Creep Damage Evaluation 9 - 12 Cr Steels

Life expectations and inspection strategies for parent materials and welds of 9-12% Cr-steels

Part Part Creep damage development and evaluation in parent materialCreep damage development and evaluation in parent material

Karl MaileKarl Maile

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ContentContent

IntroductionExperimental workCreep damage development

Metallographic findingsInfluence of stress state (multiaxiality)Influence of temperature and steel grade

Life time assessmentCreep damage assessmentSummary and conclusion

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IntroductionIntroduction

AimEnlargment of knowledge basis about the evolution of creep damage in components made of new martensitic steels

Prevention of premature failure of componentOptimization of inspection intervalls

AVIF Projects Damage Evolution I – III (2000 – 2009)

Supported by FDBR and VGB

MPA Universität Stuttgart and SPG Dresden

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Experimental Experimental workwork

Creep tests P91, E911575, 600°C 50000 hQuotient of multiaxiality h=σH/σV0.33 (uniaxial) – 0.5 (internal pressure, circumf. notch, tension

force)Measuring of creep strainsReplica

Metallographic investigationsOptical microscopy, SEM, TEM

Numerical simulationstrains, stressesEvolution of microstructure (precipitates)

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Creep damage development Creep damage development

Creep cavities – metallographic appearance

Polished by vibration technology etched Labor microsection

Replica

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Parameters on formation of creep cavities – number, sizeMultiaxiality of stress state

Notch tip Inner surface of hollow cylinder

p

Creep damage developmentCreep damage development

E911, notched, 600°C, 32000 h, integral strain over notch =1,6 %

Gradient due to the influence of loading situation/multiaxialityVerification by Computertomography

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smooth cylinderinternal pressure

Notched cylinderinternal pressure/tension

Trend to an increase of number of cavities with increasing multiaxiality

0,2 0,4 0,60

200

400

600t/tB > 0,9 E911t/tB < 0,75 E911 t/tB > 0,9 P91t/tB < 0,75 P91

num

ber o

f cav

ities

1/m

m2

Quotient of multiaxiality h

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0,5 1,0 1,5 2,00

200

400

600

800 t/tB> 0,9 E911, 600°C t/tB< 0,75 E911, 600°C t/tB > 0,9 P91, 600°C t/tB < 0,75 P91, 575°C

num

ber o

f cav

ities

1/m

m2

creep strain %


smooth cylinderinternal pressure/tension

smooth cylinderinternal pressure

Trend to a decrease of number of cavities with increasing creep strain


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Larger number of cavities at 575° in comparison to that at 600°C

Parameters on formation of creep cavities – number, sizeTemperature

Specimens with test durations > 10000 hNo failureLife time expenditure max. 80%Different melts

0,2 0,4 0,60

500

1000 P91 - 575°C E911- 575°C P91 - 600°C E911- 600°C

num

ber o

f cav

ities

1/m

m2

quotient of multiaxiality h

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Life time assessment

Life time assessment should be based on findings/data of

Replica (optical microscopy)assessment of the microstructure number of cavities (qualitatively, quantitatively) and/or porosityorientation of cavities, microcracks to maximum principal stress

Strain measurementevolution of time-dependent local creep strain

Extraction replica assessment of precipitations

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0,2 0,4 0,6 0,8 1,00

500

1000

1500

2000

P91 E911

num

ber o

f cav

ities

1/m

m2

life time expenditure t/tB

Analysis:P91, E911 – 575 °C, test duration min 10200 h, max. 23000 h


Clear indication of cavities

Increase number of cavities

Validation by ongoing tests with duration > 30000h

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0,4 0,6 0,8 1,00

200

400

1500

2000

E911 P91

num

ber o

f cav

ities

1/m

m2

life time expenditure t/tB

Analysis:P91, E911 – 600 °C, test duration min 10200 h, max. 48000 h


Uncertainty (artefactes)

Clear indication of cavities limited number of cavities < 500

notched, low rupture strain

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Richtreihe zur Bewertung der Gefügeausbildung und –schädigung zeitstandbeanspruchter Werkstoffe von Hochdruckrohrleitungen und Kesselbauteilen. VGB-TW 507 2. Ausgabe 2005; VGB PowerTech Service GmbH, Essen

Rating class Structural or damage condition Limit criteria

1 Subject to creep loading without creep cavities

2a Individual creep cavities Up to 150 voids per mm² 2b Numerous creep cavities, randomly

oriented More than 150 voids per mm²

3a Numerous creep cavities, specific orientation

See remarks

3b Chains of creep cavities; individual grain boundary separations

At least 2 successive grain boundaries with at least 3 voids

4 Microcracks More than one grain boundary length

5 Macrocracks

Smaller amount of creep cavities in comparison with low alloy ferritic steels

At more than 50 hollow cylinders investigated with test durations up to 50000 h no orientation, no chains of creep cavities (1 exception) were found

