High Temperature Alloy 617 Properties for

Engineering Design:

Creep, Fatigue & Creep-Fatigue

Thomas Lillo

Idaho National Laboratory

Program Lead: Dr. R.N. Wright

Technical Meeting on High-Temperature Qualification of High Temperature Gas Cooled Reactor Materials

10-13 June 2014

Creep and Onset of Tertiary Creep

Lead: Thomas Lillo

Team Members: Julian Benz, J.K. Wright

Idaho National Laboratory

Technical Meeting on High-Temperature Qualification of High Temperature Gas Cooled Reactor Materials

10-13 June 2014

Relevance to Elevated Temperature

Design

Heat exchanger may operate up to homologous temperature of 0.7 Tm ≈ 950 oC

Anticipated stresses will be high enough to cause creep

Creep testing is part of effort to qualify Alloy 617 for Subsection NH of ASME code

– Data helps form basis of isochronous stress strain curves

– Rupture life data is used for Larsen-Miller plot

– Tertiary creep is arbitrarily limited to 1% tertiary creep in the code with the intent to prevent leakage

through interconnected cavitation of heat exchanger tubes due to void nucleation/growth

Understanding of creep mechanism(s) - dislocation creep for all temperatures

– Develop constitutive equation across the entire temperature range

– Provides the basis for the onset of the tertiary creep

3

Time (h)

0 1000 2000 3000 4000 5000

Str

ain

(%

)

0

10

20

30

40

50

800 – 1000 oC creep curves

Creep curve for Alloy 617 generally show the three expected creep regimes

– Primary: decreasing strain rate, strain hardening

– Steady state (secondary): linear region where minimum creep rate, , is measured and often used

as design parameter for long life applications

– Tertiary: increasing strain rate, often thought to be due to cavitation/cracking

4

1000 °C, 16 MPa

Primary

Tertiary

Steady state

(minimum)

min

Time (h)

0 1000 2000 3000 4000 5000 6000

Str

ain

(%

)

0

5

10

15

20

25

121 MPa

145 MPa

750 oC creep curves

Large fraction of creep curve is tertiary creep

General shape of curve does not change significantly as stress is varied at this

temperature

– Minimal secondary creep regime

– Most of the creep life is spent in tertiary creep

5

Minimum Creep Rates

6

Minimum creep at 750 oC

Minimum creep rates obtained within the first 200 - 500 hours at 750 oC

Strain rate vs. strain highlights similarity of shape and the continually increasing

strain rate after the minimum creep rate is reached at small strains

7

Time (h)

0 200 400 600 800 1000

Str

ain

(%

)

0.0

0.5

1.0

1.5

2.0

121 MPa

145 MPa

min. creep

Strain (%)

0 5 10 15 20

(s-1

)

10-10

10-9

10-8

10-7

10-6

121 MPa

145 MPa

Zener-Holloman Plot –

all creep temperatures

Minimum creep rate can be described through Norton Equation:

where

800 – 1000 oC fits on same line with exponent of n = 5.6 → power law creep

However, 750 oC data does not fall on line

8

/E (Pa/Pa)

10-5 10-4 10-3 10-2

Z (s

-1)

108

109

1010

1011

1012

1013

1014

1015

1016

1017

1023 K (750 °C)

1073 K (800 °C)

1173 K (900 °C)

1273 K (1000 °C)

5.6

RTQ

exp

Activation energy for creep

Q = 410 kJ/mol

𝜀 min = 𝜀 0 𝜎

𝜎0 𝑛

𝜀 0 = 𝐵 exp −

𝑄𝑐𝑅𝑇

Microstructure at 750 ̊C

← Creep at 750 °C,

121 MPa →

9

← Static aging → 750 oC, 20,000 hrs

Precipitates not

present from

800 - 1000 °C

/E (Pa/Pa)

10-5 10-4 10-3 10-2

Z (

s-1

)

