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 65Cr 19 17Ni 9 13C .06 .06

Mn .8 1.8P .02 .02S .02 .02Si .5 .3B .0005 .0005N .03

Mo .2 2.2Co .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 98C .2 .3

Mn 1.3 .8P .01 .01Si .3 .3S .02 .02Cr .2 .2Ni .2 .2Mo .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