SCC Crack initiation in nickel based alloy welds in hydrogenated steam at 400°c

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SCC Crack initiation in nickel based alloy welds in hydrogenated steam at 400°c

E. Chaumun, C. Guerre, J. Crepin, C. Duhamel, M. Sennour, E. Heripre, I. de Curieres

To cite this version:

E. Chaumun, C. Guerre, J. Crepin, C. Duhamel, M. Sennour, et al.. SCC Crack initiation in

nickel based alloy welds in hydrogenated steam at 400°c. International Cooperative Group on

Environmentally-Assisted Cracking of Water Reactor Materials (ICG-EAC) annual meeting, Insti-

tute of Metal Research, Chinese Academy of Sciences (IMR, CAS), May 2016, Qindao, China. �hal-

02442236�

SCC CRACK INITIATION IN NICKEL BASED ALLOY

WELDS IN

HYDROGENATED STEAM AT 400°C.

E. Chaumun - Deneuvillers, CEA/DEN/DPC/SCCME/LECA, Mines Paristech and Ecole Polytechnique, France

C. Guerre, CEA/DEN/DPC/SCCME/LECA, Gif-sur-Yvette, France

J. Crépin, C. Duhamel, M. Sennour, Mines Paristech, Evry, France

E. Héripré, LMS, Ecole Polytechnique, Palaiseau, France

I. de Curières, IRSN, Fontenay-aux-Roses, France

9 MAI 2016 May 2016 | PAGE 2

• In France, all DMW are stress-relieved

• Alloy 82 is used for DMW and to repair A182 welds which are not stress-relieved after the repair

Dissimilar Metal Welds

Buttering Low alloy steel

Alloy 82

Stainless Steel

Clad

Context

C umul ated nu m be r of cr ack ed w el ds

True operating time (hours)

10 000 100 000 1 000 000

 3 cases out of 300

(cladding, a DMW in A182/A82)

SCC cracks in Alloy 82 welds in J-Groove weld

of Ringhals Steam Generator [Efsing2005]

 Alloy 82 used in Dissimilar Metal Welds

 Focus on the primary loop of Pressurized Water Reactor

SG 1

SG 3

[Efsing2005] P. Efsing, B. Forssgren, R. Kilian "Root cause failure analysis of defected J-groove welds in steam generator drainage nozzles", Proceedings of 12^th International Conference on Environmental Degradation of Materials in Nuclear Power Systems– Water Reactors”, TMS 2005.

9 MAI 2016

| PAGE 3

 Chemical composition (wt %)

 Welding Process

 Multi-pass V-groove weld

Materials : Alloy 82

C Si Mn P S Cu Mo Ni Cr Co Nb Ti Fe

French

spec. <0.1 <0.5 2.5/3.5 <0.03 <0.015 <0.5 - >67 18/22 <0.1 2/3 <0.75 <3 Weld A 0.014 0.17 2.88 0.002 0.017 <0.01 0.05 72.9 18.15 0.01 2.83 <0.01 2.3 Weld B 0.025 0.07 2.57 0.004 <0.001 <0.01 - 71.7 19.12 0.04 2.41 0.1 3.07

S

L T

304L 304L

A82

20 mm

1 mm

Dendrite growth direction Wire Welding

process

Metallurgical

state weld name

A FCAW As-welded Weld A/AW

B GTAW

As-welded Weld B/AW

Heat-treated (stress-relieved)

7 hr at 600°C

Weld B/HT

| PAGE 4

Microstructure

Weld B as-welded

111

100 110

IPF

Weld A as-welded

• Heterogeneous grain size and elongated grains along the S direction

• Morphology and texture depends on the weld (on the welding process)

• Representative elementary volume close to 1 cm ³

L

T

S

T

2 mm

2 mm

{100} {110} {111}

T

2 mm

Weld B as welded

S

T

9 MAI 2016 | PAGE 5

Approach

Microstructural analyses

Local mechanical behavior

Initiation sites

To identify which microstructural parameters associated with local mechanical behavior

enable SCC initiation

Macroscopic behavior Local behavior

U-bend specimen tested in hydrogenated steam

at 400°C (200 bars)

10 µm

1 µm

Strain fields

DIC using gold grids

Stress (Finite elements)

