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Because the TVA Gallatin Number 2 steam turbine rotor burst resulted from creep-low cycle fatigue interaction, the present study was undertaken to establish the effect of strain range, increasing hold times, and degree of segregation in the intermediate temperature range corresponding to Gallatin origin conditions. Manganese-sulfide (MnS) inclusions were found to be preferential initiation sites independent of hold-time duration, surface oxide spiking, and most strain ranges, but no significant decrease in specimen cyclic lives was observed to occur with variable MnS concentrations corresponding to different locations in the rotor. Fractographic simulation of the Gallatin origin was obtained by imposing a 23-h hold time on bore segregated material at 427°C and a 2.98 percent strain range. Periphery material or decreasing hold times to a 4-h cycle caused a change from intergranular to transgranular fracture at 427°C. When selected hold-time tests were interrupted at less than 20 percent of expected cyclic life, early cracking was observed with the largest MnS linkup distance occurring in bore segregated material.
fractography, chromium-molybdenum-vanadium, creep-low cycle fatigue, rotors, turbines, inclusions, life (durability), separation, hold time, linkup, topography, oxide spikes
Fellow materials engineer, Lester, Pa.