Materials Science and Engineering A 510511 (2009) 5863

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Materials Science and Engineering A

journa l homepage: www.e lsev ier .co

Long-te i

K. KimurNational Institu

a r t i c l

Article history:Received 9 JanReceived in reAccepted 30 A

Keywords:Creep deformaModied 9CrCreep data sheTransient creeAccelerating c

form(Natibouteformweenlot. IBlac

ing crractiocreahighhoweeep stress

1. Introduction

Creep-stused in atributed toincreasingcreep-ruptuin comparithe short-ttion methohave beenevaluation ovalue of sevter understmicrostructing creep exgrowth of Zhas been fothe Z-phasepotential camany of thprecipitatio

CorresponE-mail add

It is useful to investigate creep deformation properties for theevaluation of long-term creep strength property [18], since degra-

0921-5093/$ doi:10.1016/j.mrength-enhanced ferritic (CSEF) steelshavebeenwidelymodern thermal power plant and those have con-improve energy efciency of the plant by means of

steam temperature and pressure. Unexpected drop inre strength, however, was observed in the long-termson with the anticipated creep-rupture strength fromerm properties. In order to obtain a reliable estima-d of long-term creep strength, several new approachesinvestigated on CSEF steels [15]. According to a re-f long-term creep strength, the allowable tensile stresseral CSEF steels was reduced [610]. In order to bet-

and long-term creep strength properties of CSEF steels,ural evolution and the degradation mechanism dur-posure have been investigated. Precipitation and rapid-phase during creep exposure at elevated temperaturesund and many investigations have been conducted on[1117]. This phase has been considered as one of theuses of degradation, since growth of Z-phase consumese ne particles of MX carbonitride and reduces theirn strengthening effect.

ding author. Tel.: +81 29 859 2229; fax: +81 29 859 2201.ress: kimura.kazuhiro@nims.go.jp (K. Kimura).

dation due tomicrostructural change during creep exposure shouldbe reected on creep deformation. In March 2007, the rst vol-ume of Atlas of Creep Deformation Properties on 9Cr1MoVNbsteel (ASME Grade 91), that is a typical CSEF steel was publishedas a series of NIMS Creep Data Sheets [19]. In the present study,creep deformation of 9Cr1MoVNb steels (ASME SA-213 T91)is investigated and the stress dependence of creep deformation isdiscussed.

2. Experimental procedure

Three melts, hereinafter referred to as heats, of modied9Cr1Mosteel (ASMESA-213T91)wereused in this study. Chemicalcomposition and heat treatment condition of the steels are shownin Tables 1 and 2, respectively. The steels have been produced bydifferent manufacturers and the contents of minor elements andheat treatment condition indicate a little difference, however, noobvious difference has been observed on grain size, hardness andcreep strength. Tensile test was conducted under a constant nom-inal strain rate of 5105 s1 up to about 2% of total strain, and itwas increased to 1.25103 s1. Strain rate of the tensile test wascontrolledbyadifferential transformerwhose resolutionwas1m,with an extensometer attached to the gauge portion of the speci-men. Flow stress was evaluated under a constant nominal strainrate of 5105 s1, as well as 0.2% offset yield stress, in addition to

see front matter 2009 Elsevier B.V. All rights reserved.sea.2008.04.095rm creep deformation property of mod

a , H. Kushima, K. Sawadate for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan

e i n f o

uary 2008vised form 10 March 2008pril 2008

tion1Mo steeletpreep

a b s t r a c t

The rst volume of Atlas of Creep Dein March 2007, as a part of the NIMSCreep deformation properties up to astage has been observed, and creep dstages. Good linear relationships betwithin a transient stage in a loglog pexponential law, logarithmic law andcreep strain at the onset of acceleratthan 1% in the long-term region. Life fstrain, to time to rupture tended to inan important parameter for design ofcreep stage in the short-term regime,regime. For evaluation of long-term crextrapolated in consideration of the sm/locate /msea

ed 9Cr1Mo steel

ation Properties was published on modied 9Cr1Mo steelsonal Institute for Materials Science) Creep Data Sheet series.70,000h have been investigated. No clear steady-state creepation of the steel consists of transient and accelerating creepcreep strain vs. time and creep rate vs. time were observedt was appropriately expressed by a power law rather than ankburns equation. With decrease in stress, the magnitude ofeep stage decreased from about 2% in the short-term to lessn of the time to specic strain of 1% creep strain and 1% totalse with decrease in stress. The time to 1% total strain, that istemperature components, was observed to lie in the transientver, it shifted to the accelerating creep stage in the long-termtrength properties, an experimental creep test data should bedependence of creep deformation properties.

