Heat resistant materials &

Creep behavior

20160502 2nd Seminar : General Topic

Min-Gu Jo

[CSSM]

Contents

• Thermal power station & Heat resistant materials

• Creep deformation

• HEA for Creep-resistant material

• ‘14 Korea 1st Energy supplement ratio• IEO Energy consumption prospect

• CCT (Clean Coal Technology) HELE coal technology (High Efficiency Low

Emission coal technology) CCS(CO2 Capture & Storage)

Energy consumption & Thermal power station

• Thermal power stationInternational Energy Outlook 2014

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2015 Korea Energy Handbook

Coal thermal power station & Heat resistant materials

SC(Supercritical): 538℃↓USC(Ultra-supercritical): 566℃↑HSC(Advanced-USC): 704℃↑

• Current state of Korea- SC USC- 6000 ton CO2/MW- 30% efficiency (ave)

T ↑ Efficiency ↑ CO2 emission ↓

• Heat resistant materials

2/24

- Boiler (tube, pipe)

- Turbine- Welding part

- Bending part - Corrosion - Oxidation

• Coal thermal power station

TurbineBoiler Generator

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• Definition: Phenomenon which materials gradual are changed(are deformed) under a constant applied load(stress)

Normally, under a constant applied load at an elevated temperature (T > 0.4 Tm) a time dependent dimensional changeJet engine (1% in 10,000 hrs), steam tube (1% in 100,000 hrs)

Creep

To determine the engineering creep curve of metal, a constant load is applied to a tensile specimenmaintained at constant temperature and the strain of the specimen is determined as a function of time

Tensile test: constant displacement determined stress

Creep test: constant stress determined strain and time

4/24

• Creep stages (3 steps)Primary creep (Transient): strain rate decreases with increasing time work hardeningSecondary creep (Steady-state): balance between work hardening and recoveryTertiary creep (Acceleration): strain rate increases due to creep damage (effective area decrease, metallurgical change)

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• Creep mechanisms

- Dislocation glide (σ/G > 10-2)Involves dislocation moving along slip planes and overcoming barriers by thermal activationOccurs at high stress and low T

- Dislocation creep (climb, 10-1> σ/G >10-4)Involves dislocation movement to overcome barriers by thermally assisted mechanisms involving diffusion of vacancies or interstitials

- Diffusion creep (10-4 > σ/G)Involves the flow of vacancies and interstitials through a crystal under the influence of applied stressFavored at high T and low stressBulk diffusion (Nabarro-Herring creep), GB diffusion (Coble creep)

- Grain boundary slidingInvolves the sliding of grains past each other

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C.M. Hu, Materials Characterization 61 (2010) 1043 – 1053

Josh Kacher, Acta Materialia 60 (2012) 6657–6672

Strengthening mechanism

Solid Solution HardeningSubstitutional solid solution (Mo, W, Co..)Interstitial solid solution (C, N, B)

Precipitation(Dispersion) Hardeningσ = 0.8MGb/λ

Work Hardening(Dislocation Hardening)σ0 = σi + αGbρ1/2

Grain Size Refinement Hardeningσy = σ0 + kD-1/2

Secondary Phase Hardeningσave = f1σ1 + f2σ2 f : volume fraction

Microstructural degradation heterogeneous failure

But, Creep is time-dependent deformationMicrostructure changes are effective

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M23C6 ((Cr,Fe,Ni,Mo)23C6 )Crystal structure : FCC (a=10.57~10.68Å)Precipitation site : Grain boundary, Twin boundaryShape : Globular, PlateTypical Size : 200~500nm

Initial stage : Precipitation hardening Precipitate at grain boundary depletion zone Coarsening Decreasing strength

MX precipitates (MC, MN, M(CN), M1M2(CN))Alloy : Ti, Nb, V, Zr, Ta.. (strong carbide/nitride former)Crystal structure : FCC (a=4.40~4.47Å)Precipitation site : dislocation, stacking faults. Twin or grain boundaryShape : Cuboidal shape after sufficient aging

Providing good strengthening effect Stabilizing the alloy against intergranular corrosion

Carbo-Nitride

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Z-Phase (CrNbN)Crystal structure: Tetragonal (a=3.037Å, C=7.391Å)Precipitation site : grain boundaries, twin boundary, dislocation Shape : Rodlike, CuboidalTypical Size : 20~50nm

Z phase forms Nb stabilised steel with N at low temperature than MX particles.Modified Z- phase : (Cr(Nb,V)N), CrVNMX precipitates Z-phase after long time annealing (stable phase below 1000oC)

M2N (Cr2N)Crystal structure: Hexagonal close packed (a=4.78Å c=4.44Å)Precipitation site : grain boundaries, twin boundary, dislocation Shape : Rodlike, CuboidalTypical Size : 20~50nm

10/24

Laves phase (Fe2Mo, Fe2Nb, Fe2W, Fe2Ta, Fe2Ti)Crystal structure : Hexagonal (a=4.73Å, C=7.72Å)Precipitation site : Grain boundary,Mo added steel : formation after a minimum of 1000h between 625~800oCNb stabilized steel : after long time aging, 5000~10000h between 625~800oC However, NbC and Z phase are more stable than Laves phase

