Long-term creep rupture strength assessment: Development of the European Collaborative Creep Committee post-assessment tests

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International Journal of Pressure Vessels and Piping 85 (2008) 2–13 www.elsevier.com/locate/ijpvp

Long-term creep rupture strength assessment: Development of the European Collaborative Creep Committee post-assessment tests$ G. Merckling, On behalf of the European Collaborative Creep Committee (ECCC) RTM BREDA, Milan, Italy

Abstract The European Collaborative Creep Committee was founded in 1991/1992 by European Industrial and Research Organisations to harmonise and exchange the scientific activities in the field of long-duration creep. Besides the creep strength assessment activity, during the whole period of its existence ECCC supported the Working Group 1 ‘‘Creep Data Generation and Assessment Procedures’’ that was responsible for the set-up, maintenance and adaptation to new situations of the ECCC Creep Strength Assessment Procedure, the core of which are the post-assessment tests (PATs). The innovative view and the from this derived reversed approach on data assessment, allowing all procedures to be potentially applied as long as a written guidance is available and as long as it does fulfil all PATs, permitted outstanding progress and substantial gain in reliability of the ECCC strength determination. The present contribution summarises the ECCC history from the point of view of data assessment procedure optimisation, application to different data set types for different assessment purposes and evolution up to the latest computerised versions, which simplify and accelerate the check on PATs. r 2007 Elsevier Ltd. All rights reserved.

1. Introduction In a time in which reduction of national and community research budgets and of the resources available to single companies becomes constantly more severe, in which safety, reliability and foreseeability-related technical requirements are becoming more stringent, in which flexible service conditions introduce new hazards and push plants in operation conditions not covered by experience, the availability of reliable creep design values based on a possibly broad, high-quality experimental basis including long-term data, assessed with sound, reproducible and reliable procedures, becomes fundamental. The European Collaborative Creep Committee (ECCC) wanted to offer the forum for the leading European Industries and Research Organisations and has succeeded over the past 14 years in coordinating and harmonising the effort to commonly keep and possibly enlarge the technical lead in high-temperature creep properties production. One $ This article appeared in its original form in Creep & Fracture in High Temperature Components: Design & Life Assessment Issues, 2005. Lancaster, PA: DEStech Publications, Inc. Corresponding author. E-mail address: [email protected]

0308-0161/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2007.06.007

of the achieved results, perhaps the most significant in terms of innovativeness and most characteristic in terms of effort harmonisation, is the development of an evaluation method to understand and to weight the quality of the result of specific data assessments, which allows one to produce reliable and credible creep strength predictions, suitable for inclusion into European standards. The present report will try to recall and outline the development and its motivations up to the newest computer-based semi-automatic procedures of the ECCC post-assessment tests (PATs), which have become a fundamental basis to the ECCC data sheets and the therein included creep strengths. 2. The ECCC The ECCC was founded in 1991 by a large number of industrial and research companies, and includes up to today around 48 organisations from the United Kingdom, Germany, Italy, France, Switzerland, Austria, The Netherlands, Belgium, Sweden, Denmark, Finland, Portugal, The Czech Republic and Slovakia. The companies participating in ECCC cover the whole scope of industries and research organisations dealing with

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high-temperature problems: steel makers, steel product manufacturers, boiler, turbine, plant and equipment builders, utilities and end users, inspection bodies, research institutes, technical universities and testing houses. ECCC’s common purposes were laid down in a memorandum of understanding (MoU), which was jointly agreed, signed by all members and is still the fundamental basis of the cooperation. The MoU establishes that ECCC is a voluntary association of organisations representing their nations, led by industry with the aim to improve, reinforce and enhance the position of European industry in the high-temperature application market. The MoU defines which targets are to be addressed by ECCC to support European research in the creep-related area, which includes, among other topics, collation, exchange and joint assessment for creep data, support to European standardisation, European-wide coordination of creep data generation and definition of common procedures for data generation and assessment.

In this paper, no further consideration will be given to the first two aims, the results of which have been published elsewhere [1,2]. The third goal of WG1, vice versa, is the motivation for PAT birth. 4. The ideal assessment procedure 4.1. The status quo ante During the first WG1 meetings, it appeared clearly that all participants, relying on long-term successful experiences, intended to promote a favourite data assessment method to determine and to extrapolate long-term creep strengths. The bouquet of proposed methods ranged

 

from non-computerised graphical methods [3], and ‘‘traditional’’ parameter-based procedures, like [4–6], to automated and user-friendly optimisation routines, which allow a skilled user to select among a variety of different creep equations, like [7,8], to innovative, advanced statistics-based methods, trying to minimise a subjective assessor’s interference, like [9,10],1 to less-known models, partially pretending to base on physically consistent assumptions, like [11–13].



