ALLOY DEVELOPMENT FOR HOT GAS FILTERS

摘要

The use of metallic filters for trapping fine (1-20 μm) paniculate from hot (850 C) coal combustion exhausts can offer improved toughness and extended life over ceramic filters. Promising yield strength (200 MPa at 850 C), ductility, permeability, and corrosion resistance were obtained in porous (70% dense) samples that were vacuum sintered from tap-densified spherical powders of a Ni-Cr-Al-Fe alloy. To further improve the corrosion/oxidation resistance of this alloy, additional Al was added to enhance formation of the protective Al_2O_3 scale and to increase the reservoir of Al for sustained growth of the Al_2O_3 scale which, in turn, would extend filter life. Two new Ni-Cr- Al-Fe alloys consisting of up to three times the Al content relative to the base Ni-Cr-Al-Fe alloy were developed and tested. The corrosion resistance of these aluminum- enriched alloys was evaluated, in both "as-cast" and "pre-oxidized" conditions, utilizing an oxidizmg/sulfidizing gas environment designed to simulate a typical pressurized fluidized bed combustion (PFBC) process. An asymmetrical four-point bend (AFPB) test was also used to determine the yield strength of these alloys, in cast form, at room temperature, 600℃, and 850℃. Based on the corrosion test results, a Ni-Cr-Al-Fe alloy containing twice the aluminum content as the Ni-Cr-Al-Fe base alloy was selected as a viable filter alloy and was atomized using the Ames Laboratory high pressure gas atomizer for the purpose of further testing in filter form. The Ni-Cr-2xAl-Fe powder was sieved to the desired size of 25 - 45 microns and sintered to produce 0.5 mm thick porous sheets. Resistance welding trials were performed on the porous Ni-Cr-Al-Fe and Ni-Cr-2xAl-Fe sheets and the resulting microstructural changes in the heat-affected zone (HAZ) of the weld due to different welding conditions were characterized. The successful outcome of this work resulted in the ability to produce a resistance spot welded porous cylinder of the Ni-Cr-2xAl-Fe alloy, suitable for extended exposure tests in a PFBC facility.