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International Journal of Pressure Vessels and Piping 87 (2010) 282e288

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International Journal of Pressure Vessels and Piping

journal homepage: www.elsevier .com/locate/ i jpvp

Creep strength of high chromium steel with ferrite matrix

K. Kimura a,*, Y. Toda b, H. Kushima a, K. Sawada c

aMaterials Data Sheet Station, National Institute for Materials Science, Japanb Structural Metals Center, National Institute for Materials Science, JapancMaterials Reliability Center, National Institute for Materials Science, Japan

a r t i c l e i n f o

Article history:Received 30 November 2008Accepted 16 January 2010

* Corresponding author.E-mail address: kimura.kazuhiro@nims.go.jp (K. K

0308-0161/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.ijpvp.2010.03.016

a b s t r a c t

Long-term creep strength of material in the low-stress regime below elastic limit is difficult to predict byan extrapolation of short-term creep strength in the high-stress regime above elastic limit. Long-termcreep strength of fully annealed ferrite-pearlite microstructure of low alloy CreMo steel is higher thanthat of martensite and bainite microstructures. It is explained by lower dislocation density of fullyannealed microstructure. According to the above concept, creep strength of high chromium steel withferrite matrix is investigated. Creep rupture life of 15CreMoeWeCo steel with ferrite matrix which islonger than that of ASME Grade 92 steel is obtained at 650 �C by controlling the chemical compositionand heat treatment condition.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Research anddevelopment of high strength creep resistant steelshave been widely conducted, in order to improve an efficiency ofthermal power plant and to save fossil fuels. It contributes toaccomplish a harmonization of energy demand and global envi-ronmental issues. Creep strength enhanced ferritic (CSEF) steelswith tempered martensitic microstructure have been developed,and those have been utilized as large and thick components ofthermal power plant. These materials made it possible to increasesteam temperature from 566 �C to about 600 �C, and many UltraSupercritical (USC) thermal power plants have been operating.Unforeseeable strength drop in the long-term, however, has beenobserved onCSEF steels, and it has raised fears of prematuredamagedue to overestimation of long-term creep strength [1e3]. In order toimprove accuracy of evaluation and prediction of long-term creepstrength of CSEF steels, several new approaches have been investi-gated [4e8]. A region splitting analysismethod proposed by Kimuraet al. [4e6] evaluates creep strength independently for high-stressand low-stress regimes with a boundary condition of 50% of 0.2%offset yield stress under strain rate of 5 � 10�5 s�1 at the creeptemperature. Multi region analysis of creep rupture data in consid-eration of change in activation energy for creep rupture life wasproposed byMaruyamaet al. [7]. A rationalization and extrapolationmethod of creep fracture data based on relationships which involvethe activation energy for matrix diffusion and the ultimate tensile

imura).

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stress values at the creep temperature was proposed by Wilshireet al. [8]. A common importance of these newmethods is that creepdeformationmechanism and/or controlling factor of creep strengthis not constant throughout a range of stress fromshort-term to long-term. It is also important to consider the controlling factor of long-term creep strength for material design of high strength creepresistant steel.

The long-term creep strength is reduced by microstructuralchange during long-term creep exposure at the elevated temper-atures, since a coarsening of precipitates decreases precipitationstrengthening effect and a decrease in dislocation density lowerswork hardening effect. As a result of microstructural change, creepstrength decreases to an Inherent Creep Strength that is a creepstrength of matrix with an equilibrium chemical composition [9].There are two possible approaches to increase long-term creepstrength. First one is a stabilization of microstructure to retarda decrease in creep strength and second one is an extension ofcreep rupture life in the inherent creep strength regime. Accordingto the above concept, creep strength of high chromium steels withferrite matrix instead of tempered martensite has been investi-gated. In this paper, a newmaterial design concept of high strengthcreep resistant steel and a potential of creep resistant steel withferrite matrix are discussed.

2. Concept of material design

2.1. Role of stress on creep strength

Creep rupture strength of ASMEGrades P/T92 steels over a rangeof temperatures from 550 to 700 �C is shown in Fig.1 [10]. The slope

Fig. 1. Stress vs. time to rupture curves of ASME P/T92 steels [10]. Fig. 3. Relation between 0% offset yield stress and a boundary stress between low-stress and high-stress regimes where the magnitude of stress exponent changes ofASME P/T92 steels [10].