2009 Elsevier B.V. All rights reserved.

K. Kimura et al. / Materials Science and Engineering A 510511 (2009) 5863 59

Table 1Chemical composition (mass%) of the steels studied.

Heat C Si Mn P S Ni Cr

MGA 0.10 0.38 0.40 0.015 0.001 0.12 8.53MGB 0.09 0.34 0.45 0.015 0.001 0.20 8.51MGC 0.09 0.29 0.35 0.009 0.002 0.28 8.70SA-213, T91 0.080.12 0.200.50 0.300.60 0.020 0.010 0.40 8.009.50

Heat Mo Cu V Nb Al N

MGA 0.96 0.022 0.21 0.076 0.010 0.050MGB 0.90 0.026 0.205 0.076 0.02 0.042MGC 0.90 0.032 0.22 0.072 0.001 0.044SA-213, T91 0.851.05 0.180.25 0.060.10 0.04 0.0300.070

Table 2Heat treatment condition, grain size and hardness of the steels studied.

Heat Normalizing Tempering Austenite grainsize number

Vickershardness

MGA 1045 C/10min AC 780 C/60min AC 9.2 221MGB 1050 C/60min AC 760 C/60min AC 8.8 221MGC 1050 C/10min AC 765 C/30min AC 9.2 230

tensile stress that is a ow stress under a constant nominal strainrate of 1.25103 s1.

Creep dathe steels wProperties,deformatioinstantaneotiation of terate were eFig. 1.

3. Results

3.1. Creep s

Stress vsfrom 500 tslight differand 30MPathe same inis observedlong-term aat 550 and

tress vs. time to rupture curves over a range of temperatures from 500 tof 9Cr1MoVNb steel.ta over a range of temperatures from 500 to 700 C ofhich was reported in the Atlas of Creep Deformation

No.D-1 of NIMS Creep Data Sheets [19], was used. Creepn data at 550 and 600 C of the steels was analyzed andus strain, i, creep strain, c, total strain, t, time to ini-rtiary creep, tT, time to rupture, tR, andminimum creepvaluated. Denition of the parameters is described in

and discussion

trength

. time to rupture curves over a range of temperatureso 700 C of the steels are shown in Fig. 2. Althoughence in creep-rupture strength is observed at 700 C, creep-rupture strength of three steels are essentiallythe tested condition. At 500 C, rectilinear relationship, however, slope of the curves becomes steeper in thet 550 C and above. Minimum creep rate of the steels600 C are plotted against stress and shown in Fig. 3.

Fig. 2. S700 C oFig. 1. Creep curve and denition of parameters.Fig. 3. Stress vs. minimum creep rate curves and ow stresses obtained by tensiletest at 550 and 600 C of 9Cr1MoVNb steel.

60 K. Kimura et al. / Materials Science and Engineering A 510511 (2009) 5863

Tensile strength, that is a ow stress under a nominal strain rateof 1.25103 s1, and ow stress under a nominal strain rate of5105 s1 are also plotted in the same gure. Stress exponent, nof the minimum creep rate is 16 at 550 C and 12 in the high-stressregime at 6in stress atrate at 550to strain-rasimilar to Ation of Z-phafter creepat 650 C [1not consistlong-term [rate is consnism fromregime [20,

Creep-rushown in Firupture eloabove, is obit decreasetion of areais shown inanalysis areis observedother.

3.2. Creep d

Initial stted againstat 600 C is0.1% at 100smaller thait decreases

Creep cu100 to 200of creep strthe slope ofbeginning oship is obselinear creepplot, relatiostage was a(2) exponenequation [2

Power law

Exponentia

Logarithmic

Blackburns

where C isstants obtaiinclude instneous strainthe purposesion analys600 C of Mboth stressand time inlinear relatisented by th

reep rupture ductility over a range of temperatures from 500 to 700 C ofoVNb steel.