3. The challenge

 In 1992, the ECCC Management Committee (MC) agreed to support the CEN Technical Committees preparing the first generation of standards dealing with hightemperature materials (to that time future EN 10028, EN 10216, EN 10217 and EN 10222, etc.) by forwarding recommendations for the creep rupture strengths that for the first time should be included in the new European standards. As several National standards in Europe included or recommended creep strength values, so that simple acceptance of already assessed values seemed difficult, the MC organised Working Group 3 (WG3) to jointly, i.e. European wide, collate and assess all available data for the materials, candidate to the new EN standards. In order to standardise and guarantee a comparable technical quality to all WG3 collation and assessment activities, Working Group 1 (WG1) was additionally established under the leadership of Dr. Stuart Holdsworth (Alstom Power UK), including some of the most recognised European experts in the field of creep data generation and assessment, coming from Germany, the UK, Austria, Sweden, Italy, Finland, etc. At this early stage, the MC clearly underestimated the challenge that it charged WG1 with. In 2 years, WG1 should have prepared

  

a common standard for the experimental creep data generation minimum requirements (which later became the basis of EN 10291), a common creep data collation protocol, against which WG3 should have collated with a minimum pedigree the European-wide available creep data, and finally a commonly agreed creep strength assessment procedure, possibly unique for all materials, or at least a set of assessment procedures, if a unique one would not have been applicable universally.

3



In these days there was the clear assumption and expectation that either all ‘‘acceptable’’ methods would— in the hand of their most skilled users—predict the same strengths or one of them will overwhelmingly show its superiority. And during the methods presentation and explanation at the WG1 meetings, given by the related supporter, it also became clear that each user expected his method to be the superior one, trying to prove this by prior experiences, theoretical motivation, successful applications and victory in former round robin tests. 4.2. Methods comparison As by expert discussion and former result comparison no commonly acceptable solution was obtainable, WG1 decided to clarify the situation by a new round robin context, which challenged all experts promoting methods to show the quality of their prediction applying their favourite procedure to the same data sets. As ECCC’s goal was the supply to all involved European standards of strength values derived from all over Europe collated, probably large-sized data sets, WG1 organised, with the support of the related WG3 group, four big data sets, belonging to four different materials (Table 1). After careful metadata analyses, the data sets were distributed among the participants, who made huge efforts in best assessing them and producing most credible results. 1

Derived during the development of the ECCC WG1 PATs.

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G. Merckling / International Journal of Pressure Vessels and Piping 85 (2008) 2–13

Table 1 Data sets in WG1’s first round robin [1] Material (European designation)

Material (conventional designation)

Temperatures

Casts

Test points

Unbroken test points

Longest duration (kh)

Points with duration 4100 kh

Data contributing countries

10 CrMo 9 10 X19CrMoVNbN 11 1 X6CrNi 18 10 X5NiCrAlTi 31 20 X9NiCrAlTi 32 21

214 Cr 1 Mo 11CrMoVNb 304H 800

23 6 24 12

98 33 96 33

1117 369 843 552

99 45 47 57

141 129 111 84

20 8 7 0

5 2 2 3

4.3. Innovative view on assessment When assessing the round robin results with its large scatter on the predicted strength values, already the most simple response (which prediction is right?) appeared difficult to an objective solution. But in this discussion, the main evaluation points for the judgment of a creep data assessment, were generally agreed to and finally formed the basis of the PATs. These basic principles are:

  

Fig. 1. An example of the round robin assessments and the bid scatter in the predicted long-term behaviour [1].

An example overview of the data population and the predicted trend lines is given in Fig. 1. A perhaps even more significant, numerical summary on predicted strengths is reported in Table 2. The results varied considerably among the participants and it was not really clear which assessment was the ‘‘right’’ one. Moreover, it became quickly obvious that assessment acceptability may depend on the material investigated, the assessor’s knowledge about the material behaviour and undoubtedly on the user’s experience in applying the assessment procedure: the attempt to duplicate results by having two participants using the same assessment method failed evidently. The final conclusion of this round robin exercise was the definitive burial of the idea that there could be the ideal creep strength assessment method, valid for all applications, materials and data sets. Vice versa, it was the starting point for an innovative view on the data assessment task as a whole.

Physical credibility, i.e. the model must predict trend lines compatible with the expected physically reasonable and understandable material behaviour. Good data description, i.e. re-computed rupture times of the data used to calculate the model equation must come sufficiently close to the experimental ones. Stable extrapolation, i.e. the addition of a few new data, even if long term, should not revolutionise the prediction.

The application of these principles and the gradual definition of the PATs on the one hand led to a drastic reduction in result scatter of the assessments and some disappointment among the participants (Table 3) and on the other hand showed:





 

Although some assessment methods succeeded in more than one condition, no assessment procedure and no assessor will have a priori the right prediction, i.e. the ideal assessment procedure is an illusion. Different procedures can come to the same acceptable conclusions, if suitably handled. And both the expert judgment and the as far as possible objective assessment procedure may fail or may succeed in giving the best predictions. Reliable prediction is difficult in the hand of an assessor who is not familiar with the properties of the material assessed. There is an evident link between a procedure and the experience of the assessor in using it: experienced assessors testing procedures they are not used to showed a higher risk to fail than when using their ‘‘suited’’ one.

