INTRODUCTION
While alloy 825 was introduced to the market over fifty years ago, questions still abound about the proper methods for fabricating, welding and heat treating the alloy. Despite the fact that alloy 825 is a conventional iron-nickel-chromium-molybdenum alloy, it requires special attention to detail during processing to ensure that the product develops the optimum metallurgical structure, mechanical properties and corrosion resistance for the given application. This paper explores the various ways that alloy 825 products are produced and fabricated to try to define optimum processing techniques for the various applications in which the alloy is utilized.
Introduced to the market in the 1950's under the trade name, NI-O-NEL , INCOLOY® alloy 825 (UNS N08825) was developed at the R&D laboratory of the Huntington Alloys Products Division of International Nickel Company in Huntington, West Virginia as a material for sulfuric acid service (Figure 1). It was quickly found that alloy 825 offered resistance to a variety of corrosive media. With 42% nickel, the alloy offers excellent resistance to chloride-induced stress corrosion cracking. Dr. Harry Copson had determined earlier that this content of nickel results in resistance to stress cracking in a boiling magnesium chloride solution (Figure 2).1,2 With its contents of chromium and molybdenum, alloy 825 offers resistance to localized corrosion (pitting and crevice corrosion) in a variety of media in the chemical processing and marine industries. And, the alloy is stabilized against sensitization by its content of titanium, somewhat in the manner of grade 321 stainless steel. The metallurgy of the alloy is not vastly different from that of alloy 20 (UNS N08020) which is stabilized by its content of niobium in the same manner as grade 347 stainless steel. In practice alloys 825 and 20 are used in many similar applications. Corrosion-resistant, nickel-chromium-molybdenum alloys are often solution annealed at a high temperature prior to service. For example, alloys C-276 and 22 are annealed at temperatures over 2100°F. However, stabilized alloys offer optimum corrosion resistance after annealing at a lower temperature commonly referred to as a stabilization anneal. This is the standard condition of supply for alloy 825 products.
Intergranular corrosion (localized attack adjacent to grain boundaries) can occur when materials that are improperly heat treated are exposed to corrosive oxidizing media. While sensitization of stainless steels can occur at temperatures between 700 and 1500°F (370 to 815°C), nickel alloys are most subject to intergranular corrosive attack after exposure to temperatures in the 1200 to 1400°F (649 to 760°C) range. This reaction denudes the metal adjacent to the grain boundary of chromium, one of the key elements in the corrosion resistance of the material. As a result, intergranular attack (Figure 3) can occur when the sensitized product is exposed to an aggressive corrodent. While the Huey corrosion test (ASTM A262, Method C) is designed for use with austenitic stainless steel, it is also commonly used to evaluate the susceptibility of nickel alloys corrosion-resistant materials such as alloys 825 and 20 to this type of attack.
