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THERMAL AGING OF CAST AUSTENITIC STAINLESS STEEL

EVOLUTION OF MICROSTRUCTURE AND

MECHANICAL PROPERTIES

Martin Bjurman

Materials technology

2Materials Technology

Active metals

laboratory

Transports

FA (fuel storage) Hot cell laboratory

Microscopy and analysis

Corrosion and water

chemistry lab

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

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

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

• Ferrite

• Hardness increases

• More brittle

• Austenite minimally affected

=> Reduction of macroscopicalfracture toughness

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Mechanical properties aging behaviour

J kN

/m

Constant strain rate tensile test

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Brittle fracture behaviourDuctile fracture behaviour

Strain localization and creep effect at phase boundary

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

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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