereas the assessed LM-values were distinctly higher, especially at 600 C. It was considered that the LM-assessment overestimated the creep rupture strength of Grade 91. On the other hand the ECCC post assessment test PAT 2.1 [14] gave the indication that the other two assessments might be too conservative, especially at temperatures <600 C. Therefore it was decided by ECCC to make an additional assessment in order to improve the prediction in this most important temperature range for the steel grade. The new assessment was performed at Salzgitter Mannesmann Forschung (SZMF) using the ISO CRDA method [15]. A detailed description is given below.
3. New assessments on X10CrMoVNb9-1 (Grade 91) More than ten years later a re-assessment of creep rupture strength of Grade 91 was initiated by ECCC. Available long term tests had passed 100,000 h in the range 575e625 C, which would allow the establishment of valid extrapolations to 200,000 h. Furthermore some deviations from previously extrapolated curves from 1995 gave indications that a re-assessment would be reasonable. For these reasons a new ECCC database was generated. The data were collated from Japan, USA and major European Countries (mainly from Germany, France, England and Italy). Centro Sviluppo Materiali (CSM) and Vallourec Research Center (CEV) were officially designated as ECCC assessors of the new creep rupture database. Two parametric assessments were performed by CSM, one according to PD6605 procedure using the MendelsoneRobertseManson (MRM) equation as timeetemperature parameter [11] and a second according to the Larson-Miller (LM) parameter method. CEV used the Minimum Commitment (MC) equation [12]. Details of the assessments were published in 2005 at the 1st Int. Conf. of Super High
3.1. Data base used for assessment The new assessment was based on practically the same data collation which has been used for the assessments by CSM and CEV. First the meta-data was checked to be in accordance with the European Standard EN 10216-2, in terms of chemical composition, heat treatment and mechanical properties. After this check a total of 2195 creep data (including running tests) remained with cumulative test duration of nearly 18 million hours. A further data reduction was necessary because of the assessment procedure which has been used (the reason is given below). An overview on the used test data is shown in Table 2. An overwhelming number of test results came from tube and pipe material. Less than 300 data points were from bars, plates or
Table 2 Overview on evaluated Grade 91 creep test data. Temp C
450 500 538 550 575 593 600 625 649 650 700
Number of tests with durations specified <10 kh
10e20 kh
20e30 kh
8 89 (6) 13 253 (5) 52 26 (1) 515 (6) 73 55 377 (5) 39
2 7 (3) 3 37 (9) 13 9 101 (13) 3 (1) 2 42 (5) 4
2 (1) 2 (11) (2) 21 (10) 1 2 38 (9) (1) 3 16 (5)
(): Unbroken tests, tR (max): longest rupture time.
tR (max) h 30e50 kh
50e70 kh
70e100 kh
(1) 11 (3)
1 (4) (5) 5
(1) 1 (1) 2 (1)
3 (1) 27 (5)
9 (5)
(1) 1 (6)
6
6
(2)
>100 kh
1 (1)
2 (1) 1 (2)
30,343 59,492 84,309 110,301 24,202 49,514 113,431 12,447 29,363 62,532 14,106
306
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Heat A Heat B Regression Heat A Regression Heat B
Log Stress (MPa)
2,6 2,4
600°C
2,2 2,0 100 h
1,8
300 h
1000 h
3000 h
10000
30000 h
20000 h 40000 h
1,6 1,4 1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
Log Time (h)
Fig. 2. Creep rupture strength of individual heats at 600 C.
Fig. 5. Iso stress series extracted from averaged isotherms given in Fig. 4.
2,8 600°C
Log Stress (MPa)
2,6 2,4 2,2 2,0
600°C-Data 600°C-Mean
1,8 1,6 1,4 1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
Log Time (h)
Fig. 3. Determination of mean 600 C isotherm by heat-wise averaging. Fig. 6. Optimised description of the iso stress series (log ta ¼ 17.5 and Ta ¼ 550 C).
forgings; 550 C, 600 C and 649/650 C can be identified as main testing temperatures covering 81% of the rupture points. 3.2. Assessment procedure The ISO CRD method is a creep data assessment that involves a two steps process. In the first step the experimental data is used to determine mean isotherms separately for each test temperature. This should be the most direct information from the test data on the creep rupture strength at specific temperatures. Although mathematical procedures are used, the material is not forced to behave according to specific mathematical relationships at this step. In the second step a timeetemperature parameter is used to describe the behaviour of the individual isotherms in the whole timee temperatureestress-range by a master-curve. An optimum assessment should be achieved, if the master-curve can describe the
Fig. 4. Mean isotherms determined by heat-wise averaging.
