Thermal Stability of Hastelloy Alloy C-276

Abstract

Abstract Hastelloy Alloy C has long been of major importance to the chemical process industry, but in many applications, vessels fabricated from Alloy C had to be solution heat treated to remove harmful weld heat affected zone precipitates which reduced corrosion resistance. Modification of the chemical composition produced Hastelloy Alloy C-276, which is more resistant to the precipitation of grain boundary particles than Alloy C. In order to fully utilize the improved alloy, and to understand its responses to fabrication techniques, it is helpful to understand the time-temperature-transformation characteristics. Grain boundary precipitates can form in Alloy C-276 when it is exposed to temperatures from 1200 to 2000 F (649 to 1093 C). The precipitation characteristics of Alloy C-276 compared to those of Alloy C can be shown through the use of a time-temperature-transformation curve (T-T-T). The precipitates have been identified as either “P” or Ni7Mo6-type phases. No M6C precipitate was found in Alloy C-276, whereas a considerable amount is normally found in Alloy C. The maximum precipitation occurs at 1600 F (871 C) with a sharp drop at higher and lower temperatures. However, the times required to form these precipitates are at least 30 times longer than those of Alloy C. When precipitates form in Alloy C-276, they decrease its corrosion resistance in both oxidizing and reducing environments. In a ferric sulfate-sulfuric acid environment, the greatest loss in corrosion resistance was found in material heat treated at 1600 F (871 C). This is the temperature of maximum precipitation and indicates an impoverishment of chromium in the areas surrounding the precipitate. When a hydrochloric acid test is used, maximum loss in corrosion resistance occurs in material heat treated at 1400 F (760 C). This indicates a difference in the precipitation phenomenon at this temperature when compared to higher temperatures, and is confirmed through electron microscopy. Impoverishment of molybdenum in the matrix areas surrounding the precipitate could account for the loss of corrosion resistance in this environment. Even though precipitation may occur under certain conditions, the alloy can be used in the as-fabricated condition without solution heat treatment if the time-temperature relationships used during fabrication are controlled to prevent precipitation. Heavy plate of Alloy C-276 has been welded without loss of corrosion resistance. Vessel “heads” made of heavy plate have been hot formed without loss of corrosion resistance using the best practice determined from the T-T-T diagram. Similarly brazing, stress relieving of clad composites, roll bonding and braze bonding can be successfully adapted to Alloy C-276.