Microstructure Rating charts for Evaluating the Microstructure and Creep Damage of High-Temperature Steels for High-Pressure piping and Boiler Components VGB – TW 507e 2nd edition

Damage assessment

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Comparison E911 with 14MoV6-3

14MoV6-3; Replica pipe bend, Rating chart 2b

Specimen EH8, E911, 600°C, t/tB= 0,7t=37700 h, 268 C/mm2

14MoV6-3; Replica pipe bend, Rating chart 3b

Specimen EH17, E911, 600°C, t/tB= 0,94t=24410 h, 140 C/mm2

Damage assessment

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Comparison E911 with 14MoV6-3

14MoV6-3; Replica pipe bend, Rating chart 4

Specimen EH22, E911, 600°C, t/tB= 1 tB=32000 h, 167 C/mm2,

Available results show different course of creep damage development in E911 in comparison to that for low alloy ferritic steels reported in TW 507

Damage assessment

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High degree of multiaxiality – notched specimen, 48.000 h internalpressure and additional axial tension load, 600 °C, steel E911 Shortly bevor rupture: indications of chains of cavities with orientation to max principal stressComparable with rating chart 3b/4 TW 507

Damage assessmentHowever…… exceptions have been observed

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Cavity size and porosity

specimen Cavity sizeMin – Max

μm

average number of

cavities

Porosity Temperature°C

Ex-haustiont/tB (%)

Creep strain

ε %

Test du-ration

h

P5 smooth;

P91

0,56 – 3,45 638 1,0094 575 >70 0,45 10196

EH24 notched;

E911

0,77 – 3,29 156 1,0079 600 100 3,9 7250

EH7 smooth,

E911

1,09 – 7,69 278 1,0482 600 90 0,9 38000

• Cavities of P91 at 575°C seems to be smaller than that observed in E911 specimens at 600°C

• If the damaged area as parameter „Porosity“ is used, it is obvious that specimen P5 shows a lower damage degree although the number of cavities in this specimen is larger in comparison to specimen EH7

• The comparison of specimen EH24 with EH7 concludes that cavity growth is influenced by the creep strain

Damage assessment

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Summary and conclusionsSummary and conclusions

The available data with longterm creep tests under multiaxial load show:

Creep cavity formation is a clear indication for creep damage also in 9%Cr-steels

Replica technique therefore has to be considered as an essentialbasis for life time assessment of critical components

The number of creep cavities is lower than in low alloy ferritic steels –however the influence of temperature and casts has to be analyzed in detail

At vast majority of the specimens no orientation of the cavities could be found. Also the formation of chains of cavities were rarely observed even at the end of life time (90%).

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Criteria used for low alloy ferritic steels in the rating charts of VGB TW 507 have to be adjusted to the specific situation with 9%Cr-steels (no reduced creep rupture ductility):

Rating class Structural or damage condition Limit criteria (approximately - only for 600 °C)

1 Subject to creep loading without creep cavities, Up to 50 cavities per mm2

2 Individual creep cavities Up to 100 cavities per mm2

3 Increased number of cavities without orientation

More than 100 cavities up to 200 cavities per mm2

4 numerous number of cavities without orientation More than 200 cavities cavities per mm2

5 numerous number of cavities without orientation, single chains of cavities, individual grain boundary separations

Rating class for 9-11%Cr steels at 600°C:No class 2b und 3b


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Based on the findings and knowledge from the AVIF-project „Damage Evolution“ the following criteria for life time assessment of components made of 9%Cr-steels could be suggested:

Condition: homogenous stress stateno fatigue interaction

• Global total strain 1%• Local creep strain 2%• Increase of creep velocity of 10%• Life time exhaustion on basis of Robinson-Rule of max 60%


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Attaining these values special examimations have to be taken:

• Surface replica – identification of cavities• Extraction replica – evaluation of state of precipitation

• Inelastic stress analysis taking into account thereal loading conditions and melt specific material (creep) characteristics


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Thanks for your attention

Diese Arbeit wurde gefördert von der Stiftung Stahlanwendungsforschung im Stifterverband für die Deutsche Wissenschaft e.V. unter Nummer A 229 gefördert, wofür an dieser Stelle gedankt sei. Ferner wird dem VGB für die finanzielle Unterstützung sowie dem Lenkungskreis unter dem Vorsitz von Herrn Hahn (V&M) für die fachliche Unterstützung gedankt.

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Creep Damage Evaluation 9 - 12 Cr Steels