individual isotherms in a most accurate way. The main difference to other parametric assessment procedures is that the ISO CRD method applies the timeetemperature parameter to pre-assessed data (individual isotherms) and not to experimental raw data. The first assessment step shall be explained for test data at 600 C, serving as an example. A straight forward method of determining the mean 600 C isotherm is shown in Fig. 1. All the experimental test data at 600 C are plotted in a stressetimediagram with logarithmic axes. An optimum mean line is received by computerized regression analysis. In an ideal situation this mean line should represent the mean creep rupture strength of Grade 91 at 600 C. However, an inhomogeneous data set can cause severe distortions. For example this can happen, when a tested material is strongly over-represented and dominates the
Fig. 7. MansoneHaferd plot of the evaluated mean isotherms.
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307
Table 3 Coefficients of the evaluated MansoneHaferd master-curve. log ta
Ta
a0
a1
a2
a3
a4
R2
17.5
550
8.6363 E 01
1.8227 E þ 00
1.4795 E þ 00
5.3004 E 01
7.1486 E 02
0.9957
whole data set. Therefore a better choice is to make the isothermal averaging on a heat by heat basis which is described in Figs. 2 and 3. Fig. 2 shows the creep test results of two different heats. The creep rupture strength of Heat B is higher than that of Heat A. In this case Heat B is only represented by 4 data points, whereas Heat A has 25 data points. An averaging over both heats would shift the mean line towards the creep rupture strength of Heat A. The heat by heat averaging tries to diminish this distortion by describing the each heat by data points taken from an averaged line through the experimental points. The points are taken at fixed time intervals of 100, 300, 1000, 3000 and 10,000 h, followed by constant steps of 10,000 h. In the example shown in Fig. 2 both heat will be represented by almost the same number of data points. The final averaging overall heats is performed in Fig. 3. Heat-wise averaging implies that only heats could be considered with at least three data points at three different stress levels per test temperature. This was only fulfilled by the temperatures given in Table 2. Isolated test results from other temperatures could not be used. Fig. 4 gives an overview on all isotherms that have been determined separately by heat-wise averaging. The first step of the ISO CRD procedure does not involve any extrapolation. That is why the curves in Fig. 4 have different lengths corresponding to the respective longest testing times. As
Fig. 8. Evaluated mean creep isotherm and PAT 2.1 evaluation for 550 C.
a second step the ISO CRD procedure uses the extracted mean data points from the averaged isotherms to perform an analysis with a timeetemperature parameter, in most cases the MansoneHaferd parameter [15]. The MansoneHaferd description of creep rupture values refers to the following equation:
MH Parameter ¼
log t log ta ¼ f ðsÞ T Ta
(1)
with designations: t ¼ rupture time in h, T ¼ temperature in K and ta, Ta ¼ constants. The stress function is usually given by fourth order polynomial in log s:
f ðsÞ ¼ a0 þ a1 log s þ a2 ðlog sÞ2 þa3 ðlog sÞ3 þa4 ðlog sÞ4
(2)
Since the MH-Parameter should be a constant for a constant stress, iso stress plots received from the mean isotherm in Fig. 4 should principally be able to determine the constants log ta and Ta. Such plots are represented in Fig. 5. The ideal situation would be that all extrapolated lines intersect at one point with the coordinates log ta and Ta. In reality four intersection points were found. Starting with Fig. 5 a systematic survey of possible intersection points has been made in order to find the most optimum set of constants. Guidance was given by the determined overall
Fig. 9. Evaluated mean creep isotherm and PAT 2.1 evaluation for 600 C.
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Table 4 Newly assessed long term creep rupture strength values for X10CrMoVNb9-1 (Grade 91).
650°C
Temperature C
Stress (MPa)
104 h 100
Data 650°C-Mean +/- 20%
Open Symbol: Running Test
10 10
100
1000
10000
100000
1000000
Time (h)
1,E+06 650°C Data Mean Line
tR (calculated), h
1,E+05
1,E+04
1,E+03
289 270 251 233 216 200 183 167 152 137 122 109 97 86 76 68 61 54
105 h 255 236 217 199 182 164 148 132 117 103 90 79 70 62 55 48 42 36
Values according EN 10216-2 (MPa) 2 105 h a
245 225a 206a 188 170 153 136 121 106 93 81 71 63 56a 49a 43a 36a
104 h 289 271 254 234 216 199 182 166 151 136 123 110 99 89 79 70 62 55
105 h a
258 239a 220a 201a 183a 166 150 134 120 106 94 83 73 65 56 49 42 36
2 105 h 246a 227a 208a 189a 171a 154a 139a 124a 110a 97a 86a 75a 65a 57a 49a 42a 35a
Extended time extrapolation.
