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Review Article

Progress in Creep-Resistant Steels for High Efficiency Coal-Fired Power Plants

[+] Author and Article Information
Fujio Abe

National Institute for Materials Science (NIMS),
1-2-1 Sengen,
Tsukuba 305-0047, Japan
e-mail: ABE.Fujio@nims.go.jp

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received August 6, 2015; final manuscript received December 20, 2015; published online April 28, 2016. Assoc. Editor: Marina Ruggles-Wrenn.

J. Pressure Vessel Technol 138(4), 040804 (Apr 28, 2016) (21 pages) Paper No: PVT-15-1181; doi: 10.1115/1.4032372 History: Received August 06, 2015; Revised December 20, 2015

Recent progress in creep-resistant bainitic, martensitic, and austenitic steels for high efficiency coal-fired power plants is comprehensively reviewed with emphasis on long-term creep strength and microstructure stability at grain boundaries (GBs). The creep strength enhanced ferritic (CSEF) steels, such as Grade 91 (9Cr–1Mo–0.2V–0.05Nb), Grade 92 (9Cr–0.5Mo–1.8W–VNb), and Grade 122 (11Cr–0.4Mo–2W–1CuVNb), can offer the highest potential to meet the required flexibility for ultra-supercritical (USC) power plants operating at around 600 °C, because of their smaller thermal expansion and larger thermal conductivity than austenitic steels and Ni base alloys. Further improvement of creep strength of martensitic 9 to 12Cr steels has been achieved by substituting a part or all of Mo with W and also by the addition of Co, V, Nb, and boron. A martensitic 9Cr–3W–3Co–VNb steel strengthened by boron and MX nitrides, designated MARBN, exhibits not only much higher creep strength of base metal than Grade 91, Grade 92, and Grade 122 but also substantially no degradation in creep strength due to type IV fracture in welded joints at 650 °C. High-strength bainitic 2.25 to 3Cr steels have been developed by enhancing solid solution hardening due to W and precipitation hardening due to (V,Nb)C carbides in bainitic microstructure. The improvement of creep strength of austenitic steels has been achieved by solid solution hardening due to the addition of Mo, W, and nitrogen and by precipitation hardening due to the formation of fine MX (M = Ti, Nb, X = C, N), NbCrN, M23C6, Cu phase, and Fe2(Mo,W) Laves phase. The boundary and sub-boundary hardening are shown to be the most important strengthening mechanism in creep of creep-resistant steels and is enhanced by fine dispersions of precipitates along boundaries.

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References

Figures

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Fig. 1

Increase in steam temperature of coal-fired power plants in Japan and intended steam temperature in several A-USC projects

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Fig. 2

(a) Amount of Fe2(Mo,W) Laves phase at 650 °C as a function of 1/2W/(1/2W + Mo) and (b) creep rupture data for 0.08C–9Cr–(W/Mo)–0.2V–0.05Nb–0.05N–0.005B steels with 3W–0Mo, 1.8W–0.6Mo, and 0W–1.5Mo at 600–700 °C

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Fig. 3

Change in number density of MX and Z-phase particles in Grade 91 during creep at 600 °C and 70 MPa

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Fig. 4

Schematic illustration of tempered martensitic microstructure

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Fig. 5

(a) Amount of M23C6 and MX at tempering temperature of 800 °C and (b) time to rupture of 9Cr–3W–3Co–VNb steel at 650 °C and 140 MPa, as a function of carbon concentration

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Fig. 6

TEM micrographs of 9Cr–3W–3Co–VNb steel after tempering: (a) 0.002% carbon and (b) 0.078% carbon

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Fig. 7

Creep rate versus time curves of 9Cr–3W–3Co–0.2V–0.05Nb steel with 0.002% and 0.078% carbon at 650 °C and 140 MPa

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Fig. 8

(a) Mean size of M23C6 carbides after tempering and after aging at 650 °C for 10,300 hrs as a function of boron concentration and (b) effect of boron on creep rupture data for 9Cr–3W–3Co–0.2V–0.05Nb steel at 650 °C

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Fig. 9

(a) Creep rate versus time curves of 9Cr–3W–3Co–VNb steel with 0–139 ppm boron at 650 °C and 80 MPa and (b) schematic illustrations showing mechanism of effect of boron for improvement of time to rupture

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Fig. 10

Schematic drawings of microstructure in (a) W-steel and (b) Mo-steel and (c) creep rupture data for austenitic 0.08 C–23Cr–43Ni–(W, Mo)–0.2Nb–0.1Ti–0.003B steel at 700–800 °C

