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The isothermal oxidation behavior of 17Cr-0.85Si-0.5Nb-1.2Cu ferritic stainless steel in air was studied from 850°C to 1050°C by analyzing its weight gain after oxidation. The kinetic curves were plotted using the oxidation weight-gain data, and the structure, surface morphology, and element distribution of the oxide films were analyzed by XRD, SEM, and EDS. The results showed that the oxidation kinetics curves at 850°C and 950°C followed a parabolic law, and a continuous and dense oxide film composed of Cr2O3 and MnCr2O4, FeCr2O4, and Cu-Cr rich spinel was formed, which reveals that the steel displayed good oxidation resistance. When the temperature was increased to 1050°C, the oxidation kinetics curves gradually changed from parabolic to linear after 40 h exposure, which indicated that the oxidation resistance significantly worsened. A lower oxidation resistance was observed at 1050°C due to the formation of a large amount of Fe2O3 on the surface and the volatilization of the inner Cr2O3 layer. [ABSTRACT FROM AUTHOR]

The influence of temperature and strain rate on the hot tensile properties of 0Cr18AlSi ferritic stainless steel, a potential structural material in the ultra-supercritical generation industry, was investigated at temperatures ranging from 873 to 1123 K and strain rates of 1.7 × 10−4–1.7 × 10−2 s−1. The microstructural evolution linked to the hot deformation mechanism was characterized by electron backscatter diffraction (EBSD). At the same strain rate, the yield strength and ultimate tensile strength decrease rapidly from 873 K to 1023 K and then gradually to 1123 K. Meanwhile, both yield strength and ultimate tensile strength increase with the increase in strain rate. At high temperatures and low strain rates, the prolonged necking deformation can be observed, which determines the ductility of the steel to some extent. The maximum elongation is obtained at 1023 K for the strain rates of 1.7 × 10−3 and 1.7 × 10−2 s−1, while this temperature is postponed to 1073 K once decreasing the strain rate to 1.7 × 10−4 s−1. Dynamic recovery (DRV) and continuous dynamic recrystallization (CDRX) are found to be the main softening mechanisms during the hot tensile deformation. With the increase of temperature and the decrease of strain rate (i.e., 1123 K and 1.7 × 10−4 s−1), the sub-grain coalescence becomes the main mode of CDRX that evolved from the sub-grain rotation. The gradual decrease in strength above 1023 K is related to the limited increase of dynamic recrystallization and the sufficient DRV. The area around the new small recrystallized grains on the coarse grain boundaries provides the nucleation site for cavity, which generally results in a reduction in ductility. Constitutive analysis shows that the stress exponent and the deformation activation energy are 5.9 and 355 kJ·mol−1 respectively, indicating that the dominant deformation mechanism is the dislocations motion controlled by climb. This work makes a deeply understanding of the hot deformation behavior and its mechanism of the Al-bearing ferritic stainless steel and thus provides a basal design consideration for its extensive application. [ABSTRACT FROM AUTHOR]

The oxidation behaviors of Al-free and 1Al ferritic heat-resistant stainless steels (FHSSs) at 1100 °C were investigated. After oxidation for 100 h, the Al-free FHSS had triple oxide layers of a first layer Fe 2 O 3 + MnCr 2 O 4 , a second layer MnCr 2 O 4 + Cr 2 O 3 + MnO, a third layer SiO 2 and oxides of Nb and Ti. The 1Al FHSS had double oxide layers of a first layer Ti-bearing (Al, Cr) 2 O 3 particles, a second layer dense (Al, Cr) 2 O 3. The (Al, Cr) 2 O 3 layer with self-repairing feature can reduce the thickness of oxide layer by 515 times, and reduce the oxidation weight gain rate of the FHSS by 5 orders of magnitude. • The addition of Al reduces the oxidation weight gain rate by 5 orders of magnitude at 1100 °C. • The addition of Al reduces the thickness of oxide layer by 515 times at 1100 °C. • The addition of Al produces an oxide layer with self-repairing characteristics at 1100 °C. [ABSTRACT FROM AUTHOR]

