Application of stress relaxation testing in metallurgical life assessment evaluations of GTD111 alloy turbine buckets

Stress relaxation and constant displacement rate tensile tests were perform ed on polycrystalline GTD111 alloy material removed from General Electric M S6001B first stage combustion turbine buckets. Samples were examined in the standard heat treated condition, thermally exposed at 900 degrees C for 50 00 hours and from service run buckets. Creep rates of the material were mea sured and evaluated directly in terms of temperature capability at 850 degr ees C and 900 degrees C. Stress relaxation tests done at 0.8 percent total strain indicated that the creep rate properties in the service exposed airf oil Mere an order of magnitude higher than the material properties in the s tandard heat treated condition measured in the root form. In terms of tempe rature capability, the creep rate properties of the service nln airfoil mat erial had decreased by the equivalent of almost 40 degrees C. The stress re laxation test method was demonstrated to be a very useful tool in quantifyi ng the degradation of creep properties in service run components. Creep dat a that would require years to gather using conventional creep tests was gen erated in a few days. This now makes realistic life assessment and repair/r eplace decisions possible during turbine overhauls. The test method's uniqu e ability to measure changes in creep rate over a large stress range, enabl ed the technique to distinguish between changes in creep strength due to (n ormal) microstructural evolution from the combined effects of microstructur al evolution and strain related creep damage. A method for estimating stand ard constant load creep rupture life from the stress relaxation creep rate data is also presented along with time-temperature parameter correlations T he data sets examined in this study indicate that creep rupture lives can b e estimated within a factor of three from the stress relaxation data. The i nformation and analysis techniques described in this paper are directly app licable to metallurgical life assessment evaluations and the requalificatio n of repaired General Electric buckets in Frame 3, 5, 6 7, and 9 engine mod els.