Post on 13-Jan-2016
NEEP 541 – Radiation Damage in Steels
Fall 2002Jake Blanchard
Outline Damage in Steels
Steels in Reactors Requirements
High temperature operation High strength Inexpensive Low corrosion
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
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
Steel Types Ferritic Steels
Primarily ferrite – BCC Cheaper than austenitic steels Susceptible to DBTT increases
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
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
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
Other structures Above 350 C, point defects are
mobile Loops become predominant Voids also form
Microstructure of Unirradiated SS
Loops in Irradiated SS
Voids in SS
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
Hardening of Austenitic Steels High Fluence
Loops and Voids grow Annealing is slower
316 SS
316 SS
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
Transition Temp. for different batches of steel
Differences due to structure Damage differences can result
from: grain size, texture, etc. Saturation of damage can also be
sensitive to microstructure
Saturation
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
Effect of radiation on DBTTin steel containing Cu
316 SS, 400 C, 130 dpa
Helium Some steels have B in them B has a high He production cross
section He can lead to embrittlement
He Production Cross Sections
Damage in pure Fe Pure iron: defects are
Small black spots (small loops or planar clusters)
Loops cavities
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
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
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