Materials Science and Engineering A 387–389 (2004) 628–632

Influence of high pressure normalizing heat treatment on microstructureand creep strength of high Cr steels

K. Kimuraa,∗, S. Yamaokab

a Materials Information Technology Station, National Institute for Materials Science, 2-2-54 Nakameguro, Meguro-ku, Tokyo 153-0061, Japanb Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

Received 25 August 2003; received in revised form 1 June 2004

Abstract

Chromium is an effective element for improvement in the oxidation resistance of steels. However, chromium addition to ferritic creepresistant steel is restricted to around 9–10 wt.%, since excess addition of chromium results in formation of delta-ferrite which decreases creepstrength and toughness. In this study, influence of normalizing heat treatment under high pressure on microstructure and creep strength hasbeen investigated in a steel containing 15 wt.% of chromium, since austenite single phase region is extended up to about 20 wt.% of chromiumunder high pressure of about 4 GPa. In contrast to a ferritic microstructure of the steel normalized under atmospheric pressure of 100 kPa, themartensitic one was found in steel normalized under high pressure of 4 GPa. Hardness in the as tempered condition of the steels subjectedto normalizing heat treatment at 1273 and 1173 K under 4 GPa was 290 and 289 HV, respectively. This is significantly higher than the valueof 182 HV for the steel normalized at 1273 K under atmospheric pressure of 100 kPa. Creep rate of the steel normalized under high pressureof 4 GPa was significantly smaller than that of the steel normalized under pressure of 100 kPa. It has been concluded that high pressurenormalizing heat treatment is effective to suppress a formation of delta-ferrite and to improve both creep strength and oxidation resistance.© 2004 Elsevier B.V. All rights reserved.

Keywords: High Cr steel; Creep resistant steel; Creep; Oxidation resistance; High pressure

1. Introduction

Modern fossil power plants with higher energy efficiencyare demanded for reducing both emission of CO2 gas andconsumption of natural resources. Energy efficiency of suchpower plant strongly depends on steam condition, and it in-creases with increase in temperature and pressure of steam.Research and development, therefore, has been widely con-ducted on high strength ferritic creep resistant steels, whichare used for high temperature structural components suchas header and main steam pipe[1]. For development ofhigh strength ferritic creep resistant steel, not only creepstrength, but also oxidation resistance of the material mustbe improved. Chromium is a most effective alloying ele-ment for improving oxidation resistance and chromium con-tents above 11 wt.% are necessary for adequate resistance tosteam oxidation at and above 873 K[2]. However, chromium

∗ Corresponding author. Tel.:+81 3 3719 2493; fax:+81 3 3719 2177.E-mail address: kimura.kazuhiro@nims.go.p (K. Kimura).

contents are limited around 9–10 wt.%, which is in a rangeof austenite single phase region[3], in order to obtain fullymartensitic microstructure without delta-ferrite, which is adetrimental phase to creep strength and toughness. Additionof austenite phase stabilizing elements such as nickel, copperand cobalt, has been investigated on high chromium ferriticcreep resistant steel, in order to increase chromium concen-tration without formation of delta-ferrite[4]. An excellentsteam oxidation resistance was found in 9CrMoWNbV steelwith addition of 3 wt.% palladium[5]. Addition of palla-dium promoted chromium segregation at the surface to formchromium rich oxide. However, the cost of palladium is tooexpensive. Chromizing is widely used for steels to improvetheir steam oxidation resistance[6]. Thermal spray coatingsof 80Ni–20Cr [7] and 50Ni–50Cr[8] on 9Cr–1Mo steelproduced by high velocity fuel spray process showed a bet-ter steam oxidation resistance till 1000 h at 873 and 923 K.However, there are limitations to apply these surface modifi-cation techniques for the large component. An effectivenessof protective surface layer is lost by defect and cracking ofit. Improvement of steam oxidation resistance of steel itself,

0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.msea.2004.01.097

K. Kimura, S. Yamaoka / Materials Science and Engineering A 387–389 (2004) 628–632 629

Table 1Chemical compositions (wt.%) of the steels studied

Steels C Si Mn P S Ni Cr Mo W V Nb Al N B Fe

15CR 0.110 0.24 0.49 0.001 0.003 0.02 15.21 0.98 2.95 0.20 0.051<0.001 0.072 0.0028 BalanceT91 0.09 0.29 0.35 0.009 0.002 0.28 8.70 0.90 – 0.22 0.072 0.001 0.044 – BalanceP92 0.110 0.10 0.41 0.012 0.0037 0.17 9.26 0.42 1.67 0.16 0.057 0.01 0.0462 0.002 Balance

therefore, has advantage to long-term stability rather thansurface modification.

