Rejuvenation of Service-Exposed in 738 Turbine Blades

Abstract

A HIP rejuvenation study for an aluminide coated, internally cooled IN 738, turbine blade from an aero engine is described. The study,which involved assessing changes in blade shape, microstructure and creep properties of the airfoil portions of the blades as a function of service time, has shown that airfoils have a tendency to lengthen along their longitudinal axis during service and also that the rate of lengthening increases with increasing service life. The paper discusses on that basis, when to apply HIP rejuvenation for cost-effective blade life extension. It is also shown that the microstructure of the blades is substantially modified by service. The 7' particles coarsen through agglomeration, continuous networks of M23c6 carbides form along the grain boundaries and the blade surface along the internal cooling passages oxidizes. The design of a HIP rejuvenation/heat treatment cycle to recover microstructures and creep properties is discussed. Finally, a rejuvenation cycle that incorporates a diffusion treatment for recoating the blade is proposed that is shown to restore the loss of creep ductility induced by service, while improving time to rupture by a factor of 3 relative to new blades. Superalloys 1988 Edited by S. Reichman, D.N. Duhl, G. Maurer, S. Antolovich and C. Lund The Metallurgical Society, 1988