108

109

1010

1011

1012

1013

1014

1015

1016

1017

750 °C

800 °C

900 °C

1000 °C

σ0 = 65 MPa

RTQ

exp

Threshold stress at 750 oC

Addition of threshold stress to account for climb/bowing around γ’ precipitates gives

consistent exponent (n = 5.6) and allows use of same activation energy for creep

Power law creep represents entire temperature range well

Additional aging causes coarsening in γ’ and the addition of strength/creep

resistance up to γ’ dissolution temperature (750-800 °C) is eventually lost

10

/E (Pa/Pa)

10-5 10-4 10-3 10-2

Z

(

s-1

)

108

109

1010

1011

1012

1013

1014

1015

1016

1017

750 °C

800 °C

900 °C

1000 °C

750 °C, aged 20 kh

RTQ

exp

Onset of Tertiary Creep

Onset of tertiary creep

In other alloys, tertiary creep generally initiates after significant time in steady state

(i.e. minimum) creep – a substantial portion of the creep life is spent in secondary

creep regime

Tertiary creep is usually correlated with the formation of creep cavities → less cross

section to carry load so stress increases and leads to increased strain rates

Creep voids can interconnect and cause cracking/rupture

Creep curves for our heat of Alloy 617 have shown that tertiary creep is established

very quickly at many conditions, sometimes at only few % strain (e.g. creep test at

1000 oC , 12 MPa → tertiary creep initiates at 1~2 % total strain,800 h but specimen

ruptures above 60% total strain)

Limit of 1% tertiary creep in subsection NH of ASME code may be overly conservative

for Alloy 617

12

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 5.0 10.0 15.0 20.0 25.0

Ave

rage

Po

rosi

ty in

th

e G

age

Sect

ion

(%

)

Interrupted Creep Strain (%)

750 C, 121 MPa

750 C, 145 MPa

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 5.0 10.0 15.0 20.0 25.0

Ave

rage

Po

rosi

ty in

th

e G

age

Sect

ion

(%

)

Interrupted Creep Strain (%)

1000 C, 10 MPa

1000 C, 16 MPa

1000 C, 20 MPa

1000 C, 28 MPa

1000 C, 41 MPa

1000 C, 56 MPa

1000 C, 70 MPa

Creep porosity at 750 and 1000 ̊C

Porosity in creep samples interrupted between 2 – 20% strain characterized by

extensive optical metallography and automated image analysis

750 oC does not show any significant porosity out to 20%

No significant porosity until ~10% at 1000 oC

13

750 °C 1000 °C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 5.0 10.0 15.0 20.0

Ave

rage

Po

rosi

ty in

th

e G

age

Se

ctio

n, %

Tertiary Creep Strain, %

1000 C, 10 MPa

1000 C, 16 MPa

1000 C, 20 MPa

1000 C, 28 MPa

1000 C, 41 MPa

1000 C, 56 MPa

1000 C, 70 MPa

Porosity after onset of tertiary creep

Still very small value of average porosity in crept samples even after initiation of tertiary

creep

Porosity at 1% tertiary creep is negligible

Generally, > 7-8% porosity is considered limit for interconnected porosity (i.e. gas tight

structural integrity) based on powder metallurgical practices

14

Interrupted creep at 1000 oC –

small strains

Tertiary creep has not initiated at these strains due to relatively high stresses at

1000 oC

Dislocations are generally disorganized and only are seen on slip planes and

occasionally interacting with second phase particles and other dislocations

15

16 MPa, 2% strain 20 MPa, 2% strain

Interrupted creep at 1000 oC –

intermediate strains

No tertiary creep at 16 and 20 MPa with 5% creep strain

Dislocations starting to multiply and tangle but no organized structure

16

16 MPa, 5% strain

20 MPa,

5% strain

Interrupted creep at 1000 °C –

high strains

Tertiary creep has initiated at 10% total strain (~2.4% tertiary creep strain)

Dislocations rearranging to form organized structures – subgrain boundary formation