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of the curves at 550 �C of the steels is almost constant from short-term to long-term, however, it becomes steeper in the long-term at600 �C and above. Stress dependence of creep rupture life in thelong-term is different from that in the short-term. Stress versusminimum creep rate curves of ASME P/T92 steels are shown inFig. 2(a) and (b), respectively. Tensile strength and a flow stress arealso plotted at the nominal strain rate used for the evaluation in thesame figures [10]. Numerical values in the figures indicate the stressexponent, n value of the power law. The stress dependence of theminimum creep rate is clearly divided into two regimes, and that inthe high-stress regime is equivalent to the strain rate dependenceof the flow stress. The magnitude of the stress exponent, n in thehigh-stress regime is larger than that in the low-stress regime andit decreases with increase in temperature from about 20 at 550 �Cto about 10 at 700 �C. On the other hand, that in the low-stressregime is in the range of 4e8. It clearly indicates that creepdeformation mechanism in the low-stress regime is different fromthat in the high-stress regime where the stress dependence ofminimum creep rate is equivalent to the strain rate dependence offlow stress. Overestimation of long-term creep strength is recog-nized as a result of extrapolation of short-term creep rupture data

Fig. 2. Stress vs. minimum creep rate

in the high-stress regime regardless of the change in stressdependence of the creep deformation property.

Relation between 0% offset yield stress and a boundary stressbetween the low-stress and the high-stress regimes where themagnitude of stress exponent changes of ASME P/T92 steels isshown in Fig. 3. There is a good correspondence between 0% offsetyield stress and a boundary stress between the low-stress and thehigh-stress regimes. This indicates that the low-stress regime,where the stress exponent value is small, is equivalent to an elasticrange below a proportional limit, on the other hand, the high-stressregime corresponds to a plastic range above a proportional limit.The large stress dependence of creep rupture life and minimumcreep rate in the high-stress regime is regarded to be caused bya contribution of considerable plastic deformation. On the otherhand, creep deformation in the low-stress regime is considered tobe governed by diffusion controlled phenomena and, therefore,degradation in the low-stress regime is caused by microstructuralchange controlled by diffusion. Consequently, it is important toimprove a stability of microstructure at the elevated temperature,

curves of ASME P/T92 steels [10].

Fig. 4. Creep rupture strength of ferritic creep resistant steels [11,12].

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in order to retard degradation and to obtain high creep strength inthe long-term.

2.2. Inherent creep strength

Creep strength decreases as a result of microstructural change,such as coarsening of precipitate and decrease in dislocationdensity. It becomes to constant creep strength, however, whichcorresponds to strength of matrix itself after long-term creepexposure at the elevated temperatures, since strengthening effectsdepending on microstructural morphology disappears due tomicrostructural change [9]. Creep rupture strength of multi heats of0.2C steels and 10 types ferritic creep resistant steels are plotted inthe same figurewith a Larson-Miller parameter (C¼ 20) and shownin Fig. 4 [11,12]. Wide distribution of creep rupture strength, whichcorresponds to 4 or 5 orders of magnitude in creep rupture life isobserved in the high-stress regime. However, creep rupturestrength of ferritic creep resistant steels shows a tendency ofconvergence to a common strength level in the long-term inde-pendent of chemical composition and short-term creep rupturestrength. On the other hand, creep rupture strength of 0.2C steels islower than those of ferritic creep resistant steels. The large spreadof creep rupture strength of the 0.2C steels is caused by differencein molybdenum content of the steels varied from 0.005 to0.019 mass%, since creep strength of ferrite matrix is increased bymolybdenum in solid solution. Strengthening effect of molyb-denum on ferrite matrix, however, saturates at about 0.03 mass%

Fig. 5. Schematic illustration of creep strength mechanism map.

[11,12]. Creep strength of carbon steel containing 0.03 mass% ofmolybdenum is considered to be the highest creep strength offerrite matrix itself and, therefore, creep strength of ferritic creepresistant steels decreases to a common creep strength level whichis equivalent to the highest creep strength of ferrite matrix afterlong-term creep exposure. The constant creep strength of ferritematrix itself observed in the long-term was proposed as inherentcreep strength [9,11,12]. From engineering point of view, it is veryimportant that an inherent creep strength of ferritic creep resistantsteels is almost the same each other, regardless of chemicalcomposition, initial microstructure and short-term creep strength.