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Table 2 Examples of predicted 10,0000 and 70,000 h (Alloy 800) strength values at significant temperatures: results of WG1’s first round robin [1] Material (European designation)

Material (conventional designation)

Temperature (1C)

Test points

Longest duration (kh)

Points with duration 4100 kh

Number of participating methods

Range of predicted strength values (MPa)

Variability (%)

10 CrMo 9 10

214 Cr 1 Mo

11CrMoVNb

X6CrNi 18 10

304H

X5NiCrAlTi 31 20 X9NiCrAlTi 32 21

800

195 254 192 91 149 79 186 271 122 74 93 70

121 118 113 128 129 94 77 99 111 73 64 44

5 4 2 3 3 1 0 0 1 0 0 0

13

X19CrMoVNbN 11 1

500 550 600 500 550 600 600 650 700 600 700 800

113–133 61–78 32–23 247–327 168–122 24–64 79–109 51–69 30–46 97–118 41–50 18–25

718 727 748 732 737 7165 739 740 752 722 722 739

14

13

8

Table 3 Predicted 100,000 and 70,000 h (Alloy 800) strength values at significant temperatures: results of WG1’s first round robin before and after PATs [1] Material (European designation)

Material (conventional designation)

Number of participating methods

Range of predicted strength values (MPa)

Variability (%)

Methods succeeding PATs

Range of predicted strength values for methods succeeding PATs (MPa)

Variability of methods succeeding PATs (%)

10 CrMo 9 10

214 Cr 1 Mo

13

11CrMoVNb

14

X6CrNi 18 10

304H

13

X5NiCrAlTi 31 20 X9NiCrAlTi 32 21

800

718 727 748 732 737 7165 739 740 752 722

5

X19CrMoVNbN 11 1

113–133 61–78 32–23 247–327 168–122 24–64 79–109 51–69 30–46 97–118

120–127 65–68 29–33 294–314 146–167 54–64 109 67 39 115–118

76 76 712 77 714 718 Na Na Na 73

41–50 18–25

722 739

50 23–25

70 78



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A reliable procedure needs written guidance, in order to allow others to come to the same results on the same data set or to trace back the predictions.

The stepwise assessment and refinement of these basic principles led to the details codified and proceduralised in the ECCC Recommendations Volume 5 part 1a [1,14,15] and became the core of the ECCC data assessment procedure. With the PATs becoming the core, a more omni comprehensive procedure was also defined and applied. Its flow diagram is given in Fig. 2. It includes recommendations for a careful data pre-assessment, including checks on pedigree data, experimental soundness, data distribution and minimum requirements for the data population in terms of duration, present temperatures, amount of data points, casts and dominant, i.e. best-tested, casts. Mainly

3

1

2

the latter point has been re-assessed several times, in the light of the withdrawal of ISO 6303, the newly issued EN 12952 and on the basis of increasing experience with assessment results produced in the last few years according to the ECCC criteria. The final data amount and distribution minimum requirements for ECCC assessments are reported in Table 4. 5. Application of PATs After the ECCC MC discussed and approved the PATs in 1996, published the same year in Volume 5, now part 1a of [1], all ECCC data assessments were checked and validated through the application of the new procedure. The data sheets including strength values available in 2005 [16] are based on this approach and gain their credibility and reliability not only from the reputation of assessors

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SET MATERIAL SPECIFICATION

PRE-ASSESSMENT (Sect.2.3)

RE-SET MATERIAL SPEC'N

YES YES

NO NO

n>2

NO

CRDA 1 ECCC-CRDA PROCEDURE* (App.D)

CRDA 2 ECCC-CRDAs PREFERRED BUT OTHER CRDAs ACCEPTABLE

SATISFY PAT REQUIREMENTS (Sect.2.4)

SATISFY PAT REQUIREMENTS (Sect.2.4)

YES

NO

nth REPEAT CRDA

YES

SATISFY STRENGTH COMPARISON REQUIREMENTS (Rec.5, Sect.2.2)

NO

YES REPORT (APP.E1) Fig. 2. The ECCC procedure for the production of creep strengths suitable for standards. The references given in the boxes apply to [1]. (Source: [1])

and laboratories’ skill but also from the soundness of the ECCC assessment procedures. But with time going by and new targets arising for ECCC, also other topics, expecting assessment by skilled experts, needed to be addressed by WG1. 5.1. Creep strain strength Already in Volume 5 part 1b of [1], the application of PATs to creep strain strengths is shown. To that time, nevertheless, just the assessment of times to specific strains via methods usually applied to rupture data, like [3,6,20], were considered.

