thermal aging of cast austenitic stainless steel
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THERMAL AGING OF CAST AUSTENITIC STAINLESS STEEL
EVOLUTION OF MICROSTRUCTURE AND
MECHANICAL PROPERTIES
Martin Bjurman
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Materials technology
2Materials Technology
Active metals
laboratory
Transports
FA (fuel storage) Hot cell laboratory
Microscopy and analysis
Corrosion and water
chemistry lab
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Thermal aging of cast and welded stainless steels
• Large components are often often cast, e.g.
• Joints predominantly welded
• Cast and welded SS contain typically 5-15% d-ferrite
• Diffusion drives spinodal decomposition of the d-ferrite and precipitation of secondary phases
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Thermal ageing of cast and welded SS degrades the mechanical properties and is an important issue for long term operation
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Goal of the PhD-project
•Microstructurally and mechanically characterize long term in-service aged SS components
•Model macroscopic mechanical properties starting at a microstructural level
• Find a small specimen mechanical testing technique targeting relevant parameters for TA of reactor components
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Ferrite structure of cast and welded SS from R2
Weld (308L) Castings (CF8M)
• Two phases ferrite (~10%) and austeniteMicrostructural variations are large• Sizes• Shapes• compositions
ASTM E8 meeting San Antonio 4 May 16
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Ferrite decomposition quantified by APTAtomic maps of projected volumes (20x20x5 nm³ slices)
325°C for 74kh
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Chromium
Nickel
Manganese
Silicon
α'-phaseα-phaseG-phase
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• Ferrite
• Hardness increases
• More brittle
• Austenite minimally affected
=> Reduction of macroscopicalfracture toughness
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Mechanical properties aging behaviour
J kN
/m
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Constant strain rate tensile test
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Brittle fracture behaviourDuctile fracture behaviour
Strain localization and creep effect at phase boundary
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Mechanical properties – constant load tensile test
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Significant creep/relaxation measured already at low temperature
0,002
0,02
100 1000 10000 100000 1000000
Elongation
Time [s]
Creep at RT 92% of Rp0,2
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Summary
• Spinodal decomposition and G-phase formation is quantified by Atom Probe Tomography
• Mechanical properties change with aging and as expected are
• Ferrite hardnesses significantly increased
• Basic mechanical properties are changed, e.g. Fracture toughness and tensile properties
• But also
• Tensile strain rate dependence is affected by aging e.g. time dependent behaviour
• Creep/relaxation occurs already at low temperature
• Mechanical properties are sensitive to variations in microstructure and this is increased by aging of the ferrite as phase property mismatch increases
• Crystal plasticity modelling is used to target these parameters
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