NSF I/UCRC Behavior of A-USC Alloy Fusion Welds...740GTAW - SA Microstructural Evolution and...
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740GTAW - SA Microstructural Evolution and Creep-Rupture Behavior of A-USC Alloy Fusion Welds Daniel Bechetti and John DuPont, Lehigh University John Shingledecker and John Siefert, EPRI Objectives and Approach NSF I/UCRC Manufacturing and Materials Joining Innovation Center • Advanced Ultrasupercritical (A-USC) process cycle targeted for use in next generation of coal fired power plants • A-USC operating conditions (steam temperatures up to 760°C and steam pressures up to 5800psi) require use of precipitation strengthened Ni alloys – Candidates include INCONEL ® Alloy 740/740H ® , NIMONIC ® Alloy 263, and Haynes ® Alloy 282 ® • Welds involving all candidate alloys exhibit weld strength reductions (WSRs) in creep of up to 30% as compared to wrought alloy 740 – As-welded alloy 740 performs the worst, but improvements are seen by changing to alloy 263 or alloy 282 filler metal (Fig. 1) – Post-weld solution treatment of alloy 740 produces the best average WSR (Fig. 2) • WSR in alloy 740H specimens attributed to strain localization in γ’ denuded zones along grain boundaries, which form via discontinuous coarsening of γ’ Background • Project goals and objectives – Do all candidate A-USC alloys form grain boundary denuded zones similar to those observed in alloy 740H? – If so, do these denuded zones form by the same mechanism as in alloy 740H? – If so, are they associated with creep damage localization and therefore premature weld creep failure? Results and Discussion 263GMAW Conclusions Results and Discussion • Location of failure in A-USC weld creep specimens is dependent on filler metal and post- weld heat treatment • Clear difference in denuded zone susceptibility between A-USC alloys – Alloys that form phases which stabilize at the expense of γ’ are likely to form denuded zones and have reduced long-term creep rupture performance – Slow precipitation kinetics of these deleterious phases may cause alloy 263 and solution annealed alloy 740 filler metal rupture performance to appear improved at short times but drop at longer exposure times (Fig. 9) • When present, grain boundary denuded zones are clear sites of localized creep deformation • Microstructural modeling is well aligned with experimental results, so future design of A- USC alloys should be guided by such materials models in order to predict the microstructural gradients and long-term microstructural evolution in fusion welds 20000 21000 22000 23000 24000 25000 26000 27000 28000 100 740GMAW 263GMAW 740GTAW 740GTAW-SA 282GTAW 740BM Ave. -30% Stress (MPa) Larson-Miller Parameter (C = 19.392) Stress. MPa Fig. 1: Creep rupture performance of A-USC alloy cross-weld specimens Fig 2: Typical grain boundary γ’ denuded zone in alloy 740H weld metal after creep rupture Creep voids Denuded zone Coarse γ’ 1μm • Specimen matrix and techniques used – Specimens with rupture lives >5000hrs chosen to ensure representation of long-term exposure – Most important compositional differences are γ’ former content and Mo content – Stereomicroscopy, light optical microscopy, and scanning electron microscopy with X-Ray energy dispersive spectrometry used to characterize microstructures – ThermoCalc and DICTRA used to predict microstructural evolution Filler Metal Composition, wt pct Specimen ID Ni Cr Co Al Ti Nb Fe Mn Mo C 263GMAW Bal 21 20 0.5 2.2 0.0 0.37 0.21 5.9 0.06 740GTAW-SA Bal 24 20 1.0 1.8 2.0 0.46 0.26 0.51 0 282GTAW Bal 19 10 1.5 2.2 0.0 0.68 0.05 8.6 0.06 Specimen ID Creep Temperature, °C Stress, MPa Rupture Life, h 263GMAW 750 140 12,741 740GTAW-SA 800 75 25,237 282GTAW 800 120 5,103 • Failure and primary damage accumulation occurred in the fusion zone • Large γ’ denuded zones present throughout the weld metal, and creep damage is clearly associated with them • Thermodynamic and kinetic modeling predict that microsegregation during solidification will produce regions where η is stable instead of γ’ after long-term aging • With Ti-rich η stabilized, the low Al content in the alloy is insufficient to maintain γ’ Mechanism of denuded zone formation in alloy 263 weld metal is dissolution of γ’ because of the stabilization of η, which is rich in γ’-forming Ti • Post-weld solution treatment applied before creep • Failure and primary damage accumulation occurred in the base metal, likely due to recrystallization of weld metal during solution treatment • Large γ’ denuded zones present throughout the weld metal and base metal • Thermodynamic and kinetic modeling predict that both η and γ’ will be stable after aging in both weld metal and base metal • η and G identified throughout microstructure and within denuded zones 10μm 40μm Creep crack Denuded Zones Denuded Zones σ η M 23 C 6 Fig. 3: Micrographs of alloy 263 filler metal in specimen 263GMAW after creep rupture. Left: Stereo, Center: Light optical, Right: Scanning electron. Phases indicated on the SEM micrograph identified via X-Ray EDS. BM FZ Fig. 5: Micrographs of solution annealed alloy 740 specimen after creep rupture. Left: Stereo, Center: Light optical of base metal, Right: Light optical of weld metal. 