RECRYSTALLIZATION BEHAVIOUR OF A NICKEL-BASED SUPERALLOY

The thermomechanical processing of a nickel-based superalloy is the way to considerably influence the grain size. A uniform coarse-grain size increases the creep strength and the crack-growth resistance. In this work, the processing for achieving a uniform recrystallized-grain structure with a variation in the thermomechanical parameters is investigated. The MoNiCr alloy is intended for modern designs of nuclear reactors, in which molten fluoride salts are used as the coolants in the primary and/or secondary circuit. It represents a material alternative with a high corrosion resistance in the area of fluoride salts and it has very good creep properties in the temperature range of 650-750 degrees C as well. The manufacture of vessels and fittings from the MoNiCr alloy requires the mastering of the technology for forming this high-alloyed material. The key step seems to be the transition of the cast state of the material to the state of a cast recrystallized microstructure with homogenous fine grains. A particular stress condition is, besides the temperature, very important during hot forming. Nickel alloys are able to accept a significantly higher deformation level if the compressive stress prevails. A forming process involving the compression state of stress increases the probability that the material will reach, without failure, a level of deformation that allows recovery and recrystallization processes. A particular deformation process was carried out on a physical simulator. The preceding cold deformation essentially accelerates the recrystallization process of a deformed cast structure.