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Microstructural Stability of Strong 9-12Cr Steels

This article discusses the microstructural stability of strong 9-12Cr steels, focusing on the role of precipitates in achieving long-term stability. The study explores the effects of tempered martensite, tempered bainite, and other metastable phases on the stability of the steel. It also analyzes the influence of interfacial energy and diffusion on coarsening and precipitation. The results suggest that precipitates close to equilibrium, such as Laves phases and intermetallics, are required for long-term stability. The article also highlights the use of neural networks for extrapolation and the challenges of achieving long-term stability with fine or metastable precipitates.

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Microstructural Stability of Strong 9-12Cr Steels

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  1. Microstructural Stability of Strong 9-12Cr Steels www.msm.cam.ac.uk/phase-trans 650°C

  2. Tempered martensite Tempered bainite Nam (1999)

  3. Kimura et al., 2001

  4. Thermodynamic stability 650°CExtrapolation of short-term data Fe-0.2C-1.5Mn wt%Stability: stored energy

  5. martensite

  6. martensite

  7. Long term stability requires precipitates which are close to equilibriumLaves, intermetallics, MXMetastable phases not appropriate Interfacial energy?

  8. Coarsening diffusion r r 2 1 flux a q q concentration aq c r aq 1 c r 2 distance

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

  10. 0.8 Cr Mo 0.6 0.4 0.2 0.0 9Cr1Mo 1CrMoV 3Cr1.5Mo 12CrMoV 2.25Cr1Mo 0.25CrMoV 12CrMoVW 3.5NiCrMoV 12CrMoVNb Mod. 9Cr1Mo 9Cr0.5MoWV Mod. 2.25Cr1Mo Mole fraction 565 °C

  11. 0.12 0.10 0.08 0.06 0.04 0.02 0.00 9Cr1Mo 1CrMoV 3Cr1.5Mo 12CrMoV 2.25Cr1Mo 0.25CrMoV 12CrMoVW 12CrMoVNb 3.5NiCrMoV 9Cr0.5MoWV Mod. 9Cr1Mo Mod. 2.25Cr1Mo Cr concentration in ferrite Mole fraction Cr 565 °C

  12. 2 caq s Va 1 - caq kT r cqa-caq craq = caq + Coarsening reduced if last term small qa c Concentration aq c Distance

  13. 0.20 0.15 0.10 0.05 0.00 9Cr1Mo 1CrMoV 3Cr1.5Mo 12CrMoV 2.25Cr1Mo 0.25CrMoV 12CrMoVW 3.5NiCrMoV Mod. 9Cr1Mo 12CrMoVNb 9Cr0.5MoWV Mod. 2.25Cr1Mo caq (1 - caq ) cqa-caq Stability parameter = Stability parameter

  14. 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

  15. 200 150 2.25Cr1Mo Creep rupture stress/ MPa 100 50 9Cr1Mo 0 2 3 4 5 6 log(time/ h)

  16. Equilibrium precipitatesSmall interfacial energy (?)Small volume fraction Which precipitates are effective? Short term --> Long term data?

  17. non-linear functions

  18. Brun, Robson, Narayan, MacKay & Bhadeshia, 1998

  19. 105 h Creep Strength, 2.25Cr1Mo iron + microstructure 550 °C solid solution 600 °C precipitates Murugananth & Bhadeshia, 2001

  20. elements in solution Murugananth & Bhadeshia, 2001

  21. Kimura et al., 2001

  22. Muneki, Obuko, Abe (2005) Paper 42, this conference Fe-12Ni-9Co-10W-5Cr-B

  23. Sourmail & Bhadeshia, 2004

  24. Data from Abe, Masuyama, Sawaragi and Kimura, 2004

  25. Difficult to achieve long-term stability using fine or metastable precipitates. • Way forward is to avoid microstructure (Kimura, Abe) • Extrapolation is optimal with neural networks

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