Low dislocation density in the cell interiors

17

20 MPa, 10% strain 20 MPa, 10% strain

Subgrain formation and relation to

tertiary creep

Subgrain boundaries made of organized dislocation networks results in lower

energies configurations than random distributions of dislocations and tangles → total

strain energy of system is reduced

Dislocations can easily be absorbed in subgrain boundaries and climb past these

networks resulting in increasing strain rates (i.e. tertiary creep)

Overall effect is material softening

Result:

– Porosity is shown to be negligible after initiation of tertiary creep

– Subgrain formation does initiate when tertiary creep begins

18

Summary

Creep testing has been completed from 750 – 1000 oC

– long term, very low stress tests for ASME code qualification still ongoing

A single constitutive equation was established to describe the steady state (i.e.

minimum) creep rates for all temperatures tested

– Power law creep, related to dislocation/climb mechanisms, dominates (n ≈ 5)

– A threshold stress for creep at 750 oC exists but the positive effect eventually disappears due to

microstructure changes after long times

Porosity was not found at any significant values even at large strains

– The porosity measurements will be used to argue that the 1% tertiary creep limit in the code for Alloy

617 can be relaxed

– Tertiary creep does not initiate from void formation/cracking but likely arises from the formation of

ordered dislocation networks or subgrains

19

Fatigue of Alloy 617 at Low and High Temperatures

Richard Wright

Idaho National Laboratory

Technical Meeting on High-Temperature Qualification of High Temperature Gas Cooled Reactor Materials

10-13 June 2014

Fatigue in the ASME Code

Section III – Nuclear Design

– Subsection NB

• Temperatures less than 427ºC (800ºF) for austenitic alloys

• Elastic deformation only

• Includes fatigue effects in the form of a fatigue design curve

– Subsection NH – Elevated Temperature Design

• Temperatures greater than 427ºC

• Inelastic deformation is permitted

• Interaction of creep and fatigue is believed to be most severe structural failure mode

21

Alloy 617 ASME Section III NB Code

Qualification

Provide fatigue design curves derived from experiments and analysis of historical

treatment of austenitic alloys

Subsection NB typically uses one curve to represent 20- 427C

The fatigue design curve is the only information missing to complete Alloy 617 Code

Case for Subsection NB in May of 2014

22

10

100

1000

10000

1.E+02 1.E+04 1.E+06 1.E+08 1.E+10

All

ow

ab

le S

tres

s, S

a (

MP

a)

Cycles to Failure, Nf

with zero mean stress

with maximum mean stress

I-9.5M from ASME Section

III Appendices

(UNS N06003, N06007,

N06455, and N10276 for

T≤425C)

ASME Fatigue Procedure

Testing is fully-reversed using strain-control

Although it has been demonstrated that Alloy

617 has no strain rate sensitivity at low

temperatures

– ASME method for fatigue testing specifies a strain

rate of 10-3 /s for strain control or load control test

at higher frequency for strain range where material

is elastic

– Fatigue tests are required for millions of cycles; we

have developed procedure with higher strain rate

strain control test as compromise method

Propose to test only one heat and one

product form

Test at ambient temperature and 427C to

demonstrate comparability with historical

data

Show existing design curves for Ni-Cr alloys

are adequately conservative based on

comparison to data to avoid complexity of

non-zero mean stress analysis

23

Definition of Parameters Used to Calculate

Design Curve

24

S is the “pseudostress amplitude,” a fictitious value

Greater than the measured stress amplitude, Ds

Proportional to the total strain range, Dt

– S = 0.5 E(Dt)

Useful for fatigue under multiaxial loading

E is the room temperature elastic modulus

Dt is the sum of the elastic and inelastic strain

Dt = in + e

in is the width of the hysteresis loop

Ds-

S

Ds+

in Dt

e

Process for Developing a Design Curve

Numerical factors are used to cover the effects of size, environment, surface finish

and scatter of data

Divide S by a factor of 2 (controlling for high Nf)

Divide Nf by a factor of 20 (controlling for high Sa)

Draw smooth curve

25

S (

MP

a)