ep rate vs. time curves of MGC heat at 600 C over a range ofs from 100 to 200MPa are shown in Fig. 9. Creep deforma-the steel consists of transient and tertiary creep stages andious steady state is observed. In the transient creep stage,ate decreases linearly with increase in time in a double log-ic plot. Creep rate in the transient creep stage at 200 anda is calculated from the predicted creep curves shown inand those are described in Fig. 10 for 200 and 100MPa. Lin-tionship between creep rate and timewithin transient creepn a loglog plot is accurately represented by a power law, asreep curve. Creep deformation in the transient creep stagesteel could not be expressed by any of the other expres-Similar results have also been observed on carbon steel, lowrMo steels and SUS 316 austenitic steel in this issue [24]. Itbeuseful to analyze creepdeformationpropertieswithin thent creep stage with a power law for several creep-resistantals.00 C, however, it decreases to about 4 with decrease600 C. Large stress dependence of the minimum creepC and in the high-stress regime at 600 C is equivalentte dependence of ow stress evaluated by tensile test,SME Grade 122 and Grade 92 steels [20,21]. Precipita-ase was investigated on MGC heat and it was observedexposure for about 10,000h at 600 C and about 5000h4,15]. However, the beginning of Z-phase formation didwith inection of the stress-time to rupture curve in the17]. The change in stress dependence ofminimumcreepidered to be caused by change in deformation mecha-low stress and elastic regime to high-stress and plastic21].pture elongation and reduction of area of the steels areg. 4 for MGA, MGB and MGC heat. High ductility whosengation is 20 to 40% and reduction of area is 80% andserved for all the steels in the short-term, however,

s in the long-term, especially pertaining to a reduc-at 650 C. MonkmanGrant relationship of the steelsFig. 5. Parameters of A and B obtained by regressionalso indicated in the gure. Good linear relationshipfor all the steels and those are equivalent to each

eformation properties

rain values of the steels at 550 and 600 C are plot-stress as shown in Fig. 6. Magnitude of initial strainabout 0.2% at 200MPa, and it decreases to less thanMPa with decrease in stress. At 550 C, initial strain isn that at 600 C at the same stress condition, however,with decrease in stress similar to that at 600 C.rves ofMGC heat at 600 C over a range of stresses fromMPa are shown in Fig. 7 in a loglog plot. Magnitudeain at 0.1h decreases with decrease in stress. Althoughthe creep curve slightly decreases with timewithin thef creep deformation less than 1h, rectilinear relation-rved in the transient creep stage. In order to expresscurve within the transient creep stage in a loglog

n between creep strain and time in the transient creepnalyzed with the following equations of (1) power law,tial law, (3) logarithmic law [22] and (4) Blackburns3].

C = atb (1)

l law C = 0 + a{1 exp(bt)} (2)

law C = 0 + a ln(1 + bt) (3)

eq. C = 0 + a{1 exp(bt)} + c{1 exp(dt)} (4)

a creep strain and t is a time. 0, a, b, c and d are con-ned from analysis. Since C is a creep strain, it does notantaneous strain. Consequently, 0 is not an instanta-and it is used for the analysis except for power law, forof improvement of tting accuracy. Results of regres-

is on a transient creep stage for 200 and 100MPa atGC heat are described in Fig. 8 for 200 and 100MPa. Forconditions, a linear relationship between creep straina loglog plot precisely corresponds to a power law. Aonship in the transient creep stage could not be repre-e other three equations.

Fig. 4. C9Cr1M

Crestressetion ofno obvcreep rarithm100MPFig. 8,ear relastage iis the cof thesions.alloy Cshouldtransiemateri

K. Kimura et al. / Materials Science and Engineering A 510511 (2009) 5863 61

Fig. 5. MonkmanGrant plot steel over a range of temperatures from 500 to 700 Cof 9Cr1MoVNb steel.

Fig. 6. Stress dependence of initial strain at 550 and 600 C of 9Cr1MoVNb steel.

Fig. 7. Creep curves at 600 C of MGC heat of 9Cr1MoVNb steel.

Fig. 8. Compaat (a) 600 C: 2

3.3. Stress d

Creep rarange of strous stress dand creep rof creep destrain curvshowsminidependencvs. creep strate indicatminimum c1% at a lowe

Fig. 9. Creep rrison of observed and predicted creep curves in a transient creep stage00MPa and (b) 600 C: 100MPa ofMGC heat of 9Cr1MoVNb steel.

ependence of creep deformation

te vs. creep strain curves of MGC heat at 600 C over aesses from 100 to 200MPa are shown in Fig. 11. No obvi-ependence has been observed on creep curve in Fig. 7

ate vs. time curve in Fig. 9, however, stress dependenceformation is clearly recognized on creep rate vs. creepes. The magnitude of creep strain where a creep ratemumvalue decreaseswith decrease in stress, and stresse of creep deformation is clearly detected in creep raterain curve. At stress range of 140MPa and above, creepes minimum value at a creep strain of 23%, however, areep rate is observed at a small creep strain of less thanr stress condition of 120MPa and below. In addition to

ate vs. time curves at 600 C of MGC heat of 9Cr1MoVNb steel.

62 K. Kimura et al. / Materials Science and Engineering A 510511 (2009) 5863

Fig. 10. Comp600 C: 200M

power-lawstage, stressstage shoulmation procomponentalso creep-dstruction ofBoiler and PNH [25], a tSt value isfollowing.

