EFFECTS OF HYDROGEN ANNEALING, SULFUR SEGREGATION AND DIFFUSION ON THE CYCLIC OXIDATION RESISTANCE OF SUPERALLOYS - A REVIEW

This review is based on the phenomenon of improved oxide scale adhesio n for desulfurized superalloys. The proposed adhesion mechanism involv es sulfur interfacial segregation and scale-metal bond weakening. Sulf ur surface segregation on superalloys is examined as a function of tem perature and sulfur content, and is related to the classical behaviour predicted by the McLean isotherm. Effective desulfurization to less t han 1 ppmw can be accomplished by hydrogen annealing and is described by sulfur diffusion kinetics in nickel. Hydrogen annealing results in excellent cyclic oxidation resistance for a number of advanced superal loys. The concept of a critical sulfur content is discussed in terms o f practical annealing conditions and section thicknesses.