Investigation of microstructure and mechanical properties of thermally aged Alloy 600

Seung Chang Yoo

with significant contributions from

KJ Choi T Kim S Kim and JHKim Corresponding author Ji Hyun Kim (kimjhunistackr)

School of Mechanical and Nuclear Engineering Ulsan National Institute of Science and Technology (UNIST) Ulsan Republic of Korea

Presented at 17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems ndash Water Reactors

Fairmont Chateau Laurier Ottawa Ontario Canada August 10 2015

Outlook

1

2

3

4

Background

Objective and Approach

Experiment

Results and discussions

5 Summary

Materials used in the primary circuit of nuclear power plants (NPPs) are exposed to a very challenging environment of high temperature stress vibration radiation and corrosive water

Head penetration nozzles for the control rod drive mechanism (CRDM) of the reactor pressure vessel suffers from a number of cracking and coolant leak incidents in recent years mainly due to primary water stress corrosion cracking (PWSCC)

Addition to this the issue of material degradation by long-term thermal aging by increased operational life of NPPs make it issued as an important factor in evaluating the safety and reliability of NPP in long-term operation

However the effect of long-term exposure to relatively low temperature (300~400) to material degradation which is similar to real situation in NPPs have not been investigated fully

The potential for material degradation of head penetration nozzles have been emphasized and the influence of long-term thermal aging to the susceptibility of material degradation for this part have not been clarified Therefore to understand the change of material properties and PWSCC resistance by long-term thermal aging basic microstructures and mechanical properties are investigated in this research

3

Background (11)

Introduction

Objective Investigate the effect of long-term thermal aging to PWSCC initiation resistance of material (preliminary study) Investigate the variation of detailed microstructural and mechanical

properties of the Alloy 600 subjected to long-term thermal aging in an operating NPP to evaluate the influence of long-term thermal aging to material property

Approach Thermal aging simulation of light water reactor environment at accelerated temperature

Microstructural analysis with Electron Microscopy Electron Backscattered Diffraction (EBSD)

Mechanical property analysis with Tensile test Nanoindentation

IGSCC resistance analysis with huey test

4

Objective and Approach

Objective and Approach

Alloy 600 thick rod specified by ASTM B166 was provided by Doosan Heavy Industries amp Construction Annealed at 1060 for 35 h and water quenched

Accelerated temperature was determined as 400oC and for activation energy of Cr diffusion at grain boundary 180 kJmol was used for Ni base alloy which containing 15 to 30 wt chromium [1]

5

Experiment (12)

Material amp Thermal aging

Chemical compositions (wt)

Element C Si Mn Cr Cu Ni S Fe

wt 007 033 056 1583 002 7479 lt0001 840

Heat treatment and aging conditions of each specimen Specimen name Simulated aging time and temperature Heat treatment time and temperature

As-received - - HT400_Y10 10 years at 320 1142 hours at 400 HT400_Y20 20 years at 320 2284 hours at 400

119905119886119886119886119886119886119905119903119903119903

= 119890119890119890 minus119876 1

119879119903119903119903minus 1119879119886119886119886119886119886

119877

119905119886119886119886119886119886 119905119903119903119903 119879119886119886119886119886119886 119879119903119903119903

R 119876

= Heat treatment time [h] = Simulated aging time [h] = Heat treatment temperature [K] = Simulated aging temperature [K] = Gas Constance [=8314 JmolK] = Activation Energy for Cr diffusion [kJmol]

Diffusion equation for thermal aging

[1] JM Boursier et al ASME PVP (2004)

6

Experiment (22)

Conditions Experiment conditions

EBSD Acceleration voltage 10 kV Current 064 nA Tilt angle 70deg Step size 2 μm Scanned area 298 μm x 881 μm to cover several number of grains

Nanoindentation Poisson`s ratio 03 Strain rate 005 sec-1

Displacement into surface 2000 nm using berkovich tip To improve the reliability of the results an average was taken from multiple indentations

at random position of matrix Tensile test

Multiple tests were performed at strain rate of 04 sec-1 at room temperature Test were prepared and performed based on ASTM E8-E8m

Huey test 20 cm3 was exposed to 80oC 65 HNO3 solution for 96 hours and weight loss of each

specimens were measured Test were prepared and performed based on ASTM A262

Proportionally reduced specimen from ASTM standard used in tensile test

Specimen As-received HT400_Y10 HT400_Y20

Young`s modulus [GPa] 2113 plusmn 120 2907 plusmn 119 2567 plusmn 464

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

7

Results and discussions (18)

Tensile test

bull At the engineering stress-strain curve hardening occurred by 10 years thermal aging while softening occurred by 20 years thermal aging

Representative stress-strain curves of each specimen

8

Results and discussions (28)

Tensile test

bull At fracture surface of each specimen typical ductile failure mode with many dimple were observed It is hard to quantify the size of dimple since there size is not unique as small dimples observed in large dimples

bull There exist grain boundary fracture in thermally aged specimens

50μm 50μm 50μm

Fracture surface of As-received Fracture surface of HT400_Y10 Fracture surface of HT400_Y20

30μm 30μm 30μm

9

Results and discussions (38)

Hardness

bull In nanoindentation test same trend with tensile test were observed The hardness was increased by 10 years thermal aging and decreased by 20 years thermal aging

bull Hardness at grain boundary have slightly higher value for all specimens however the difference was not significant in order of 005 GPa It is because the plastic deformation zone of indentation is large (~25um) enough to cover several grain boundaries so influence of precipitates to hardness is almost constant

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 247 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 259 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)Indentation depth (nm)

Mean 281 GPa

Nanoindentation result of As-received Nanoindentation result of HT400_Y10 Nanoindentation result of HT400_Y20

Investigated location As-received HT400_Y10 HT400_Y20

Inside grain [GPa] 244 plusmn 003 275 plusmn 008 255 plusmn 009

Cover grain boundary [GPa] 25 plusmn 002 276 plusmn 005 26 plusmn 01

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

30μm 30μm 30μm

10

Results and discussions (48)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

OM image of As-received OM image of HT400_Y10 OM image of HT400_Y20

200μm 200μm 200μm

11

Results and discussions (58)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

30μm 30μm 30μm

10μm 10μm 10μm

bull The morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

12

Results and discussions (68)

Microstructure

bull The morphology of precipitates were known to have significant influence to material`s mechanical properties And this feature is the main reason for mechanical property changes due to thermal aging

bull The feature of calculated material strength according to Orowan mechanism which explain the amount of hardening due to precipiates shows same trend with that of mechanical test which was conducted in this study hardening by 10 years thermal aging and softening by 20 years thermal aging Therefore it could be thought that the aspect of mechanical properties due to thermal aging is closely related to the morphology of precipitates

Material hardening and softening behavior depending on the number fraction and length of precipitates

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates [] 036 plusmn 004 12 plusmn 01 143 plusmn 014

Precipitate length [μm] 080 plusmn 030 092 plusmn 031 186 plusmn 119

Spacing between precipitates [μm] 278 plusmn 022 192 plusmn 014 328 plusmn 031

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

Morphology of precipitates Discrete Semi-continuous Continuous

Summary of mechanical and microstructural characteristics of each specimen

120533 =119918119918

119923 minus 2119955

[1] Z Guo et al Mater Trans (2002) [2] R Hayes et al Acta Metal (1982)

13

Results and discussions (78)

Microstructure Kernel Average Misorientation

Grain boundary misorientation

Grain boundary misorientation

Grain boundary misorientation

Kernel Average Misorientation

Kernel Average Misorientation

EBSD analysis results of HT400_Y20 EBSD analysis results of HT400_Y10 EBSD analysis results of As-received

200μm 200μm 200μm 200μm 200μm 200μm

bull Kernel average misorientation (KAM) which is proportional to residual strain induced by dislocation density was not much changed by thermal aging

bull Grain boundary misorientation however significantly changed Coincidence site lattice boundary which is more resistant to material degradation was decreased by thermal aging The formation of precipitates could be a reason for this however it could be verified with other tools like TEM

14

Results and discussions (88)

Microstructure

bull Major changes induced by thermal aging were length and morphology of precipitates bull HT400_Y10 have semi-continuous precipitates HT400_Y20 have continuous precipitates and As-received

specimen have discontinuous precipitates bull Several previous studies suggest that material with continuous feature of precipitates have lower

susceptibility to SCC than materials with both semi-continuous and discontinuous precipitates bull With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to

formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates []

036 plusmn 004

12 plusmn 01

143 plusmn 014

Precipitate length [μm] 0801 plusmn 0297

0923 plusmn 0312

1864 plusmn 119

Grain size [μm] 2667 2461 2539

Fraction of CSL boundary []

