Ongoing work has developed high chromium nickel-based alloys suitable for use in shipboard incinerators operating between 704 °C (1300 °F) and 980 °C (1800 °F) and has developed the science and technology base for the manufacture of these advanced alloys. Nickel alloys containing more than 30 percent chromium are extremely difficult to fabricate by conventional hot working processes because of the development of a brittle alpha chromium phase. In addition, high-chromium nickel- based alloys are susceptible to a decrease in ductility after exposure to elevated service temperatures.
The hot corrosion resistance and thermal stability of various spray-formed, grain-refined fabricable Ni-Cr alloys containing 30 to 50 weight percent chromium and alloys with niobium additions will be examined.
The US Navy currently utilizes shipboard incinerators on several ship classes for blackwater disposal. The amount of blackwater produced on a typical ship ranges from 6 to 26 gallons/man/day, and liquid domestic wastes average 30 gallons/man/day. Because of shipboard space and weight limitations for containing this waste and current and projected environmental regulations, it is necessary to consider on-board waste treatment processes coupled with discharge o f environmentally acceptable effluents. Current shipboard incinerators used for the disposal of blackwater utilize Alloy 690 (Ni-30Cr-9.5Fe) operating at a peak temperature of 760 °C (1400 °F). Higher temperatures around 980 °C (1800 °F) are required for "multi-functional" incinerators that will process additional waste streams.
It is known that increasing the Cr content of alloys can increase their operational capability in many high temperature environments [1,2]. McDowell and Mihalisin showed that a 50Ni-50Cr alloy was clearly superior to 25Cr-20Ni-Fe in a mixture of vanadium pentoxide and sodium sulfate [3]. Later work confirmed that the maximum corrosion resistance to simulated residual fuel oil ash was afforded by the highest possible combination of nickel and chromium [4]. Increasing the chromium content was shown to reduce the rate of corrosion in pure H2S or SO2 [5]. The reducing environment was much more aggressive than the oxidizing condition.
However, Ni alloys containing more than 30 % Cr are extremely difficult to produce and fabricate by conventional hot working processes because of development of a brittle alpha chromium phase [6]. In addition, high chromimn-nickel alloys are susceptible to a decrease in ductility after exposure to service temperatures. In general, the ductility of the NiCr alloys decreases with increasing chromium content. Once ot-Cr is present, the alloy is more difficult to work, because (z-Cr is hard to deform and less ductile than ,{-Ni. Poor workability is often attributed to the presence of nitrogen in the o~-Cr phase. Because of this factor, nitride-forming elements are added to chromium to reduce the ductile/brittle transition temperature and improve workability. Reactive elements have shown to significantly improve hot workability. Cold workability can be improved by heat treatment, but heat treatment times required for improved ductility are impractical without the presence of nitride-forming elements.
The present process to fabricate liners requires a cast alloy to be rolled into sheet form, bent into a tubular shape and then welded. Alloy 671 (52Ni-48Cr) was one commercial high-chromium alloy that was developed, but it required several intermediate annealing processes to avoid work hardening and cracking [7]. Except for its use as a cladding material, the production of Alloy 671 has been virtually discontinued.
The addition of niobium improves the high temperature streng