Cycles to Failure

S vs. Nf

Nf/20

S/2

design curve I-9.5M

427 C

427 C, suspended

RT

RT, in progress

Data and Fatigue Design curve for

Austenitic Alloys with Alloy 617 Data

Superimposed

Comparison of Alloy 617 Data With

Other Ni Based Alloys in Code

27

100

1000

10000

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

S (M

Pa)

Cycles to Failure

Alloy 617 427 C

Alloy 617 RT

Hastelloy X RT

Hast C-276 RT

Hast C-276 427 C

Hastelloy G RT

Hast G 427 C

Hastelloy C-4 RT

Elevated Temperature Fatigue

28

Low cycle fatigue testing of the Next

Generation Nuclear Plant (NGNP) heat of

Alloy 617 at 850 and 950ºC

Contributes to American Society of

Mechanical Engineers (ASME) Section

III, Subsection NH Code Case for Alloy

617

Provides verification that a heat

representative of modern melt practice

has properties comparable to historical

Alloy 617 data sets

Provides detailed data for development

of constitutive models

Contributes to a mechanistic

understanding of fatigue behavior to

facilitate fatigue life prediction outside

the range of conditions considered

experimentally

Procedure

Fully-reversed strain-controlled low cycle fatigue testing in accordance with

American Society for Testing and Materials Standard E606

850C, 0.3 – 3.0% total strain range, 10-3 /s strain rate

950C, 0.3 – 2.0% total strain range, 10-3 /s strain rate with limited testing at 10-4

and 10-5 /s

Procedure (cont.)

Ni, cycles to crack initiation and Nf, cycles to failure defined using max/min

stress ratio

Repeatability

0.4% total strain 1.0% total strain

Comparison with Previously Published Data

Fatigue data from the current experiments are in good agreement with

previously published data for Alloy 617 compiled from numerous sources

5 5

Coffin-Manson Relation

Log-log plot of inelastic strain range as a function of cycles to failure typically

results in a straight line with a slope of -0.5

For Alloy 617 the slope is close to -1.0

Deformation in the creep regime or the environmental component of crack

growth may account for the different slope

Summary

Fatigue testing to support low temperature (Section III NB) Code Case is

complete

Comparison of current test results with data used to develop existing Code

fatigue design curves supports the argument that Alloy 617 behavior is well

described by those curves

Low cycle fatigue testing of NGNP heat of Alloy 617 at 850 and 950ºC has been

completed

Low cycle fatigue properties are comparable to previously published data for

Alloy 617

Observed behavior for Alloy 617 follows a Coffin-Manson relationship

Alloy 617 Creep-Fatigue Characterization

Lead: Laura Carroll

Team Members: J. Simpson, Q.R. Lloyd, J.K. Wright

Idaho National Laboratory

Technical Meeting on High-Temperature Qualification of High Temperature Gas Cooled Reactor Materials

10-13 June 2014

Motivation

36

Creep-fatigue (CF) is the dominant deformation mechanism expected for an IHX of a

VHTR-type reactor

– CF combines creep and fatigue deformation into a single cycle

– Accelerated failures occur as a result of interaction between the two deformation modes

– Focus on the higher temperature regime where creep is active

• 850 C

• 950 C

High priority Draft Alloy 617 Code Case recommendations

– Improved understanding of CF behavior

• Saturation – does an increasing hold time in the creep portion of the cycle continue to degrade failure life?

– Saturation allows extrapolation of experimental CF data to reactor conditions

• Environment – are the CF properties generated in air for the ASME Code representative of those in impure

helium?

– NRC licensing

– Develop a CF interaction diagram

• ASME Code uses a CF interaction diagram to assess CF interactions

– Is this method representative for Alloy 617?

– Weldments

• Weldments are known to be detrimental to CF

– What is the extent of this degradation?

Background: Fatigue and CF

Fatigue

Constant strain hold

CF

Background: CF

Figure from Miller, Hamm, Phillips, Materials Science &

Engineering, vol. 53, p.234.