100μm Creep crack Denuded Zones Denuded Zones 100μm Creep crack FZ BM Mechanism of denuded zone formation in alloy 740 base metal and weld metal is dissolution of γ’ because of the stabilization of η and G, which are both rich in γ’-forming Ti and Nb Fig. 4: Calculated microstructural variation in alloy 263 fusion zone due to microsegregation γ’ η σ M 23 C 6 Fig. 6: SEM micrograph from base metal in specimen 740GTAW-SA. Phases indicated identified via X-Ray EDS 20μm G M 23 C 6 282GTAW • Failure and primary damage accumulation occurred in the heat affected zone (HAZ) • Minimal evidence of large γ’ denuded zones in the alloy 282 weld metal • Thermodynamic and kinetic modeling predict that microsegregation will stabilize secondary phases during aging but will not have a negative effect on γ’ content • M 23 C 6 and μ identified through out the microstructure, as predicted by modeling – Both phases rich in Mo and Cr but not Al or Ti Alloy 282 weld metal does not form large γ’ denuded zones because the phases that are stable after longer term aging are not rich in the γ’-forming elements 10μm 40μm Creep crack Second Phase (not denuded zones) μ M 23 C 6 Fig. 7: Micrographs of alloy 282 filler metal in specimen 282GTAW after creep rupture. Left: Stereo, Center: Light optical, Right: Scanning electron. Phases indicated on the SEM micrograph identified via X-Ray EDS. HAZ FZ 10 100 1000 10000 100000 0.5 0.6 0.7 0.8 0.9 1.0 740GMAW 263GMAW 740GTAW 740GTAW-SA 282GTAW WSF = Weldment-Actual tr / BM-Average tr [LMP=19.392] Weld Strength Reduction (relative to alloy 740 BM) Rupture Life, h γ’ μ M 23 C 6 Fig. 8: Calculated microstructural variation in alloy 282 fusion zone due to microsegregation Fig. 9: WSR values for the specimens shown in Fig. 1. η
Transcript of NSF I/UCRC Behavior of A-USC Alloy Fusion Welds...740GTAW - SA Microstructural Evolution and...
740GTAW - SA
Microstructural Evolution and Creep-Rupture Behavior of A-USC Alloy Fusion Welds
Daniel Bechetti and John DuPont, Lehigh University John Shingledecker and John Siefert, EPRI
Objectives and Approach
NSF I/UCRC Manufacturing and Materials
Joining Innovation
Center
• Advanced Ultrasupercritical (A-USC) process cycle targeted for use in next generation of coal fired power plants
• A-USC operating conditions (steam temperatures up to 760°C and steam pressures up to 5800psi) require use of precipitation strengthened Ni alloys
– Candidates include INCONEL® Alloy 740/740H®, NIMONIC® Alloy 263, and Haynes® Alloy 282®
• Welds involving all candidate alloys exhibit weld strength reductions (WSRs) in creep of up to 30% as compared to wrought alloy 740
– As-welded alloy 740 performs the worst, but improvements are seen by changing to alloy 263 or alloy 282 filler metal (Fig. 1)
– Post-weld solution treatment of alloy 740 produces the best average WSR (Fig. 2)
• WSR in alloy 740H specimens attributed to strain localization in γ’ denuded zones along grain boundaries, which form via discontinuous coarsening of γ’
Background
• Project goals and objectives – Do all candidate A-USC alloys form grain boundary denuded zones similar to those observed in
alloy 740H?
– If so, do these denuded zones form by the same mechanism as in alloy 740H?
– If so, are they associated with creep damage localization and therefore premature weld creep failure?
Results and Discussion
263GMAW
Conclusions Results and Discussion
• Location of failure in A-USC weld creep specimens is dependent on filler metal and post-weld heat treatment
• Clear difference in denuded zone susceptibility between A-USC alloys
– Alloys that form phases which stabilize at the expense of γ’ are likely to form denuded zones and have reduced long-term creep rupture performance
– Slow precipitation kinetics of these deleterious phases may cause alloy 263 and solution annealed alloy 740 filler metal rupture performance to appear improved at short times but drop at longer exposure times (Fig. 9)
• When present, grain boundary denuded zones are clear sites of localized creep deformation
• Microstructural modeling is well aligned with experimental results, so future design of A-USC alloys should be guided by such materials models in order to predict the microstructural gradients and long-term microstructural evolution in fusion welds
20000 21000 22000 23000 24000 25000 26000 27000 28000
100
740GMAW
263GMAW
740GTAW
740GTAW-SA
282GTAW
740BM Ave.