3.3. Discussion of results Slope: 0,93 Outlyers: 1,4%
1,E+01 1,E+01
1,E+02
1,E+03
1,E+04
1,E+05
1,E+06
tR (observed), h
Fig. 10. Evaluated mean creep isotherm and PAT 2.1 evaluation for 650 C.
regression coefficient R2. Optimum constants were found to be log ta ¼ 17.5 and Ta ¼ 550 C (Fig. 6). The optimised constants were used for parameterisation of the evaluated mean isotherms according to equation (1). The results are plotted in Fig. 7. A master-curve according to Equation (2) was obtained by regression analysis. There is a good overlap of results from different temperatures and the scattering is rather low. The coefficients of the master-curve are listed in Table 3; the calculated 105 h value at 600 C is 90 MPa.
1000 650°C
100 Tube/Pipe Plate Bar/Forging 650°C-Mean +/- 20% 10 10
500 510 520 530 540 550 560 570 580 590 600 610 620 630 640 650 660 670 a
1,E+02
Stress (MPa)
Values of new assessment (MPa)
100
1000
10000
100000
1000000
Time (h)
Fig. 11. Test results from different product forms at 650 C (only rupture data).
The master-curve is able to predict average creep rupture strength values within the range of evaluated temperatures and time scales. As a first step a comparison with the original test data has to be performed in order to show whether the assessment gives a good description of the data set. In addition post assessment tests (PATs) are recommended by ECCC. The PAT 2.1, described in the ECCC recommendations, compares the predicted rupture times from calculation with the actually observed rupture times. Figs. 8 to 10 show combined plots of creep isotherm and the corresponding PAT 2.1 evaluations for the main testing temperatures 550 C, 600 C and 650 C. The test results of the regarded isotherms are well represented by the calculated average line and the vast majority of data points are situated within the normal scatter-band of 20% around the average line. Concerning the PAT 2.1 evaluation, the ideal behaviour would be that all points are situated within a small scatter-band around the center line with a slope of 1. According to the description of PAT 2.1, the allowed width of the scatter-band is specified as 2.5 times the standard deviation (outer broken lines). Not more than 1.5% of the data points should fall outside this scatter-band. The mean line for the data set should lie within the inner broken lines having a slope between 0.78 and 1.22. As shown in the figures, the present assessment fulfils the requirements of PAT 2.1 very well. The mean line comes very close to the ideal line at 105 h for 550 C and 600 C, which covers the normal application range for Grade 91. For 650 C the assessment gives results for 105 h which are somewhat lower than the observed ones. As has been mentioned before, an overwhelming number of data points comes from tubes and pipes. The question of whether product form has an influence on creep rupture strength could best be investigated for 650 C, since many data points from plates, bars and forgings were available at this temperature (Fig. 11). The figure shows that a distinct difference in creep rupture strength for different product forms cannot be observed. As final result of the new assessment on X10CrMoVNb9-1 (Grade 91), creep rupture strength values for 104 h, 105 h and 2 105 h are tabulated in Table 4. These long term values are published in a new ECCC data sheet for X10CrMoVNb9-1 (Grade 91). For comparison the X10CrMoVNb9-1 values included in the European standard EN 10216-2 are also
W. Bendick et al. / International Journal of Pressure Vessels and Piping 87 (2010) 304e309
presented in Table 4. The number of values with extended time extrapolation decreased due to the achieved longer testing times. The 104 h-values did not differ from the older EN values, whereas a slight decrease up to 4 and 5 MPa was obtained for the 105 h and 2 105 h-values, respectively. This is equivalent to a decrease of 4e6%. The new ECCC values shall also be introduced in EN 10216-2. 4. Summary and conclusions More than ten years after the first ECCC assessment of creep rupture strength for Grade 91 a re-assessment was made due to available long term tests values >100,000 h in the range 575e625 C. Also some deviations from previously extrapolated curves gave indications that a re-assessment would be reasonable. With respect to the former values a slight decrease up to 4 and 5 MPa was obtained for the 105 h and 2 105 h-values, respectively. The ISO CRD method was used for creep data assessment which involves a two steps process. In the first step the experimental data is used to determine mean isotherms separately for each test temperature. Then a timeetemperature parameter is used to describe the behaviour of the individual isotherms in the whole timeetemperatureestress-range by a mastercurve. The main difference to other parametric assessment procedures is that the ISO CRD method applies the timeetemperature parameter to pre-assessed data (individual isotherms) and not to experimental raw data. This procedure has proved to be an excellent method for creep data assessment for other 9% Cr-steels, too [16].
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