NEEP 541 – Radiation Damage in Steels

1. NEEP 541 – Radiation Damage in Steels Fall 2002 Jake Blanchard

2. Outline • Damage in Steels

3. Steels in Reactors • Requirements • High temperature operation • High strength • Inexpensive • Low corrosion

4. 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

5. Composition

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

7. Composition

9. 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

10. 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

11. Other structures • Above 350 C, point defects are mobile • Loops become predominant • Voids also form

12. Microstructure of Unirradiated SS

13. Loops in Irradiated SS

14. Voids in SS

15. 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

16. Hardening of Austenitic Steels • High Fluence • Loops and Voids grow • Annealing is slower

17. 316 SS

18. 316 SS

19. 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

20. Transition Temp. for different batches of steel

21. Differences due to structure • Damage differences can result from: grain size, texture, etc. • Saturation of damage can also be sensitive to microstructure

22. Saturation

23. 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

24. Effect of radiation on DBTTin steel containing Cu

25. 316 SS, 400 C, 130 dpa

26. Helium • Some steels have B in them • B has a high He production cross section • He can lead to embrittlement

27. He Production Cross Sections

28. Damage in pure Fe • Pure iron: defects are • Small black spots (small loops or planar clusters) • Loops • cavities

29. 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

30. 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

31. 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