CF

interaction

Fatigue

dominated

Creep

dominated

850 C CF - Saturation

39

Goal: Determine if there is saturation at 850 C

– Outcome: Cycles to failure do not saturate for the investigated hold times

• Stresses continue to relax throughout hold time

– Plan forward: Longer hold time creep-fatigue testing at 850 C

0

50

100

150

200

0 500 1000 1500 2000

Str

ess

(M

Pa

)

Time (s)

3 min

10 min

30 min

0.3% total strain

Creep damage present as intergranular grain boundary cracks

– Increasing hold times = increasing amounts of grain boundary damage

– Promotes intergranular type failures

CF behavior is expected to saturate with the stress relaxation behavior

850 C CF - Saturation

Creep-fatigue

interaction regime

Interior grain boundary cracking

0.3% total, 10 min CF 0.3% total strain

Longer hold time=

Increased interior grain boundary cracking

950 C CF - Saturation

41

Goal: Determine if there is saturation at 950 C

– Outcome: Cycles to failure saturate

• Stresses fully relax during short hold times

0.3% total strain

NRC licensing: Comparison of CF in air and impure helium

– Outcome: The CF behavior in air is representative of that in impure helium

– Limited environmental influence consistent with failures mode and saturation at 950 C

950 C CF - Environment

950 C, 0.3% total strain

CF Interaction Diagram

Analysis completed by

S. Sham (ORNL)

Goal: Develop a CF interaction diagram

– CF diagram is necessary for design in the high temperature regime

– ASME Code uses a time fraction method to assess CF damage

– Outcome: The proposed D-diagram for Alloy 617 is representative of average behavior

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Tota

l C

reep

Dam

age,

DC

Total Fatigue Damage, DF

Draft Alloy 617 Code Case

850 °C

950 °C

950 C CF Interaction Diagram

100

1000

10000

100 1000 10000

Ob

serv

ed

N2%

Predicted N2%

BE SMDE & Df by DT with Cyc by Cyc

Relax Strain

Cont. Cycling

3 min Dwell

10 min Dwell

30 min Dwell

150 min Dwell

1:1

x2

/2

100

1000

10000

100 1000 10000

Ob

serv

ed

N2%

Predicted N2%

BE time f ract & Df by DT with Cyc by Cyc

Relax Strain

Cont. Cycling

3 min Dwell

10 min Dwell

30 min Dwell

150 min Dwell

1:1

x2

/2

Goal: Compare R5 (stress modified ductility exhaustion (SMDE)) and ASME (time

fraction) analysis method

Outcome: Both the R5 and ASME Code are excellent at predicting failure

Analysis completed by M. Spindler (EDF)

R5 – British Code ASME Code

Test method B-

base + weld + base

45

950 C CF - Weldments

Test method A -

weld only

GTA weld w/ Alloy

617 filler metal

Goal: Understand CF behavior of weldments

– Outcome: CF testing of GTA welded Alloy 617 completed with test method A and in progress with

test method B

specimen

axis

950 C CF - Weldments

Test method A failure -

1.0% total strain, 10 min hold

Goal: Understand CF behavior of weldments

– Outcome: CF cycles to failure of weldments is reduced relative to base material

Conclusions

CF saturation (ASME)

– 850 C

• Saturation not observed for hold times of as long as 30 min

• Longer hold time testing at durations sufficient to allow full stress relaxation is in progress

– 950 C

• Low strain range: saturation at hold times as short as 18 s

• High strain range: saturation at hold times of approximately 10 min

Environment: air v. impure helium (NRC)

– 850 C

• Likely creep-dominated based on interior grain boundary cracking analysis

• TBD from longer hold time CF tests

– 950 C

• Low strain range: negligible

• High strain range: saturation suggests limited influence

Creep-fatigue interaction diagram (ASME)

– Representative of average creep-fatigue behavior

Weldments (ASME)

– Cycles to failure of Alloy 617 GTA welds is reduced relative to base material

– Testing of configuration B is in progress

• Gage contains both the weld and base metal in the gage section