Advanced Ultra-supercritical Coal-fired Power Plants Research Articles

The mechanism and regulation of hot cracking in a new complex-concentrated alloy by a novel integrated directed energy deposition

Improving thermal efficiency in advanced ultra-supercritical coal-fired power plants relies on increased steam temperature and pressure, accelerating the emergence of newly complex-concentrated alloys (CCAs). However, the new developed CCA have a combined Al and Ti content for 4.87 wt%, which indicates a high susceptibility to hot cracking. In this work, the hot cracking problem for the CCA during the laser deposition process was studied. The eutectic of the matrix and carbides with a low melting point liquefied during reheating process. The hot cracking occurred with local unbearable stress concentration. Based on the above analysis of hot cracking, a novel integrated directed laser deposition method was proposed to flexibly adjust the microstructure evolution and stress distribution by thermal cycles to achieve the control of hot cracks. The microstructure evolution and the control of residual stress are realized by gas quenching and preheating substrates, respectively. The evidence was obtained to show that the coupled process with synchronous gas quenching during preheating can eliminate the hot cracks effectively. More specifically, the preheating process relieved the residual stress, and the local gas quenching simultaneously accelerated the liquid film solidification and reduced carbide segregation. Finally, the sample prepared by the integrated directed energy deposition presented superior overall performance with ultimate tensile strength of 991.84 MPa and elongation of 18.28 %. This work provides a meaningful reference for producing crack-free superalloys in metal additive manufacturing.

Comprehensive technical review of the high-efficiency low-emission technology in advanced coal-fired power plants

Abstract Advancements in supercritical (SC), ultrasupercritical (USC), and advanced USC coal-fired power plants have been achieved through the development of enhanced materials utilized in advanced steam cycles and through the deployment of advanced emission control systems. These are referred to as high-efficiency low-emission (HELE) technologies, which may solve numerous issues associated with coal-based power generation. There is a clear global transition from subcritical to advanced power plant types and significant R&D work on HELE technologies. Therefore, this comprehensive review covers the latest HELE technology deployment in major coal-consuming countries and their R&D roadmaps to advance HELE technologies. In spite of the various advantages of HELE technologies, there have been numerous technical challenges relevant to achieving the HELE steam conditions and deploying low emission control technologies in the HELE systems. Hence, this review covers the technical challenges and the relevant recent research by using various coal combustion test facilities. The current focus for the progression from USC boilers to advanced USC boilers is a successful demonstration of the developed high-performance alloys under the advanced steam conditions. This review covers the current status of research and development of advanced USC (A-USC) materials and challenges based on the major material research programs.

High-Temperature Air and Steam Oxidation and Oxide Layer Characteristics of Alloy 617

Nickel-based superalloys are the candidate materials for many high-temperature applications, especially in advanced ultra-supercritical coal-fired power plants and in the next-generation nuclear power plants. This paper reports the results of a comprehensive study on chemical composition, structure and kinetics of oxide film formation of a nickel-based superalloy, alloy 617, in ambient air and steam at 750 °C for a duration of 500 h. The morphology and nature of oxide scales have been characterized by scanning electron microscope with energy-dispersive x-ray attachment, grazing incidence x-ray diffraction technique and glow discharge optical emission spectrometry (GDOES). The mass gain of alloy 617 in steam was higher than that in air, and followed a parabolic rate law for both atmospheres. The depth profiles of the oxide films obtained by GDOES showed the presence of an outer chromium oxide scale with mixed oxides of nickel and aluminum at the interface. The oxidation kinetics of alloy 617 and the nature of oxide scales formed during the steam and air oxidation are discussed.

Solidification characteristics and segregation behavior of a P-containing Ni–Fe–Cr-based alloy

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.

Microstructural stability of long-term aged wrought Ni–Fe-based superalloys

ABSTRACTThe influence of long-term thermal exposure on the microstructure and microhardness of a new Ni–Fe-based superalloy, for advanced ultra-supercritical coal-fired power plants, was studied. Spherical γ′ particles were coarsened with the increase of aging time, while the volume fraction of γ′ was decreased. The amount of M23C6 carbides was increased with aging time at 750°C, and the coarsening rate of the width of M23C6 was consistent with that of the LSW model. The microhardness of the alloy was maintained following 935 h of aging time and then was slightly decreased. It was found that the transition from the strongly coupled dislocations model to the Orowan model was dependent on the size and volume fraction of γ′.

Effect of heat treatment on the microstructure of a Ni–Fe based superalloy for advanced ultra-supercritical power plant applications

The effect of heat treatment on the microstructure and microhardness of a Ni–Fe based superalloy for 700°C advanced ultra-supercritical coal-fired power plants was investigated. Results showed that the main phases in the alloy were γ, γ′, MC and M23C6, and no harmful phase was observed in the alloy. M23C6-type carbides discretely distributed nearby grain boundaries as the alloy was aged at above 840°C. The microhardness decreased with increasing aging temperature. The coarsening of γ′ led to the increment of microhardness at 780°C and 810°C for a short aging time, and a significant decrease in microhardness after aging at 840°C. The aging temperature had more significant role on the microstructure than holding time. Therefore, to obtain optimum strengthening effect for this alloy, the aging temperature should not exceed 810°C.