5.2. Stress relaxation Under the need to supply to the new EN 10269, related to bolting materials also for high temperature, WG1 tested the applicability of the PATs (with the additional PAT1.4) to stress relaxation data sets, when they are assessed by procedures derived from creep strength models. Although in some cases the model efficiency was relatively reduced, due to different mechanisms becoming responsible at different temperatures for the phenomenon, the PATs (for example in Fig. 3) again showed their applicability in indicating the most credible among all available predictions (see Volume 5, part 1c in [1]).

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Table 4 Data minimum requirements for ECCC assessments [1] Interim-minimum requirements

Target-minimum requirements Original (TM1)

For X3 casts, there should be tu(T,s0) observations from:  X3 tests at each of X3 temperatures, at intervals of 50–100 1C J X3 tests per temperature (different s0) with tu,maxX10 kh

For X6 casts, there should be tu(T,s0) observations from:  X5 tests at each of X3 temperatures in the design application range at intervals of 25–50 1C J X4 tests per temperature (different s0) with tup40 kh J X1 tests per temperature with tu,maxX40 kh

TM2

TM3

For data sets with X300 observations, originating from X10 casts, at X5 temperatures covering the range TMAIN7X50 1C

For data sets with X500 observations, originating from X20 casts, at X5 temperatures covering the range TMAIN7X50 1C

For X5 casts, there should be tu(T,s0) observations from:  X5 tests at each of X2 temperatures in the design application range at an interval(s) of 25–50 1C J X4 tests per temperature (different s0) with tup35 kh J X1 tests per temperature with tu,maxX35 kh

For X5 casts, there should be tu(T,s0) observations from:  X5 tests X1 temperature(s) in the design application range (at intervals of 25–50 1C) J X4 tests per temperature (different s0) with tup35 kh J X1 tests per temperature with tu,maxX35 kh

Predicted strength values determined from an interim-minimum data set shall be regarded as tentative until the data requirements defined in one of the target-minimum columns are obtained

for the assessment of smaller data sets, typical to the new material types of interest. This time from the beginning there was the expectation that the PATs still could be an option in supporting the evaluation of competitive assessments. A series of round robins were again set in place:

PAT2.1 2.5

predicted stress

2

1.5

 1

log(S_calc)

WH-LOG2

wh

WH+2.5sarlt

WH+LOG2

WH-2.5sarlt

Regr

0.5 1

1.5

2

2.5

exp. stress



Fig. 3. PAT example: PAT 2.1 on relaxation data set: test successful.

5.3. Smaller data sets The original goal of ECCC was the common assessment of creep data sets for standard purposes, i.e. data sets supposed to include a large amount of data points, belonging to many casts and test temperatures and having—at least partially— long, i.e. service-relevant, durations. With the publication of the ECCC data sheets in 1996 [17], the new ECCC interest in welds and post-exposure materials and the foundation of a WG3 subgroup dealing with superalloys, WG1 was requested to issue guidelines

Two based on sub-groups of the data sets of the previous round robin. From all four materials sub-size data sets were extracted and commonly assessed. As a particular thrill, in some cases the sub-size data sets were particularly dominated by a test temperature or cast, as may happen for new materials at the beginning of their application. Another group of round robins included data sets for welds, which commonly are relatively small and additionally included mechanical and metallurgical complex issues.

The discussion during these activities showed that in principle the application of the ECCC assessment procedure centred on the PATs was fully applicable and that the success or failure in applying the PATs was an excellent indicator about the credibility of the assessment. Nevertheless, the assessors’ worries focussed on the credibility of the long-term, i.e. extended, extrapolation required to obtain creep strength values at 100 kh or more. The following became clear:



There is no way to substitute long-term data by mathematical predictions, as long as there is no detailed

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and motivated knowledge about the material development with time and temperature. To underpin partially the credibility of long-term extrapolation from too short duration data sets, significant effort still needs to be applied and research in this field should be encouraged. If no other way is possible than to have long-term prediction from short-term data, then these strengths must be considered as preliminary and their drastic, maybe non-conservative, change must be envisaged, as

soon as long-term data are available. Nevertheless, a few tools could be used to enhance confidence in prediction: J The use of data factors may in some situations allow one to define a lower bound of the long-term behaviour [18,19]. J Statistical modelling including unbroken points consideration, for instance, based on maximum likelihood or survival statistics have been already applied on large data sets [10] and may allow successful predictions also on smaller sets, although no application is yet known.

