Creep-Fatigue-Oxidation Interactions:Predicting Alloy Lifetimes under Fossil

Energy Service Conditions

Sebastien Dryepondt

Amit Shyam

Oak Ridge National Laboratory

2016 NETL Crosscutting Research Review Meeting

April 18-22 2016, Pittsburgh

2

Power Plants Will Need to be Capable of Flexible Operation

- Frequent (~daily) load cycling will result in significant creep-fatigue interaction- Project will focus on:

- Long term creep fatigue testing and lifetime modeling- Study of microstructurally small cracks under creep-fatigue loading- Interactions among creep fatigue and oxidation

week1 week2 week3 week4 week1 week2 week3 week4

* Ralf Mohrmann, Proceedings Liege conference 2014

*

3

Creep-Fatigue Behavior of Gr.91 Steel- Gr.91 creep performance is significantly affected by cyclic loading due to microstructure changes (Fournier et al. 2008)

- No significant effect of cyclic creep (unloading/loading) due to limited cyclic strain ( 3500h, ~50 cycles)

Creep-Fatigue Tests at 550ºC- sub grain coarsening- dislocation density decrease As Received

4

0

2

4

6

8

10

12

14

0 200 400 600 800 1000

Strain

(%)

Time (h)

Decrease of lifetime at 650ºC, 90MPa with 10h cycle

650ºC

Gr 91, ORNL heat 30176

0.0013%/h0.0029%/h

90MPa referenceNo cycling

9MPa

90MPa10h

5

0

2

4

6

8

10

12

14

0 500 1000 1500 2000 2500 3000 3500 4000

Strain

(%)

Time (h)

Increase of creep rate at 600ºC, 150MPa with 10h cycle

15MPa

150MPa10h

10min

Gr 91, ORNL heat 30176

0.00066%/h

0.0012%/h

6

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 1000 2000 3000 4000 5000 6000 7000

Strain(%

)

Time(h)

Significant increase of creep rate at 550ºC, 210MPa with 10h cycle

21MPa

210MPa10h

10min

Gr 91, ORNL heat 30176

0.000041%/h0.00024%/h

Ongoing

7

Significant Increase of Creep Rate for Longer Creep Tests

- Systematic increase (~X2) of creep rate due to cycling- Need to focus on long term test / low creep rate

8

0

40

80

120

160

1.9

2

2.1

2.2

2.3

1344 1344.5 1345 1345.5 1346

Stress

(MPa

)

Strain

(%)

Time (h)

Recovery Observed During Unloading at Each Cycle

Recovery

Decrease of dislocation density?Coarsening of sub-grain structure?

Stress

Strain600ºC/150MPa

9

Creep-Fatigue Behavior of Gr.91 Steel- Decrease of cycle to rupture with hold time

- No cavity observed after testing

- Main effect of hold time is related to the formation of multi-layer thick scale vs thin scale on pure fatigue specimens

- Need for longer creep fatigue test duration

Fournier et al. (2008)

Fatigue Creep-Fatigue

10

Decrease of Nf with Hold Time for Creep-fatigue Tests at 625ºC, ±0.25%

Cycle 5No hold

Cycle 510 min

Gr.91, 625ºC10 min hold time

Stre

ss (M

PA)

−400

−300

−200

−100

0

100

200

300

Strain (%)−0.2 0 0.2

Faster softening due to 10min hold time (500h test)

11

Decrease of Nf with Hold Time for Creep-fatigue Tests at 625ºC, ±0.5%?

10min

10h

Gr.91, 625ºC

Cycle 5

No hold

Stre

ss (M

Pa)

−400

−300

−200

−100

0

100

200

300

Strain (%)−0.5 0 0.5

-Faster softening due to hold time but not as drastic as for ±0.25%-Increase of noise for 10h hold test (>2100h)

12

Significant Decrease of Stress During 10h Hold Time

- Close to a steady stress after ~2h- Creep lifetime at 625ºC, 100MPa ~ 5000h

10min

5 CyclesGr.91, 625ºC

10hStre

ss (M

Pa)

0

50

100

150

200

250

300

Time (s)0 100 200 300 400 500 600

5 CyclesGr.91, 625ºC

10h

10min

Stre

ss (M

Pa)

0

50

100

150

200

Time (s)0 10,000 20,000 30,000

2h

13

Slow Evolution of “Steady Stress” for 10h Hold Time Test

10hGr.91, 625ºC

5 Cycles

50 CyclesStre

ss (M

Pa)

0

50

100

150

200

Time (s)0 10,000 20,000 30,000

5 Cycles

10 min hold timeGr.91, 625ºC

50 cyclesStre

ss (M

Pa)

0

50

100

150

200

250

300

Time (s)0 100 200 300 400 500 600

14

Thicker Oxide Scale for Tests Performed at ±0.5%, 10min Hold Time

±0.25%,10min ±0.25%,No hold ±0.5%,10min ±0.5%,No hold

15

Higher Strain Leads to Oxide Scale Cracking and Re-oxidation

±0.25%,10min

±0.5%,10min

2 mm

2 mm

80μm

-40μm

30μm

-30μm

16

New Set up to Study MicrostructurallySmall Crack Growth at High TºC

• Sumit Bahl’s work (Indian Institute of Science)

• Slower propagation for small cracks

• Fournier et Al.(2008)

