Yomei Yoshioka

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

Crack Initiation Behavior and its Estimation for a Gas Turbine Rotor Based on the EBSD Analysis

Gas turbine rotors made of Ni-base superalloys were sometimes found to have cracks at the dovetail of the first and second stage wheel. In order to maintain the reliability of gas turbine rotors over the long term, in addition to the application of countermeasures such as the shot-peening process, it is essential to confirm the characteristics of crack initiation mechanisms and to predict the possibility of new cracking.In this paper, first, the case study of crack initiation concerning a wheel dovetail crack has been carried out. Second, to reveal the characteristics of the crack, comparative evaluation between the actual crack and various mechanical fracture samples were conducted by using a scanning electron microscope (SEM) and the electron back-scatter diffraction (EBSD) method which can analyze crystallographic misorientation. As a result, it was found that even in relatively low temperatures, Inconel® Alloy 706 is subjected to brittle grain boundary oxidation when under constant high stress, i.e. a similar phenomenon to stress accelerated grain boundary oxidation (SAGBO), so called hold-time cracking (HTC).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

Lifing issues in hot gas path components of heavy-duty gas turbines

Abstract Gas turbine hot-gas-path components, which include combustion liners, transition pieces, turbine nozzles and turbine buckets, are exposed to hot gases discharged from combustion systems and suffer from severe materials degradation and damage even in the early stage of operation. The severity of the damage and degradation increases with increasing inlet temperature and size of the gas turbines, which also increase the maintenance cost. ‘Lifing’ of components is, therefore, becoming a very critical issue. This paper describes several kinds of component damage and material degradation occurring in the 1,100°C- and 1,300°C-class heavy-duty gas turbines and then shows how we revised those component lives from the original design ones. Analytical-based assessment methods associated with condition-based assessment ones, some examples of assessment results, and component life extension technologies are also described.

Rafting of Single Crystal Superalloy by High Thermal Stress

Rafting of the nickel-base single crystal superalloy CMSX-2 was investigated under the condition of high thermal stress using a newly constructed experimental equipment. The test piece with the diameter of 25 mm was tested at the maximum material temperature of 1 000 degree Celsius for 80 hours. The results showed that the raft structure occurred only under the influence of thermal stress. The raft structure developed parallel to the direction of the compressive thermal stress because of negative lattice misfit. The thickness of gamma phase and gamma prime phase in rafting increase with the increase in the temperature and the thermal stress.

Fretting Fatigue in Slot Dovetail of Turbo-generator Rotor Shaft

This paper describes the fretting fatigue of slot-dovetails in turbo-generator rotor including typical examples and repairs of fretting fatigue cracks, results of fretting fatigue tests, factors affecting fretting fatigue strength, fretting fatigue preventive technologies, and UT inspection methods.

Fretting Fatigue of Slot-dovetail in Turbo-generator Rotor Shaft

This paper describes the fretting fatigue of slot-dovetails in turbo-generator rotor including typical examples and repairs of fretting fatigue cracks, results of fretting fatigue tests, factors affecting fretting fatigue strength, fretting fatigue preventive technologies, and UT inspection methods

A New Ni-Base Single Crystal Superalloy TMS-82+

A new single crystal (SC) superalloy TMS-82+ which contains only 2.4% Re has been developed by using a computer aided alloy design program (ADP) developed in the National Institute for Materials Science (NIMS). The compositions were designed as follows; a large negative lattice misfit at operated temperature, 60 vol.% of γ′, solution heat treatment window of >50°C, and creep strength better than 2nd generation superalloys. Corrosion resistance was also considered. A brief explanation of the NIMS-ADP and the whole development works of SC alloy TMS-82+, such as alloy selection, metallurgical and mechanical tests, a full-scale production test and an engine test in the 15MW gas turbine, will be described here.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

Effect of Cooled Turbine Blade Surface Roughness on Heat Transfer Rate and Metal Temperature in Gas Turbine Field Operations.

This paper describes the effect of turbine blade surface roughness on the heat transfer rate and metal temperature in gas turbine long term field operations. The metal temperature of Ni based super-alloys can be estimated from changes in microstructure due to the aging. Applying this technology to the rotating blades used in gas turbine field operations, long term operational metal temperature were higher than during short term operation. Also, it was observed that the surface roughness of rotating blades increased during long term operations compared to new blades, which corresponds to the increase in metal temperature. The increased heat transfer rate on the rough surface of the rotating blades was predicted analytically. From these results, the relationship of the rotating turbine blade surface roughness and the increase in the metal temperature in gas turbine field operations can be estimated.