Table 5 Results of the latest post-exposure material round robin, including all predictions CRL assessment name

Used CRL method

Used data

Pipe D: further service for 50 kh

Estimate of true life end (h)

PATs not successful Only Only Only Only

D, D, E D SIEM D SIEM2

Parametric Parametric Parametric Parametric

PE data only of pipe D

Si Si Si Si

2.8 M 32 M 17 M 17 M

All SIEM

Parametric

PE data of power utility steam pipes

Si

6.5 M

All2 A All1 All SIEM2 All E

Parametric PD6605 Parametric Parametric Parametric

All PE data

Si Si Si Si Si

800 k 1.4 M 1.7 M 6.9 M

All ECCC

LDAR based on ECCC+parametric

All PE data after suitable ‘‘assimilation’’ process

All ASTM

LDAR based on ASTM+parametric

Omega Poli

Strain-based MPC Omega method (polynomial descr.) Strain based MPC Omega method (parametric descr.) Strain-based modified MPC Omega method

Omega Para Omega E Omega E2 Omega E4 Omega E5 Omega E6

330 k

Si

200 k

Si

790 k

Si

2.5 M

Si

Na

Si

90 k

PE data with To650 1C 2021 data+low-stress PE data 2021 data+low-stress PE data with To650 1C 2021 data+PE data with so70 MPa

Si Si Si

120 k 175 k 182 k

Si

240 k

Only pipe D 520/560 1C

Si Si Si Si

32 M 105 M 152 M 400 k

Si

Na 204 k

All PE creep strain data

Omega I Omega I2 Omega I2-ref Omega PoliLDAR Omega Para LDAR

Strain-based API RP 579 Omega method Full API RP 579 LDAR/ECCC +Omega Poli

New ASTM

ISO 6303

Virgin material ASTM

Si

New ECCC

DESA

Virgin material ECCC

Si

New Omega E3

Strain-based modified MPC Omega method

Virgin material 2021 project

Si

203 k

IS

circolare ISPESL 15/92

Creep strength acc. DIN 17175

Si

1.2 M

Original E

Limite di accettabilita`

Virgin ASTM+PE data pipe D

Si

Variability

All PE creep strain after suitable ‘‘assimilation’’

LDAR/ECCC+Omega Para

PATs successful

Na

Na

240 k ?

450 k Factor 1000

Factor 1.65

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If and where creep strain data are available and applicable, this information could be used to increase that of the rupture data set. Methods such as graphical cross plotting [3], curve family assessment with DESA [20], the 4y [21] or the MPC-Omega method [22] can have a benefit from this. J In some cases, such as welds and post-exposure data, reference master curves can be used to ensure the prediction and to underpin the shape of a given curve. J In other situations, like using modified grades where modification is not expected to change the creep behaviour, comparable data, i.e. data belonging to the non-modified grade, may be included to enlarge the available data sets. In other cases also the concept of similar curves [1] can be used, if the actual data are expected to be stronger or weaker but essentially behaves like another ‘‘similar’’ grade. J

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slightly adjusted to the specific characteristics of the residual life assessment, allowed one to reduce this scatter to a factor of 1.65, i.e. to a range between 200,000 and 330,000 h. It appeared that out of 29 assessment attempts using all kinds of offered data options, only 4 were successful in all PATs. In particular, PAT 1.1b—examples are given in Figs. 4 and 5, which requests comparison with the same grade of virgin material, turned out to be a severe obstacle to the majority of the assessments. In the mean time, the application of PATs to assessment results during remnant life computation has been included in the informative appendix to the new Italian Regulation and has been recommended by the pressure vessel authority [23].

PAT 1.1b + Omega 560°C 2.4 2.2 log (stress)

The acknowledgment of these additional tools encouraged ECCC WG1 to include an additional PAT1.1b (a comparison between the small data set prediction and a comparable or reference material; for instance, weld behaviour prediction against suited parent material or post-exposure versus virgin material, etc.), which in many cases turned out to be very discriminating. Additionally, the requirements for extrapolation repeatability were slightly loosened, due to the manifest extrapolation weakness of small and short-term data sets. Nevertheless, at the end the ECCC data assessment procedure, enhanced with suited PATs for small data sets, weld data sets and post-exposure material data sets, was confirmed, although long-term tests were acknowledged to be essential.

2 1.8 1.6 1.4 1.2 1E+1

1E+2

5.4. Post-service exposure strength

1E+4

1E+5

1E+6

poli

ASTM+20

para

ASTM-20

ASTM

ECCC+20

ECCC

ECCC-20

Fig. 4. PAT 1.1b applied to the Omega para and Omega poli method at 560 1C compared with the ASTM and the ECCC virgin material scatter band: both fail (see Table 5).

SPG LDAR PAT 1.1b 580°C 1000

stress

As an example and a particular condition of small creep data sets which require long-term extrapolations, a few results out of the post-exposure creep data round robin, performed in 1998/1999 and upgraded in 2002/2005 by a WG1 subgroup named PEDS (Post Exposure creep Data Subgroup, since 2001 ‘‘WG1.1’’), are shown in Table 5. The task to the assessors was to verify whether a serviced 10 CrMo 9 10 (214 Cr 1 Mo) pipe would be suitable for another 50,000 h in service, and to make a guess about its remnant exploitable service life. The data set—typical to residual life assessment tasks—was made up of 10 creep curves with maximum duration 8000 h. In addition, each assessor was allowed to use other ‘‘similar’’ post-exposure materials, base materials and standard strength values. The results in Table 5 agree fortunately on the allowance for Pipe D to continue service for another 50,000 h, but the remnant life prediction derived from these assessments had an enormous scatter of approximately a factor 1000, i.e. the predictions ranged from 92,000 h to over several millions. In these circumstances, the application of the PATs as laid down in ECCC Recommendations Volume 5 part III,

1E+3

time to rupture [h]

virg+20% virg-20% virgin LDAR

100

10 1E+1

1E+2

1E+3

1E+4

1E+5

1E+6

time

Fig. 5. PAT 1.1b applied to a linear damage accumulation rule enhanced assessment based on the concept of similar curves at 580 1C: test successful (see Table 5).

