NEEP 541 – Radiation Damage in Steels Fall 2002 Jake 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

Composition Element304 (wt %)316 (wt %) Fe7065 Cr1917 Ni913 C.06 Mn.81.8 P.02 S Si.5.3 B.0005 N.03 Mo.22.2 Co.2.3

Steel Types Ferritic Steels Primarily ferrite – BCC Cheaper than austenitic steels Susceptible to DBTT increases

Composition ElementA 302-BA 212-B Fe9798 C.2.3 Mn1.3.8 P.01 Si.3 S.02 Cr.2 Ni.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

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 DBTT in 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>4x10 23 n/m 2 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 C, cavities observed Sizes are up to 12 nm in diameter Concentration up to /m 3 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