255 plusmn 38

153 plusmn 488

68 plusmn 441

Morphology of precipitates Discrete Semi-

continuous Continuous

Corrosion resistance by huey test [mmyr] 504 697 523

Average properties in electron microscopy

[1] G S Was Corrosion (1990) [2] J R Crum et al 4th EDM-NPS (1990)

SEM images of each specimen (a) as-received (b) HT400_Y10 and (c) HT400_Y20

(a)

(b) (c)

Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

Summary

• Slide Number 1
• Slide Number 2
• Slide Number 3
• Slide Number 4
• Slide Number 5
• Slide Number 6
• Slide Number 7
• Slide Number 8
• Slide Number 9
• Slide Number 10
• Slide Number 11
• Slide Number 12
• Slide Number 13
• Slide Number 14
• Slide Number 15
• Slide Number 16

Materials used in the primary circuit of nuclear power plants (NPPs) are exposed to a very challenging environment of high temperature stress vibration radiation and corrosive water

Head penetration nozzles for the control rod drive mechanism (CRDM) of the reactor pressure vessel suffers from a number of cracking and coolant leak incidents in recent years mainly due to primary water stress corrosion cracking (PWSCC)

Addition to this the issue of material degradation by long-term thermal aging by increased operational life of NPPs make it issued as an important factor in evaluating the safety and reliability of NPP in long-term operation

However the effect of long-term exposure to relatively low temperature (300~400) to material degradation which is similar to real situation in NPPs have not been investigated fully

The potential for material degradation of head penetration nozzles have been emphasized and the influence of long-term thermal aging to the susceptibility of material degradation for this part have not been clarified Therefore to understand the change of material properties and PWSCC resistance by long-term thermal aging basic microstructures and mechanical properties are investigated in this research

3

Background (11)

Introduction

Objective Investigate the effect of long-term thermal aging to PWSCC initiation resistance of material (preliminary study) Investigate the variation of detailed microstructural and mechanical

properties of the Alloy 600 subjected to long-term thermal aging in an operating NPP to evaluate the influence of long-term thermal aging to material property

Approach Thermal aging simulation of light water reactor environment at accelerated temperature

Microstructural analysis with Electron Microscopy Electron Backscattered Diffraction (EBSD)

Mechanical property analysis with Tensile test Nanoindentation

IGSCC resistance analysis with huey test

4

Objective and Approach

Objective and Approach

Alloy 600 thick rod specified by ASTM B166 was provided by Doosan Heavy Industries amp Construction Annealed at 1060 for 35 h and water quenched

Accelerated temperature was determined as 400oC and for activation energy of Cr diffusion at grain boundary 180 kJmol was used for Ni base alloy which containing 15 to 30 wt chromium [1]

5

Experiment (12)

Material amp Thermal aging

Chemical compositions (wt)

Element C Si Mn Cr Cu Ni S Fe

wt 007 033 056 1583 002 7479 lt0001 840

Heat treatment and aging conditions of each specimen Specimen name Simulated aging time and temperature Heat treatment time and temperature

As-received - - HT400_Y10 10 years at 320 1142 hours at 400 HT400_Y20 20 years at 320 2284 hours at 400

119905119886119886119886119886119886119905119903119903119903

= 119890119890119890 minus119876 1

119879119903119903119903minus 1119879119886119886119886119886119886

119877

119905119886119886119886119886119886 119905119903119903119903 119879119886119886119886119886119886 119879119903119903119903

R 119876

= Heat treatment time [h] = Simulated aging time [h] = Heat treatment temperature [K] = Simulated aging temperature [K] = Gas Constance [=8314 JmolK] = Activation Energy for Cr diffusion [kJmol]

Diffusion equation for thermal aging

[1] JM Boursier et al ASME PVP (2004)

6

Experiment (22)

Conditions Experiment conditions

EBSD Acceleration voltage 10 kV Current 064 nA Tilt angle 70deg Step size 2 μm Scanned area 298 μm x 881 μm to cover several number of grains

Nanoindentation Poisson`s ratio 03 Strain rate 005 sec-1

Displacement into surface 2000 nm using berkovich tip To improve the reliability of the results an average was taken from multiple indentations

at random position of matrix Tensile test

Multiple tests were performed at strain rate of 04 sec-1 at room temperature Test were prepared and performed based on ASTM E8-E8m

Huey test 20 cm3 was exposed to 80oC 65 HNO3 solution for 96 hours and weight loss of each

specimens were measured Test were prepared and performed based on ASTM A262

Proportionally reduced specimen from ASTM standard used in tensile test

Specimen As-received HT400_Y10 HT400_Y20

Young`s modulus [GPa] 2113 plusmn 120 2907 plusmn 119 2567 plusmn 464

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

7

Results and discussions (18)

Tensile test

bull At the engineering stress-strain curve hardening occurred by 10 years thermal aging while softening occurred by 20 years thermal aging

Representative stress-strain curves of each specimen

8

Results and discussions (28)

Tensile test

bull At fracture surface of each specimen typical ductile failure mode with many dimple were observed It is hard to quantify the size of dimple since there size is not unique as small dimples observed in large dimples

bull There exist grain boundary fracture in thermally aged specimens

50μm 50μm 50μm

Fracture surface of As-received Fracture surface of HT400_Y10 Fracture surface of HT400_Y20

30μm 30μm 30μm

9

Results and discussions (38)

Hardness

bull In nanoindentation test same trend with tensile test were observed The hardness was increased by 10 years thermal aging and decreased by 20 years thermal aging

bull Hardness at grain boundary have slightly higher value for all specimens however the difference was not significant in order of 005 GPa It is because the plastic deformation zone of indentation is large (~25um) enough to cover several grain boundaries so influence of precipitates to hardness is almost constant

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 247 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 259 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)Indentation depth (nm)

Mean 281 GPa

Nanoindentation result of As-received Nanoindentation result of HT400_Y10 Nanoindentation result of HT400_Y20

Investigated location As-received HT400_Y10 HT400_Y20

Inside grain [GPa] 244 plusmn 003 275 plusmn 008 255 plusmn 009

Cover grain boundary [GPa] 25 plusmn 002 276 plusmn 005 26 plusmn 01

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

30μm 30μm 30μm

10

Results and discussions (48)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

OM image of As-received OM image of HT400_Y10 OM image of HT400_Y20

200μm 200μm 200μm

11

Results and discussions (58)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

30μm 30μm 30μm

10μm 10μm 10μm

bull The morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

12

Results and discussions (68)

Microstructure

bull The morphology of precipitates were known to have significant influence to material`s mechanical properties And this feature is the main reason for mechanical property changes due to thermal aging

bull The feature of calculated material strength according to Orowan mechanism which explain the amount of hardening due to precipiates shows same trend with that of mechanical test which was conducted in this study hardening by 10 years thermal aging and softening by 20 years thermal aging Therefore it could be thought that the aspect of mechanical properties due to thermal aging is closely related to the morphology of precipitates

Material hardening and softening behavior depending on the number fraction and length of precipitates

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates [] 036 plusmn 004 12 plusmn 01 143 plusmn 014

Precipitate length [μm] 080 plusmn 030 092 plusmn 031 186 plusmn 119

Spacing between precipitates [μm] 278 plusmn 022 192 plusmn 014 328 plusmn 031

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

Morphology of precipitates Discrete Semi-continuous Continuous

Summary of mechanical and microstructural characteristics of each specimen

120533 =119918119918

119923 minus 2119955

[1] Z Guo et al Mater Trans (2002) [2] R Hayes et al Acta Metal (1982)

13

Results and discussions (78)

Microstructure Kernel Average Misorientation

Grain boundary misorientation

Grain boundary misorientation

Grain boundary misorientation

Kernel Average Misorientation

Kernel Average Misorientation

EBSD analysis results of HT400_Y20 EBSD analysis results of HT400_Y10 EBSD analysis results of As-received

200μm 200μm 200μm 200μm 200μm 200μm

bull Kernel average misorientation (KAM) which is proportional to residual strain induced by dislocation density was not much changed by thermal aging

bull Grain boundary misorientation however significantly changed Coincidence site lattice boundary which is more resistant to material degradation was decreased by thermal aging The formation of precipitates could be a reason for this however it could be verified with other tools like TEM

14

Results and discussions (88)

Microstructure

bull Major changes induced by thermal aging were length and morphology of precipitates bull HT400_Y10 have semi-continuous precipitates HT400_Y20 have continuous precipitates and As-received

specimen have discontinuous precipitates bull Several previous studies suggest that material with continuous feature of precipitates have lower

susceptibility to SCC than materials with both semi-continuous and discontinuous precipitates bull With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to

formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates []

