Issue 24

A. A. Shanyavskiy, Frattura ed Integrità Strutturale, 24 (2013) 13-25; DOI: 10.3221/IGF-ESIS.24.03 15 (a) (b) Figure 2 : (a) General view of the Stage-III turbine disk of a NK8-2u engine; (b) Scheme of the disk cracking sequence. Arrows indicate the disk section in that initial fatigue crack propagated. I NVESTIGATION PROCEDURE Material t was well-known superalloy EI698 is strengthened primarily by precipitation of a gamma prime phase (γ’), Fig. 3. The grain size was measured using the line intercept method on optical micrographs. The distribution of grain size has shown that the mean grain size, excluding the presence of annealing twins, is about 38 μm. Large grains on the order of two to three times of the average grain size and large grain clusters exist in the microstructure. The disk material—EI698 superalloy—was tested using standard test specimens. The latter were made from the disks, broken in service, and subjected to shirt-term tensile tests and impact-toughness tests at 20  C and to creep-rupture tests at 750  C with 420 MPa applied stress (smooth specimens), 650  C, 720 MPa applied stress (smooth specimens) and 650  C, 850 MPa applied stress (notched specimens with a 0.15-mm curvature radius of the notch tip). The specimens were cut out of the under-rim part of a disk along a chord direction and out of the hub part. At 20  C both disks exhibited mechanical properties that satisfied all standard requirements to the new disks. Ultimate tension stress, Yield stress and elongation were respectively not less than 1150 MPa, 720 MPa and 19 %. The time to fracture shown by the smooth and notched specimens tested for creep-rupture strength appeared well above the smallest standard one (not below 50 hours), i.e., was indicative of a quite good high-temperature strength of the disk material. Metallographic investigation of both investigated disks in area of crack origination has shown material structure in accordance with certificate for manufactured turbine disks (see Fig. 3) without any evidence of faults. (a) (b) Figure 3 : Typical microstructure of EI698 with gamma prime phase (  ’) under (a) , (b) different magnification in electron microscope. Non-destructive inspection data In-service, after the failure discovering of two turbine disks, there was performed non-destructive inspection of all operated turbine disks. In many cases there were discovered cracks being different in size for different disks. Disks with I