9 MAI 2016 | PAGE 6

-0,01 -0,005 0 0,005 0,01 0,015 0,02 0,025 0,03 0,035 0,04

0 1000 2000 3000 4000

Ra tio

Temps en heures -0,01

-0,005 0 0,005 0,01 0,015 0,02 0,025 0,03 0,035 0,04

0 1000 2000 3000 4000

Ra tio

Temps en heures -0,01

-0,005 1E-17 0,005 0,01 0,015 0,02 0,025 0,03 0,035 0,04

0 1000 2000 3000 4000

Ra tio

Temps en heures -0,01

-0,005 0 0,005 0,01 0,015 0,02 0,025 0,03 0,035 0,04

0 1000 2000 3000 4000

Number of cracks / number of perpendicular grain boundaries

Alloy B/AW : A82 18%Cr, FCAW, as-welded Alloy A/AW : A82 19%Cr, GTAW, as-welded Alloy A/HT : A82 19%Cr, GTAW, heat-treated

No crack

[Sennour2013] M. Sennour et al. JNM, 2013

Initiation tests : macroscopic behavior

Time (hr)

- Weld B/AW is less susceptible to SCC than weld A/AW

- Weld B/HT is less susceptible than weld B/AW

 “weld to weld” variability

 beneficial effect of the heat treatment assumed to be due to the formation of intergranular chromium carbides

[Sennour2013]

 but some scattering for the same weld + specimen size versus REV

 a local approach can be used.

| PAGE 7

Initiation tests : scattering ?

Weld A

4 specimens

| PAGE 7

2 mm

2 mm

2 mm

2 mm

Apex of the U-bend

specimen

-0,01 -0,005 0 0,005 0,01 0,015 0,02 0,025 0,03 0,035 0,04

0 1000 2000 3000 4000

111

100 110

IPF suivant la direction S Number of cracks / number of

perpendicular grain boundaries

Chemical heterogeneities in the weld passes

| PAGE 8

SIMS analysis at different locations in weld passes (weld A):

at the root of the weld pass (with small grains or large grains ) and in the middle of the pass

111

100 110

IPF suivant la direction S

S T

Area fraction of phases containing Al, Ti or S

0 5 10 15 20 25 30 35 40 45 50

Pied gros grains Milieu Pied petits grains

Milieu Pied

A lumi nium et Tit an e ( % )

Al S Ti

Root

Large grains

Middle Root Small grains

More impurities at the roots of the weld passes (small grains or large grains).

SCC initiation and microstructure ?

| PAGE 9

Depending of the welding process, the chemical composition or the thermal treatment

 more impurities can be found in some places in the welds (roots of the welds passes for instance)

 chromium carbides can precipitate in the grain boundaries

 modify the grain boundary cohesion energy

Weld A – as-welded

2 mm

But for the same chemistry and / or precipitation, not all the grain boundaries crack.

What about the mechanical fields ?

 Strain is not a sufficient parameter to model the SCC initiation behavior [Chaumun et al., 2015] [Chaumun, 2016]

 Stress close to the grain boundary (finite

elements analysis) ?

9 MAI 2016 | PAGE 10

SCC initiation and stress ?

Finite elements computations

 Finite elements computation around selected cracked and uncracked grain boundary

cracked grain boundaries

uncracked grain boundaries

Grain boundary

• Crystallographic orientation (EBSD) and the experimental displacement (DIC) are

applied to the bi-crystal

system.

Parameters in the calculations :

• Cracked and uncracked GB are selected.

• GB are considered as cracked or uncracked depending on FIB characterizations (on each calculated grain).

• Displacement field.

• Crystallographic orientation.

• Angle q.

Results :

• σ ^N _gb = Maximum normal stress σ _gb normalized by the average normal stress of all

computations Grain

Boundary

S T

L

SCC initiation and stress ?

q

 The average of the normalized maximum normal stress is higher for the cracked GB than for the uncracked ones.

SCC initiation and stress ?

0 0,5 1 1,5 2

σ ^N _{gb max} 2,5

vraie

0 0,5 1 1,5 2

σ ^N _{gb max} 2,5

Average = 1,70

Average = 0,67

Cracked GB Uncracked GB

| PAGE 12

Conclusions

9 MAI 2016 | PAGE 13

Alloy 82 is susceptible to SCC initiation in hydrogenated steam at 400°C.

Its susceptibility depends on the welding process, chemical composition and thermal treatment

 the heat treatment can induce intergranular chromium carbides formation that are beneficial. The formation of chromium carbides also depends on the chemical composition (available C and Cr).

The susceptibility depends on the location in the weld passes

 the roots of the weld passes can contain more impurities (correlation with the weld process and with the chemical composition).

