The Effect of High-temperature Exposure on the Interfacial Microstructure of Co-based Coatings (1)

8/9/2019 The Effect of High-temperature Exposure on the Interfacial Microstructure of Co-based Coatings (1)

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Materials Sci ence & Technolog y

The effect of highThe effect of high--temperature exposure on thetemperature exposure on the

interfacial microstructure of Cointerfacial microstructure of Co--based coatingsbased coatingson a 9%on a 9% --Cr steel for steam turbineCr steel for steam turbine

applicationsapplications

Reinhard Hitzek, Andrea Al-Badri, Dr. Christian Leinenbach

Power Gen India & Central Asia 2012

19.-21. April 2012

Hilfe2



Slide 1

Hilfe2 Diese Folie enthält zwei Mastergruppen (Master und Titelmaster), welche den Corporate-Design-konformen Auftritt definieren. Der jetztzugewiesene Empa-Master 1 sieht für die Titelfolie das Empa-Logo vor. Den weiteren Folien ist kein Logo zugewiesen. Für längere

Vorträge mit Zwischentiteln empfehlen wir, den Folien mit Zwischentiteln den Empa-Master 2 (mit Logo unten rechts) zuzuweisen. Dazuöffnen Sie via Ansicht > Aufgabenbereich > Foliendesign-Entwurfsvorlage rechts die Masterauswahl. Nun markieren Sie im linken Ansichtsfenster die Folien, denen Empa-Master 2 zugewiesen werden soll (mindestens zwei, ansonsten für den ganzen SatzEmpa-Master 1 verwendet wird). Weitere Hilfe erhalten Sie bei Monika Ernst, 4995 (Empa, Dübendorf)M. Ernst, 2/4/2005



Stellba, Power Gen India & Central Asia 2012 2

OutlineOutline

Introduction Stellba History of materials

- From steel to nickel based materials

Why special treatments are necessary

- High temperature exposure

- Failure of coatings occurring at these temperature

Materials and Method

- Base and coating materials

- Coating technique and heat treatment

Material Characterisation

Results

- SEM investigations, element distribution,

hardness measurements, TEM investigation

Phase diagram and thermodynamic calculations

Conclusions



Founded 1957 in SwitzerlandCertified ISO 9001, 14001, 3834-2

GTS Certification for Thermal Spraying

Address:

Stellba Schweisstechnik AGWohlerstrasse 51

5605 Dottikon / Switzerland

Phone +41 56 201 43 43

Fax +41 56 201 43 41

[email protected], www.stellba.ch

Introduction of company StellbaIntroduction of company Stellba

3Stellba, Power Gen India & Central Asia 2012



History of materialsHistory of materials

Steam turbine valves and materials

Year Temp. °C Materials / Coatings HP-control HP-stop IP-control IP-stop

1960 460°C -Low alloy steels-Nitrided

-Some Stellit

X X X X

538°C -Low alloy creep resistant steels-Stellit

X X X X

565°C -High alloy creep resistant steels(12%Cr)

-Stellit-Chromium carbide

X X X X

585°C -High alloy creep resistant steels(9-12%Cr)

-Stellit-Chromium carbide

X X X X

600°C + -High alloy creep resistant steels(9-12%Cr)

-Nickel based materials-Special coatings

X X X X

2020 700°C -Nickel based materials-Special coatings

R&DParticipate in

Comtes 700project

R&DParticipate in

Comtes 700project

R&DParticipate in

Comtes 700project

R&DParticipate in

Comtes 700project

X = produced by Stellba X = produced by Stellba

HP = high pressure IP = intermediate pressure /reheat

Remarks:All type of valves including actuators and casings can be produced at Stellba

Our Customers are all major OEM’s world wide

4Stellba, Power Gen India & Central Asia 2012




Why special treatments are necessaryWhy special treatments are necessary

SIEMENS AG

Alstom

- Reduction of CO2 emission into atmosphere- Therefore improvement of efficiency of steam power plants

- One important parameter is the working temperature in turbines(efficiency increase 1% per 20°C increase of steam temperature)

- valve seat coatings are used for temperatures up to 550°CProblems at temperatures T > 600°C: • Delaminating of Stellite coatings on

stainless steel on valve seats

WHY SO?WHY SO?




Materials and MethodMaterials and Method

Base and coating material

Base: 9% Cr-steel X10CrMoVNb 9-1

Coating: Co-based hardfacing alloys

Stellite 6 (Co-29Cr-4W-1C)

Stellite 21 (Co-27Cr-5Mo-0.3C)

Ultimet (Co-26Cr-9Ni-5Mo-0.04C)

Coating technique and heat treatment

X10CrMoVNb9-1 - Co-based alloy → Plasma Transferred Arc (PTA) welding

After coating process→

heat treatment at 690°C for 4 h to minimize residual stress

Heat treatment was performed at 650°C, respectively for 1’000, 3’000 and

10’000 hours in a furnace in standard air atmosphere.