036 plusmn 004

12 plusmn 01

143 plusmn 014

Precipitate length [μm] 0801 plusmn 0297

0923 plusmn 0312

1864 plusmn 119

Grain size [μm] 2667 2461 2539

Fraction of CSL boundary []

255 plusmn 38

153 plusmn 488

68 plusmn 441

Morphology of precipitates Discrete Semi-

continuous Continuous

Corrosion resistance by huey test [mmyr] 504 697 523

Average properties in electron microscopy

[1] G S Was Corrosion (1990) [2] J R Crum et al 4th EDM-NPS (1990)

SEM images of each specimen (a) as-received (b) HT400_Y10 and (c) HT400_Y20

(a)

(b) (c)

Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

Summary

• Slide Number 1
• Slide Number 2
• Slide Number 3
• Slide Number 4
• Slide Number 5
• Slide Number 6
• Slide Number 7
• Slide Number 8
• Slide Number 9
• Slide Number 10
• Slide Number 11
• Slide Number 12
• Slide Number 13
• Slide Number 14
• Slide Number 15
• Slide Number 16

Objective Investigate the effect of long-term thermal aging to PWSCC initiation resistance of material (preliminary study) Investigate the variation of detailed microstructural and mechanical

properties of the Alloy 600 subjected to long-term thermal aging in an operating NPP to evaluate the influence of long-term thermal aging to material property

Approach Thermal aging simulation of light water reactor environment at accelerated temperature

Microstructural analysis with Electron Microscopy Electron Backscattered Diffraction (EBSD)

Mechanical property analysis with Tensile test Nanoindentation

IGSCC resistance analysis with huey test

4

Objective and Approach

Objective and Approach

Alloy 600 thick rod specified by ASTM B166 was provided by Doosan Heavy Industries amp Construction Annealed at 1060 for 35 h and water quenched

Accelerated temperature was determined as 400oC and for activation energy of Cr diffusion at grain boundary 180 kJmol was used for Ni base alloy which containing 15 to 30 wt chromium [1]

5

Experiment (12)

Material amp Thermal aging

Chemical compositions (wt)

Element C Si Mn Cr Cu Ni S Fe

wt 007 033 056 1583 002 7479 lt0001 840

Heat treatment and aging conditions of each specimen Specimen name Simulated aging time and temperature Heat treatment time and temperature

As-received - - HT400_Y10 10 years at 320 1142 hours at 400 HT400_Y20 20 years at 320 2284 hours at 400

119905119886119886119886119886119886119905119903119903119903

= 119890119890119890 minus119876 1

119879119903119903119903minus 1119879119886119886119886119886119886

119877

119905119886119886119886119886119886 119905119903119903119903 119879119886119886119886119886119886 119879119903119903119903

R 119876

= Heat treatment time [h] = Simulated aging time [h] = Heat treatment temperature [K] = Simulated aging temperature [K] = Gas Constance [=8314 JmolK] = Activation Energy for Cr diffusion [kJmol]

Diffusion equation for thermal aging

[1] JM Boursier et al ASME PVP (2004)

6

Experiment (22)

Conditions Experiment conditions

EBSD Acceleration voltage 10 kV Current 064 nA Tilt angle 70deg Step size 2 μm Scanned area 298 μm x 881 μm to cover several number of grains

Nanoindentation Poisson`s ratio 03 Strain rate 005 sec-1

Displacement into surface 2000 nm using berkovich tip To improve the reliability of the results an average was taken from multiple indentations

at random position of matrix Tensile test

Multiple tests were performed at strain rate of 04 sec-1 at room temperature Test were prepared and performed based on ASTM E8-E8m

Huey test 20 cm3 was exposed to 80oC 65 HNO3 solution for 96 hours and weight loss of each

specimens were measured Test were prepared and performed based on ASTM A262

Proportionally reduced specimen from ASTM standard used in tensile test

Specimen As-received HT400_Y10 HT400_Y20

Young`s modulus [GPa] 2113 plusmn 120 2907 plusmn 119 2567 plusmn 464

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

7

Results and discussions (18)

Tensile test

bull At the engineering stress-strain curve hardening occurred by 10 years thermal aging while softening occurred by 20 years thermal aging

Representative stress-strain curves of each specimen

8

Results and discussions (28)

Tensile test

bull At fracture surface of each specimen typical ductile failure mode with many dimple were observed It is hard to quantify the size of dimple since there size is not unique as small dimples observed in large dimples

bull There exist grain boundary fracture in thermally aged specimens

50μm 50μm 50μm

Fracture surface of As-received Fracture surface of HT400_Y10 Fracture surface of HT400_Y20

30μm 30μm 30μm

9

Results and discussions (38)

Hardness

bull In nanoindentation test same trend with tensile test were observed The hardness was increased by 10 years thermal aging and decreased by 20 years thermal aging

bull Hardness at grain boundary have slightly higher value for all specimens however the difference was not significant in order of 005 GPa It is because the plastic deformation zone of indentation is large (~25um) enough to cover several grain boundaries so influence of precipitates to hardness is almost constant

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 247 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 259 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)Indentation depth (nm)

Mean 281 GPa

Nanoindentation result of As-received Nanoindentation result of HT400_Y10 Nanoindentation result of HT400_Y20

Investigated location As-received HT400_Y10 HT400_Y20

Inside grain [GPa] 244 plusmn 003 275 plusmn 008 255 plusmn 009

Cover grain boundary [GPa] 25 plusmn 002 276 plusmn 005 26 plusmn 01

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

30μm 30μm 30μm

10

Results and discussions (48)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

OM image of As-received OM image of HT400_Y10 OM image of HT400_Y20

200μm 200μm 200μm

11

Results and discussions (58)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

30μm 30μm 30μm

10μm 10μm 10μm

bull The morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

12

Results and discussions (68)

Microstructure

bull The morphology of precipitates were known to have significant influence to material`s mechanical properties And this feature is the main reason for mechanical property changes due to thermal aging

bull The feature of calculated material strength according to Orowan mechanism which explain the amount of hardening due to precipiates shows same trend with that of mechanical test which was conducted in this study hardening by 10 years thermal aging and softening by 20 years thermal aging Therefore it could be thought that the aspect of mechanical properties due to thermal aging is closely related to the morphology of precipitates

Material hardening and softening behavior depending on the number fraction and length of precipitates

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates [] 036 plusmn 004 12 plusmn 01 143 plusmn 014

Precipitate length [μm] 080 plusmn 030 092 plusmn 031 186 plusmn 119

Spacing between precipitates [μm] 278 plusmn 022 192 plusmn 014 328 plusmn 031

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

Morphology of precipitates Discrete Semi-continuous Continuous

Summary of mechanical and microstructural characteristics of each specimen

120533 =119918119918

119923 minus 2119955

[1] Z Guo et al Mater Trans (2002) [2] R Hayes et al Acta Metal (1982)

13

Results and discussions (78)

Microstructure Kernel Average Misorientation

Grain boundary misorientation

Grain boundary misorientation

Grain boundary misorientation

Kernel Average Misorientation

Kernel Average Misorientation

EBSD analysis results of HT400_Y20 EBSD analysis results of HT400_Y10 EBSD analysis results of As-received

200μm 200μm 200μm 200μm 200μm 200μm

bull Kernel average misorientation (KAM) which is proportional to residual strain induced by dislocation density was not much changed by thermal aging

bull Grain boundary misorientation however significantly changed Coincidence site lattice boundary which is more resistant to material degradation was decreased by thermal aging The formation of precipitates could be a reason for this however it could be verified with other tools like TEM

14

Results and discussions (88)

Microstructure

bull Major changes induced by thermal aging were length and morphology of precipitates bull HT400_Y10 have semi-continuous precipitates HT400_Y20 have continuous precipitates and As-received

specimen have discontinuous precipitates bull Several previous studies suggest that material with continuous feature of precipitates have lower

susceptibility to SCC than materials with both semi-continuous and discontinuous precipitates bull With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to

formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates []

036 plusmn 004

12 plusmn 01

143 plusmn 014

Precipitate length [μm] 0801 plusmn 0297

0923 plusmn 0312

1864 plusmn 119

Grain size [μm] 2667 2461 2539

Fraction of CSL boundary []

255 plusmn 38

153 plusmn 488

68 plusmn 441

Morphology of precipitates Discrete Semi-

continuous Continuous

Corrosion resistance by huey test [mmyr] 504 697 523

Average properties in electron microscopy

[1] G S Was Corrosion (1990) [2] J R Crum et al 4th EDM-NPS (1990)

