Jintao Lu

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 iron and chromium on coal-ash corrosion behavior of Ni-Cr-Co based alloy for advanced-ultrasupercritical power plants

The influence of alloyed iron and chromium on the coal ash corrosion behavior of Ni-Cr-Co based alloy was investigated at 800°C to 850°C. Results show that the alloyed iron accelerates the corrosio...

Fireside Corrosion Behaviors of Inconel 740H Superalloy in Various SO 2 Contents

Fireside corrosion behavior of Inconel 740H superalloy was studied at 750 °C in simulated coal ash/flue gas environments by means of XRD, SEM and EDS. The results indicated that the corrosion behavior was strongly related to the SO2 levels and was significantly affected by NaCl additions. In presence of the atmospheres with 0.1%SO2, the alloy exhibited the highest corrosion resistance due to formation of a stable and dense Cr2O3 film. In present of the atmosphere with 1.5%SO2, however, a non-coherent and porous Cr2O3 film was formed. The thickness of film and internal sulfides were substantially increased. The mechanism of ash corrosion formation was also discussed.

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.