Design and Development of Lightly Alloyed Ferritic Fire-Resistant Structural Steels
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To improve safety in case of building fires, stricter building codes have been proposed requiring structural steels to maintain two-thirds of their room-temperature yield strength after exposure to 873 K (600 °C) for longer than 20 minutes. To address this need, we have designed lightly alloyed structural steels, employing computational thermodynamics in combination with fundamental principles of precipitation strengthening and its temperature dependence, precipitate stability, characterization by optical microscopy and atom probe tomography (APT), and mechanical testing at room and elevated temperatures. The design process resulted in low-carbon ferritic steels with small alloying additions of V, Nb, and Mo that maintain over 80 pct of room-temperature yield strength in compression, and nearly 70 pct in tension, after 2 hours of exposure at 873 K (600 °C). APT demonstrates the formation of nanoscale MX and M2X (where M = V + Nb + Mo and X = C + N) precipitates after exposure to 873 K (600 °C). The favorable high-temperature mechanical properties are discussed with a model of precipitation strengthening by detachment-stress-mediated dislocation pinning at nanoscale semi-coherent MX precipitates.
The authors would like to acknowledge the NSF CMMI Division [Grant Numbers NSF-CMMI-1130000 and CMMI-1462850] for providing funding for this research and Nucor for supplying one of the alloys gratis used in this research. 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. APT was performed at the Northwestern University Center for Atom Probe Tomography (NUCAPT). The local-electrode atom probe tomograph at NUCAPT was acquired and upgraded with equipment grants from the MRI Program of the National Science Foundation (NSF DMR-0420532) and the DURIP Program of the Office of Naval Research (N00014-0400798, N00014-0610539, N00014-0910781, N00014-1712870). NUCAPT received support from the MRSEC Program (NSF DMR-1720139) at the Materials Research Center, the SHyNE Resource (NSF ECCS-1542205), and the Institute for Sustainability and Energy at Northwestern (ISEN).
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