Microstructure Stability: Optimization of 263 Ni

0164
Microstructure Stability: Optimization of 263 Ni-based Superalloy
Coraline Crozet, Alexandre Devaux, Denis Béchet – Aubert&Duval, Site des Ancizes, 63770 Les Ancizes, France
[email protected]
Introduction
Alloy Design – Theoritical approach
To improve efficiency, future generation of steam power plants plans to operate at temperature higher
than 700°C. Nimonic C-263 [1] (19-21Co,19-21Cr, 5.6-6.1Mo, 1.9-2.4Ti, ≤0.6Al, 0.04-0.08C, bal.Ni wt%)
is considered to be one of the best candidate materials [2].
Conditions to optimize C-263 composition:
• To improve microstructure stability at 750°C
1/ To remove ɳ phase formation
Two strengthening mechanisms occur in C-263 alloy:
• Precipitation hardening : precipitation of gamma-prime (ɣ’) phase during the hardening heat treatment.
• Solid solution strengthening : elements such as molybdenum provide high temperature strengthening
through lattice distortion due to its atomic size difference from the matrix.
• Process properties (workability, weldability…) as good as C-263
2/ ɣ’ solvus temperature similar to C-263 ɣ’ solvus temperature
3/ ɣ’ content at 750°C similar to C-263 ɣ’ content at 750°C
Strength
• Elevated temperature strength and creep
resistance
• high corrosion and oxidation resistance
• good process properties: low sensitivity for
segregation, high workability and weldability
Weakness
• ɳ phase precipitates at the expense of ɣ’ in
long-time overaged sample [3]
• Detrimental for long-term creep strength [4]
Thermo-Calc simulations: %wt Ti = f (%wt Al)
• Stability area of ɳ phase
• C-263: ɣ’ solvus temperature equal to 900°C
and ɣ’ content at 750°C equal to 10at%
• Ti-Al couples which allow 900°C as ɣ’ solvus
temperature
(b) TTT diagram [2]:
ɳ phase will precipitate at temperatures of interest for the
future power plants (700°C-900°C)
• Ti-Al couples which allow 9, 10 and 11 at%
of ɣ’ content at 750°C
Best compromise
%Al : 0.83 +/- 0.05
%Ti: 1.53 +/- 0.05
New superalloy based on C-263 composition [5] with
higher microstructure stability at 750°C and good
process properties as C-263
(a) Thermo-Calc (TCNi5) simulations
Optimized Alloy composition: 20Co, 20Cr, 5.9Mo, 1.53Ti, 0.83Al, 0.05C, bal. Ni wt%
Experimental study
Industrial test (6 tons): Optimized Alloy
Trial tests (150kg): C-263 and Optimized Alloy
• VIM + VAR
• Homogenization above 1150°C
• Forging to billet Ø80mm
Good workability! - No surface cracks
• Thermal heat treatment
Similar grain size (around 2 ASTM)
Only one population of secondary ɣ’ precipitates
(around 22 nm)
C-263
• VIM + VAR
• Homogenization above 1150°C
• Forging to billet Ø5000mm
• Thermal heat treatment
Homogeneous grain size (around 2 ASTM)
Good workability - No segregation issue
Optimized alloy
Only one population of secondary ɣ’
precipitates (around 23 nm) similar to trial
test
Core
Rim
(a) C-263 and Optimized Alloy bars (Ø 80mm)
• Chemical analysis
(c) Optical micrographs after heat treatment :
1150°C/2h/Air + 800°C/8h/Air
wt%
Ni Cr Co Mo Ti
Al
C
C-263
Bal 19.7 20 5.8 2.2 0.42 0.05
Optimized alloy Bal 19.7 20 5.9 1.53 0.79 0.05
500µm
(e) Optimized Alloy bars (Ø 500mm)
• Dilatometry tests: ɣ’ solvus temperature
C-263
500µm
(f) Optical micrographs after heat treatment 1130°C/2h/WQ + 800°C/8h/Air
Optimized Alloy
Chemical analysis:
20 nm
20 nm
wt%
Ni Cr
Co Mo Ti
Al
C
Optimized alloy Bal 19.8 20 6.1 1.51 0.78 0.05
20 nm
100 nm
100 nm
200 nm
(g) FEG-SEM micrograph after heat
treatment - secondary ɣ’ precipitates (around
23 nm)
(d) FEG-SEM micrographs after heat treatment - secondary
ɣ’ precipitates (around 22 nm)
(b) Dilatometry curves – Second cycle (3°C/min)
Similar ɣ’ solvus temperatures
Alloy Properties and Discussion
Properties after standard heat treatment
Trial alloys: 1150°C/2h/Air + 800°C/8h/Air
Tensile properties for Optimized Alloy are
slightly lower than those of C-263
Could be due to an Al content in the lower
range of the expected composition
Conclusions
Properties after long-term ageing: Ageing of 3000h at 750°C
Microstructure: No ɳ phase precipitation on Optimized Alloy
C-263
Trial
Industrial
No plates of ɳ phase
Plates of ɳ phase
20 nm
Optimized Alloy
20 nm
20 nm
(c) FEG-SEM micrographs after 3000h/750°C
Optimized Alloy has higher microctructure stability
Mechanical properties on Optimized Alloy:
No effect on impact strength of long-term ageing at 750°C contrary to C-263
Lower influence on creep rupture life than C-263
(a) Tensile strength on C-263 and Optimized alloy
after standard HT
Aubert&Duval has optimized C-263 grade based on:
Higher microstructure stability regarding ɳ phase (1)
Similar workability and properties as C-263:
Similar ɣ’ solvus temperature (2)
Similar ɣ’ content at 750°C (3)
Trial and industrial tests were performed and demonstrated that
chosen composition of Optimized Alloy satisfies the three
criteria of material design.
Optimized Alloy versus C-263
Microstructure:
After standard Heat treatment
Similar grain size and same population of secondary ɣ’
precipitates
After long-term ageing
No ɳ phase precipitation after long-term ageing
Mechanical properties:
After standard Heat treatment
Similar properties, tensile properties could be improved with a
better arrangement of Ti and Al contents
After long-term ageing
No decrease of impact strength and creep rupture life for
Optimized Alloy
(d) Impact strengths and creep results on C-263 and Optimized alloy without and with over-ageing at 750°C/3000h
(b) Creep results on C-263 and Optimized
alloy after standard HT
Optimized Alloy has higher mechanical properties after long-term ageing
Promising new superalloy with high potential
for applications at high temperature and long
time as future A-USC steam turbines.
References
[1] S.A.Smith, G.D West, K.Chi, W.Gamble, R.C Thomson, Adv. in materials technology for Fossil Power Plants. Proc. from the sixth int. conf., 110-126, (2010)
[2] Special Metal: Nimonic alloy 263. SMC-054, (2004)
[3] J.C Zhao, V.Ravikumar, A.M Beltran, Met.and Met.Trans.A, 32(6), 1271-1282 (2001)
[4] C.T Sims, N.S Stoloff , W.C Hagel. Superalloys II High-temperature materials for aerospace and industrial power. 110-111, (1987)
[5]Patent application is pending
This project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant
agreement no ENER/FP7EN/249745/"NEXTGENPOWER”

Download Report