(1) 100% of(2) 80% of t(3) 67% of t

Fig. 11. Creepsteel.

Fig. 12. Stress vs. times to 1% total strain, 1% creep strain, initiation of tertiary creepand time to rupture at 600 C of MGC heat of 9Cr1MoVNb steel.

Stress vs. times to 1% total strain, 1% creep strain, initiationiary creep and time to rupture at 600 C of MGC heat ofoVNb steel is shown in Fig. 12. At 200MPa, time to ini-of tertiary creep is about 10 times longer than times to 1%rain and 1% creep strain, however, difference between thoseeters decreasing with decrease in stress and it becomes neg-small at 100MPa.nges in life fraction of times to 1% total strain, 1% creep strainitiation of tertiary creep with increase in time to rupture atd 600 C of 9Cr1MoVNb steels are shown in Fig. 13. Lifen of the time to initiation of tertiary creep is within a range0.7 at 550 C and a range of 0.50.7 at 600 C, and it is almostnt inn of% ofe inarison of observed and predicted creep rate vs. time curves at (a)Pa and (b) 600 C: 100MPa of MGC heat of 9Cr1MoVNb steel.

creep deformation behaviourwithin the transient creepdependence of the onset creep strain of tertiary creep

d be considered for evaluation of long-term creep defor-perties, because design of high-temperature structurals is controlled not only by creep-rupture strength, buteformation properties. According to the rules for con-

of tert9Cr1Mtiationtotal stparamligibly

Chaand in550 anfractioof 0.6constafractiothan 10increasnuclear power plant components regulated in ASMEressure Vessel Code Section III, Division 1, Subsectionemperature and time-dependent stress intensity limit,determined for each specic time by the lesser of the

the average stress required to obtain a total strain of 1%,he minimum stress to cause initiation of tertiary creep,he minimum stress to cause rupture.

rate vs. creep strain curves at 600 C of MGC heat of 9Cr1MoVNbFig. 13. Chaninitiation of te9Cr1MoVNdependent of time to rupture. On the other hand, lifetimes to 1% total strain and 1% creep strain is smallertime to rupture in the short-term, and it increases withtime to rupture. Life fraction of times to 1% total strainges in life fraction of times to 1% total strain, 1% creep strain andrtiary creep with increase in time to rupture at 550 and 600 C ofb steels.

K. Kimura et al. / Materials Science and Engineering A 510511 (2009) 5863 63

and 1% creep strain increases to about 50% of time to rupture inthe long-term at 600 C. It is almost the same as the life fraction ofinitiation of tertiary creep. It indicates that an initiation of tertiarycreep is more important parameter for the stress intensity limit, St,than time to a total strain of 1%, since 80% of theminimum stress tocause initiation of tertiary creep is denitely smaller than 100% ofthe average stress required to obtain a total strain of 1% in the long-term. In the short-term, 1% creep strain is observed in a transientcreep stage and it is attained in a tertiary creep stage in the long-term as shown in Fig. 11. Increase in life fraction of the times to 1%total strain and 1% creep strain in the long-term should be derivedfrom the onset of tertiary creep stage within a smaller strain thanthat in the short-term. Inection of stress vs. time to rupture curveof the steel is considered to be caused by the stress dependence ofcreep deformation property. The effect of stress on creep deforma-tion property, especially on long-term creep deformation, shouldbe taken into account in consideration of microstructural changeduring creep exposure.

4. Conclusions

Creep deformation property of 9Cr1MoVNb steels (ASMESA-213 T91) was investigated and stress dependence of the creepdeformation property was discussed. Large stress dependence ofcreep rupture life andminimumcreep rate in thehigh-stress regimedecreased ithe minimulent to stratest. It wasmechanismplastic regitime and cobserved. Ithan exponA magnitudvalue decreto 1% totalin creep ruabout 50%an initiatiothe stressThe effecton long-terin considersure.

Acknowledgement

Part of this study was nancially supported by the Budget forNuclear Research of the Ministry of Education, Culture, Sports, Sci-ence and Technology, based on the screening and counseling by theAtomic Energy Commission of Japan.

References

[1] K. Kimura, H. Kushima, F. Abe, in: R. Viswanathan (Ed.), Advances in Life Assess-ment and Optimization of Fossil Power Plants, EPRI, California, 2002.

[2] K. Kimura, H. Kushima, K. Sawada, in: A. Strang, et al. (Eds.), Engineering Issuesin Turbine Machinery, Power Plant and Renewables, MANEY, London, 2003, p.443.

