During the industrial turbine engine operation of the Siemens V84.2, Row 2 vanes, and the Alstom Tornado, 2nd stage stator segments, “craze-cracking” and isolated thermal fatigue cracks develop during engine operation. Other damages found include pitting and dents resulting from corrosion/erosion and FOD (foreign object damage), respectively. Erosion and oxidation damage is also commonly found on the airfoils. This paper describes the vacuum LPDB (liquid phase diffusion bond) repair process used to repair all of the above-mentioned damage.
As a means of qualifying the high temperature diffusion bond process, both metallurgical and mechanical property evaluations were carried out. The metallurgical evaluation consisted of optical and scanning electron microscopy. The wide gap diffusion bonded area consisted of a fine-grained structure with intermetallic phases dispersed both intergranularly and intragranularly. An Energy Dispersive X-ray analysis was also conducted and the results are reported.
The chemistry of the repaired area is similar to the base metal which may explain why mechanical tests revealed properties equivalent to that of the base metal. The mechanical evaluations undertaken were tensile tests at room temperature and elevated temperature, as well as stress rupture tests. These results were equivalent to mechanical properties of the IN738 and IN939 Ni-based superalloys, which is the base metal that the above mentioned vanes and stator segments are manufactured from.