SEM images of each specimen (a) as-received (b) HT400_Y10 and (c) HT400_Y20

(a)

(b) (c)

Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

Summary

• Slide Number 1
• Slide Number 2
• Slide Number 3
• Slide Number 4
• Slide Number 5
• Slide Number 6
• Slide Number 7
• Slide Number 8
• Slide Number 9
• Slide Number 10
• Slide Number 11
• Slide Number 12
• Slide Number 13
• Slide Number 14
• Slide Number 15
• Slide Number 16

Alloy 600 thick rod specified by ASTM B166 was provided by Doosan Heavy Industries amp Construction Annealed at 1060 for 35 h and water quenched

Accelerated temperature was determined as 400oC and for activation energy of Cr diffusion at grain boundary 180 kJmol was used for Ni base alloy which containing 15 to 30 wt chromium [1]

5

Experiment (12)

Material amp Thermal aging

Chemical compositions (wt)

Element C Si Mn Cr Cu Ni S Fe

wt 007 033 056 1583 002 7479 lt0001 840

Heat treatment and aging conditions of each specimen Specimen name Simulated aging time and temperature Heat treatment time and temperature

As-received - - HT400_Y10 10 years at 320 1142 hours at 400 HT400_Y20 20 years at 320 2284 hours at 400

119905119886119886119886119886119886119905119903119903119903

= 119890119890119890 minus119876 1

119879119903119903119903minus 1119879119886119886119886119886119886

119877

119905119886119886119886119886119886 119905119903119903119903 119879119886119886119886119886119886 119879119903119903119903

R 119876

= Heat treatment time [h] = Simulated aging time [h] = Heat treatment temperature [K] = Simulated aging temperature [K] = Gas Constance [=8314 JmolK] = Activation Energy for Cr diffusion [kJmol]

Diffusion equation for thermal aging

[1] JM Boursier et al ASME PVP (2004)

6

Experiment (22)

Conditions Experiment conditions

EBSD Acceleration voltage 10 kV Current 064 nA Tilt angle 70deg Step size 2 μm Scanned area 298 μm x 881 μm to cover several number of grains

Nanoindentation Poisson`s ratio 03 Strain rate 005 sec-1

Displacement into surface 2000 nm using berkovich tip To improve the reliability of the results an average was taken from multiple indentations

at random position of matrix Tensile test

Multiple tests were performed at strain rate of 04 sec-1 at room temperature Test were prepared and performed based on ASTM E8-E8m

Huey test 20 cm3 was exposed to 80oC 65 HNO3 solution for 96 hours and weight loss of each

specimens were measured Test were prepared and performed based on ASTM A262

Proportionally reduced specimen from ASTM standard used in tensile test

Specimen As-received HT400_Y10 HT400_Y20

Young`s modulus [GPa] 2113 plusmn 120 2907 plusmn 119 2567 plusmn 464

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

7

Results and discussions (18)

Tensile test

bull At the engineering stress-strain curve hardening occurred by 10 years thermal aging while softening occurred by 20 years thermal aging

Representative stress-strain curves of each specimen

8

Results and discussions (28)

Tensile test

bull At fracture surface of each specimen typical ductile failure mode with many dimple were observed It is hard to quantify the size of dimple since there size is not unique as small dimples observed in large dimples

bull There exist grain boundary fracture in thermally aged specimens

50μm 50μm 50μm

Fracture surface of As-received Fracture surface of HT400_Y10 Fracture surface of HT400_Y20

30μm 30μm 30μm

9

Results and discussions (38)

Hardness

bull In nanoindentation test same trend with tensile test were observed The hardness was increased by 10 years thermal aging and decreased by 20 years thermal aging

bull Hardness at grain boundary have slightly higher value for all specimens however the difference was not significant in order of 005 GPa It is because the plastic deformation zone of indentation is large (~25um) enough to cover several grain boundaries so influence of precipitates to hardness is almost constant

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 247 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 259 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)Indentation depth (nm)

Mean 281 GPa

Nanoindentation result of As-received Nanoindentation result of HT400_Y10 Nanoindentation result of HT400_Y20

Investigated location As-received HT400_Y10 HT400_Y20

Inside grain [GPa] 244 plusmn 003 275 plusmn 008 255 plusmn 009

Cover grain boundary [GPa] 25 plusmn 002 276 plusmn 005 26 plusmn 01

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

30μm 30μm 30μm

10

Results and discussions (48)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

OM image of As-received OM image of HT400_Y10 OM image of HT400_Y20

200μm 200μm 200μm

11

Results and discussions (58)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

30μm 30μm 30μm

10μm 10μm 10μm

bull The morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

12

Results and discussions (68)

Microstructure

bull The morphology of precipitates were known to have significant influence to material`s mechanical properties And this feature is the main reason for mechanical property changes due to thermal aging

bull The feature of calculated material strength according to Orowan mechanism which explain the amount of hardening due to precipiates shows same trend with that of mechanical test which was conducted in this study hardening by 10 years thermal aging and softening by 20 years thermal aging Therefore it could be thought that the aspect of mechanical properties due to thermal aging is closely related to the morphology of precipitates

Material hardening and softening behavior depending on the number fraction and length of precipitates

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates [] 036 plusmn 004 12 plusmn 01 143 plusmn 014

Precipitate length [μm] 080 plusmn 030 092 plusmn 031 186 plusmn 119

Spacing between precipitates [μm] 278 plusmn 022 192 plusmn 014 328 plusmn 031

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

Morphology of precipitates Discrete Semi-continuous Continuous

Summary of mechanical and microstructural characteristics of each specimen

120533 =119918119918

119923 minus 2119955

[1] Z Guo et al Mater Trans (2002) [2] R Hayes et al Acta Metal (1982)

13

Results and discussions (78)

Microstructure Kernel Average Misorientation

Grain boundary misorientation

Grain boundary misorientation

Grain boundary misorientation

Kernel Average Misorientation

Kernel Average Misorientation

EBSD analysis results of HT400_Y20 EBSD analysis results of HT400_Y10 EBSD analysis results of As-received

200μm 200μm 200μm 200μm 200μm 200μm

bull Kernel average misorientation (KAM) which is proportional to residual strain induced by dislocation density was not much changed by thermal aging

bull Grain boundary misorientation however significantly changed Coincidence site lattice boundary which is more resistant to material degradation was decreased by thermal aging The formation of precipitates could be a reason for this however it could be verified with other tools like TEM

14

Results and discussions (88)

Microstructure

bull Major changes induced by thermal aging were length and morphology of precipitates bull HT400_Y10 have semi-continuous precipitates HT400_Y20 have continuous precipitates and As-received

specimen have discontinuous precipitates bull Several previous studies suggest that material with continuous feature of precipitates have lower

susceptibility to SCC than materials with both semi-continuous and discontinuous precipitates bull With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to

formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates []

036 plusmn 004

12 plusmn 01

143 plusmn 014

Precipitate length [μm] 0801 plusmn 0297

0923 plusmn 0312

1864 plusmn 119

Grain size [μm] 2667 2461 2539

Fraction of CSL boundary []

255 plusmn 38

153 plusmn 488

68 plusmn 441

Morphology of precipitates Discrete Semi-

continuous Continuous

Corrosion resistance by huey test [mmyr] 504 697 523

Average properties in electron microscopy

[1] G S Was Corrosion (1990) [2] J R Crum et al 4th EDM-NPS (1990)

SEM images of each specimen (a) as-received (b) HT400_Y10 and (c) HT400_Y20

(a)

(b) (c)

Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

Summary

• Slide Number 1
• Slide Number 2
• Slide Number 3
• Slide Number 4
• Slide Number 5
• Slide Number 6
• Slide Number 7
• Slide Number 8
• Slide Number 9
• Slide Number 10
• Slide Number 11
• Slide Number 12
• Slide Number 13
• Slide Number 14
• Slide Number 15
• Slide Number 16

6

Experiment (22)

Conditions Experiment conditions

EBSD Acceleration voltage 10 kV Current 064 nA Tilt angle 70deg Step size 2 μm Scanned area 298 μm x 881 μm to cover several number of grains

Nanoindentation Poisson`s ratio 03 Strain rate 005 sec-1

Displacement into surface 2000 nm using berkovich tip To improve the reliability of the results an average was taken from multiple indentations

at random position of matrix Tensile test

Multiple tests were performed at strain rate of 04 sec-1 at room temperature Test were prepared and performed based on ASTM E8-E8m