-30%
Str
ess (
MP
a)
LMP C=19.392Larson-Miller Parameter (C = 19.392)
Str
ess
. M
Pa
Fig. 1: Creep rupture
performance of A-USC alloy cross-weld specimens
Fig 2: Typical grain boundary γ’ denuded zone in alloy 740H weld metal after creep rupture
Creep voids
Denuded zone
Coarse γ’
1μm
• Specimen matrix and techniques used
– Specimens with rupture lives >5000hrs chosen to ensure representation of long-term exposure
– Most important compositional differences are γ’ former content and Mo content
– Stereomicroscopy, light optical microscopy, and scanning electron microscopy with X-Ray energy dispersive spectrometry used to characterize microstructures
– ThermoCalc and DICTRA used to predict microstructural evolution
Filler Metal Composition, wt pct
Specimen ID Ni Cr Co Al Ti Nb Fe Mn Mo C
263GMAW Bal 21 20 0.5 2.2 0.0 0.37 0.21 5.9 0.06
740GTAW-SA Bal 24 20 1.0 1.8 2.0 0.46 0.26 0.51 0
282GTAW Bal 19 10 1.5 2.2 0.0 0.68 0.05 8.6 0.06
Specimen ID Creep Temperature, °C Stress, MPa Rupture Life, h
263GMAW 750 140 12,741
740GTAW-SA 800 75 25,237
282GTAW 800 120 5,103
• Failure and primary damage accumulation occurred in the fusion zone
• Large γ’ denuded zones present throughout the weld metal, and creep damage is clearly associated with them
• Thermodynamic and kinetic modeling predict that microsegregation during solidification will produce regions where η is stable instead of γ’ after long-term aging
• With Ti-rich η stabilized, the low Al content in the alloy is insufficient to maintain γ’
Mechanism of denuded zone formation in alloy 263 weld metal is dissolution of γ’ because of the stabilization of η, which is rich in γ’-forming Ti
• Post-weld solution treatment applied before creep
• Failure and primary damage accumulation occurred in the base metal, likely due to recrystallization of weld metal during solution treatment
• Large γ’ denuded zones present throughout the weld metal and base metal
• Thermodynamic and kinetic modeling predict that both η and γ’ will be stable after aging in both weld metal and base metal
• η and G identified throughout microstructure and within denuded zones
10μm 40μm
Creep crack
Denuded Zones
Denuded Zones
σ
η
M23C6
Fig. 3: Micrographs of alloy 263 filler metal in specimen 263GMAW after creep rupture. Left: Stereo, Center: Light optical, Right: Scanning electron. Phases indicated on the SEM micrograph identified via X-Ray EDS.
BM
FZ
Fig. 5: Micrographs of solution annealed alloy 740 specimen after creep rupture. Left: Stereo, Center: Light optical of base metal, Right: Light optical of weld metal.
100μm
Creep crack
Denuded Zones
Denuded Zones
100μm
Creep crack
FZ BM
Mechanism of denuded zone formation in alloy 740 base metal and weld metal is dissolution of γ’ because of the stabilization of η and G, which are both rich in γ’-forming Ti and Nb
Fig. 4: Calculated microstructural variation in alloy 263 fusion zone due to microsegregation
γ’
η
σ M23C6
Fig. 6: SEM micrograph from base metal in specimen 740GTAW-SA. Phases indicated
identified via X-Ray EDS
20μm
G M23C6
282GTAW
• Failure and primary damage accumulation occurred in the heat affected zone (HAZ)
• Minimal evidence of large γ’ denuded zones in the alloy 282 weld metal
• Thermodynamic and kinetic modeling predict that microsegregation will stabilize secondary phases during aging but will not have a negative effect on γ’ content
• M23C6 and μ identified through out the microstructure, as predicted by modeling
– Both phases rich in Mo and Cr but not Al or Ti
Alloy 282 weld metal does not form large γ’ denuded zones because the phases that are stable after longer term aging are not rich in the γ’-forming elements
10μm 40μm
Creep crack
Second Phase (not denuded
zones)
μ M23C6
Fig. 7: Micrographs of alloy 282 filler metal in specimen 282GTAW after creep rupture. Left: Stereo, Center: Light optical, Right: Scanning electron. Phases indicated on the SEM micrograph identified via X-Ray EDS.
HAZ FZ
10 100 1000 10000 100000
0.5
0.6
0.7
0.8
0.9
1.0 740GMAW
263GMAW
740GTAW
740GTAW-SA
282GTAW
WS
F =
W
eld
ment-
Actu
al
tr /
BM
-Avera
ge
tr [LM
P=
19.3
92]
Weldment Rupture Life (hrs)
Weld
Str
ength
Reduct
ion (
rela
tive t
o
allo
y 7
40 B
M)
Rupture Life, h
γ’ μ
M23C6
Fig. 8: Calculated microstructural variation in alloy 282 fusion zone due to microsegregation
Fig. 9: WSR values for the specimens shown
in Fig. 1.
η
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