Abstract
The long-term performance of current heat-resistant ferritic steels used in steam generators is primarily limited by microstructure degradation as a result of coarsening or transformation of precipitates. To improve the microstructural stability, a heat-resistant ferritic steel was designed via computational thermodynamics to exclusively contain metal carbonitride (MX) precipitates as the primary means for strengthening at elevated temperatures. The volume fraction of precipitates is 0.35 vol pct, about one-seventh of P91. These MX precipitates are either V-rich or Nb-rich with plate-like or spheroidal morphology, respectively. The precipitate size remains almost constant at 973 K for up to 3000 hours aging. Electron diffraction analysis revealed a Baker-Nutting orientation relationship between the precipitate and the matrix. Consistent with the thermodynamics-based design, M23C6-, Laves-, and Z phase were not detected. The creep threshold stress, derived from high-temperature compressive creep tests, are evaluated to be 63 ± 1 and 43 ± 2 MPa at 923 K and 973 K, respectively, on par with or slightly better than P91. This study reveals that MX precipitates in ferritic steels coarsen slowly at temperatures up to 973 K and that a relatively small volume fraction of MX precipitates can provide effective long-term creep performance at elevated temperatures.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 638-647 |
| Number of pages | 10 |
| Journal | Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science |
| Volume | 51 |
| Issue number | 2 |
| DOIs | |
| State | Published - Feb 1 2020 |
Funding
The authors would like to acknowledge the support from the US National Science Foundation (Grant no. CMMI-1462850) and the Chinese Scholarship Counsel (CSC). This work made use of the EPIC facility of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the MRSEC program (NSF DMR-1720139) at the Materials Research Center, the International Institute for Nanotechnology (IIN), the Keck Foundation, and the State of Illinois, through the IIN. This work used instrumentation at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). NUCAPT received support from the NSF-MRI (DMR-0420532), ONR-DURIP (N00014-0400798, N00014-0610539, N00014-0910781, N00014-1712870) programs, the MRSEC program (NSF DMR-1720139) at the Materials Research Center, the SHyNE Resource (NSF ECCS-1542205), and the Initiative for Sustainability and Energy (ISEN) at Northwestern University. This work made use of the MatCI Facility which receives support from the MRSEC Program (NSF DMR- 1720139) of the Materials Research Center at Northwestern University. This work made use of the CLaMMP Facility at Northwestern University. We thank Prof. David Dunand of Northwestern University for providing access to the creep frame at his laboratory and for his critical reading of this manuscript and many helpful discussions. We would also like to thank Dr. Shrikant Bhat of ArcelorMittal for insightful comments on this manuscript.
ASJC Scopus subject areas
- Condensed Matter Physics
- Mechanics of Materials
- Metals and Alloys