Jingbo Yan

On precipitates in Fe–Ni base alloys used for USC boilers

Abstract This paper overviews the effects of some precipitates in Fe–Ni base boilers materials. The volume fraction of these precipitates is usually below 10% for maintaining the microstructure stability and manufacture ability. They appear in a number of morphologies, but all of them improve the creep strength when they exist in small size. For the alloys serviced below 650°C, the strengthening precipitates disperse within the matrix and maintain their diameter below 100 nm during long time exposure. For the alloys serviced above 650°C, the strengthening precipitates are normally larger than 100 nm in equivalent diameter. They may experience long growth duration and thus keep their size below 500 nm. Meanwhile, the precipitates may improve creep strength when they are dispersed over the grain boundaries.

Role of Grain Boundaries on the Cyclic Steam Oxidation Behaviour of 18-8 Austenitic Stainless Steel

This work investigated the oxidation behaviour of 18-8 austenitic stainless steel under cyclic steam oxidation condition. The results indicate that grain boundaries not only promote the chromium outward diffusion, but also provide the fast diffusion paths for the oxygen penetration. Thus, oxide inward growth occurs when the chromium content at scale-matrix interface cannot sustain the continuous chromia scale. In addition, the grain boundaries promote the Fe outward diffusion, accompanied with the fast growth of interfacial voids between two oxide layers. As a result, the oxide scale exfoliates once the voids grow to a critical value.

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

ABSTRACT The 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′.

Impact of Aging Temperature on the Performance of a Nickel-Iron-Based Superalloy

In this study, the effects of aging temperature on the microstructure and properties of a nickel-iron (Ni-Fe)-based superalloy were investigated. On the one hand, owing to the increase in the size and particle spacing of Ni3Al (γ′) precipitate, long-term aging induced a significant drop in the alloy strength. Moreover, the increasing aging temperature from 700 °C to 750 °C further induced more than 75 MPa decline in the alloy yield strength. Furthermore, it led to a decrease in the critical stress because of dynamic recrystallization. On the other hand, the long-term aging increased the alloy’s ductility. The crack propagation along the grain boundary was inhibited, because of the decreasing grain boundary brittleness. Although the grain boundary precipitates changed from carbide to γ′ when the aging temperature increased, a distinct change in the alloy’s ductility was not observed. The transmission electron microscopy results showed that both precipitates were sheared by the grain boundary during the alloy deformation. These results confirm that aging temperature has less effect on alloy’s ductility.

Effect of Alloying Additions on Oxidation Behaviors of Ni‐Fe Based Superalloy for Ultra‐Supercritical Boiler Applications

A new kind of Ni-Fe-based superalloy is designed recently for 750 °C-class A-USC boiler tube. The oxidation behavior of the designed alloys with various combinations of anti-oxidation additions, Cr, Al and Si, was investigated at 750 °C and 850 °C, respectively. The results indicated that the oxidation rate of tested alloys decreased with the increase of the sum of additions. Cr addition may drop the relative constant of parabolic rate greatly when temperature is raised. But the oxide scale, mainly consisted of NiCr spinel at 750 °C and NiCrMn spinel at 850 °C, was similar while the Cr content is in a range of 20–25 wt.% at tested temperatures. Al addition, however, showed the best effective to reduce the oxidation rates. Internal Al-rich oxide was observed at the scale/metal interface for alloys added with high content of Al and was increased with Al content increase. Very tiny difference between the oxide scales of the Si-added alloys was identified when Si content varies among 0.02–0.05 wt.%. Basing on these results, this presentation discussed the optimum combination of anti-oxidation additions as well as oxidation mechanisms in the designed Ni-Fe-base superalloy.