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  1. 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Coarsening during tempering martensite Cementite size / µm bainite 100 1000 1000 100000 Tempering time / s Nam (1999)

  2. Tempered martensite Tempered bainite Nam (1999)

  3. 300 250 200 150 100 50 0 200 300 400 500 Embrittlement virgin Impact energy / J service exposed Test temperature / K Wignarajah, Masumoto & Hara (1990)

  4. two-stage treatment? 720°C 500°C Temperature Time

  5. Role of matrix microstructure ?

  6. 750 750 650 650 550 550 450 450 .1 1 10 100 1000 .1 1 10 100 1000 Martensite Bainite M C M C 23 6 23 6 M C M C 23 6 23 6 + M C + M C 6 6 + M C 7 3 + M C Fe C + M C 7 3 3 2 + M C Fe C + M C 7 3 3 2 + M C 7 3 Fe C + M C Tempering temperature / °C Tempering temperature / °C Fe C + M C 3 2 3 2 Fe C Fe C 3 3 Tempering time / h Tempering time / h Baker & Nutting (1959)

  7. 800 700 600 500 400 300 200 0 1 2 3 4 5 6 7 8 9 10 10 10 10 10 10 10 10 10 10 a) 2.3Cr 1Mo b) 4.3Cr 1Mo c) 9.3Cr 1Mo ferrite a Temperature / °C b bainite c martensite Time / s

  8. Mechanical Properties of Martensite in Heat-Resistant Steels • Why are modern steels martensitic? • What makes martensite desirable? • Can the martensite be optimised?

  9. Why martensitic? • Greater Cr needed for oxidation, corrosion resistance • Cr must be balanced by other elements to avoid ∂-ferrite • Therefore, greater hardenability.

  10. Conclusions • If creep controlled by dislocation climb, then refine matrix grains • Coarsening should be worse in high Cr steels • For equivalent conditions 2.25Cr1Mo is better than 9Cr1Mo

  11. 0.06 M23C6 M2X Laves 0.05 0.04 0.03 0.02 0.01 0.00 9Cr1Mo 1CrMoV 9CrMoWV 12CrMoVW 3.5NiCrMoV 2.25CrMo Fraction 565 °C

  12. 0.8 Cr Mo 0.6 0.4 0.2 0.0 9Cr1Mo 1CrMoV 12CrMoV 3Cr1.5Mo 2.25Cr1Mo 0.25CrMoV 12CrMoVW 3.5NiCrMoV 12CrMoVNb 9Cr0.5MoWV Mod. 9Cr1Mo Mod. 2.25Cr1Mo Mole fraction 565 °C

  13. 0.12 0.10 0.08 0.06 0.04 0.02 0.00 9Cr1Mo 1CrMoV 12CrMoV 3Cr1.5Mo 2.25Cr1Mo 0.25CrMoV 12CrMoVW 3.5NiCrMoV 12CrMoVNb 9Cr0.5MoWV Mod. 9Cr1Mo Mod. 2.25Cr1Mo Cr concentration in ferrite Mole fraction Cr 565 °C

  14. 2 caq s Va 1 - caq kT r cqa-caq craq = caq + Coarsening reduced if last term small qa c Concentration aq c Distance

  15. 0.20 0.15 0.10 0.05 0.00 9Cr1Mo 1CrMoV 12CrMoV 3Cr1.5Mo 2.25Cr1Mo 0.25CrMoV 12CrMoVW 3.5NiCrMoV 12CrMoVNb 9Cr0.5MoWV Mod. 9Cr1Mo Mod. 2.25Cr1Mo caq (1 - caq ) cqa-caq Stability parameter = Stability parameter

  16. 200 150 2.25Cr1Mo Creep rupture stress/ MPa 100 50 9Cr1Mo 0 2 3 4 5 6 log(time/ h)

  17. Comparison • 0.15C-0.25Si-0.50Mn-2.3Cr-1Mo- 0.10Ni • 0.10C-0.60Si-0.40Mn-9.0Cr-1Mo-0.00Ni • 1056 °C for 12 h, 740 °C for 13 h