METALLURGICAL ANALYSIS OF IN SERVICE CMSX-2 SINGLE CRYSTAL GAS TURBINE BUCKETS

摘要

The microstructural characterization of a single crystal CMSX-2 bucket, in service for 500 hours with a 15MW heavy-duty gas turbine, was conducted in order to understand the type, distribution and direction of the stresses. Before the destructive evaluation of the bucket, tension and compression creep tests using [001] specimens were conducted and γ' rafting perpendicular to the tensile stress and parallel to the compressive stress were observed. Thermo-elastic FE analyses of the bucket were also carried out in order to determine the stress and temperature distributions in the airfoil. Based on those tests and analyses, a microstructural evaluation of the bucket, which was sectioned longitudinally ([001] direction) and transversely, was conducted. The raft structure of the γ' phases was observed around the area of 50-100μm below the airfoil external surface. Investigations of the rafting morphology were conducted in order to ascertain the stress state during operation. The results were compared with those of the FE stress-temperature analyses. The main factor to cause the observed rafting morphology was found to be the dominant compressive thermal stresses that were induced by the temperature gradients across the walls of the cooled bucket and, less so, the contribution of the tensile centrifugal stresses.