An affordable creep-resistantnickel-base alloy for power plant

Franck Tancret

Harry Bhadeshia

The problem

Future power plant: 750°C

But:

Commercial superalloys are too expensive(Nb, Ta, Co, Mo…)

New steels: 650°C

=> Use of Ni-base alloys

=> Design an affordable creep-resistant Ni-base alloy

Industrial requirements

- Affordable

- 100 000 h creep lifetime under 100 MPa at 750°C

- Stable at service temperature

- Forgeable

- Weldable

- Corrosion resistance

- Toughness

Design procedure

- Empirical nonlinear multiparametric modelling of mechanical properties as a function of composition and processing conditions (Gaussian processes)

Based on a huge database on the properties of many existing alloys => captures trends and interactions

- Phase diagram and segregation simulation (Thermo-Calc)- Processability- General metallurgy principles

Materials Science & Technology, 19 (2003)

Ni – 20Cr – 3.5 W – 2.3 Al – 2.1 Ti – 5 Fe – 0.4 Si – 0.07 C – 0.005 B

Experimental results

1 2 3 4 50

100

200

300

400

500

600

700

Experimentalresults

Target

Prediction anderror bounds

617

625

Creep rupture stressat 750°C (MPa)

log (lifetime) (h)

Next issue:PROCESSING

Melting and solidification (primary chemical segregation) Forging (’-free temperature window) Heat treatment (precipiation hardening…) Welding

How modelling can be used to address these issues?

Is the designed alloy easy to process?

Phase diagram simulation (Thermo-Calc)

600 800 1000 1200 14000.00

0.02

0.04

MB

2

'

liquid

. %mol

(° )Temperature C

0

2

α-CrM23C6

M7C3

. %mol

0

20

40

60

80

100

. %mol forgingwindow

melting

No undesirable phasesat service temperature

Primary segregation simulation(Thermo-Calc)

Scheil’s approximation: homogeneous liquid diffusion-free solids LIQUID

T = T – 1 K

SOLID(S) LIQUID

V andcomposition

Primary segregation simulation(Thermo-Calc)

Simple dendrite geometrical models=> composition profiles

2r

Vr

∝

sphere

rV

r

∝

cylinder

Vr ∝

plate

Primary segregation simulation(Thermo-Calc)

EDS analysisprofile

Primary segregation simulation(Thermo-Calc)

0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

80

100

0

1

2

3

4

Ti K

α ( )counts

Relative dendrite thickness

sphere

cylinder

plate ( %)Ti concentration mol

Precipitation hardening kinetics

Solutionising1175°C, WQ

Isothermal heat treatmentsbelow ’ solvus, WQ

Vickers hardness

0.1 1 10 100 1000 10000

2.5

3.0

3.5

4.0

Avrami model+ Friedel-type hardening:

H = H0 + A V

f

1/2

850°C

800°C

750°C

700°CV

icke

rs h

ard

nes

s (G

Pa)

Ageing time (h)

Precipitation hardening kineticsDiffusion-controlled growth model:

3 Ni + diffusing (Al, Ti) => Ni3(Al,Ti)

Before ageing:solutionised

During ageing: ’

Cav

Precipitation hardening kinetics

Diffusion-controlled growth model (Al + Ti)

dN(i)dN(i-1)dNp

distance fromprecipitate

C(1)=

Ceq

C(2) C(i-1) C(i) C(i+1)… …

/ ’interface

surface S dtdx

)1i(C)i(CDS)i(dN

Sdx

)i(dN

Sdx

)1i(dN)i(C)i(C tdtt

adjustableparameter

Precipitation hardening kinetics

Diffusion-controlled growth model (Al + Ti)

C

d

Cav

Ceq

t = 0

C

d

Cav

Ceq

t

C

d

Cav

Ceq

t =

Precipitation hardening kinetics

Diffusion-controlled growth model (Al + Ti)

0.1 1 10 100 1000 10000

2.5

3.0

3.5

4.0

4.5

Friedel-type hardening:

H = H0 + A N

p

1/2

850°C

800°C

750°C

700°C

Vic

kers

har

dn

ess

(GP

a)

Ageing time (h)

Precipitation hardening kinetics

Diffusion-controlled growth model (Al + Ti)

8.5x10-4 9.0x10-4 9.5x10-4 1.0x10-3 1.0x10-3-6

-5

-4

-3

-2

-1

0

1

Q = 348 kJ/mol

D = D0 exp(-Q/RT)

ln D

(n

m2 /s

)

1/T (K-1)

CONCLUSIONS

Design of an affordable Ni-base alloy for power-plant 100 000 h creep lifetime under 100 MPa at 750°C Corrosion-resistant, stable, forgeable, weldable…

Extensive use of modelling:• Mechanical properties (Gaussian processes)• Phase diagram simulation

- Stability- Forgeability- Weldability- Age-hardening- Solidification segregation (Scheil’s model)

• Precipitation-hardening kinetics (diffusion model)

Present and future work

Multicomponent diffusion growth model

Welding

High temperature ductility / forgeability / recrystallisation…

Corrosion-resistance