• - Initiation: Tanaka and Mura model

• - Propagation: Tomkins model

• Crack initiation at room temperature

• High cyclic Fatigue & Creep Fatigue Testing

• In Situ imaging of crack propagation

• Tests conducted at Room and 550ºC

17

Crack Growth Imaging at Room TºC

300μm

18

No Frequency Effect on Crack Propagation

19

No Effect of Hold Time on Crack Propagation

Test initiated with 10min hold time

Results consistent with decrease of dislocation

density at crack tip

20

BCS* Theory Crack Growth Characterization

−= 1)

2sec(r max

pmys

aσ

σπ

( )

−−=

−−= 1))1(

4sec(1)

2)1(

2sec(r maxmax

pcysys

RaRaσ

σπσ

σπ

Monotonic Plastic Zone Size*

Cyclic Plastic Zone Size

rpc

rpm

Monotonic and Cyclic Plasticity are Related by Load RatioSubstitute σmax σmax – σmin = σmax(1-R) and σys 2 σys

( )

−=

ys

ysm E

aσ

πσπ

νσφ

2secln

18 max2

R = -1, rpc = rpm; R > 0, rpc < rpm

Crack tip displacements (for distribution of edge dislocations at crack tip)**

Monotonic Crack Tip Displacement

( ) ( )

−−=

ys

ysc

RE

aσ

πσπ

νσφ

41secln

116 max2

Cyclic Crack Tip Displacement

* Bilby BA, Cottrell AH, Swinden KH. Proc Royal Soc A 1963;272:304.** Weertman J. “Dislocations Based Fracture Mechanics”

21

Small Fatigue Crack Growth Model

c

crN φ

φf

=Δ

ysmσαφ=Δa

ysσφμφ mc dNda =

wherecrφαμ f=

Number of cycles to failure

Crack extension at failure

Crack-extension force on dislocations in monotonic plastic zone

φc – cyclic crack-tip displacementφm – monotonic crack-tip displacementf – slip irreversibility factorφcr – critical opening displacementα – geometric factor

22

Results at room and 550ºC Consistent with Crack Growth Model

Room and 550ºC data

Tensile testing at different deformation ratesMicrostructure characterization to evaluate dislocation density

23

Creep-Fatigue-Oxidation Interaction in Steam?

In Air: From protective scale to non-protective scale due to cracking and re-oxidationIn Steam:Effect of strain on Non-protective scale?

±0.25%,10min

±0.5%,10min

2 mm

2 mm

80μm

-40μm

30μm

-30μm

24

Martensitic Gr. 91 and fully ferritic 9Cr alloys creep tested at 650ºC in air and

steam

Similar composition but different microstructures:-Gr.91-1 = standard commercial heat treatmentNormalization: 1040ºC and Tempering: 730-760ºC - Gr.91-2 = standard commercial heat treatmentNormalization: 1080ºC and Tempering: 760ºC - 9Cr = Non heat treated materialFully ferritic as-fabricated microstructure

Alloys Fe Cr Mo Si Mn V CGr.91-1 Bal. 8.31 0.9 0.13 0.34 0.26 0.08

Gr.91-2 (or 9Cr) Bal. 8.61 0.89 0.11 0.27 0.21 0.08

25

Effect of Load-Bearing Scale for Thin 9Cr Specimens

- Thinner specimen to increase the volume to surface ratio

Tim

e to

rupt

ure

(h)

9Cr

Oxide

F =σoxideSoxide

S +σ9Cr

S9CrS

9Cr

26

Stress Calculation Is Very Consistent With Load Bearing Scale

- Average oxide scale based on parabolic growthto calculate Soxide and S9Cr

− σ9Cr estimated from lifetime in steam

σ9Cr

Specimen (mm) Scale (μm) σ oxide (MPa)2 38 1271.5 62 1321 51 134

Thickness

27

No Effect or Small Increase of Lifetime in Steam vs Air for Gr.91 Alloys

- Again consistent with Load-Bearing Scale compensating for metal loss

Ongoing

28

Healed cracks lead to a Continuous Bearing Inner Oxide scale

Gr91-1, Steam, 350h,100MPa

50μmGr91-1

Inner Layer

Outer Layer

Healed Crack

29

Decrease of Lifetime in Steam due to Thermal Cycling

On going

650ºC

10h

250ºC

2h 3.5h

30

Similar Oxide Scale Microstructure for Specimens Thermally Cycled

- Local cracking of the scale leads to sudden increase of stress and rupture?- Ongoing cyclic air test to verify Gr.91 microstructure is not affect by thermal cycling

50μm

a) b)

Inner Layer

Outer Layer

Gr.91-1, 855h Gr.91-2, 2512h

31

Future Activities

- Conduct microstructure characterization of cyclic creep, creep fatigue and fatigue crack growth specimens

- Sub grain coarsening

- Decrease of dislocation density

- Cavity formation

- Continue long-term tests

- Conduct fatigue and creep-fatigue testing in steam.

- Design is Ready

- Effect of cyclic loading on a thick non protective scale?

32

Acknowledgements

- D. Erdman, C.S. Hawkins, T. Lowe, T. Jordan, J. Turan, J. Arnold and D. McClurg for assistance with the experimental work- B. Pint, P. Tortorelli, for exciting scientific discussions- This research was sponsored by the U.S. Department of Energy, Office of Fossil Energy under the supervision of Vito Cedro III