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Solidification characteristics and segregation behavior of a P-containing Ni-Fe-Cr-based alloy, considered as boiler and turbine materials in 700 °C advanced ultra-supercritical coal-fired power plants, have been investigated by differential thermal analysis and directional solidification quenching technique. Results reveal that P decreases the solidus temperature, but only has negligible influence on liquidus temperature. After P was added, the solidification sequence has no apparent change, but the width of the mushy zone increases and dendritic structures become coarser. Moreover, P increases the amount and changes the morphology of MC carbide. Energy-dispersive spectroscopy analysis reveals that P has obvious influence on the segregation behavior of the constitute elements with equilibrium partition coefficients ( $$k_{i}$$ ) far away from unity, whereas has negligible effect on the constituent elements with $$k_{i}$$ close to unity and has more influence on the final stage of solidification than at early stage. The distribution profiles reveal that P atoms pile up ahead of the solid/liquid (S/L) interface and strongly segregate to the interdendritic liquid region. The influence of P on solidification characteristics and segregation behavior of Ni-Fe-Cr-based alloy could be attributed to the accumulation of P ahead of the S/L interface during solidification. [ABSTRACT FROM AUTHOR]

Microstructural changes and their effect on tensile properties of a Ni-Fe based alloy, considered as boiler materials in 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants, were investigated during exposure at 700–800 °C for up to 30000 h. The results show that the major precipitates are still spherical γ׳, blocky MC and discrete particle-like M 23 C 6 after long-term thermal exposure. No η or σ and other detrimental phases were observed. The main changes in microstructure are γ′ coarsening and the precipitation and growth of M 23 C 6 carbide. The coarsening behavior of γ′ precipitates is in good accordance with the prediction of the Lifshitz-Slozov-Wagner (LSW) theory during exposure at 700 °C. However, it deviates from the prediction of LSW theory at 750 and 800 °C and an anomalous coarsening behavior of γ′ precipitates was observed. For M 23 C 6 carbide, the size and amount first increase, and then keep almost constant with increasing exposure time at 700 and 750 °C, but the dissolution of M 23 C 6 carbide appears at 800 °C. The yield strength first increases and then slightly decreases with increasing exposure time at 700 °C, while it decreases gradually at 750 °C and decreases drastically and then keeps almost constant at 800 °C. However, the yield strength almost keeps the same level as that of the samples after standard heat treatment even if after exposure at 700 °C for 30000 h. The elongation has no obvious change with increasing exposure time at 700 and 750 °C, but it increases gradually at 800 °C. The changes in yield strength and ductility can be attributed to the γ′ coarsening and the evolution of M 23 C 6 carbide. [ABSTRACT FROM AUTHOR]

Ni–Fe based superalloys are being considered as boiler materials in 700 °C advanced ultra-supercritical (A-USC) coal fired power plants due to their excellent oxidation and hot corrosion resistance, outstanding workability and low cost. In this paper, the microstructural stability and mechanical properties of a boron (B) modified Ni–Fe based superalloy designed for 700 °C A-USC during thermal exposure at 650–750 °C for up to 5000 h were investigated. The results show that adding boron has no apparent influence on the major precipitates, including spherical γ′ and blocky MC. However, the amount of M 23 C 6 decreases markedly after standard heat treatment. During long-term thermal exposure, the addition of boron has no influence on γ′ coarsening, η phase precipitation and primary MC degeneration, but decreases the growth rate of M 23 C 6 along grain boundary. The stress rupture life and ductility are obviously improved after the addition of B. Meanwhile, the yield strength of B-doped alloy almost keeps the same level as that without boron addition. The fracture surface characterization exhibits that the dimples increase significantly after adding boron. During long-term thermal exposure, the elongation of the alloy with B addition increases slightly, but, for the alloy without B addition, the elongation obviously increases. The improvement of the stress rupture life and ductility can be attributed to the increase of grain boundary strength and the optimization of M 23 C 6 carbide distribution at grain boundary. [ABSTRACT FROM AUTHOR]

The effect of initial alloy state on hot deformation and dynamic recrystallization (DRX) of GH984G alloy, used to manufacture large scale forgings for 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants, was studied by isothermal compression test. The experimental results show that the apparent activation energy of hot deformation for the as-cast alloy is larger than that of the hot-rolled alloy. The reasonable ranges of processing parameters for the as-cast and hot-rolled alloys are 1100–1200 °C/0.01–0.5 s−1 and 1100–1200 °C/1.0–10 s−1, respectively, based on the processing maps. The flow instability domain of the as-cast alloy is larger than that of the hot-rolled alloy. The different hot deformation behaviors of the as-cast and hot-rolled alloys result from the various initial grain sizes and DRX mechanisms. The main DRX nucleation mode of the as-cast alloy with large grains is continuous dynamic recrystallization (CDRX). However, for the hot-rolled alloy with fine grains, the discontinuous dynamic recrystallization (DDRX) is dominant. • Investigate the effect of initial state on hot deformation of a Ni-Fe based alloy. • The main DRX mechanism of as-cast and hot-rolled alloys is CDRX and DDRX. • Establish the relationship between the optimal deformation parameter and DRX mechanism. • Confine the initial alloy state in the isothermal compression test. [ABSTRACT FROM AUTHOR]