PHASE SEPARATION STUDY OF IN-SERVICE THERMALLY AGED CAST STAINLESS STEEL

ATOM PROBE TOMOGRAPHY

Martin Bjurman1,3, Mattias Thuvander2, Fang Liu2, Pål Efsing3,4. 1 Studsvik Nuclear AB

2 Chalmers University of Technology

3 Royal Institute of Technology (KTH)

4 Ringhals AB

• Thermal ageing of cast and welded austenitic stainless steels is an important issue in LTO-programs

• The cast component are generally large and replacements are costly.

• Variations in overall composition and solidification rates affects. • Ferrite content, size and morphology.

• Overall microstructure.

• Segregation and compositional variations within each phase.

• Aging mainly occurs in δ−ferrite where • Cr-rich α´- and Fe rich α-phase forms by spinodal decomposition.

• G-phase forms by nucleation at the α´ to α phase boundary.

Introduction

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• In-service thermal aging of ferrite from CF8M was investigated by APT for spinodal decomposition and precipitation.

• Two specimens were analysed from ageing at 325°C and 291°C in both laser and voltage pulse modes.

• Limited work has previously been conducted using 3D-APT on in service aged material below 325°C for large production castings.

• This work is part of a base characterisation for various ongoing mechanical testing.

Work content and motivation

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Materials investigated Component

Full power time ~70 000h ~22 000h

hot leg 325ºC 303ºC

Cold leg 291ºC 274ºC

∆T 34ºC 29ºC

Total Full Power time 92 000h

• Material: ASTM 351 CF8M • Ringhals 2 old steam generator (SG) elbows

• inlet (hot) • crossover (SG-outlet) to reactor coolant pump

Inlet Crossover

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Unaged Aged tangetial Aged axial

Crossover 137 69 71

Hot 141 33 31

Charpy Impact test results in Joule (KCU) at 20ºC

Materials investigated Composition and microstructure

C Si Mn P S Cr Ni Mo Ti Cu Co N Ferrite content

Hot leg 0.037 1.03 0.77 0.022 0.008 20.0 10.6 2.09 0.004 0.17 0.040 0.044 20.1

Crossover 0.039 1.11 0.82 0.020 0.012 19.6 10.5 2.08 0.004 0.08 0.035 0.037 19.8

• Local Ferrite-content measured by Ferritescope varies 1.5-22.5% across the component.

Atom % Fe Cr Ni Mn Si Mo

Hot-leg Ferrite 63.1 26.4 5.57 0.82 2.00 2.05

Austenite 65.7 20.8 9.41 0.96 1.82 1.30 Crossover-leg Ferrite 62.9 26.5 5.69 0.87 1.91 2.11

Austenite 65.5 20.5 10.26 0.80 1.82 1.14

Table 4. Results from EDS-analysis of ferrite and austenite in as received material.

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• Two solidification routes occur • Liquid→Liquid+δ→Liquid+δ+γ→δ+γ Generally SS- weld materials • pure δ-solidification Generally large CF8M-castings

• CF8M castings of both routes are found in the literature.

• Scheil-ThermoCalc Calphad modelling and simplified equations1 are used for verification.

• Time independent techniques assuming diffusion of species to be • infinite in the liquid phase, • equilibrium at the interface and • zero diffusion in the solid phase.

• The present castings are close to the limit between these routes

• Final annealing at 1050°C and quenching. • Quenching rates have previously been shown to affect the rate of spinodal decomposition • local ferrite content variations of 1.5 - 22.5 % are indications

• A possible contributor to the decomposition, especially of importance for the small decomposition of the crossover-leg, is decomposition that might have occurred during cooling of the original casting.

1 Suutala N. , Met. Trans. A, vol. 14A, 191–197, 1983

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Manufacturing

• Sample extracted approximately 10 mm below outer surface.

• Limited Ferrite content => Sample lift out by Focused Ion Beam/ SEM using a FEI Versa 3D DualBeam.

