The nature of nano-sized precipitates in ferritic/martensitic steel P92 produced by thermomechanical treatment

Journal Article · Thu Sep 15 04:00:00 EDT 2016 · Materials Characterization

DOI:https://doi.org/10.1016/J.MATCHAR.2016.06.035· OSTI ID:22689607

Shen, Yinzhong ^[1]; Shang, Zhongxia ^[2]; Xu, Zhiqiang; Liu, Wenwen; Huang, Xi; Liu, Huan ^[1]

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1. School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240 (China)
2. School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240 (China)

Thermomechanical treatment (TMT) can effectively improve the mechanical properties of high-Cr ferritic/martensitic (F/M) steels, which has been mainly attributed to a dense dispersion of nano-sized precipitates. Precipitate phases in high-Cr F/M steels produced by TMT require further investigations. Precipitates in commercial F/M steel P92 produced by a TMT process, warm-rolled at 650 °C plus tempered at 650 °C for 1 h, were investigated by transmission electron microscopy. Nano-sized precipitates with a high number density in the steel after the TMT were found to be Cr-rich M{sub 2}(C,N) carbonitride, rather than MX or M{sub 23}C{sub 6} phase. The M{sub 2}(C,N) carbonitride has a hexagonal lattice with the lattice parameters about a/c = 0.299/0.463 nm. These M{sub 2}(C,N) carbonitrides with a typical composition of (Cr{sub 0.85}V{sub 0.06}Fe{sub 0.06}Mo{sub 0.03}){sub 2}(C,N) have an average diameter smaller than 30 nm, and mainly distribute on dislocations and at the boundaries of equiaxed ferrite grains in the TMT steel. The TMT process inhibits the precipitation of M{sub 23}C{sub 6} and M{sub 5}C{sub 2} phases. Enhanced creep properties of the P92 steel after the TMT, as reported previously, were considered to be mainly attributed to plenty of nano-sized Cr-rich M{sub 2}(C,N) carbonitrides produced by the TMT rather than to MX and M{sub 23}C{sub 6} precipitates. - Graphical abstract: TEM micrographs of precipitates on extraction carbon replicas prepared from ferritic/martensitic (F/M) steel P92. (a) After conventional heat treatment, normalized at 1050 °C for 30 min plus tempered at 765 °C for 1 h. (b) After a thermomechanical treatment (TMT), warm-rolled at 650 °C plus tempered at 650 °C for 1 h. Nano-sized precipitates with a high number density in the steel produced by the TMT were found to be Cr-rich M{sub 2}(C,N) carbonitride, rather than MX or M{sub 23}C{sub 6} phase. The TMT process inhibits the precipitation of M{sub 23}C{sub 6} and M{sub 5}C{sub 2} phases. Enhanced creep properties of the steel by the TMT are mainly attributed to plenty of nano-sized Cr-rich M{sub 2}(C,N) carbonitrides, rather than to MX and M{sub 23}C{sub 6} precipitates. It is suggested that plenty of nano-sized Cr-rich M{sub 2}(C,N) carbonitrides produced by the TMT are more helpful to improve the creep properties of the steel than M{sub 23}C{sub 6} plus MX precipitates obtained by the conventional heat treatment. - Highlights: •Carbon replicas were used for precisely identifying nano-sized phases in steel produced by TMT. •Densely nano-sized precipitates in P92 steel after TMT were found to be Cr-rich M{sub 2}(C,N) phase. •Fine precipitates produced by TMT can be M{sub 2}X phase instead of MX and/or M{sub 23}C{sub 6} phase. •Enhanced creep properties of P92 steel by TMT is attributed to fine M{sub 2}X, rather than to MX and M{sub 23}C{sub 6}. •TMT involving a tempering at 650 °C inhibits the formation of M{sub 23}C{sub 6}/M{sub 5}C{sub 2} phases in high-Cr steels.

• OSTI ID: 22689607
• Journal Information: Materials Characterization, Journal Name: Materials Characterization Vol. 119; ISSN 1044-5803; ISSN MACHEX
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
CARBON
CARBONITRIDES
CHROMIUM STEELS
CREEP
DENSITY
DISLOCATIONS
DISPERSIONS
EXTRACTION
FERRITE
FERRITIC STEELS
HEAT
HEXAGONAL LATTICES
LATTICE PARAMETERS
MARTENSITIC STEELS
NANOSTRUCTURES
PRECIPITATION
REPLICAS
TEMPERING
THERMOMECHANICAL TREATMENTS
TRANSMISSION ELECTRON MICROSCOPY