Investigation of carbides in gas turbine disks produced from EP428 grade steel after long-term service exposure

TEM microdiffraction, EDS analysis and light microscopy using general-purpose, selective- and color-etching techniques were used to show that the tempered martensite and carbide phase composition, size and abundance at different locations in the gas turbine disk, produced from martensitic stainless steel grade EP428 exposed to long-term service (100,000 h) at different temperatures (350 and 500 °C), has differences. M23C6 carbide in two morphologies were present in both hub and rim locations. A small concentration of particles of the hexagonal M7C3 carbide was observed in both the hub and the rim sections of the disk. A small concentration of particles of alloyed M3C cementite was found in the hub section of the disk. However, M3C was not observed in the rim section. Because the cementite has a strengthening effect, the mechanical properties of parts of the disk remained the same.