Kazutoshi Ishibashi

Damage and Degradation Assessment of Stage 1 Bucket Coatings in a 1,100° C-Class Gas Turbine

According to increasing the inlet temperature of gas turbines, the role of coating is getting more and more important for the bucket life. It is, therefore, very important to understand the actual phenomena of the bucket coatings under the operation. Four different kinds of new and serviced stage 1 bucket coatings for 1,100°C-class gas turbines, such as a platinum-aluminide diffusion coating and overlay-type coatings of CoCrAlY, over-aluminized CoCrAlY and CoNiCrAlY, were selected and evaluated nondestructively and destructively at every inspection times. The platinum-aluminide and over-aluminized CoCrAlY coatings suffered from heavy cracking that propagated from the root of the airfoil to the tip during operations, but the CoCrAlY and CoNiCrAlY coatings did show little or no cracking. The degrees of oxidation and microstructure degradation were also different not only coating by coating, but also the locations of the airfoil surface due to the surface temperature distributions. To understand precisely the actual phenomena, hardness and 3-point bend tests at room and elevated temperatures were conducted by using the new and serviced bucket coatings and figured out there were good correlation between the hardness and the strain to cracking, which imply higher hardness coating at certain temperature has more sensitive to the cracking. The degrees of the coating crack conditions of the serviced buckets were the same as these fundamental test results. The effects of microstructure degradation on the hardness and the strain to cracking were also evaluated to understand why the cracking propagate to the higher temperature and lower centrifugal stress portion of mid-span and tip of the airfoil. The test results showed the microstructure degradation reduced the hardness and also reduced the sensitivity to the cracking, but still enough sensitivity to cause cracking. Formation of oxide layer was also found to enhance the cracking sensitivity by the further microstructure observation of the serviced buckets.Copyright

Effects of Environments and Shot Peening on Creep and Creep-Fatigue Behaviors of Ni-Fe-Base Superalloy Inconel®Alloy 706

Fleet leader machines with non-shot-peened discs made of Inconel® alloy 706 were experienced to have cracks in the first stage gas turbine wheels. These were inter-granular cracking and observed to be highly stressed locations with less potential for oxidation, which is thought to be quasi-brittle inter-granular cracking due to stress induced atmospheric oxygen penetration, so called, hold-time cracking. To recognize this phenomenon, creep and creep-fatigue tests with smooth and notched specimens were conducted at 600 and 650°C in air and vacuum and confirmed the environmental effects on those lives and fracture modes.The effectiveness of shot peening which was used as one of the countermeasures for this phenomenon was verified by using the creep-fatigue tests. The durability was also evaluated by thermal and stress aging tests at 450 and 500°C up to around 104 hours. Little relaxations were observed during the thermal agings after the initial rapid relaxation of the surface residual stress, but the effects of the loading stresses were observed above the yielding stress at each temperature.Copyright

Life-Refurbishment of Service-Degraded Gas Turbine Buckets

Various methods have been developed, and used in practice, to reduce the maintenance cost of gas turbine hot parts by extending the replacement life. The life-refurbishment process by Hot Isostatic Pressing (HIP) is one of our accomplishments for the buckets retired due to the scheduled life and was confirmed to rejuvenate the microstructures and the tensile, Charpy impact, creep and low cycle fatigue properties to a condition equivalent to or even better than the new ones under the development program. According to above mentioned accomplishments, a number of HIP refurbished buckets were reinstalled in a Chubu Electric Power Co. Inc. (CEPCO) 1100°C-class gas turbine rotor on December 1999 and operated successfully for 2 years under the joint program. Those buckets also served for destructive and non-destructive evaluations after operation. From those tests, we reconfirmed the reliability of the HIP refurbished buckets and also validated that the life-consumption rate was the same as new ones.Copyright