Schematic illustration of creep strength mechanism map isshown in Fig. 5. Basically, stress condition is divided into threeregimes of high, middle and low-stresses. In the high-stress regimeH, which is higher than elastic limit, creep strength is controlled bytensile and/or yield strength. In the middle stress regime M, creepdeformation is governed by diffusion controlled phenomena. It iscontrolled by strength of matrix that is inherent creep strength inthe stress regime L. In order to increase creep rupture strength inthe stress regime M, stability of microstructure should beimproved. On the other hand, stabilization of microstructure is nouse to increase creep rupture strength in the stress regime L, andthe different approach should be applied to extend creep rupturelife in the stress regime L.

2.3. Role of initial microstructure

In order to investigate a possibility of extension of creep rupturelife in the stress regime L where creep strength is controlled byinherent creep strength, role of initial microstructure on the long-term creep strength has been examined. Several initial micro-structures of martensite (MT), tempered martensite (TS and TL),bainite (BT) and amixture of ferrite and pearlite (FP) were preparedfor the 0.5Cre0.5Mo steel by different heat treatment condition,and bright field TEM images of those are shown in Fig. 6 [13].Vickers hardness of the initial microstructure in the as heat treatedcondition is HV408, HV270, HV171, HV237 andHV114 forMT, TS, TL,BT and FP, respectively. Stress versus time to rupture curves at575 �C of the steels with different initial microstructure are shownin Fig. 7. According to a stability of microstructure, creep rupture lifeat 60 MPa and below is considered to be controlled by its inherentcreep strength, and creep rupture life is almost the same formartensitic microstructure regardless of tempering. On the otherhand, creep rupture life of a mixture of ferrite and pearlite micro-structure is the longest in the long-term, and it is speculated to beobtained by the lowest dislocation density of full annealed micro-structure. In general, creep rate is described by a multiplication ofmobile dislocation density, r, Burger’s vector, b, and velocity ofdislocation, v. Almost all strengthening mechanisms reducevelocity of dislocation and it is thought to be no use in the stressregime L, since strengthening effect depending on microstructuralmorphology disappears. If a mobile dislocation density is reduced,creep rate may be lowered and creep rupture life may be extendedeven in the low-stress regime L, where creep strength is controlledby its inherent creep strength. Even if tensile strength and short-term creep strength of full annealed microstructure is poor, it hasadvantage for long-term creep strength in the stress regime L forthe reason of its low dislocation density.

3. Experimental procedures

The chemical composition of the 15CreMoeWeCo steelsinvestigated is shown in Table 1. In order to obtain ferrite matrix,chromium concentration was increased to 15 mass%. It was foundthat an addition of tungsten and cobalt was effective to improve

Fig. 6. Bright field TEM images of the 0.5Cre0.5Mo steels in the as heat treated condition [13].

Fig. 7. Stress vs. time to rupture curves at 575 �C of the 0.5Cre0.5Mo steels with thedifferent initial microstructures.

Table 1Chemical composition (mass%) of the 15CreMoeWeCo steels studied.

Steel C Ni Cr Mo W

Base 0.046 <0.01 15.00 1.00 6.070.4Ni 0.047 0.42 14.93 1.00 6.050.8Ni 0.048 0.78 15.00 1.00 6.051.2Ni 0.049 1.21 15.02 1.00 6.041.6Ni 0.048 1.60 15.00 1.00 6.032.0Ni 0.048 2.00 14.96 0.99 6.07

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creep strength [14,15], and creep strength was also influenced byconcentration of carbon and nitrogen [16]. According to the aboveprevious work [14e16], the base composition of 0.05C-15Cre1Moe6We0.2Ve0.05Nbe3Coe0.04Ne0.003B was selected.The other five steels were prepared by nickel addition of 0.4, 0.8,1.2, 1.6 and 2.0 mass%, which was effective to improve impacttoughness [17,18]. These steels were melted in a vacuum inductionfurnace, and ingots with a weight of 10 kg were prepared. Theingots were hot forged into round bars with a diameter of 15 mm.The bars were solution treated for 30 min at 1200 �C followed byfurnace cooling or water quenching. Creep test was conducted at650 �C up to about 50,000 h. Charpy impact test was carried outusing V-notch type specimen.