σ-Phase (Fe,Ni)x(Cr,Mo)yCrystal structure : Tetragonal (a=8.80Å, C=4.54Å)Precipitation site : Grain boundary, Twin boundary, InclusionThe precipitates form after long term aging at high temperature (10,000~15,000h, 600oC)

G Phase (Ni16Nb6Si, Ni16Ti6Si7, (Ni,Fe,Cr)16(Nb,Ti)6Si7) (austenitic stainless steel)Crystal structure : FCC Phase (a=1.115~1.12Å)

Intermetallic compound

T. Sourmail, Precipitation in creep resistant austenitic stainless steels, Materials Science and Technology, 17 (2001) 1-14

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10-1 100 101 102 103 104 105

0.050.100.150.200.250.300.350.400.450.50

80MPa 100MPa 120MPa 150MPa

Stra

in (m

m/m

m)

Time (h)10-1 100 101 102 103 104 105

0.05

0.10

0.15

0.20

0.25

0.30 150MPa 200MPa 250MPa 300MPa

Stra

in (m

m/m

m)

Time (h)

Typical creep curve

650oC 800oC

10-1 100 101 102 103 104 10510-810-710-610-510-410-310-210-1100

Cree

p ra

te (h

-1)

Time (h)

150MPa 200MPa 250MPa 300MPa

650oC

10-1 100 101 102 103 104 10510-7

10-6

10-5

10-4

10-3

10-2

10-1

100

80MPa 100MPa 120MPa 150MPa

Cree

p ra

te (h

-1)

Time (h)

800oC

12/24

D-B Park, PhD thesis, Korea univ (2013)

Stress dependences of minimum creep rates

100 200 300 40010-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

800°C 750°C 700°C 650°C

Min

imum

cre

ep ra

te (h

-1)

Stress (MPa)

33.8221088.5 σε −×= 59.6191098.1 σε −×=

02.6191095.1 σε −×=6.8271008.7 σε −×=

nm Aσε =

Dislocation creep

n=3~5: pure metal, simple alloy

n=3~12austenitic stainless steel

In this studyn=6~8.6

Norton law (Power law)

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Dae-Bum Park, Materials Characterization 93 (2014) 52 – 61

Activation energy for dislocation creep

100 120 140 160 180 200 220 240 260280320340360380400420440460480500520540560

Activ

atio

n en

ergy

(kJ/

mol⋅K

)

Applied stress (MPa)0.92 0.94 0.96 0.98 1.00 1.02 1.04 1.06 1.08 1.10

10-8

10-7

10-6

10-5

10-4

10-3

10-2

120MPa 150MPa 200MPa 250MPa

Min

imum

cre

ep ra

te (h

-1)

1000/T (K-1)

−=

RTQA n expσε

γ-Fe self diffusion activation energy : 295kJ/mol14Cr-15Ni-Ti austenitic stainless : 460kJ/mol347H austenitic stainless steel : 441~461kJ/molTP347H austenitic stainless steel : 385~463kJ/mol

The similarly value of apparent activation energy (465~485kJ/mol)

14/24

Dae-Bum Park, Materials Characterization 93 (2014) 52 – 61

15/24

Fig. 4. (a) Creep rate versus time tested at 866 K and (b) creep-rupture plot of the Steel Aand Steel B acquired at various creep conditions.

Fig. 5. XRD profiles of the powders electrolytically extracted from the gauge part of creep specimens of (a) Steel A and (b) Steel B tested at 866 K.

Kyu-Ho Lee, Materials Characterization 102 (2015) 79–84

Fig. 7. Equilibrium phase fraction of high-Cr martensitic heat-resistant steels, (a) Steel A and (b) Steel B calculated by MatCalc.

Fig. 8. Development of phase fraction of the precipitates in (a) Steel A, (b) Steel B and (c) chemical composition ofM2N in Steel B during aging at 866 K calculated by thermo-kinetic simulation.

Kyu-Ho Lee, Materials Characterization 102 (2015) 79–84

Monkman-Grant relationship (predict the rupture time)

184.0006.1 =− rm tε

10-8 10-7 10-6 10-5 10-4 10-3 10-210-1

100

101

102

103

104

105

800°C 750°C 700°C 650°C

Tim

e to

rupt

ure

(h)

Minimum creep rate (h-1)

Ktrnm =ε

10-1 100 101 102 103 104 10510-810-710-610-510-410-310-210-1100

Cree

p ra

te (h

-1)

Time (h)

150MPa 200MPa 250MPa 300MPa

650oC

10-1 100 101 102 103 104 105

0.05

0.10

0.15

0.20

0.25

0.30 150MPa 200MPa 250MPa 300MPa

Stra

in (m

m/m

m)

Time (h)

650oC

16/24

Dae-Bum Park, Materials Characterization 93 (2014) 52 – 61

Larson-Miller plot (predict the rupture time)