Material CharacterisationMaterial Characterisation

Microstructural characterisation

- analysed by scanning electron microscopy (SEM) in BSE mode

Element distribution

- taken perpendicular to the interface by energy dispersive spectrometry (EDS)

- three spot measurements at a defined distance from Steel- Co-based-

interface were taken and averaged

Hardness measurement

- a Fisherscope HM 2000 with WIN- HCU ® - software was used for micro

hardness measurements

- Vickers hardness was measured perpendicular to the interface on 5 different

spots at a defined distance and averaged




TEM investigation

a transmission electron microscopic (TEM) image was taken in the interlayer

region with the assumed interface between steel and Stellite 6

preparation of a thin semicircular disc with 3mm diameter and 80 µm thickness

thinned to approx. 30 µm thickness by grinding

and polishing

a section with a width of 20 µm across the

interface was thinned by Focus IonBeam (FIB) milling, using a gallium ion beam

Thermodynamic simulations

CALPHAD based software ThermoCalc version S

database TCFE3 including thermodynamic data on Co-Cr and

Co-Cr-C systems




ResultsResults

1’000h

3’000h

SEM Investigations

10’000h

as-coated

X10CrMoVNb 9-1 – Stellite 6

Stellite 6

Steel




3’000h 10’000has-coated

X10CrMoVNb 9-1 – Stellite 21

as-coated 1’000h 3’000h

X10CrMoVNb 9-1 – Ultimet

Stellite 21

Steel

Ultimet

Steel




Interlayer thickness as function of annealing time

20 30 40 50 60 70 80 90 100 1100

5

10

15

20

25

30

35

Stellite 6

Stellite 21

Ultimet

d i n t e r l a y e r

[ µ m ]

t

0.5

[h

0.5]

]




Element DistributionX10CrMoVNb 9-1 – Stellite 6

-20 -10 0 10 200

10

20

30

40

50

60

70

80

90

100as-coated

Stellite 6X10CrMoVNb9-1

Fe

Cr

Co

Ni

W

c [ m a . -

% ]

d [µm]

-20 -10 0 10 20 30 40 500

10

20

30

40

50

60

70

80

90

1003'000h

Stellite 6InterlayerX10CrMoVNb9-1

Fe

Cr Co

Ni

W

c [ m a . -

% ]

d [µm]

-30 -20 -10 0 10 20 30 40 50 600

10

20

30

40

50

60

70

80

90

100Interlayer

10'000h

X10CrMoVNb9-1 Stellite 6

Fe

Cr Co

Ni

W

c [ m a . -

% ]

d [µm]

0

Stellite 6

Steel

0

10

20

-10

-20

Stellite 6

Steel

-10

-20

10

20

30




Hardness measurement

-60 -40 -20 0 20 40 60250

300350

400

450

500

550

600

650

700

750

Interlayer Stellite 21X10CrMoVNb9-1

as-coated 1'000h

3'000h

10'000h

M

i c r o h a r d n e s s

H V

d [µm]-60 -40 -20 0 20 40 60

250

300350

400

450

500

550

600

650

700

750

InterlayerX10CrMoVNb9-1

Stellite 6- 10'000h

Ultimet- 3'000h

M

i c r o h a r d n e s s

H V

d [µm]




TEM investigation

Stellite 6 annealed at 625°C

EDS mapping:

Cyan: Cobalt, Co

Green: Iron, Fe

Purple: Chromium, Cr

Yellow: Tungsten, W

point Fe Co Cr W C possible phase

1 28.2 38.7 32.0 0.8 <0.3

2 45.8 45.6 7.5 0.8 <0.3 α‘ – ordered bcc

3 8.4 7.3 71.7 8.1 5.1 M23C6

4 7.7 25.6 42.0 20.0 4.7 M23C6

spot measurement [wt%]




Phase diagramPhase diagram

Massalski

point Fe Co Cr W C possible phase

2 45.8 45.6 7.5 0.8 <0.3 α‘ – ordered bcc

αα‘‘ (B2(B2--type):type): believed to

result in intergranular brittlefracture

E.P. George, A.N. Gubbi, I. Baker, L.Robertson, Material Science

Engineering A329-331 (2002) 325-333




Thermodynamic calculationsThermodynamic calculations

Thermodynamic simulations

Stellite 6 at 650°C

44Fe-42Co-9Cr-1Ni-0.45Mo-3W-1C

Average composition at 20 µmfrom 0-point in the interlayer:

0

Possible phases:• M23C6

• µ- phase

• M7C3

• α

- phase




ConclusionConclusion

Failure of Co-based coatings on 9% Cr – steel attemperatures >600°C:

development of a thin interlayer with a high hardness

interlayer thickness depends on the annealing time

Fe diffuses into the Co-based coating

TEM investigations assume the development of abrittle B2 interlayer and M23C6-carbides

thermodynamic simulations support the developmentof M23C6-carbides and α-phase, as well as thedevelopment of µ precipitates

but development of µ precipitates could not directlyobserved




Thank you for your attention!

The project (N°8523.2 E) is financially supported by

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The Effect of High-temperature Exposure on the Interfacial Microstructure of Co-based Coatings (1)