Microstructural deterioration assessment of ex-service power plant steel using concentric backscatter imaging - Presented By: Melody van Rooyen ...
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Microstructural deterioration assessment of ex-service power plant steel using concentric backscatter imaging Presented By: Melody van Rooyen, Supervisor: Prof TH Becker In collaboration with: CHRTEM, NMU (Dr. J Westraadt and Dr. G Marx)
Background – Power stations and creep 350 Max. operating hours Exceed design life Design life 300 Life exhaustion ratio 250 Life exhaustion ratio OL/DL (%) 200 150 100 50 0 High temperature (> 550 °C), high Creep steam pressures (>15 MPa) iosrjournals.org MSSA Conference, Langebaan, 4 December 2019 1
Background – Material assessment Microcracks 1.0 Oriented cavities Isolated cavities Relative strain, No damage Macrocracks 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Life fraction, Damage state Cavities/mm2 12 % Cr steel: New Ideally none X20CrMo12-1 Low 60-90 (X20 for short) Ex- Medium ≈200 service EPPEI X20 High >220-690 MSSA Conference, Langebaan, 4 December 2019 2
Background – Microstructural analysis Micro-grain boundary Block Laths of boundary two variants Packet boundary Packet Cr23C6 / Laves phase (Fe2Mo) Lath MX Prior austenite grain boundary Mobile dislocations Dipole dislocations MSSA Conference, Langebaan, 4 December 2019 3
Experimental description - Materials Virgin Low Medium High Damage Material (New) Damage Damage Type: X20 CrMoV12-1 Cavity density measured from 250 BSE-SEM (cavity/mm2) Operating - 545 545 543 temperature (°C) Operating steam - 17.0 19.4 18.1 pressure (MPa) Operating life - 130 000 130 000 156 000 (hours) MSSA Conference, Langebaan, 4 December 2019 4
Experimental description – Creep testing (a) (b) Disc sampling locations in gauge region x=0x=5 Longitudinal centreline 5 mm Specimen extraction Disc sampling Gauge location in grip region Loading direction (c) Cameras (d) Specimen (d) DIC: Digital Image Correlation Water-cooled grips White LED Gleeble vacuum chamber MSSA Conference, Langebaan, 4 December 2019 5
Experimental description – Creep testing (cont.) MSSA Conference, Langebaan, 4 December 2019 6
Experimental description – Creep testing (cont.) MSSA Conference, Langebaan, 4 December 2019 7
Experimental description – Microstructure CBS-SEM: Concentric Backscatter- CBS detector Specimen Scanning Electron Microscopy settings preparation • FIB-SEM • Twin-jet • 5 kV electropolishin • 0.20 nA g • 4 mm WD • 5 % HClO4 • Total of 4 solution images per • 21-30 V site (680 μm2) • -20 °C FEI MSSA Conference, Langebaan, 4 December 2019 8
Experimental description – Micro. (cont.) Micro-grains 5 μm Precipitates Diameter Interparticle Cr23C6 spacing Laves w phase Number density 2.5 μm MSSA Conference, Langebaan, 4 December 2019 9
Results - Qualitative Low - Grip High - Grip Medium - Grip High – Fracture (gauge) MSSA Conference, Langebaan, 4 December 2019 10
Results – Quantitative (micro-grains) 25 0.8 Low EXL - Grip m) EXH - Grip 0.75 20 High EXL - Grip lognormal fit Low EXH - Grip lognormal fit 0.7 15 High Micro-grain short width ( Frequency (%) 0.65 10 0.6 5 0.55 0 0.5 0 1 2 3 4 Low Med. High Subgrain short width ( m) Similar Larger sized grains grains MSSA Conference, Langebaan, 4 December 2019 11
Results – Quantitative (Precipitates) Particle Inter-particle Volume number density size spacing Size or inter-particle spacing (nm) Volume number density (μm-3) :Gauge Cr23C6 :Grip Low Medium High MSSA Conference, Langebaan, 4 December 2019 12
Results – Quantitative (Precipitates) Particle Inter-particle Volume number density size spacing Size or inter-particle spacing (nm) Size or inter-particle spacing (nm) Volume number density (μm-3) Volume number density (μm-3) :Gauge Cr23C6 Laves :Grip Low Medium High Low Medium High Dense carbide Smaller micro- Higher Laves distribution grains phase distribution MSSA Conference, Langebaan, 4 December 2019 13
Results – Hardness Medium: high hardness Low Medium High Longitudinal distance from central gauge location MSSA Conference, Langebaan, 4 December 2019 14
Mechanical response High 0.1 600 °C 0.08 Low Med. 0.06 Creep strain 0.04 0.02 0 New 0 1 2 3 4 5 Time (hours) MSSA Conference, Langebaan, 4 December 2019 15
Mechanical response Confirms CBS results: high Cr23C6 density enhances creep resistance despite cavity density Faster creep rates for High low compared to 0.1 medium 600 °C -3 °C 0.08 10 600 Low Med. -4 0.06 ) 10 -1 Creep strain Low Med. Creep rate (s 0.04 -5 10 0.02 -6 10 0 New -7 10 0 1 2 3 4 5 0 1 2 3 4 5 Time (hours) Time (hours) MSSA Conference, Langebaan, 4 December 2019 16
Conclusion • CBS is a convenient microstructural method for micro-grain and large particle characterisation of ex-service material • Highly damage material has large polygonal micro-grains, low density of carbides and high fraction of Laves phase especially near voids. • Low damage has less Laves phase but had a lower density of carbides than medium • Medium displays relatively smaller grains due to dense carbide distribution. Medium and low damage display similar creep rates despite differences in void distributions • Using CBS allows faster damage assessment through a single microstructural analysis stream – faster maintenance decisions MSSA Conference, Langebaan, 4 December 2019 17
Conclusion “No SINGLE measurement is sufficiently comprehensive to describe the [damage state of] the steel with all requisite completeness” ~ Bhadesia et al. (1998) MSSA Conference, Langebaan, 4 December 2019 18
Acknowledgements and questions Thank you to the following: Supervision: Prof Thorsten Becker and Materials Engineering Research Group at SU CBS-SEM imaging: NMU CHRTEM team, Dr J Westraadt and Dr G Marx Gleeble testing/specimen preparation facilities: Prof Rob Knutsen and S. von Willingh at CME Stakeholder guidance and material provision: EPPEI staff and fellow students Funding: NRF (THRIP) and THUTHUKA, Frank Wilhelm Bursary Fund MATENG group www0.sun.ac.za/mateng ResearchGate MSSA Conference, Langebaan, 4 December 2019 19
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