The susceptibility depends on the GB binding energy which depends on the GB chemistry, on the strain discrepancy [Wehbi2014], on the precipitation, …

 Not only one local parameter can explain the susceptibility of grain boundaries  BUT a coupling of parameters

Tend to a initiation criterion = mechanical behavior (maximal normal stress and deformation discrepancy) + chemical parameter (intergranular oxide, grain boundary cohesion energy)

Ma cro sco p ic b e h a vio r L o ca l b e h a vio r

Conclusions

| PAGE 14

GB binding

energy

IG chromium

carbides

GB impurities

GB oxidation

rate

….

Strain discrepancy Welding

process Chemical

composition Heat

treatment Normal

stress

400 µm

DEN DPC SCCME Commissariat à l’énergie atomique et aux énergies alternatives

Centre de Saclay| 91191 Gif-sur-Yvette Cedex T. +33 (0)1 69 08 16 27|

Etablissement public à caractère industriel et commercial |R.C.S Paris B 775 685 019

9 MAI 2016

| PAGE 15

CEA | 10 AVRIL 2012

References

M. Sennour, E. Chaumun, J. Crépin, C. Duhamel, F. Gaslain, C. Guerre, I.

de Curières, TEM investigation on the effect of chromium content and of stress relief treatment on precipitation in Alloy 82, Journal of Nuclear Materials, Volume 442, Issues 1–3, November 2013, Pages 262–269 C. Guerre et al., ICG-EAC meeting 2014 and 2015

E. Chaumun, J. Crépin, C. Duhamel, C. Guerre, E. Héripré, M. Sennour, I.

de Curières, SCC crack initiation in nickel based alloy welds in hydrogenated steam at 400°C, 17th International Conference on

Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors, August 9-12, 2015, Ottawa, Ontario, Canada

E. Deneuvillers – Chaumun, PhD thesis, Mines Paristech, 2016

WELDING PROCESS USING FOR GTAW AND FCAW

9 MAI 2016 Environmental Degradation 2015 | August 2015 | PAGE 16

Conditions GTAW FCAW

Wire diameter 1 – 1,2 mm 1,2 mm Current 180A, 13-14V 200-210A, 28-

29V

Welding speed 10cm/mn 30cm/mn

Current polarity DC negative DC positive Heating between

welding passes

120°C max 120°C max

Passes number 90 90

| PAGE 17 Environmental Degradation 2015 | August 2015

CHEMICAL HETEROGENEITIES IN WELD PASSES ?

 Most of the cracks are located in small grain zones

 Majority of small grains are located at the bottom of weld passes

1 mm

 Distribution of chemical

elements inside weld passes ?

 SIMS analyses performed in the middle and bottom of weld passes

1.5 mm

L S T S

L T

2 mm

2 mm

2 mm

APEX zone on each U-bend specimen

2 mm

cracks

| PAGE 18

 Plus de précipitation d’impuretés dans le Moule A

que dans les moules B et B’

 Les moules B et B’ : peu de différence entre pieds et milieux de passes Analyses chimiques intragranulaires par Microsonde de Castaing

)

Soutenance Elizabeth CHAUMUN – 6 Avril 2016

Observation de plus de précipitation intragranulaire en pieds de passes

 l’hypothèse d’une présence importante d’impuretés intergranulaires

II) Corrélation sites d’amorçage de fissures / microstructure

Hétérogénéité chimique au sein des passes des moules A/A’ et B/B’

Moule A Moule B

S

Al Ti

Moule B’

0 1 2 3 4 5 6 7

0 2 4 6 8 10 12 14 16 18 20

Pied gros grains

Milieu Pied petits grains

Milieu Pied Milieu Pied

Frac ti on su rf ac iqu e S ou fre ( % )

Frac ti on su rf ac iqu e A lumi nium et T it an e ( % )

| PAGE 19

 Analyses par SIMS en augmentant le rapport signal/bruit (cas du soufre)

 Joints de grains (MET) :

Moule A Moule A’

Pas de mise en évidence de ségrégation de soufre aux joints de grains par SIMS (dans nos conditions d’analyse)

Mais plus de soufre en pieds de passe

Soutenance Elizabeth CHAUMUN – 6 Avril 2016

II) Corrélation sites d’amorçage de fissures / microstructure

Analyses chimiques au niveau des joints de grains

 Moule B précipitation intergranulaire plus dense que le Moule A

 Moule A’ : pas de formation de

carbures de chrome après traitement thermique ≠ Moule B’

Moule A - NbC (≈ quelques dizaines de nm)

Moule A’ - NbC (≈ quelques dizaines de nm)

Moule B - NbC (≈ quelques dizaines de nm) - NbC (≈ plusieurs µm)

