Rejuvenation Heat Treatment Development to Extend the Service Life of René N4 Single Crystal Gas Turbine Blades

Single crystal superalloys are widely used for gas turbine blades exposed to high stresses, high temperatures in aggressive environments. When these components reach their serviceable limit, they must be replaced by new ones, with an important impact in terms of maintenance costs. Significant savings can be achieved through the application of a rejuvenation heat treatment, able to extend the life of a serviced turbine blade beyond its original design intent and avoid parts scrap. The success of such a technology for SX-superalloys is related to the effective restoration of the original γ/γ’ microstructure, which is linked to the material mechanical properties, without impairing the original single crystal structure. Indeed, one challenge in designing a rejuvenation heat treatment for single crystal superalloys is the risk of recrystallization, which must be kept under control. This work is focused on the development of a rejuvenation heat treatment for René N4 alloy. Different rejuvenation cycles have been explored through an in-depth material characterization, using samples taken from both new material and real serviced gas turbine blades. Several solutioning heat treatments have been investigated in a window ranging from the theoretical γ’ solvus temperature and the original solutioning temperature used during blades manufacturing. The assessment of the candidate rejuvenation heat treatments effectiveness is based on the evaluation of microstructure γ/γ’restoration and the presence of recrystallized grains.