Huey test 20 cm3 was exposed to 80oC 65 HNO3 solution for 96 hours and weight loss of each

specimens were measured Test were prepared and performed based on ASTM A262

Proportionally reduced specimen from ASTM standard used in tensile test

Specimen As-received HT400_Y10 HT400_Y20

Young`s modulus [GPa] 2113 plusmn 120 2907 plusmn 119 2567 plusmn 464

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

7

Results and discussions (18)

Tensile test

bull At the engineering stress-strain curve hardening occurred by 10 years thermal aging while softening occurred by 20 years thermal aging

Representative stress-strain curves of each specimen

8

Results and discussions (28)

Tensile test

bull At fracture surface of each specimen typical ductile failure mode with many dimple were observed It is hard to quantify the size of dimple since there size is not unique as small dimples observed in large dimples

bull There exist grain boundary fracture in thermally aged specimens

50μm 50μm 50μm

Fracture surface of As-received Fracture surface of HT400_Y10 Fracture surface of HT400_Y20

30μm 30μm 30μm

9

Results and discussions (38)

Hardness

bull In nanoindentation test same trend with tensile test were observed The hardness was increased by 10 years thermal aging and decreased by 20 years thermal aging

bull Hardness at grain boundary have slightly higher value for all specimens however the difference was not significant in order of 005 GPa It is because the plastic deformation zone of indentation is large (~25um) enough to cover several grain boundaries so influence of precipitates to hardness is almost constant

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 247 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 259 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)Indentation depth (nm)

Mean 281 GPa

Nanoindentation result of As-received Nanoindentation result of HT400_Y10 Nanoindentation result of HT400_Y20

Investigated location As-received HT400_Y10 HT400_Y20

Inside grain [GPa] 244 plusmn 003 275 plusmn 008 255 plusmn 009

Cover grain boundary [GPa] 25 plusmn 002 276 plusmn 005 26 plusmn 01

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

30μm 30μm 30μm

10

Results and discussions (48)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

OM image of As-received OM image of HT400_Y10 OM image of HT400_Y20

200μm 200μm 200μm

11

Results and discussions (58)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

30μm 30μm 30μm

10μm 10μm 10μm

bull The morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

12

Results and discussions (68)

Microstructure

bull The morphology of precipitates were known to have significant influence to material`s mechanical properties And this feature is the main reason for mechanical property changes due to thermal aging

bull The feature of calculated material strength according to Orowan mechanism which explain the amount of hardening due to precipiates shows same trend with that of mechanical test which was conducted in this study hardening by 10 years thermal aging and softening by 20 years thermal aging Therefore it could be thought that the aspect of mechanical properties due to thermal aging is closely related to the morphology of precipitates

Material hardening and softening behavior depending on the number fraction and length of precipitates

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates [] 036 plusmn 004 12 plusmn 01 143 plusmn 014

Precipitate length [μm] 080 plusmn 030 092 plusmn 031 186 plusmn 119

Spacing between precipitates [μm] 278 plusmn 022 192 plusmn 014 328 plusmn 031

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

Morphology of precipitates Discrete Semi-continuous Continuous

Summary of mechanical and microstructural characteristics of each specimen

120533 =119918119918

119923 minus 2119955

[1] Z Guo et al Mater Trans (2002) [2] R Hayes et al Acta Metal (1982)

13

Results and discussions (78)

Microstructure Kernel Average Misorientation

Grain boundary misorientation

Grain boundary misorientation

Grain boundary misorientation

Kernel Average Misorientation

Kernel Average Misorientation

EBSD analysis results of HT400_Y20 EBSD analysis results of HT400_Y10 EBSD analysis results of As-received

200μm 200μm 200μm 200μm 200μm 200μm

bull Kernel average misorientation (KAM) which is proportional to residual strain induced by dislocation density was not much changed by thermal aging

bull Grain boundary misorientation however significantly changed Coincidence site lattice boundary which is more resistant to material degradation was decreased by thermal aging The formation of precipitates could be a reason for this however it could be verified with other tools like TEM

14

Results and discussions (88)

Microstructure

bull Major changes induced by thermal aging were length and morphology of precipitates bull HT400_Y10 have semi-continuous precipitates HT400_Y20 have continuous precipitates and As-received

specimen have discontinuous precipitates bull Several previous studies suggest that material with continuous feature of precipitates have lower

susceptibility to SCC than materials with both semi-continuous and discontinuous precipitates bull With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to

formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates []

036 plusmn 004

12 plusmn 01

143 plusmn 014

Precipitate length [μm] 0801 plusmn 0297

0923 plusmn 0312

1864 plusmn 119

Grain size [μm] 2667 2461 2539

Fraction of CSL boundary []

255 plusmn 38

153 plusmn 488

68 plusmn 441

Morphology of precipitates Discrete Semi-

continuous Continuous

Corrosion resistance by huey test [mmyr] 504 697 523

Average properties in electron microscopy

[1] G S Was Corrosion (1990) [2] J R Crum et al 4th EDM-NPS (1990)

SEM images of each specimen (a) as-received (b) HT400_Y10 and (c) HT400_Y20

(a)

(b) (c)

Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

Summary

• Slide Number 1
• Slide Number 2
• Slide Number 3
• Slide Number 4
• Slide Number 5
• Slide Number 6
• Slide Number 7
• Slide Number 8
• Slide Number 9
• Slide Number 10
• Slide Number 11
• Slide Number 12
• Slide Number 13
• Slide Number 14
• Slide Number 15
• Slide Number 16

Specimen As-received HT400_Y10 HT400_Y20

Young`s modulus [GPa] 2113 plusmn 120 2907 plusmn 119 2567 plusmn 464

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

7

Results and discussions (18)

Tensile test

bull At the engineering stress-strain curve hardening occurred by 10 years thermal aging while softening occurred by 20 years thermal aging

Representative stress-strain curves of each specimen

8

Results and discussions (28)

Tensile test

bull At fracture surface of each specimen typical ductile failure mode with many dimple were observed It is hard to quantify the size of dimple since there size is not unique as small dimples observed in large dimples

bull There exist grain boundary fracture in thermally aged specimens

50μm 50μm 50μm

Fracture surface of As-received Fracture surface of HT400_Y10 Fracture surface of HT400_Y20

30μm 30μm 30μm

9

Results and discussions (38)

Hardness

bull In nanoindentation test same trend with tensile test were observed The hardness was increased by 10 years thermal aging and decreased by 20 years thermal aging

bull Hardness at grain boundary have slightly higher value for all specimens however the difference was not significant in order of 005 GPa It is because the plastic deformation zone of indentation is large (~25um) enough to cover several grain boundaries so influence of precipitates to hardness is almost constant

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 247 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 259 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)Indentation depth (nm)

Mean 281 GPa

Nanoindentation result of As-received Nanoindentation result of HT400_Y10 Nanoindentation result of HT400_Y20

Investigated location As-received HT400_Y10 HT400_Y20

Inside grain [GPa] 244 plusmn 003 275 plusmn 008 255 plusmn 009

Cover grain boundary [GPa] 25 plusmn 002 276 plusmn 005 26 plusmn 01

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

30μm 30μm 30μm

10

Results and discussions (48)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

OM image of As-received OM image of HT400_Y10 OM image of HT400_Y20

200μm 200μm 200μm

11

Results and discussions (58)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

30μm 30μm 30μm

10μm 10μm 10μm

bull The morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

12

Results and discussions (68)

Microstructure

bull The morphology of precipitates were known to have significant influence to material`s mechanical properties And this feature is the main reason for mechanical property changes due to thermal aging

bull The feature of calculated material strength according to Orowan mechanism which explain the amount of hardening due to precipiates shows same trend with that of mechanical test which was conducted in this study hardening by 10 years thermal aging and softening by 20 years thermal aging Therefore it could be thought that the aspect of mechanical properties due to thermal aging is closely related to the morphology of precipitates

Material hardening and softening behavior depending on the number fraction and length of precipitates

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates [] 036 plusmn 004 12 plusmn 01 143 plusmn 014

Precipitate length [μm] 080 plusmn 030 092 plusmn 031 186 plusmn 119

Spacing between precipitates [μm] 278 plusmn 022 192 plusmn 014 328 plusmn 031

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

Morphology of precipitates Discrete Semi-continuous Continuous

Summary of mechanical and microstructural characteristics of each specimen

120533 =119918119918

119923 minus 2119955

[1] Z Guo et al Mater Trans (2002) [2] R Hayes et al Acta Metal (1982)

13

Results and discussions (78)

Microstructure Kernel Average Misorientation

Grain boundary misorientation

Grain boundary misorientation

Grain boundary misorientation

Kernel Average Misorientation

Kernel Average Misorientation

EBSD analysis results of HT400_Y20 EBSD analysis results of HT400_Y10 EBSD analysis results of As-received