[3] K. Kimura, K. Sawada, K. Kubo, H. Kushima, in: Y.Y. Wang (Ed.), Experience withCreep-strength Enhanced Ferritic Steels andNewand Emerging ComputationalMethods, PVP, vol.476, ASME, New York, 2004, p. 11.

[4] K. Maruyama, J.S. Lee, in: I.A. Shibli, et al. (Eds.), Creep & Fracture in High Tem-perature ComponentsDesign & Life Assessment Issues, DEStech Publications,Inc., Pennsylvania, 2005, p. 372.

[5] B. Wilshire, P.J. Scharning, Scripta Mater. 56 (2007) 701.[6] K. Kimura, Proceedings of the 2005 ASME Pressure Vessels and Piping Division

Conference, Denver, USA July 1721, 2005, PVP2005-71039.[7] K. Kimura, Proceedings of the 2006 ASME Pressure Vessels and Piping Division

Conference, Vancouver, Canada, July 2327, 2006, PVP2006-ICPVT11-93294.[8] ASME Boiler and Pressure Vessel Code Case 2179-6, August 4, 2006.[9] ASME Boiler and Pressure Vessel Code Case 2180-4, August 4, 2006.

[10] ECCC data sheet, Steel ASTM Grade 92, 2005.[11] A. Strang, V. Foldyna, A. Jakobova, Z. Kubon, V. Vodarek, J. Lenert, in: A. Strang,

et al. (Eds.), Advances in Turbine Materials, Design and Manufacturing, Thetitutetrangep Reateriofer, Himur

(Eds.),tituteuzuki698. Daniawadoldswas of Ctituteimuraferennd, FlimuraarofaYork

. Blackk, 197awadE Bo4.n the low stress regime. Large stress dependence ofm creep rate in the high-stress regime was equiva-

in-rate dependence of ow stress evaluated by tensileconsidered to be caused by change in deformationfrom low stress and elastic regime to high-stress and

me. Good linear relationship between creep strain vs.reep rate vs. time in a double logarithmic plot weret was appropriately expressed by a power law ratherential law, logarithmic law and Blackburns equation.e of creep strain where a creep rate shows minimumased with decrease in stress. Life fraction of timesstrain and 1% creep strain increased with increasepture life from several per cent of rupture life toof that in the long-term at 600 C. It indicates thatn of tertiary creep is more important parameter forintensity limit, St, than time to a total strain of 1%.of stress on creep deformation property, especiallym creep deformation, should be taken into accountation of microstructural change during creep expo-

Ins[12] A. S

Creof M

[13] P. H[14] K. K

al.Ins

[15] K. S691

[16] H.K[17] K. S[18] S. H[19] Atl

Ins[20] K. K

ConIsla

[21] K. K[22] F. G

New[23] L.D

Yor[24] K. S[25] ASM

200of Materials, London, 1997, p. 603., V. Vodarek, in: A. Strang, et al. (Eds.), Microstructural Stability ofsistant Alloys for High Temperature Plant Applications, The Instituteals, London, 1998, p. 117.. Cerjak, P. Warbichler, Mater. Sci. Technol. 16 (2000) 1221.

a, H. Kushima, F. Abe, K. Suzuki, S. Kumai, A. Satoh, in: A. Strang, etAdvanced Materials for 21st Century Turbines and Power Plant, Theof Materials, London, 2000, p. 590., S. Kumai, H. Kushima, K. Kimura, F. Abe, Tetsu-to-Hagane 89 (2003)(in Japanese).elsen, J. Hald, Energy Mater. 1 (2006) 49.a, H. Kushima, K. Kimura, M. Tabuchi, ISIJ Int. 47 (2007) 733.orth, Mater. High Temp. 21 (2004) 2532.reep Deformation Property, NIMS Creep Data Sheet, No.D-1, Nationalfor Materials Science, 2007., K. Sawada,H. Kushima, Y. Toda, Proceedings of the Fifth Internationalce Advances in Materials Technology for Fossil Power Plants, Marcoorida, USA, October 35, 2007., K. Sawada, H. Kushima, K. Kubo, Int. J. Mater. Res. 99 (2008) 395.

lo, Fundamentals of Creep and Creep Rupture in Metals, MacMillan,, 1965.burn, TheGenerationof Isochronous StressStrain Curves, ASME,New2, p. 15.a, M. Tabuchi, K. Kimura, this issue.iler and Pressure Vessel Code, Section III, Division 1, Subsection NH,

Long-term creep deformation property of modified 9Cr-1Mo steelIntroductionExperimental procedureResults and discussionCreep strengthCreep deformation propertiesStress dependence of creep deformation

ConclusionsAcknowledgementReferences