Tensile deformation-induced dislocation configurations at intermediate temperatures in a Ni–Fe-based superalloy for advanced ultra-supercritical coal-fired power plants

A low cost Ni–Fe-based superalloy due to its good mechanical properties and good workability has been evaluated as a promising candidate boiler material for advanced ultra-supercritical coal-fired power plants with temperature up to 700 °C. The major concerns with Ni–Fe-based alloys are the fundamental deformation mechanisms at intermediate temperatures. In this paper, great emphasis is placed on the dislocation configurations induced by tensile deformation in this low cost Ni–Fe-based superalloy. Two typical deformation-induced microstructure features, Orowan looping and precipitates shearing, are present during tensile deformation. Careful identification of dislocation-based deformation mechanisms by transmission electron microscopy was performed in tensile strained specimens at 20 °C, 500 °C, 650 °C, 700 °C and 750 °C in order to correlate the macroscopic behavior with the tension controlling mechanisms.

Phenomenological and microstructural analysis of intermediate temperatures creep in a Ni–Fe-based alloy for advanced ultra-supercritical fossil power plants

A newly developed Ni–Fe-based alloy with high-creep strength and low cost has been developed and evaluated as the promising candidate boiler materials for 700 °C advanced ultra-supercritical coal-fired power plants applications. Three electron microscopy characterization methods–scanning electron microscopy and transmission electron microscopy and high-resolution transmission electron microscopy–were combined to obtain new insights into the microstructural and fracture surface characteristics after creep rupture tests at intermediate temperatures. The alloying elements distribution characteristics have been investigated at nanoscale through EDS mapping, especially Fe element. Fractographic analysis has been also conducted with the finding that the fracture mechanism of the crept specimens at 700 °C/300 MPa and 750 °C/150 MPa are intergranular fracture model. Dislocation configurations resulting from the creep deformation have been also performed on the crept specimens. At 700 °C/300 MPa, Orowan process combining climb of a/2 <110> matrix dislocations was dominant mechanism. At 750 °C/150 MPa, the dominant mechanism is Orowan process combining slip of a/2 <110> matrix dislocations and γ′ precipitates shearing. The formation α-Cr precipitation during the creep process could act as obstacle to impede the dislocation gliding and thus increase the creep strength.

Dislocation motion in a NI-Fe-based superalloy during creep–rupture beyond 700 °C

A Ni–Fe based superalloys with excellent high temperature mechanical properties and low cost have been evaluated as the promising candidate materials for advanced ultra-supercritical coal-fired power plants. The major concerns with the Ni-Fe-based alloys are the insufficient fundamental deformation mechanisms at intermediate temperatures. In this study, dislocation-based deformation mechanisms were identified carefully in creep ruptured specimens beyond 700°C. The motion of dislocations allows plastic deformation to occur. Two types of dislocation configurations mainly occurred during the creep rupture tests beyond 700°C. Below 750°C, Orowan looping are more likely occur and considered as the predominant deformation mechanism. Above 750°C, the predominant model of deformation mechanism becomes γ′ precipitates shearing. Detailed identification of dislocation configurations were performed in the creep rupture specimens in order to correlate the macroscopic behavior with the creep controlling mechanisms.

Thermal Stability of a New Ni–Fe–Cr Base Alloy with Different Ti/Al Ratios

Influence of Ti/Al ratios on the thermal stability of a new low cost Ni–Fe–Cr base wrought alloy, designed for application at 700 °C in advanced ultra-supercritical coal-fired power plants (700 °C A-USC), was investigated both experimentally and thermodynamically. After standard heat treatment, the alloys with different Ti/Al ratios had the same microstructural characteristics. However, compared with the alloys with high Ti/Al ratio, the low Ti/Al ratio can increase the γ′-solvus temperature, decrease γ′ coarsening rate and reduce the temperature range of η phase precipitation. For the alloys with low Ti/Al ratio, the yield strength has no obvious decrease during long-term thermal exposure at 700 and 750 °C, but after thermal exposure at 750 °C for 5000 h, the yield strength of the alloys with high Ti/Al ratio obviously decreases due to the η phase precipitation. The influence of η phase on mechanical properties is related with its size. When the η phase is small, it has no obvious influence on mechanical properties, but η phase becomes the crack initiation site with the further growth of η phase. It can be concluded that the decrease in Ti/Al ratio can improve the thermal stability to meet the requirement of 700 °C A-USC coal-fired power plants.

Detrimental effect of cellular precipitation on the creep strength of Inconel740H

Cellular precipitation (also known as discontinuous precipitation) has been observed at the grain boundaries of a newly developed nickel-based Inconel740H alloy designed for use at 700 °C in advanced ultrasupercritical coal-fired power plants. By means of element mapping and selected area diffraction, the cellular precipitates were identified as Cr-rich M23C6 carbides. The onset of cellular precipitation was found to follow a pucker mechanism in Inconel740H. The cellular precipitates at the grain boundaries, even at low volume fractions, were severely detrimental to the creep strength at 750 °C. The creep rupture life of Inconel740H containing cellular precipitates at grain boundaries was only one-tenth of that for the alloy without cellular precipitates. The reason for the drastically decreased creep rupture life is attributed to the poor resistance of cellular precipitates to crack propagation during creep.