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Fig. 11

Formation of Z-phase in NF12 after aging at 650 °C for 17,000 hrs

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Fig. 12

Effect of Al on creep rupture data for (a) 12Cr–1Mo–1W–0.3V steel and (b) JIS SUS 316HTB (18Cr–12Ni–Mo steel)

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Fig. 13

TEM micrograph of Grade 91 after creep rupture testing for 34,141 hrs at 600 °C and 100 MPa

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Fig. 14

Effect of tempering temperature on (a) creep rupture strength of 12Cr–1Mo–1W–VNb steel at 600 and 650 °C and (b) change in hardness of head and gauge portions during creep at 650 °C

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Fig. 15

Effect of cold rolling and subsequent annealing on creep rupture data for Grade 91

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Fig. 16

(a) Creep rupture data and (b) hardness change in head portion under no stress (thermal aging) of Grade 91

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Fig. 17

(a) Subgrain width and (b) interparticle spacing of M23C6 and MX in Grade 91 during aging

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Fig. 18

Creep rupture data and regression curves for welded joints, comparing with those for base metals: (a) Grade 91, (b) Grade 92, and (c) Grade 122

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Fig. 19

Schematic of cross section of martensitic steel welded joint

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Fig. 20

Time to rupture of simulated-HAZ specimens of Grade 92, Grade 92N, and Grade 92NN at 650 °C and 110 MPa as a function of peak temperature in thermal cycle

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Fig. 21

Optical and scanning electron micrographs of simulated-HAZ specimens of Grade 92, Grade 92N, and Grade 92NN after thermal cycle to a peak temperature of 950 °C, near AC3, followed by PWHT

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Fig. 22

Schematics of microstructure evolution in Grade 92 during simulated-HAZ thermal cycle

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Fig. 23

Development progress of bainitic 2.25-3Cr steels. The steels with asterisk are code-approved materials.

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Fig. 24

Creep rupture strength of 100,000 hrs as a function of temperature of Grades A and B of 3Cr–1.5W–0.75Mo–0.25V steel comparing with T22, T23, T24, and T91

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Fig. 25

Creep rupture data for 2.25Cr–1Mo–0.25V–0.05Nb and 2.25Cr–1.6W–0.25V–0.05Nb steels at 650 °C, comparing with those for conventional 2.25Cr–1Mo steel

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Fig. 26

Development progress of martensitic 9–12Cr steels for main steam pipe and header. The steels with asterisk are code-approved materials.

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Fig. 27

Development progress of martensitic 12Cr steels for turbine rotor

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Fig. 28

Effect of Cr concentration on creep rupture strength of (8.5–12)Cr–3.5W–3Co–VNbB steel at 650 °C

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Fig. 29

Time to rupture and minimum creep rate of 9Cr–3W–3Co–0.2V–0.05Nb–0.08C steel with 140 ppm boron at 650 °C and 120 MPa as a function of nitrogen concentration

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Fig. 30

Composition diagram of boron and nitrogen for 9–12Cr steels at a normalizing temperature of 1050–1150 °C

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Fig. 31

Creep rupture data for MARBN, P92, and P122 at 650 °C

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Fig. 32

Effect of nitrogen concentration on reduction of area of 9Cr–3W–3Co–0.2V–0.05Nb steel containing about 140 ppm boron (solid symbols) at 650 °C, together with data for T91, T/P92, and P122

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Fig. 33

Development progress of austenitic steels for superheater tube

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Fig. 34

(a) Effect of Cu addition on time to rupture of experimental 0.07C–18Cr–10Ni–1.5Mn–0.1Si–(0–5.42)Cu–0.02N steel at 700 °C and 137 MPa and (b) very fine Cu phase in Super304H (18Cr–9Ni–3Cu–Nb–N steel) after aging at 700 °C for 10,000 hrs

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Fig. 35

Optimization of Nb and Cu concentrations in Super 304H steel for improving elevated temperature strength, toughness, and hot corrosion

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Fig. 36

(a) Developed manufacturing process to establish fine-grained microstructure in ASME TP347HFG (fine-grained 18Cr–10/12Ni–Nb steel) and (b) conventional process for TP347H steel

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Fig. 37

Larson–Miller parameter plot of CF8C-Plus (19Cr–12.5Ni–4Mn–MoNbN steel) compared to high-strength wrought austenitic steels and Ni base alloy. The top abscissa gives the estimated temperature for rupture in 100,000 hrs.

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Fig. 38

Creep rupture data for solution-treated Fe–20Cr–30Ni–2Nb (at. %) austenitic steel (base steel) and boron-doped steel (B-doped steel) at 700 °C and 120 MPa, together with those for TP347H stainless steel

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