4. Results and discussion

4.1. Creep strength

Stress versus time to rupture curves at 650 �C of the furnacecooled andwater quenched 15Cr steels are shown in Fig. 8, togetherwith those of ASME P/T92 steels. Creep rupture lives of the furnacecooled 15Cr steels are longer than those of ASME P/T92 steels.However, slope of the curves of furnace cooled 15Cr steels is slightly

V Nb Co N B

0.19 0.043 2.97 0.033 0.00300.20 0.050 2.96 0.041 0.00290.20 0.050 2.96 0.042 0.00290.20 0.051 2.96 0.042 0.00280.20 0.050 2.95 0.044 0.00260.20 0.050 2.98 0.036 0.0029

Fig. 8. Stress vs. time to rupture curves at 650 �C of the 15CreMoeWeCo steels andASME P/T92 steels.

Fig. 9. Creep rate vs. time curves at 650 �C-140 MPa of the 15CreMoeWeCo steels andASME P/T92 steels.

Fig. 10. Creep rupture elongation and reduction of area at 650

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steeper than those of ASME P/T92 steels, and the difference in creeprupture lives decreases in the long-term. Creep rupture lives of thewater quenched 15Cr steels are about ten times longer than thoseof the furnace cooled 15Cr steels and several tenth times longerthan those of ASME P/T92 steels. Although creep rupture life tendsto decrease with increase in nickel concentration, expected100,000 h creep rupture strength at 650 �C is 100 MPa and higherfor part of the water quenched 15Cr steels. Creep rate versus timecurves at 650 �C and 140 MPa of the furnace cooled and waterquenched 15Cr steels and ASME P/T92 steels are shown in Fig. 9.Creep test of the water quenched nickel free base steel is still inprogress after 47,000 h without rupture. Creep deformation of thesteels consists of transient and accelerating creep stages, and nowide steady state stage is observed. In the beginning of creepdeformation at 0.1 h, creep rate of the 15Cr steels is smaller thanthose of ASME P/T92 steels, and those of the water quenched 15Crsteels are smaller than those of the furnace cooled 15Cr steels.Minimum creep rate of the water quenched 15Cr steels are thou-sandth part of those of ASME P/T92 steels. Creep rupture elongationand reduction of area at 650 �C of the furnace cooled and waterquenched 15Cr steels and ASME P/T92 steels are shown in Fig. 10.High creep rupture ductility is observed on all the steels in theshort-term less than 1000 h, and it rapidly decreased in the long-term beyond 1000 h. However, water quenched 15Cr steels possesshigh ductility even in the long-term except for a nickel free basesteel (C) and 0.4Ni steel (:).

4.2. Charpy impact toughness

Charpy absorbed energy of the furnace cooled and waterquenched 15Cr steels at 100 �C are shown in Fig. 11 [17]. It is wellknown that toughness of the steel is improved by nickel addition[19]. Charpy absorbed energy of the nickel free base steel was verylow regardless of cooling rate, and no effect of nickel addition onCharpy absorbed energy was observed on the furnace cooled 15Crsteels. On the other hand, a significant improvement in Charpyabsorbed energy with the increase in nickel concentration wasobserved on the water quenched 15Cr steels. Temperature depen-dence of Charpy absorbed energy of thewater quenched 15Cr steelsis shown in Fig. 12 [17]. Impact toughness of the water quenched15Cr steel is clearly improved by increase in nickel concentration,and ductileebrittle transition temperature (DBTT) decreases withincrease in nickel concentration.

�C of the 15CreMoeWeCo steels and ASME P/T92 steels.

Fig. 11. Influence of nickel content on Charpy absorbed energy of the 15CreMoeWeCosteels [17].

Fig. 12. Temperature dependence of Charpy absorbed energy of the 15CreMoeWeCosteels [17].

Fig. 13. Optical micrographs of (a) base, (b) 1.2Ni and (c) 2.0Ni steels in the as furnace cooled[18]. Vf is a volume fraction of martensite phase.

