Pieces of a burst compressor wheel from a large combustion turbine revealed severe segregation of alloying constituents and impurities, high densities of sulfide inclusions, and intergranular, oxide filled microcracks. Analysis of oxidation kinetics for AISI 4140 steel indicates, however, that the service environment could not have caused the observed scale thicknesses in the microcracks. One- and two-dimensional stress analyses confirm modest cyclic stresses and maximum stresses in the range of 531 to 683 MPa (77 to 99 ksi), not the 827 MPa (120 ksi) level implied by crack growth from 0.76 mm (0.03 in.) flaws. To account for the observed cyclic life of about 1500 cycles, the initial defects would have to be 8.6 mm (0.34 in.) to 16.8 mm (0.66 in.) long, according to fracture mechanics analyses performed by an influence function method. On the basis of all the results, it is concluded that small cracks were already in the wheel when it went into service.