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5.5. Creep strain assessments In the latest period of WG1 activities, creep strain strengths modelled by complex equations were considered. Detailed results can be found in the present conference presented by the relevant protagonists (e.g. the paper by Holdsworth et al. in Session 4). Also in this case, PATs were shown to be reliably applicable for specific strains considered, although a new parameter named Z was additionally introduced, with Z ¼ 102:5sarlt oZ limit , sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn 2 n i¼1 ðlog tp;i  log tp;i Þ ; sarlt ¼ n1

ð1Þ

where tpe,i is the ith experimental time to specific strain e, tp;i the correspondent predicted time, n the number of experimental data points and Zlimit is between 4 and

probably 10, depending on the type of data set considered (single-cast, single-laboratory or multi-cast, multi-laboratory). An example for Z is given for grades 22 and 91 data sets for different strains in Figs. 6 and 7, assessed with an MPC-Omega method and a modification of it including a primary creep term. The higher low-strain Z-values are due to the fact that the original Omega model does not consider primary deformation and due to the higher scatter in the experimental data, quite high in both cases and becoming reasonably low for large strains and rupture only. Additionally, the high-strain Z values for the P91 grade are significantly higher, because here a multi-cast and multi-laboratory data set, including tests with continuous and interrupted strain measurement, were assessed, where the P22 set was just a single cast. The immediate meaning of Z is shown in Fig. 8: Z is the width between the 2.5 sarlt lines (where sarlt is the standard deviation of the residual log times for all the all data

P22

1E+7

10000 Data A

1E+5

1000

Z

tp02*

1E+3

100

1E+1 1E-1 Wbh +log2 -log2

1E-3 10

+2.5s(tp02all -2.5s(tp02all ) all

1E-5 1E- 3 1E- 2 1E- 1 1E+ 0 1E+ 1 1E+ 2 1E+ 3 1E+ 4 1E+5 tp02

1 0

5

10

15

20

25

30

strain [%]

m = 0.66

Fig. 6. Dependence of Z on strain for creep strin data of a single cast assessed with MPC Omega.

sarlt = 0.856

Z = 102.5sarlt =138

MPC Omega METHOD 6

P91 5

1000 log(Tr-cal)

Data A

Z

100

4 3 Log(Trcal) center line +log(2)

2 1

-2.5*STD

0

10

-log(2) Mean +2.5*STD

1

2

3

4

5

Log(Tr-exp)

m = 0.98

1 0

5

10

15

20

25

sarlt = 0.12

Z = 102.5sarlt =2.01

30

strain [%] Fig. 7. Dependence of Z on strain for creep strain data from six casts assessed with a modified MPC Omega, including a primary term.

Fig. 8. Examples for high-Z (top, related to 0.2% creep strain)) and low-Z (bottom, related to rupture time prediction from creep strain) results within PAT 2.1 of modified MPC-Omega method applications: Z is the interval width between the violet 72.5 sarlt lines.

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data at all temperatures in PAT 2.1 or sirlt the standard deviation of the isothermal residual log times for all the data at constant temperature in PAT 2.2). Evidently,

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the more these lines are apart, the easier the regression line stays between them, i.e. the easier it becomes to fulfil PAT 2.

PAT 2.1 - all Data Points

PAT 1.1 103

103

[M qpa]

tu [h]

104

102

103

102

3×101 1 5×100 10

103

102

104

101 5×100 1 5×100 10

105

103

102

tu[h]

tu[h]

560°C 2.4

2.2

2.2

log (stress)

log (stress)

520°C 2.4

2 1.8 1.6 1.4 1.2 1E+1

2 1.8 1.6 1.4

1E+2

1E+3

1E+4

1E+5

1.2 1E+1

1E+6

1E+2

time to rupture [h]

1E+4

102

104

103

105

5.00

0.00 101

106

102

D 520°C 105

105

104

104 tu* [h]

[M qpa]

3×102

σ [MPa]

tu[h]

103

D 560°C

103

102

102

101

1E+6

PAT 1.3

102

101

1E+5

10.00

[M qpa]

[M qpa]

1E+3

time to rupture [h]

PAT 1.2

103

101 101

105

104

102

103 tu[h]

104

105

101 101

102

103 tu[h]

Fig. 9. Example results delivered from the Automated PATs program ‘‘ePAT’’ [24].