Ferrite/austenite transformation behaviour of steel isstrongly influenced by not only addition of alloying ele-ments, but also pressure. An austenite single phase regionof Fe–Cr binary alloy is extended to about 20 wt.% ofchromium under high pressure of 4 GPa[9]. Normalizingheat treatment under high pressure has a possibility to ob-tain martensitic microstructure for steels contain higherchromium concentration without formation of delta-ferrite.In this study, influence of normalizing heat treatment underhigh pressure on microstructure and creep strength has beeninvestigated on a steel contains 15 wt.% of chromium.

2. Experimental procedures

Chemical compositions and heat treatment conditions ofthe steels are shown inTables 1 and 2, respectively. 15Crsteel was subjected to normalizing heat treatment for 1 hat 1273 K under 100 kPa (15CR-A) and 4 GPa (15CR-B),and for 2 h at 1173 K under 4 GPa (15CR-C), followed bytempering heat treatment for 1 h at 1053 K under 100 kPa.15CR-A was air cooled from the normalizing temperature.On the other hand, cooling from the normalizing tempera-ture of 15CR-B and 15CR-C was subjected by heat conduc-tion to the apparatus to produce high pressure under 4 GPa,and cooling rate was larger than 100 K/s. Conventional highchromium ferritic creep resistant steels of ASME SA-213T91 [10] and ASME SA-335 P92[11] were used for com-parison.

Compression creep test under constant load condition wasconducted on cylindrical specimens with 6 mm in diameterand 5 mm in height at 923 K and initial stress of 150 MPa.Microstructure of the steels in the as tempered condition wasinvestigated under transmission electron microscope. Vick-ers hardness was measured on the steels in the as temperedcondition under load of 98 N.

Table 2Heat treatment conditions of the steels studied

Steels Normalizing Tempering

15CR-A 1273 K/1 h 100 kPa 1053 K/1 h AC 100 kPa15CR-B 1273 K/1 h 4 GPa15CR-C 1173 K/2 hT91 1323 K/10 min AC 100 kPa 1038 K/30 min ACP92 1343 K/2 h AC 1053 K/2 h AC

AC: air cooling.

3. Results and discussion

3.1. Hardness

Vickers hardness of the steels in the as tempered conditionfor 15CR-A, 15CR-B and 15CR-C were measured. In con-trast to hardness of 15CR-A normalized under atmosphericpressure, which is 182 HV, those of the other steels subjectedto high pressure normalizing heat treatments were 290 and289 HV for 15CR-B and 15CR-C, respectively. Hardness ofthe steel normalized under high pressure of 4 GPa was sig-nificantly higher than those of the steel normalized under at-mospheric pressure and a T91 steel, since Vickers hardnessof ASME T91 steel was 235 HV in the as tempered condi-tion [12]. The higher value of Vickers hardness of the 15Crsteel normalized under 4 GPa than that of ASME T91 steelmay be caused by higher chromium content and/or marten-sitic transformation under high pressure, however, it shouldbe investigated in the future work.

3.2. Microstructure

Bright field TEM image of ASME T91 steel in the as tem-pered condition is shown inFig. 1. Microstructure of ASMET91 steel is tempered martensite with high dislocation den-sity and fine lath structure. That of ASME P92 steel is alsoessentially the same as that of ASME T91 steel.

Bright field TEM images of the steels in the as temperedcondition for (a) 15CR-A, (b) 15CR-B and (c) 15CR-C areshown inFig. 2. Microstructure of 15CR-A in the as tem-pered condition (Fig. 2a) was ferritic with small amounts ofcoarse particles, and dislocation density of it was very low.On the contrary, tempered martensitic microstructure withhigh dislocation density and lath structure was observed on15CR-B and 15CR-C in the as tempered condition (Fig. 2band c)).

From these observations, it is supposed that phase of the15CR steel at 1273 K under atmospheric pressure is ferrite,however, it is austenite at 1173 and 1273 K under high pres-sure of 4 GPa. Tempered martensitic microstructure withhigher chromium concentration can be obtained by highpressure normalizing heat treatment.

3.3. Creep strength

Creep curves for the steels 15CR-A, 15CR-B, 15CR-C,ASME T91 and ASME P92 under compression stress of150 MPa at 923 K are shown inFig. 3. Creep strains of

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Fig. 1. Bright field TEM image of ASME T91 steel in the as tempered condition.

15CR-B and 15CR-C whose microstructure was temperedmartensite were much smaller than that of 15CR-A whosemicrostructure was ferrite. Although microstructure ofASME T91 and ASME P92 steels are also tempered marten-site similar to those of 15CR-B and 15CR-C, creep strainsof 15CR-B and 15CR-C normalized under high pressure of4 GPa were significantly smaller than those of ASME T91and ASME P92 steels. Larger creep strain of ASME T91than that of ASME P92 corresponds to difference in creepstrength of those steels[10,11].