Moule B’ -Cr

C

- NbC

9 MAI 2016 Environmental Degradation 2015 | August 2015 | PAGE 20

SCC INITIATION TESTS DIRECT AND COMPLEX LOADING

 Initiation tests

 In an autoclave

 Interrupted at 500 hours, 1500 hours, 2500 hours and 3500 hours

Direct loading 0 % to +12 %

Complex loading Step 1 : 0 % to -2 % Step 2 : -2% to +12%

U-bend specimen

dies

- 2% + 12%

Samples dimensions : 50mm x 9mm

A82

Sample dimension and grain size dimension

= Representative Volume Element

9 MAI 2016 | PAGE 21

SCC INITIATION TESTS

 Initiation tests

 U – bends specimen

 Interrupted at 500 hours, 1500 hours, 2500 hours and 3500 hours

 Tests in hydrogenated steam

Conditions

Temperature 400°C Total pressure 188 bar Hydrogen partial

pressure

0.7 bar

U-bend specimen

Samples dimensions : 50mm x 9mm

A82

9 MAI 2016 Environmental Degradation 2015 | August 2015 | PAGE 22

 Majority of cracked boundaries localized in small grains zones (58% of concerned grains for A/AW TL and 90% for B/AW TL have an equivalent diameter <500µm

 Majority of cases highlight : at least one of the two grains of the couple has a equivalent diameter < 500 µm

 Cracks are intergranular (100% of cases), perpendicular to the loading direction (95%) and localized on HAGB (>15° of misorientation)

FINITE ELEMENT COMPUTATIONS : STRESS

FIELDS ALONG GRAIN BOUNDARY

9 MAI 2016 Environmental Degradation 2015 | August 2015 | PAGE 23

FINITE ELEMENTS COMPUTATIONS : STRESS FIELDS ALONG GRAIN BOUNDARY

[Méric-Cailletaud]L. Méric, P. Poubanne, and G. Cailletaud, Journal of engineering materials and technology transactions of the ASME,1991

 Simple finite elements simulation (3D) : 2 grains

 Crystalline orientations from EBSD analyses

y x

z

Displacement following X =0

Displacement following X = 12%

Grain boundary

𝛾 = 𝜎 − 𝑋 − 𝑅

𝐾 ^𝑛

𝛾 : shearing rate, 𝜎 : critical resolved shear stress, X : kinematic hardening, R : isotrope hardening et K,n Norton law parameters

front/back/top/bottom surfaces  F=0

 Crystalline elastoviscoplatic law with a non-linear isotropic hardening and a linear kinematic hardening (as [Méric-Cailletaud])

 Boundary conditions

 Quadratic mesh, thiner along grain boundary

 Macroscopic boundary conditions

9 MAI 2016 Environmental Degradation 2015 | August 2015 | PAGE 24

FINITE ELEMENTS COMPUTATIONS

 Experimental boundary conditions (ExBC)

[Méric-Cailletaud]L. Méric, P. Poubanne, and G. Cailletaud, Journal of engineering materials and technology transactions of the ASME,1991

 Finite elements computation around cracked and uncracked grain boundaries

𝛾 = 𝜎 − 𝑋 − 𝑅

𝐾 ^𝑛

Grain boundary front/back/top/bottom

surfaces  F=0

Displacement on the outline of the by-crystal system = local deformation

 Crystallographic orientation (EBSD)

 Boundary conditions applied to the outline of the bi-crystal system (experimental displacement)

 Crystalline elastoviscoplatic law with a non-linear isotropic hardening and a linear kinematic hardening (as [Méric-Cailletaud])

 Quadratic mesh, thinner along grain boundary

𝛾 : shearing rate, 𝜎 : critical resolved shear stress, X : kinematic hardening, R : isotrope hardening K,n Norton law parameters

Parameter (units) Value

E (Mpa) 164955

ʋ 0,33

n 6,505

K 31,6

Ro (MPa) 100

Q (MPa) 6000

h 5,8

C 66000

non-linear isotropic hardening

linear kinematic hardening

9 MAI 2016 Environmental Degradation 2015 | August 2015 | PAGE 25

FINITE ELEMENTS COMPUTATIONS : STRESS FIELDS ALONG GRAIN BOUNDARY

 Macroscopic boundary conditions

Normal stress normalized with the average stress

0.91 0.94 0.96 0.98 1.01 1.04 1.18

0,90 0,95 1,00 1,05 1,10

0 20 40 60

 Finite element computations made on:

- cracked grain boundaries - uncracked grain boundaries

 Normalized normal stress profile measured at Gauss point from left grain to right grain

Cracked boundaries Uncracked boundaries

 Stress localized at grain boundary in both cases (cracked and uncraked)

 No differences between cracked and uncracked boundaries

 Apply to the bi-crystal system the local and

experimental deformation