104

105

ARTICLE IN PRESS G. Merckling / International Journal of Pressure Vessels and Piping 85 (2008) 2–13

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Therefore, the minimisation of Z for the strain of interest and the requirement to fulfil all PATs is the key for a reliable assessment of creep strain strengths. 6. Latest developments The overall acceptance of the PATs, central point in the ECCC creep data assessment procedure, has also within the ECCC not yet been reached. The main points of criticism are essentially the relatively large amount of work that the PATs application requires, and the belief, that all ‘‘sufficiently skilled’’ assessments will meet them anyway. In order to overcome this situation, ECCC initiated and co-funded at the University of Darmstadt, Institut fu¨r Werkstoffkunde, the automation of the PATs [24], which from this conference on will be available. Essentially, this automation consists in a few easy steps:

  

 



Assess the creep data with the chosen assessment method. Put data (experimental and predicted by the assessment equation) into a set-up special MS-Excel spreadsheet. Check the consistency of these data with the program ‘‘cull.exe’’, which will deliver an output Excel sheet including the culled data sets on which PAT 3 should be performed. Do PAT 3 on the culled sets with the selected assessment method and enter the PAT 3 results in the Excel sheet. Do the ePAT.exe-program with the completed Excel sheet, which will J check the input data, J perform all PATs stepwise, presenting graphs of all results (which can be exported), additional information (logarithmic residuals, etc.) and numerical results, J produce a standardised report containing ‘‘neutral and objective’’ statements about successful and failed PATs and J save relevant data for PATs to a separate Excel sheet to allow additional evaluation by the user. Check ePAT results: J if all PATs were successful, the assessment is acceptable; J if one or more PAT fails, the assessment should be repeated.

A typical example of the results of ‘‘ePAT’’ is summarised in Fig. 9 for an MPC Omega method application on post-exposure material. ‘‘ePAT’’ delivers for all PATs an exportable graph and all required numerical results. The duration for an ‘‘ePAT’’ application is in the order of a few minutes, after which an objective evaluation about the assessment credibility is available.

7. Conclusions Since 1991, ECCC represents the exchange forum of the European high-temperature industry. Among the many activities and contributions ECCC gave to its members and to the European Standardisation bodies, one in particular is representative of the European collaboration and for its intent to gain reliable creep strengths for the long term: the innovative approach developed by the ECCC WG1 in assessing data, becoming an objective tool to evaluate the credibility, description quality and extrapolation stability of the model equation developed in the data assessment— the PATs. With minor detail adaptation to various assessment conditions, the main body of the PATs has proven to be sound and reliable; upgrades and additions allow its application expansion to creep strength, relaxation strength, small and weldment data sets, post-exposure remnant creep strength and creep strain models. With the development of an automated tool, the ‘‘ePAT’’ program, now also the effort to do them as well as the possibility to ‘‘direct’’ PAT success are significantly reduced. Acknowledgements The authors acknowledge gladly the contribution of all ECCC members and participants and in particular of the WG1 members, who under the lead of Stuart Holdsworth de facto developed, optimised and validated over the last 14 years the here described ECCC creep data assessment procedure and the PATs. Also, the funding support of the EC via the BRITEEuram Concerted Action BE-5524 ‘‘Creep’’ (1992–1996), the Framework V Thematic Network BET2-0509 ‘‘Welcreep’’ (1997–2001) and the Thematic Network ‘‘Advanced Creep’’ (2001–2005) is gladly recognised. References [1] ECCC Recommendations 2001. Leatherhead: ERA Technology Ltd.; Volume 1 [issue 4], 2001: creep data validation and assessment procedures—overview. In: Holdsworth SR, editor.; Volume 2, 2001: terms and terminology for use with stress rupture, creep and stress relaxation: testing, data collation and assessment. In: Morris P, Orr J, Servetto C, Seliger P, editors. (a) Part I [issue 7]: Parent material, (b) Part IIa [issue 2]: Welding, (c) Part IIb [issue 2]: Testing on welds, (d) Part III [issue 2]: Post exposure material.; Volume 3, 2001: recommendations for data acceptability criteria for creep, creep rupture, stress rupture and stress relaxation data. In: Holdsworth SR, Granacher J, Klenk A, Buchmayr B, Gariboldi E, editors. (a) Part I [issue 5]: Parent material, (b) Part II [issue 2]: Creep data for welds (c) Part III [issue 2]: Post exposure material.; Volume 4 [issue 4], 2001: guidance for the exchange and collation of creep rupture, creep strain–time and stress relaxation data for assessment purposes. In: Merckling G, Calvano F, Bullough CK, editors.; Volume 5, 2001. Guidance for the assessment of creep rupture, creep strain and stress relaxation data. In: Holdsworth SR, Merckling G, editors. (a) Part I [issue 4]: Full-size datasets, (b) Part IIa [issue 1]:

