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Carbide-Free Bainite: design principles

Carbide-Free Bainite: design principles. http://cml.postech.ac.kr. Problem: to design a bulk nanocrystalline steel which is very strong, tough, cheap …. Brenner, 1956. Scifer, 5.5 GPa and ductile. 1 Denier : weight in grams, of 9 km of fibre. 50-10 Denier. Scifer is 9 Denier.

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Carbide-Free Bainite: design principles

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  1. Carbide-Free Bainite: design principles http://cml.postech.ac.kr

  2. Problem: to design a bulk nanocrystalline steel which is very strong, tough, cheap ….

  3. Brenner, 1956

  4. Scifer, 5.5 GPa and ductile

  5. 1 Denier: weight in grams, of 9 km of fibre 50-10 Denier Scifer is 9 Denier

  6. Carbon nanotube with iron particle in the middle blobby iron particle Kinloch, 2002

  7. Claimed strength of carbon nanotube is 130 GPa Edwards, Acta Astronautica, 2000 Claimed modulus is 1.2 TPa Terrones et al., Phil. Trans. Roy. Soc., 2004

  8. Strength of whiskers decreases with size Strength of a nanotube rope 2 mm long is less than 2000 MPa

  9. Summary • Strength produced by deformation limits shape: wires, sheets... • Strength in small particles relies on perfection. Doomed as size increases.

  10. Smallest size that can be achieved in a polycrystalline substance?

  11. Yokota & Bhadeshia, 2004

  12. Summary Thermomechanical processing limited by recalescence Need to store the heat Reduce rate Transform at low temperature

  13. Courtesy of Tsuji, Ito, Saito, Minamino, Scripta Mater. 47 (2002) 893. Howe, Materials Science and Technology 16 (2000) 1264.

  14. Fine crystals by transformation Introduce work-hardening capacity Need to store the heat Reduce rate Transform at low temperature

  15. Swallow and Bhadeshia, 1996

  16. Cementite suppressed using silicon 1 µm

  17. Fe-2Si-3Mn-C wt% 800 B S 600 Temperature / K 400 M S 200 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Carbon / wt%

  18. Fe-2Si-3Mn-C wt% 1.E+08 1 year 1 month Time / s 1.E+04 1.E+00 0 0.5 1 1.5 Carbon / wt%

  19. Low transformation temperature Bainitic hardenability Reasonable transformation time Elimination of cementite Austenite grain size control Avoidance of temper embrittlement wt%

  20. Homogenisation Austenitisation Isothermal transformation 1200 C o 2 days 1000 o C 15 min Temperature Air 125 C - 325 C o o slow cooling hours - months cooling Quench Time

  21. g g a a a Caballero, Mateo, Bhadeshia 200 Å

  22. Low temperature transformation: 0.25 T/Tm Fine microstructure: 20-40 nm thick plates Harder than most martensites (710 HV) Carbide-free Designed using theory alone

  23. Caballero, Mateo, Bhadeshia

  24. Above percolation threshold Below percolation threshold

  25. Geometrical percolation threshold of overlapping ellipsoids

  26. Stress / GPa Velocity km s-1 Hammond and Cross, 2004

  27. “more serious battlefield threats”

  28. ballistic mass efficiency consider unit area of armour

  29. Very strong Huge uniform ductility g g No deformation No rapid cooling No residual stresses a Cheap Uniform in very large sections a

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