[Display omitted] • The addition of Al increases the nucleation rate of the Cu-rich phase. • The addition of Al can increase the coarsening rate of the Cu-rich phase. • The addition of Al changes the crystal structure of the Cu-rich phase. • The addition of Al hinders the coarsening of the Cu-rich phase into a rod shape. • The Al-containing Cu-rich nanoprecipitates are rich in high-density edge dislocations. In this article, scanning electron microscopy, energy dispersive spectroscopy, and transmission electron microscopy were used to characterize the precipitation behaviour of the Cu-rich phase in Al-free and 1Al Cu-bearing ferrite stainless steels; and further, the effects of Al addition on the precipitation and coarsening of the Cu-rich phase and its mechanism were analysed. The results show that Al increases the nucleation rate of the Cu-rich phase, and refines Cu-rich nanoprecipitates after ageing for 1 min. The Cu-rich phase with four crystal structures-BCC, 9R, twin FCC, and non-twin FCC were found in the Al-free sample during ageing. However, only Cu-rich phase with BCC and BCT structures were observed after Al addition. There are high-density edge dislocations in Al-containing Cu-rich nanoprecipitates. These dislocations can be used as a diffusion channel for solute atoms, increasing the coarsening rate of the Cu-rich phase. However, with the extension of the ageing time, dislocations in the Cu-rich phase will be annihilated owing to the thermal activation and the decrease of the Al concentration in the Cu-rich phase, which will lead to a decrease in the dislocation density in Al-containing Cu-rich phase. Finally, the promotion effect of Al on the coarsening of the Cu-rich phase is weakened. [ABSTRACT FROM AUTHOR]

In the present study, the evolution and strengthening effect of precipitates in 17Cr ferrite stainless steel during the creep process are investigated. The precipitates are characterized by scanning electron microscopy, energy dispersive spectroscopy and transmission electron microscopy. Moreover, the interactions between the precipitates and the grain boundary and dislocation are quantitatively analysed. The results show that Cu- and Nb-rich phases are formed during the creep process. The stress concentration at the tip of the precipitates, the stacking fault at the precipitate edge and the internal dislocation can promote the nucleation of the Nb-rich phase. The Nb-rich precipitates can provide the driving force for the nucleation of Cu-rich particles through stacking fault, dislocation entanglement and internal dislocation. At 700 °C, as the creep time increases, the pinning effect of precipitates on the grain boundary increases linearly, while the pinning effect on the dislocation decreases. As the creep temperature increases, a peak pinning effect on the grain boundary is observed at 750 °C, and the pinning effect of the precipitates on dislocation decreases gradually inside the grain. The formation of dislocation networks at grain boundaries can delay pinning effect reduction of the Nb-rich phase, and the main blocking mechanism of precipitates on dislocation movement is bypassing. The precipitation strengthening effects of Cu- and Nb-rich phases are also compared with each other. • The precipitation rule of Cu-rich phase is BCC → Non-twin 9R → FCC. • The orientation relations of the precipitates with matrix are revealed. • The Nb-rich phase mainly pins the grain boundary, while the motion of dislocation is easy to be blocked by the Cu-rich particles. • The dislocation networks promote nucleation of precipitates. • The strengthening effect of Cu- and Nb-rich phases is compared. [ABSTRACT FROM AUTHOR]

Herein, the precipitation behavior of 17Cr-0.86Si-1.2Cu-0.5Nb ferritic stainless steel during isothermal aging is studied by using scanning electron microscopy, energy dispersive spectrometer and high-resolution transmission electron microscopy. Moreover, the influence of aging temperature and time on hardness is systematically investigated. The results reveal that the Cu-rich phase mainly precipitates below 750 °C, whereas the Nb-rich phase precipitates above 750 °C. The Cu-rich phase exhibits an evolution behavior of B2-9R-FCC and the coarsening rate of Cu-rich phase increases by two orders of magnitude from 650 to 750 °C. Furthermore, the side of rod-shaped Cu-rich phase maintains incoherent interface with the matrix, however, the tip of Cu-rich phase exhibits a coherent interface with the matrix with an orientation relationship of 01 1 ¯ α / / 1 ¯ 11 Cu. The Nb-rich phase includes Fe 2 Nb, Fe 3 Nb 4 Si 5 , CrNbSi, (Fe,Cr,Si) 2 Nb, and Nb 2 C phase after thermal aging. At T < 750 °C, both sides of the Nb-rich phase provide nucleation points for the Cu-rich phase. At T = 850 °C, the tip of Cu-rich phase provides a nucleation site for Nb-rich precipitation. A coherent interface can be found between (Fe,Cr,Si) 2 Nb and Cu-rich phases, and the Cu-rich phase grows along (311) crystal planes of (Fe,Cr,Si) 2 Nb. Finally, the hardness change with aging time shows a single peak feature at T < 750 °C and a double-peak at 850 °C. The occurrence of aging peak is delayed and the peak values gradually decrease with the increase of aging temperature. • The copper rich phase has the precipitation rule of B2-9R-FCC. • The complex precipitation behavior of Nb, Cu and Si existed in the steel. • The Nb- and Cu-rich phases have interdependent growth relationship. • The coarsening rates of Cu- and Nb-rich phases were calculated. • The precipitation of the Cu-rich phases delays the occurrence time of aging peak. [ABSTRACT FROM AUTHOR]