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Sub-size datasets, (c) Part IIb [issue 1]: Weldment datasets, (d) Part III [issue 1]: Datasets for PE (ex-service) materials. ECCC Recommendations Upgrade 2003. Ashtead: European Technology Development Ltd.; Volume 1 [issue 5] 2003: Creep data validation and assessment procedures—overview. In: Holdsworth SR, editor.; Volume 2, 2003: Terms and terminology for use with stress rupture, creep and stress relaxation: testing, data collation and assessment. In: Morris P, Orr J, Servetto C, Seliger P, Holsworth SR, editors. (d) Part III [issue 3]: Post exposure material, (e) Part IV [issue 1]: Creep crack initiation.; Volume 3, 2003: recommendations for data acceptability criteria for creep, creep rupture, stress rupture and stress relaxation data. In: Holdsworth SR, Granacher J, Klenk A, Buchmayr B, Gariboldi E, Brett S, et al., editors. (c) Part III [issue 3]: Post exposure material, (d) Part IV [issue 1]: Creep crack initiation.; Volume 4 [issue 5], 2003: Guidance for the exchange and collation of creep rupture, creep strain–time and stress relaxation data for assessment purposes. In: Merckling G, Nespoli N, Calvano F, Bullough CK, editors. Bendick W, Haarmann K, Wellnitz G. Evaluation of design values for Steel 91. In: Proceedings of ECSC information day on the manufacture and properties of steel 91 for the power plant and process industries, 5/11/92, Du¨sseldorf, Germany. Larson FR, Miller J. A time–temperature relationship for rupture and creep stresses. Trans ASM 1952;74. Manson SS, Haferd AM. A linear time–temperature relation for extrapolation of creep and stress rupture data. NACA TN 1953;2890. ISO 6303: 1981 Annex ‘‘pressure vessel steels not included in ISO 2604, Parts 1 to 6—derivation of long time stress rupture properties,’’ 1981 ISO 6303: 1981 Annex ‘‘Pressure vessel steels not included in ISO 2604, Parts 1 to 6—derivation of long time stress rupture properties’’. 1981. Ivarson B. Evaluation of different methods for extrapolation of creep rupture data. SIMR-Report IM1794, 1983. Granacher J, Monsees M, Pfenning A. Anwenderhandbuch des Programmes DESA 2.2, IfW TH Darmstadt, 1995. Barraclough DR, Logsdon J. A revised method for the analysis o stress rupture data. Nuclear Electric Report TD/SEB/REP/1664/92 and ECCC WG1 doc ref 5524/WG1/58.

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[10] PD 6605, 1998. Guidance on methodology for the assessment of stress rupture data. BSI, 1998. [11] Spera DA. Calculation of elevated temperature cyclic life considering low cycle fatigue and creep, NASA Report TN D-5317. Cleveland, OH: Lewis Research Centre; 1969. [12] Merckling G. Kriech- und Ermu¨dungsverhalten ausgewa¨hlter metallischer Werkstoffe. Doctor thesis, Universita¨t Karlsruhe, 1989. [13] Manson SS, Muraldihan U. Analysis of creep rupture data for five multiheat alloys by the minimum commitment method using double heat term centring technique. Research Project 638-1, EPRI-CS-3171, 1983. [14] Holdsworth SR, Orr J, Granacher J, Merckling G, Bullogh CK, on behalf of the ECCC-WG1. European Creep Collaborative Committee Activities on creep data generation and assessment methodologies. In: Coutsouradis D, et al., editors. Materials for advanced power engineering. Liege, 3.–6.10.1994. Dordrecht: Kluwer Academic Publishers; 1994. p. 591–600. [15] Holdsworth SR. Recent developments in the assessment of creeprupture data. In: Proceedings of the eighth international conference on creep and fracture of engineering materials and structures, Tsukuba, November 1999. p. 1–8. [16] ECCC Data Sheets Issue 2005. European Technology Development Ltd. Publishers. [17] ECCC Data Sheets Issue 1996. ERA Technology Publishers. [18] Sandstro¨m RO, Linde´ L. Precision in the extrapolation of creeprupture data. J Test Eval 1999;27:203–10. [19] Holdsworth SR, Merckling G. The assessment of sub-size creeprupture datasets. In: Proceedings of the international colloquium, 50th anniversary of German Creep Committee, Du¨sseldorf, November 1999. [20] KloX KH, Granacher J, Oehl M. Beschreibung des Zeitdehnverhaltens warmfester Sta¨hle, Teil 1 und 2, Mat. wiss. u. Werstofftech. 24, 1993. p. 287–95, 331–8. [21] Evans RW, Wilshire B. In: Creep of metals and alloys. London: Publ. Inst. Metals; 1985. [22] Prager M. Development of the MPC omega method for life assessment in the creep range. J Press Vess Technnol 1995;117:95. [23] ISPESL Circolare 34/04, 2004. [24] Berger C, Scholz A, Schwienheer M, Linn S. Guidance for post assessment test input sheet release 2, May 2004.