K. Kimura et al. / International Journal of Pressure Vessels and Piping 87 (2010) 282e288 287

4.3. Microstructure

Optical micrographs of (a) base, (b) 1.2Ni and (c) 2.0Ni steels inthe as furnace cooled condition and (d) base, (e) 1.2Ni and (f) 2.0Nisteels in the as water quenched condition are shown in Fig. 13. Vfmarked on each micrograph indicates the volume fraction ofmartensite phase. In the as furnace cooled condition, a lot ofparticles precipitated along the grain boundaries and volumefraction of martensite phase increased from 0% of base steel to43.9% of 1.2Ni steel and 60.3% of 2.0Ni steel. On the other hand,precipitates were not observed in the as water quenched condition.Volume fraction of martensire phase in the as water quenchedcondition increased from 0% of base steel to 16.7% of 1.2Ni steel and31.2% of 2.0Ni steel, however, that is smaller than that in the asfurnace cooled condition. Large volume fraction of martensitephase in the as furnace cooled condition may be caused by phasetransformation during cooling from the solution treated tempera-ture of 1200 �C. Precipitation of a lot of second phase takes placeduring furnace cooling from 1200 �C, and it is suppressed by waterquenching.

Secondary electron images of the water quenched 1.2Ni steel (a)in the as water quenched condition and isothermally aged for (b)100 h and (c) 1000 h at 650 �C are shown in Fig. 14. No precipitateswas observed in the as water quenched condition. In the steel agedfor 100 h, uniformly distributed many fine precipitates with200e300 nm in size were observed within ferrite grains, but therewere few precipitates within martensite grains. Many precipitateswithin ferrite grains were still fine even after aging for 1000 h. Theprecipitates observed in the as furnace cooled condition and afteragingwere identified as Laves phase and c-phase bymeans of X-raydiffraction and EDX analysis [18]. The amount of fine particleprecipitated during creep exposure in the furnace cooled 15Crsteels was smaller than that in the water quenched steels, sincelarge amounts of particles precipitated during furnace cooling priorto creep exposure. The higher creep strength of water quenched15Cr steels than that of furnace cooled 15Cr steels was obtained bylarge amounts of fine precipitates within ferrite grain. Decrease increep strength of the water quenched 15Cr steels with increase innickel concentration was caused by increase in volume fraction ofmartensite phase, since there were few particles precipitatedwithin martensite grains.

condition and (d) base, (e) 1.2Ni and (f) 2.0Ni steels in the as water quenched condition

Fig. 14. Secondary electron images of the water quenched 1.2Ni steel (a) in the as water quenched condition and isothermally aged for (b) 100 h and (c) 1000 h at 650 �C [18].

K. Kimura et al. / International Journal of Pressure Vessels and Piping 87 (2010) 282e288288

With a combination of ferrite matrix and precipitation ofintermetallic compounds, higher creep strength than ASME P/T92steels with tempered martensitic microstructure was obtained at650 �C. Toughness of the 15Cr steel was significantly improved bynickel addition and water quenching after solution treatment,however, creep strength was slightly reduced by the addition ofnickel. Although chemical composition, heat treatment conditionand microstructure should be optimized in order to obtain bettermechanical properties, the potential of high chromium steel withferrite matrix was suggested.

5. Conclusions

Based on the concept of new material design for high strengthcreep resistant steel, a potential of creep resistant steel with ferritematrix was investigated.

1. For creep strength enhanced ferritic steels with temperedmartensiticmicrostructure, it is important to improve a stabilityofmicrostructure at the elevated temperature, in order to retarddegradation and to obtain high creep strength in the long-term.

2. Stabilizationofmicrostructure is nouse to increase creep rupturestrength in the stress regimeL,where creep strength is controlledby its inherent creep strength, and the different approach shouldbe applied to extend creep rupture life in the stress regime L.

3. Full annealed microstructure possesses advantage for long-term creep strength in the stress regime L for the reason of itslow dislocation density.

4. Higher creep strength than ASME P/T92 steels at 650 �C isobtained by the water quenched 15Cr steel with a combinationof ferrite matrix and precipitation of intermetallic compounds.

5. Chemical composition, heat treatment condition and micro-structure should be optimized in order to obtain better mechan-ical properties, however, high chromium steel with ferritematrixpossesses a potential as a high strength creep resistant steel.

Acknowledgement

A part of this workwas financially supported by Grant-in-Aid forScientific Research (KAKENHI) (B) 20360323.

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