Creep rate versus time curves for the steels 15CR-A,15CR-B, 15CR-C, ASME T91 and ASME P92 steels un-

Fig. 2. Bright field TEM images of (a) 15CR-A, (b) 15CR-B and (c) 15CR-C in the as tempered condition.

der constant compression load with initial stress of 150 MPaat 923 K are shown inFig. 4. Creep rate of 15CR-B and15CR-C was almost the same and about one order of mag-nitude smaller than that of 15CR-A, up to about 200 h. Incontrast to 15CR-A which shows monotonous decrease increep rate with time, creep rate of 15CR-B and 15CR-C in-crease after showing minimum value at about 200 h. Sincecreep test has been conducted under constant compressionload condition, stress decreases with creep deformation asa result of increase in cross sectional area. Consequently,increase in creep rate of 15CR-B and 15CR-C should becaused by decrease in creep strength due to microstructural

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0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 100 200 300 400 500

Str

ain

Time / h

923K-150 MPaConstant load

15CR-A

15CR-C15CR-B

ASME T91

ASME P92

Fig. 3. Creep curves for the steels 15CR-A, 15CR-B, 15CR-C, ASME T91 and ASME P92 under compression stress of 150 MPa at 923 K.

change. It should be evidence that creep strength of 15CR-Band 15CR-C is improved by their tempered martensitic mi-crostructure.

Creep rate of ASME T91 and ASME P92 steels werelarger than those of 15CR-B and 15CR-C. Slight increasein creep rate was observed in ASME T91 and ASME P92steels after showing minimum value at about 10 and 50 h,respectively. Precipitation of M23C6 carbide, MX carboni-

10-5

10-4

10-3

10-2

10-1

10-2 10-1 100 101 102 103 104

Cre

epra

te/ h

-1

Time / h

923K-150 MPaConstant load

15CR-B

15CR-A

15CR-C

ASME T91

ASME P92

Fig. 4. Creep rate vs. time curves for the steels 15CR-A, 15CR-B, 15CR-C, ASME T91 and ASME P92 under compression stress of 150 MPa at 923 K.

tride and Laves phase influences on creep strength of ASMET91 and ASME P92 steels[13–15]. It has been reported thatM23C6 carbide and a lot of intermetallic compound precipi-tate in the present 15Cr steel[16]. Type of the intermetalliccompounds precipitated in the 15Cr steel was identified as�-phase,�-phase and Laves phase, and the amount of thosewas about 3 wt.% at 923 K[16]. On the other hand, amountof Laves phase in ASME T91 steel is about 0.6 wt.% at 873 K

632 K. Kimura, S. Yamaoka / Materials Science and Engineering A 387–389 (2004) 628–632

and no precipitate at 923 K[13]. Although precipitation ofLaves phase takes place at 923 K in ASME P92 steel[15],the amount of it should be less than that in the 15Cr steel, be-cause of the lower concentrations of molybdenum and tung-sten in ASME P92 steel. Molybdenum and tungsten are themain elements to form Laves phase, Fe2(Mo, W), and con-centrations of those in ASME P92 steel, which is 0.42 wt.%of Mo and 1.67 wt.% of W, are about half of those in the15Cr steel, which is 0.98 wt.% of Mo and 2.95 wt.% of W.Smaller creep rate, that is higher creep strength, of the 15Crsteel than those of ASME T91 and P92 steels may be ob-tained by precipitation strengthening of a lot of intermetalliccompound. It has been concluded, consequently, that highercreep strength of 15CR-B and 15CR-C than those of con-ventional 9Cr ferritic creep resistant steels of ASME T91and ASME P92 are attained by tempered martensitic mi-crostructure produced under high pressure and precipitationof large amount of intermetallic compound.

4. Conclusions

Influence of normalizing heat treatment under high pres-sure on microstructure and creep strength was investigatedin a steel contains 15 wt.% of chromium, and the followingresults were obtained:

1. Martensitic microstructure was obtained for 15 wt.%chromium steel by normalizing heat treatment under highpressure of 4 GPa, while a ferrite with coarse particleswas found in the steel heat treated at the atmosphericpressure.

2. Creep strength of 15Cr steel subjected to high pressurenormalizing heat treatment was higher than not only the15Cr steel subjected to normalizing heat treatment atthe atmospheric pressure, but also conventional 9Cr fer-ritic creep resistant steels with tempered martensitic mi-crostructure.

3. It has been concluded that fully martensitic microstruc-ture is produced by normalizing heat treatment under highpressure of 4 GPa and higher creep strength of the 15Crsteels heat treated under high pressure than the conven-tional 9Cr ferritic creep resistant steels is obtained bytempered martensitic microstructure and large amount ofintermetallic compound precipitates.

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