Atom Probe Tomography – Sample lift out by FIB

85 mm

950 mm

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Sample extraction (hot leg)

Results – APT radial density functions Spinodal Decomposition

0.95

1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

1.4

1.45

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Bulk

Nor

mal

ized

Conc

entr

atio

n

Radial distance (nm)

0.98

1

1.02

1.04

1.06

1.08

1.1

1.12

0 1 2 3 4 5 6 7 8 9 10

Bulk

Nor

mal

ized

Conc

entr

atio

n

Radial distance (nm)

Hot-leg

Crossover-leg

Cr

Ni

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Wavelength 6.8 nm

Wavelength 4 nm

Decomposition amplitude

Atomic maps of projected volumes (20x20x5 nm³ slices)

hot-leg crossover-leg.

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Chromium-concentration

Nickel

Manganese

Silicon

Limit Si Mn Cr Ni P Mo W V Cu Co

α Fe>72 1.10 0.18 12.9 3.66 0.02 1.10 0.01 0.03 0.02 0.05

α' Cr>30 1.98 0.57 42.0 4.09 0.04 2.13 0.01 0.11 0.01 0.04

G Ni+Mn+Si >20

11.0 4.28 16.5 20.5 0.21 2.16 0.03 0.05 0.18 0.06

Iso-concentration plot of hot-leg

Composition (at.%) of the different phases of the hot-leg, balance Fe.

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The size of the box is 34×34×37 nm3.

α'-phase α-phase G-phase

Proximity Histogram of Hot leg G-phase

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0

5

10

15

20

25

-10 -8 -6 -4 -2 0 2

Conc

entr

atio

n (a

t.%)

Distance from interface (nm)

Ni

Si

Mn

Cu

P

Conc

entr

atio

n (a

t%)

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Increased levels of Cu and P in the G-phase

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

0 1 2 3 4 5Bulk

Nor

mal

ized

Conc

entr

atio

n

Radial distance (nm)

Hot-leg

Crossover-leg

Radial Density Functions – G-phase clustering

0.91

1.11.21.31.41.51.61.71.81.9

0 1 2 3 4 5

Bulk

Nor

mal

ized

Conc

entr

atio

n

Raidal distance (nm)

Hot-leg

Crossover-leg

Mn with respect to Ni as ref. point.

Si with respect to Ni as ref. point.

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Si Mn Cr Ni Mo Fe 15.2 4.6 8.9 45.7 0.9 24.5

Average comp. (at.%) of clusters in the crossover-leg.

Cluster density of 5.6×1024 m-3 Cluster size 57 atoms

• In-service thermal aging of ferrite from CF8M was investigated by APT for spinodal decomposition and precipitation.

• Two specimens were analysed from hot- and crossover-legs in both laser and voltage pulse modes.

• Hot-leg aged at 325°C • Spinodal decomposition was significant with spinodal length of 6.4 nm and a Cr-concentration amplitude of

10.8 at %.

• G-phase was estimated to 3.9×1024 m-3 and diameter 3.2 nm for the hot-leg,

• Crossover-leg aged at 291°C • Limited spinodal decomposition with spinodal length of 4 nm and amplitude 8.1 at %

• G-phase precipitation with a cluster density of 5.6×1024 m-3 of approximately 57 atoms in each position, much less than expected from the hot-leg content.

• The reduction in fracture resistance previously measured in the crossover-leg is high compared to the weak spinodal decomposition measured by APT.

Conclusions

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APT - Ferrite Composition

Si Mn Cr Ni P Mo W V Cu Co Hot 2.22 0.63 24.4 5.71 0.06 1.84 0.02 0.06 0.02 0.05

Crossover 2.10 0.62 23.9 5.80 0.05 2.20 0.001 0.06 0.03 0.03

Composition (at.%), balance Fe.

Local electrode atom probe, Imago LEAP 3000X HR.

• Sample in the shape of a needle with tip radius =10-100 nm

• Ultra-high vacuum (UHV), <10-10 torr

• Cooled sample 50 and 70 K (20-80 K)

• Standing DC voltage (V=3-15 kV)

• Short (ns) pulses at 200 kHz to achieve field evaporation • Voltage pulse (metals) to increase electrical field 15% (15-25%)

• Laser pulse (non-metals) to increase temperature 0.3 nJ or 523 nm (0.01-1.0 nJ)

• Analyzed volume typically 60x60x200 nm3

• Chemical analysis by counting atoms from TOF-MS

• Detection efficiency 37% with reflectron

APT - Equipment

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