NEEP 541 – Radiation Damage in Steels Fall 2002 Jake Blanchard.
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Transcript of NEEP 541 – Radiation Damage in Steels Fall 2002 Jake Blanchard.
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NEEP 541 – Radiation Damage in Steels
Fall 2002Jake Blanchard
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Outline Damage in Steels
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Steels in Reactors Requirements
High temperature operation High strength Inexpensive Low corrosion
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Steel Types Austenitic
Primarily austenite phase - FCC Stabilized by Ni Good creep strength Resists corrosion with sodium and
mixed oxide fuels Inexpensive High void swelling
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CompositionElement 304 (wt %) 316 (wt %)
Fe 70 65
Cr 19 17
Ni 9 13
C .06 .06
Mn .8 1.8
P .02 .02
S .02 .02
Si .5 .3
B .0005 .0005
N .03
Mo .2 2.2
Co .2 .3
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Steel Types Ferritic Steels
Primarily ferrite – BCC Cheaper than austenitic steels Susceptible to DBTT increases
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CompositionElement A 302-B A 212-B
Fe 97 98
C .2 .3
Mn 1.3 .8
P .01 .01
Si .3 .3
S .02 .02
Cr .2 .2
Ni .2 .2
Mo .5 .02
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Microstructure Evolution Transmission Electron Microscopy is
used to study damage Several hundred keV electron beam
passes through sample Some electrons transmitted, others
diffracted Only transmitted electrons are viewed Defects alters diffraction conditions When defects are oriented to transmit
better, then they appear as a dark image
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Black Dot Structure Defects produced at low
temperatures show up on TEM as black dots
Defects are too small to be resolved They are believed to be depleted
zones or small vacancy clusters Below 350 C, increased fluence
increases black dot density
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Other structures Above 350 C, point defects are
mobile Loops become predominant Voids also form
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Microstructure of Unirradiated SS
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Loops in Irradiated SS
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Voids in SS
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Hardening of Austenitic Steels Low Fluence
Hardening primarily from depleted zones
At low T (below half the melting temperature), little annealing, hardening occurs
At high T, damage anneals out, no hardening
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Hardening of Austenitic Steels High Fluence
Loops and Voids grow Annealing is slower
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316 SS
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316 SS
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Steel Type Affects Damage Large differences exist among
various types and heat treatments Weld metal is often more
susceptible than base metal Even a single type of steel can
exhibit large variations in damage effects
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Transition Temp. for different batches of steel
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Differences due to structure Damage differences can result
from: grain size, texture, etc. Saturation of damage can also be
sensitive to microstructure
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Saturation
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Chemistry Chemistry may be the most important
factor in steel embrittlement Sulphur and phosphorous are
detrimental Irradiation can form sulfides (MnS,
FeS) These nucleate segregation of copper Adding N leads to increased
hardening, either by forming clusters or collecting in loops
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Effect of radiation on DBTTin steel containing Cu
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316 SS, 400 C, 130 dpa
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Helium Some steels have B in them B has a high He production cross
section He can lead to embrittlement
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He Production Cross Sections
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Damage in pure Fe Pure iron: defects are
Small black spots (small loops or planar clusters)
Loops cavities
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Neutron Damage Must have fluence>4x1023 n/m2
Threshold is lower for less pure metals
At low fluence, defect distribution is heterogeneous
Clusters and loops are only formed near dislocations or sub-boundaries
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Damage in a low-carbon steel At 275-450 C, cavities observed Sizes are up to 12 nm in diameter Concentration up to 1021 /m3
Above 500 C, cavities only at grain boundaries
No cavities at all above 575 C
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Annealing Annealing pure Fe below 300 C has
no effect on black dots Annealing above 300 C leads to
loops Above 500 C, loops are annealed
away