200μm 200μm 200μm 200μm 200μm 200μm

bull Kernel average misorientation (KAM) which is proportional to residual strain induced by dislocation density was not much changed by thermal aging

bull Grain boundary misorientation however significantly changed Coincidence site lattice boundary which is more resistant to material degradation was decreased by thermal aging The formation of precipitates could be a reason for this however it could be verified with other tools like TEM

14

Results and discussions (88)

Microstructure

bull Major changes induced by thermal aging were length and morphology of precipitates bull HT400_Y10 have semi-continuous precipitates HT400_Y20 have continuous precipitates and As-received

specimen have discontinuous precipitates bull Several previous studies suggest that material with continuous feature of precipitates have lower

susceptibility to SCC than materials with both semi-continuous and discontinuous precipitates bull With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to

formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates []

036 plusmn 004

12 plusmn 01

143 plusmn 014

Precipitate length [μm] 0801 plusmn 0297

0923 plusmn 0312

1864 plusmn 119

Grain size [μm] 2667 2461 2539

Fraction of CSL boundary []

255 plusmn 38

153 plusmn 488

68 plusmn 441

Morphology of precipitates Discrete Semi-

continuous Continuous

Corrosion resistance by huey test [mmyr] 504 697 523

Average properties in electron microscopy

[1] G S Was Corrosion (1990) [2] J R Crum et al 4th EDM-NPS (1990)

SEM images of each specimen (a) as-received (b) HT400_Y10 and (c) HT400_Y20

(a)

(b) (c)

Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

Summary

• Slide Number 1
• Slide Number 2
• Slide Number 3
• Slide Number 4
• Slide Number 5
• Slide Number 6
• Slide Number 7
• Slide Number 8
• Slide Number 9
• Slide Number 10
• Slide Number 11
• Slide Number 12
• Slide Number 13
• Slide Number 14
• Slide Number 15
• Slide Number 16

8

Results and discussions (28)

Tensile test

bull At fracture surface of each specimen typical ductile failure mode with many dimple were observed It is hard to quantify the size of dimple since there size is not unique as small dimples observed in large dimples

bull There exist grain boundary fracture in thermally aged specimens

50μm 50μm 50μm

Fracture surface of As-received Fracture surface of HT400_Y10 Fracture surface of HT400_Y20

30μm 30μm 30μm

9

Results and discussions (38)

Hardness

bull In nanoindentation test same trend with tensile test were observed The hardness was increased by 10 years thermal aging and decreased by 20 years thermal aging

bull Hardness at grain boundary have slightly higher value for all specimens however the difference was not significant in order of 005 GPa It is because the plastic deformation zone of indentation is large (~25um) enough to cover several grain boundaries so influence of precipitates to hardness is almost constant

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 247 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 259 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)Indentation depth (nm)

Mean 281 GPa

Nanoindentation result of As-received Nanoindentation result of HT400_Y10 Nanoindentation result of HT400_Y20

Investigated location As-received HT400_Y10 HT400_Y20

Inside grain [GPa] 244 plusmn 003 275 plusmn 008 255 plusmn 009

Cover grain boundary [GPa] 25 plusmn 002 276 plusmn 005 26 plusmn 01

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

30μm 30μm 30μm

10

Results and discussions (48)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

OM image of As-received OM image of HT400_Y10 OM image of HT400_Y20

200μm 200μm 200μm

11

Results and discussions (58)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

30μm 30μm 30μm

10μm 10μm 10μm

bull The morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

12

Results and discussions (68)

Microstructure

bull The morphology of precipitates were known to have significant influence to material`s mechanical properties And this feature is the main reason for mechanical property changes due to thermal aging

bull The feature of calculated material strength according to Orowan mechanism which explain the amount of hardening due to precipiates shows same trend with that of mechanical test which was conducted in this study hardening by 10 years thermal aging and softening by 20 years thermal aging Therefore it could be thought that the aspect of mechanical properties due to thermal aging is closely related to the morphology of precipitates

Material hardening and softening behavior depending on the number fraction and length of precipitates

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates [] 036 plusmn 004 12 plusmn 01 143 plusmn 014

Precipitate length [μm] 080 plusmn 030 092 plusmn 031 186 plusmn 119

Spacing between precipitates [μm] 278 plusmn 022 192 plusmn 014 328 plusmn 031

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

Morphology of precipitates Discrete Semi-continuous Continuous

Summary of mechanical and microstructural characteristics of each specimen

120533 =119918119918

119923 minus 2119955

[1] Z Guo et al Mater Trans (2002) [2] R Hayes et al Acta Metal (1982)

13

Results and discussions (78)

Microstructure Kernel Average Misorientation

Grain boundary misorientation

Grain boundary misorientation

Grain boundary misorientation

Kernel Average Misorientation

Kernel Average Misorientation

EBSD analysis results of HT400_Y20 EBSD analysis results of HT400_Y10 EBSD analysis results of As-received

200μm 200μm 200μm 200μm 200μm 200μm

bull Kernel average misorientation (KAM) which is proportional to residual strain induced by dislocation density was not much changed by thermal aging

bull Grain boundary misorientation however significantly changed Coincidence site lattice boundary which is more resistant to material degradation was decreased by thermal aging The formation of precipitates could be a reason for this however it could be verified with other tools like TEM

14

Results and discussions (88)

Microstructure

bull Major changes induced by thermal aging were length and morphology of precipitates bull HT400_Y10 have semi-continuous precipitates HT400_Y20 have continuous precipitates and As-received

specimen have discontinuous precipitates bull Several previous studies suggest that material with continuous feature of precipitates have lower

susceptibility to SCC than materials with both semi-continuous and discontinuous precipitates bull With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to

formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates []

036 plusmn 004

12 plusmn 01

143 plusmn 014

Precipitate length [μm] 0801 plusmn 0297

0923 plusmn 0312

1864 plusmn 119

Grain size [μm] 2667 2461 2539

Fraction of CSL boundary []

255 plusmn 38

153 plusmn 488

68 plusmn 441

Morphology of precipitates Discrete Semi-

continuous Continuous

Corrosion resistance by huey test [mmyr] 504 697 523

Average properties in electron microscopy

[1] G S Was Corrosion (1990) [2] J R Crum et al 4th EDM-NPS (1990)

SEM images of each specimen (a) as-received (b) HT400_Y10 and (c) HT400_Y20

(a)

(b) (c)

Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

Summary

• Slide Number 1
• Slide Number 2
• Slide Number 3
• Slide Number 4
• Slide Number 5
• Slide Number 6
• Slide Number 7
• Slide Number 8
• Slide Number 9
• Slide Number 10
• Slide Number 11
• Slide Number 12
• Slide Number 13
• Slide Number 14
• Slide Number 15
• Slide Number 16

9

Results and discussions (38)

Hardness

bull In nanoindentation test same trend with tensile test were observed The hardness was increased by 10 years thermal aging and decreased by 20 years thermal aging

bull Hardness at grain boundary have slightly higher value for all specimens however the difference was not significant in order of 005 GPa It is because the plastic deformation zone of indentation is large (~25um) enough to cover several grain boundaries so influence of precipitates to hardness is almost constant

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 247 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)

Indentation depth (nm)

Mean 259 GPa

0 500 1000 1500 20000

2

4

6

8

10

Hard

ness

(GPa

)Indentation depth (nm)

Mean 281 GPa

Nanoindentation result of As-received Nanoindentation result of HT400_Y10 Nanoindentation result of HT400_Y20

Investigated location As-received HT400_Y10 HT400_Y20

Inside grain [GPa] 244 plusmn 003 275 plusmn 008 255 plusmn 009

Cover grain boundary [GPa] 25 plusmn 002 276 plusmn 005 26 plusmn 01

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

30μm 30μm 30μm

10

Results and discussions (48)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

OM image of As-received OM image of HT400_Y10 OM image of HT400_Y20

200μm 200μm 200μm

11

Results and discussions (58)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

30μm 30μm 30μm

10μm 10μm 10μm

bull The morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

12

Results and discussions (68)

Microstructure

bull The morphology of precipitates were known to have significant influence to material`s mechanical properties And this feature is the main reason for mechanical property changes due to thermal aging

bull The feature of calculated material strength according to Orowan mechanism which explain the amount of hardening due to precipiates shows same trend with that of mechanical test which was conducted in this study hardening by 10 years thermal aging and softening by 20 years thermal aging Therefore it could be thought that the aspect of mechanical properties due to thermal aging is closely related to the morphology of precipitates

Material hardening and softening behavior depending on the number fraction and length of precipitates