23000 24000 25000 26000 27000 28000 29000 30000 31000

100

200

300

400500

304HV Super304H[1]

SUS 347H[2]

SUS 316[3]

Appl

ied

stre

ss (M

Pa)

LMP=(T)(logtr+25) (K,h)

KCtT r =+ )(log

[1] NRIM Creep data sheep No. 56[2] NRIM Creep data sheep No. 28B[3] NRIM Creep data sheep No. 6B

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Dae-Bum Park, Materials Characterization 93 (2014) 52 – 61

High Entropy alloy(HEA) Introduction

1) Solid solution strengthening2) Distorted lattice structure3) Cocktail effect4) Sluggish effect

5) Nanoscale deformation twin (Cantor alloy)

Core effect

Daniel B. Miracle, “Exploration and Development of High Entropy Alloys for Structural Applications”, Entropy (2014), 16, 494-525

Definition: At least five major metallic element having an 5-35 at% B. Cantor, Materials Science and Engineering A 375–377 (2004) 213–218

FeCrMnNiCo

Yeh, Advanced Engineering Materials, 6, 5 (2004) 299-303 High-Entropy alloys (HEAs)

i

N

iiconf nnRS ln

1∑=

−=∆

18/24

30 40 50 60 70 80 90 100

0

5000

10000

15000

20000

25000

30000

35000

40000

Inte

nsity

Diffraction angle 2-Theta

Initial specimen information• EBSD IQ(Microstructure) • XRD

• Vickers hardness

161Hv±2.677

FCC singe phaseLattice parameter* : 3.58238 Å(*:Bragg’s law와 d-spacing으로얻은각각의면들의 lattice parameter의평균)

Element Wt% At%CrK 18.30 19.76

MnK 18.36 18.75FeK 19.95 20.80CoK 21.18 20.92NiK 19.94 19.78

Matrix Correction ZAF

• EDS

50.34㎛±19.89

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Tensile results

0.0 0.1 0.2 0.3 0.4 0.5 0.60

200

400

600

RT 500 ℃ 600 ℃ 700 ℃

Eng

Stre

ss (M

Pa)

Eng Strain0.0 0.1 0.2 0.3 0.4

0

200

400

600

800

1000

RT 500 ℃ 600 ℃ 700 ℃

True

Stre

ss (M

Pa)

True Strain

0 100 200 300 400 500 600 700 800 9000

200

400

600

800

1000

1200

Cantor Ni-based super alloy (KIST) Austenite (NIMS) High Cr (NIMS) Low Cr (NIMS)

UTS

(MPa

)

Temperature ( ℃)10-4 10-3 10-2

200

250

300

350

400

450

500

550

600

UTS

(MPa

)

Strain rate (S-1)

27 ℃ 500 ℃ 600 ℃ 700 ℃

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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

100

200

300

400

500

600

700

Cantor (This stduy) S304H (KEPCO)

Eng

Stre

ss (M

Pa)

Eng Strain

Tensile results compared with S304H

CantorStrain rate : 8.333x10-4 S304HStrain rate : 6.666x10-4

RT

RT

450℃

550℃

600℃650℃ 500℃

600℃

700℃

0 100 200 300 400 500 600 700 800

300

400

500

600

700

UTS

(MPa

)

Temperature ( ℃)

Cantor S304H

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Creep results

10-3 10-2 10-1 100 101 102 10310-5

10-4

10-3

10-2

10-1

100MPa 200MPa

T=650 ℃

Stra

in

Time (hr)10-3 10-2 10-1 100 101 102 103

10-810-710-610-510-410-310-210-1100101102103

200MPa 100MPa

T=650 ℃

Cree

p ra

te (1

/h)

Time (hr)

100 101 102 103 104 1050

50

100

150

200

250

300

350

400 Cantor Ni-based super alloy (NIMS) Austenitre (NIMS) High Cr (NIMS) Low Cr (NIMS)

Stre

ss (M

Pa)

Rupture Time (hr)

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XRD results after creep test

200MPa 100MPa Initial

40 50 60 70 80 90 100

Relat

ive In

tensit

y

Diffraction angle 2-theta

200MPa 100MPa Initial

45 48 51

Relat

ive In

tensit

y

Diffraction angle 2-theta

Specimen Initial 100MPa 200MPa

Crystalstructure FCC FCC FCC

Lattice parameter*(Å)

3.58238 3.58729 3.59578

(*:Bragg’s law와 d-spacing으로 얻은각각의면들의 lattice parameter의 평균)

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Summary

24/24

• To improve efficiency of coal thermal power station, increase of operating temperature is required. Therefore, research on heat-resistant material should be preceded.

• Materials with high heat-resistance, oxidation-resistance, corrosion-resistance, weldability and especially creep-resistance are required as the structural materials for the thermal power station application.

• Creep phenomenon is a time-dependent deformation behavior and microstructural change is dominant effect on the failure of materials.

• Through the creep test condition under high temperature and constant load compare to the real operating condition, we expect to predict the lifetime of structural materials for the thermal power station.

Thank you for your attention

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