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates [] 036 plusmn 004 12 plusmn 01 143 plusmn 014

Precipitate length [μm] 080 plusmn 030 092 plusmn 031 186 plusmn 119

Spacing between precipitates [μm] 278 plusmn 022 192 plusmn 014 328 plusmn 031

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

Morphology of precipitates Discrete Semi-continuous Continuous

Summary of mechanical and microstructural characteristics of each specimen

120533 =119918119918

119923 minus 2119955

[1] Z Guo et al Mater Trans (2002) [2] R Hayes et al Acta Metal (1982)

13

Results and discussions (78)

Microstructure Kernel Average Misorientation

Grain boundary misorientation

Grain boundary misorientation

Grain boundary misorientation

Kernel Average Misorientation

Kernel Average Misorientation

EBSD analysis results of HT400_Y20 EBSD analysis results of HT400_Y10 EBSD analysis results of As-received

200μm 200μm 200μm 200μm 200μm 200μm

bull Kernel average misorientation (KAM) which is proportional to residual strain induced by dislocation density was not much changed by thermal aging

bull Grain boundary misorientation however significantly changed Coincidence site lattice boundary which is more resistant to material degradation was decreased by thermal aging The formation of precipitates could be a reason for this however it could be verified with other tools like TEM

14

Results and discussions (88)

Microstructure

bull Major changes induced by thermal aging were length and morphology of precipitates bull HT400_Y10 have semi-continuous precipitates HT400_Y20 have continuous precipitates and As-received

specimen have discontinuous precipitates bull Several previous studies suggest that material with continuous feature of precipitates have lower

susceptibility to SCC than materials with both semi-continuous and discontinuous precipitates bull With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to

formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates []

036 plusmn 004

12 plusmn 01

143 plusmn 014

Precipitate length [μm] 0801 plusmn 0297

0923 plusmn 0312

1864 plusmn 119

Grain size [μm] 2667 2461 2539

Fraction of CSL boundary []

255 plusmn 38

153 plusmn 488

68 plusmn 441

Morphology of precipitates Discrete Semi-

continuous Continuous

Corrosion resistance by huey test [mmyr] 504 697 523

Average properties in electron microscopy

[1] G S Was Corrosion (1990) [2] J R Crum et al 4th EDM-NPS (1990)

SEM images of each specimen (a) as-received (b) HT400_Y10 and (c) HT400_Y20

(a)

(b) (c)

Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

Summary

• Slide Number 1
• Slide Number 2
• Slide Number 3
• Slide Number 4
• Slide Number 5
• Slide Number 6
• Slide Number 7
• Slide Number 8
• Slide Number 9
• Slide Number 10
• Slide Number 11
• Slide Number 12
• Slide Number 13
• Slide Number 14
• Slide Number 15
• Slide Number 16

30μm 30μm 30μm

10

Results and discussions (48)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

OM image of As-received OM image of HT400_Y10 OM image of HT400_Y20

200μm 200μm 200μm

11

Results and discussions (58)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

30μm 30μm 30μm

10μm 10μm 10μm

bull The morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

12

Results and discussions (68)

Microstructure

bull The morphology of precipitates were known to have significant influence to material`s mechanical properties And this feature is the main reason for mechanical property changes due to thermal aging

bull The feature of calculated material strength according to Orowan mechanism which explain the amount of hardening due to precipiates shows same trend with that of mechanical test which was conducted in this study hardening by 10 years thermal aging and softening by 20 years thermal aging Therefore it could be thought that the aspect of mechanical properties due to thermal aging is closely related to the morphology of precipitates

Material hardening and softening behavior depending on the number fraction and length of precipitates

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates [] 036 plusmn 004 12 plusmn 01 143 plusmn 014

Precipitate length [μm] 080 plusmn 030 092 plusmn 031 186 plusmn 119

Spacing between precipitates [μm] 278 plusmn 022 192 plusmn 014 328 plusmn 031

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

Morphology of precipitates Discrete Semi-continuous Continuous

Summary of mechanical and microstructural characteristics of each specimen

120533 =119918119918

119923 minus 2119955

[1] Z Guo et al Mater Trans (2002) [2] R Hayes et al Acta Metal (1982)

13

Results and discussions (78)

Microstructure Kernel Average Misorientation

Grain boundary misorientation

Grain boundary misorientation

Grain boundary misorientation

Kernel Average Misorientation

Kernel Average Misorientation

EBSD analysis results of HT400_Y20 EBSD analysis results of HT400_Y10 EBSD analysis results of As-received

200μm 200μm 200μm 200μm 200μm 200μm

bull Kernel average misorientation (KAM) which is proportional to residual strain induced by dislocation density was not much changed by thermal aging

bull Grain boundary misorientation however significantly changed Coincidence site lattice boundary which is more resistant to material degradation was decreased by thermal aging The formation of precipitates could be a reason for this however it could be verified with other tools like TEM

14

Results and discussions (88)

Microstructure

bull Major changes induced by thermal aging were length and morphology of precipitates bull HT400_Y10 have semi-continuous precipitates HT400_Y20 have continuous precipitates and As-received

specimen have discontinuous precipitates bull Several previous studies suggest that material with continuous feature of precipitates have lower

susceptibility to SCC than materials with both semi-continuous and discontinuous precipitates bull With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to

formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates []

036 plusmn 004

12 plusmn 01

143 plusmn 014

Precipitate length [μm] 0801 plusmn 0297

0923 plusmn 0312

1864 plusmn 119

Grain size [μm] 2667 2461 2539

Fraction of CSL boundary []

255 plusmn 38

153 plusmn 488

68 plusmn 441

Morphology of precipitates Discrete Semi-

continuous Continuous

Corrosion resistance by huey test [mmyr] 504 697 523

Average properties in electron microscopy

[1] G S Was Corrosion (1990) [2] J R Crum et al 4th EDM-NPS (1990)

SEM images of each specimen (a) as-received (b) HT400_Y10 and (c) HT400_Y20

(a)

(b) (c)

Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

Summary

• Slide Number 1
• Slide Number 2
• Slide Number 3
• Slide Number 4
• Slide Number 5
• Slide Number 6
• Slide Number 7
• Slide Number 8
• Slide Number 9
• Slide Number 10
• Slide Number 11
• Slide Number 12
• Slide Number 13
• Slide Number 14
• Slide Number 15
• Slide Number 16

11

Results and discussions (58)

Microstructure

SEM image of HT400_Y20 SEM image of HT400_Y10 SEM image of As-received

30μm 30μm 30μm

10μm 10μm 10μm

bull The morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

12

Results and discussions (68)

Microstructure

bull The morphology of precipitates were known to have significant influence to material`s mechanical properties And this feature is the main reason for mechanical property changes due to thermal aging

bull The feature of calculated material strength according to Orowan mechanism which explain the amount of hardening due to precipiates shows same trend with that of mechanical test which was conducted in this study hardening by 10 years thermal aging and softening by 20 years thermal aging Therefore it could be thought that the aspect of mechanical properties due to thermal aging is closely related to the morphology of precipitates

Material hardening and softening behavior depending on the number fraction and length of precipitates

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates [] 036 plusmn 004 12 plusmn 01 143 plusmn 014

Precipitate length [μm] 080 plusmn 030 092 plusmn 031 186 plusmn 119

Spacing between precipitates [μm] 278 plusmn 022 192 plusmn 014 328 plusmn 031

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

Morphology of precipitates Discrete Semi-continuous Continuous

Summary of mechanical and microstructural characteristics of each specimen

120533 =119918119918

119923 minus 2119955

[1] Z Guo et al Mater Trans (2002) [2] R Hayes et al Acta Metal (1982)

13

Results and discussions (78)

Microstructure Kernel Average Misorientation

Grain boundary misorientation

Grain boundary misorientation

Grain boundary misorientation

Kernel Average Misorientation

Kernel Average Misorientation

EBSD analysis results of HT400_Y20 EBSD analysis results of HT400_Y10 EBSD analysis results of As-received

200μm 200μm 200μm 200μm 200μm 200μm

bull Kernel average misorientation (KAM) which is proportional to residual strain induced by dislocation density was not much changed by thermal aging

bull Grain boundary misorientation however significantly changed Coincidence site lattice boundary which is more resistant to material degradation was decreased by thermal aging The formation of precipitates could be a reason for this however it could be verified with other tools like TEM

14

Results and discussions (88)

Microstructure

bull Major changes induced by thermal aging were length and morphology of precipitates bull HT400_Y10 have semi-continuous precipitates HT400_Y20 have continuous precipitates and As-received

specimen have discontinuous precipitates bull Several previous studies suggest that material with continuous feature of precipitates have lower

susceptibility to SCC than materials with both semi-continuous and discontinuous precipitates bull With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to

formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates []

036 plusmn 004

12 plusmn 01

143 plusmn 014

Precipitate length [μm] 0801 plusmn 0297

0923 plusmn 0312

1864 plusmn 119

Grain size [μm] 2667 2461 2539

Fraction of CSL boundary []

255 plusmn 38

153 plusmn 488

68 plusmn 441

Morphology of precipitates Discrete Semi-

continuous Continuous

Corrosion resistance by huey test [mmyr] 504 697 523

Average properties in electron microscopy

[1] G S Was Corrosion (1990) [2] J R Crum et al 4th EDM-NPS (1990)

SEM images of each specimen (a) as-received (b) HT400_Y10 and (c) HT400_Y20

(a)

(b) (c)

Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

Summary

• Slide Number 1
• Slide Number 2
• Slide Number 3
• Slide Number 4
• Slide Number 5
• Slide Number 6
• Slide Number 7
• Slide Number 8
• Slide Number 9
• Slide Number 10
• Slide Number 11
• Slide Number 12
• Slide Number 13
• Slide Number 14
• Slide Number 15
• Slide Number 16

12

Results and discussions (68)

Microstructure

bull The morphology of precipitates were known to have significant influence to material`s mechanical properties And this feature is the main reason for mechanical property changes due to thermal aging

bull The feature of calculated material strength according to Orowan mechanism which explain the amount of hardening due to precipiates shows same trend with that of mechanical test which was conducted in this study hardening by 10 years thermal aging and softening by 20 years thermal aging Therefore it could be thought that the aspect of mechanical properties due to thermal aging is closely related to the morphology of precipitates

Material hardening and softening behavior depending on the number fraction and length of precipitates

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates [] 036 plusmn 004 12 plusmn 01 143 plusmn 014

Precipitate length [μm] 080 plusmn 030 092 plusmn 031 186 plusmn 119

Spacing between precipitates [μm] 278 plusmn 022 192 plusmn 014 328 plusmn 031

Average hardness [GPa] 247 plusmn 014 281 plusmn 024 259 plusmn 031

02 offset yield strength [MPa] 4095 plusmn 109 5873 plusmn 59 4973 plusmn 22

Ultimate tensile strength [MPa] 7018 plusmn 245 7757 plusmn 247 7677 plusmn 25

Elongation [] 500 plusmn 16 360 plusmn 16 420 plusmn 31

Morphology of precipitates Discrete Semi-continuous Continuous

Summary of mechanical and microstructural characteristics of each specimen

120533 =119918119918

119923 minus 2119955

[1] Z Guo et al Mater Trans (2002) [2] R Hayes et al Acta Metal (1982)

13

Results and discussions (78)

Microstructure Kernel Average Misorientation

Grain boundary misorientation

Grain boundary misorientation

Grain boundary misorientation

Kernel Average Misorientation

Kernel Average Misorientation

EBSD analysis results of HT400_Y20 EBSD analysis results of HT400_Y10 EBSD analysis results of As-received

200μm 200μm 200μm 200μm 200μm 200μm

bull Kernel average misorientation (KAM) which is proportional to residual strain induced by dislocation density was not much changed by thermal aging

bull Grain boundary misorientation however significantly changed Coincidence site lattice boundary which is more resistant to material degradation was decreased by thermal aging The formation of precipitates could be a reason for this however it could be verified with other tools like TEM

14

Results and discussions (88)

Microstructure

bull Major changes induced by thermal aging were length and morphology of precipitates bull HT400_Y10 have semi-continuous precipitates HT400_Y20 have continuous precipitates and As-received

specimen have discontinuous precipitates bull Several previous studies suggest that material with continuous feature of precipitates have lower

susceptibility to SCC than materials with both semi-continuous and discontinuous precipitates bull With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to

formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates []

036 plusmn 004

12 plusmn 01

143 plusmn 014

Precipitate length [μm] 0801 plusmn 0297

0923 plusmn 0312

1864 plusmn 119

Grain size [μm] 2667 2461 2539

Fraction of CSL boundary []

255 plusmn 38

153 plusmn 488

68 plusmn 441

Morphology of precipitates Discrete Semi-

continuous Continuous

Corrosion resistance by huey test [mmyr] 504 697 523

Average properties in electron microscopy

[1] G S Was Corrosion (1990) [2] J R Crum et al 4th EDM-NPS (1990)

SEM images of each specimen (a) as-received (b) HT400_Y10 and (c) HT400_Y20

(a)

(b) (c)

Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

Summary

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Results and discussions (78)

Microstructure Kernel Average Misorientation

Grain boundary misorientation

Grain boundary misorientation

Grain boundary misorientation

Kernel Average Misorientation

Kernel Average Misorientation

EBSD analysis results of HT400_Y20 EBSD analysis results of HT400_Y10 EBSD analysis results of As-received

200μm 200μm 200μm 200μm 200μm 200μm

bull Kernel average misorientation (KAM) which is proportional to residual strain induced by dislocation density was not much changed by thermal aging

bull Grain boundary misorientation however significantly changed Coincidence site lattice boundary which is more resistant to material degradation was decreased by thermal aging The formation of precipitates could be a reason for this however it could be verified with other tools like TEM

14

Results and discussions (88)

Microstructure

bull Major changes induced by thermal aging were length and morphology of precipitates bull HT400_Y10 have semi-continuous precipitates HT400_Y20 have continuous precipitates and As-received

specimen have discontinuous precipitates bull Several previous studies suggest that material with continuous feature of precipitates have lower

susceptibility to SCC than materials with both semi-continuous and discontinuous precipitates bull With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to

formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates []

036 plusmn 004

12 plusmn 01

143 plusmn 014

Precipitate length [μm] 0801 plusmn 0297

0923 plusmn 0312

1864 plusmn 119

Grain size [μm] 2667 2461 2539

Fraction of CSL boundary []

255 plusmn 38

153 plusmn 488

68 plusmn 441

Morphology of precipitates Discrete Semi-

continuous Continuous

Corrosion resistance by huey test [mmyr] 504 697 523

Average properties in electron microscopy

[1] G S Was Corrosion (1990) [2] J R Crum et al 4th EDM-NPS (1990)

SEM images of each specimen (a) as-received (b) HT400_Y10 and (c) HT400_Y20

(a)

(b) (c)

Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

Summary

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14

Results and discussions (88)

Microstructure

bull Major changes induced by thermal aging were length and morphology of precipitates bull HT400_Y10 have semi-continuous precipitates HT400_Y20 have continuous precipitates and As-received

specimen have discontinuous precipitates bull Several previous studies suggest that material with continuous feature of precipitates have lower

susceptibility to SCC than materials with both semi-continuous and discontinuous precipitates bull With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to

formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation

Specimen As-received HT400_Y10 HT400_Y20

Area fraction of precipitates []

036 plusmn 004

12 plusmn 01

143 plusmn 014

Precipitate length [μm] 0801 plusmn 0297

0923 plusmn 0312

1864 plusmn 119

Grain size [μm] 2667 2461 2539

Fraction of CSL boundary []

255 plusmn 38

153 plusmn 488

68 plusmn 441

Morphology of precipitates Discrete Semi-

continuous Continuous

Corrosion resistance by huey test [mmyr] 504 697 523

Average properties in electron microscopy

[1] G S Was Corrosion (1990) [2] J R Crum et al 4th EDM-NPS (1990)

SEM images of each specimen (a) as-received (b) HT400_Y10 and (c) HT400_Y20

(a)

(b) (c)

Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

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Thermally aged Alloy 600 was investigated to understand the change of microstructural and mechanical features by long-term thermal aging

In tensile and indentation test material shows hardening at 10 years thermal aging while softening happens in 20 years thermal aging This could be explained by Orowan mechanism that there exist peak strength during precipitate strengthening and softening will occur after that point

bull The morphology of precipitates were mainly changed by thermal aging By 10 years thermal aging length was almost same with as-received but area fraction was significantly increased By 20 years thermal aging length was severally increased and area fraction was not much changed And the morphology of precipitates were discrete at as-received specimen while it was semi-continuous for 10 years thermal aging and continuous for 20 years thermal aging

With consideration of corrosion resistance influenced by chromium depletion at grain boundary due to formation of precipitates it is expected that HT400_Y10 which have large amount of semi-continuous precipitates and have high susceptibility to corrosion would have higher susceptibility to SCC initiation This will be verified with PWSCC initiation test which will be conducted in PWR environment

15

Summary

Summary

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