Yukio Kagiya

An Assessment and Estimation of the Damage Progression Behavior of IN738LC under Various Applied Stress Conditions Based on EBSD Analysis

In order to characterize the damage progression behavior of IN738LC and the influence of applied stress, the average misorientation within grains was evaluated using the electron backscattered diffraction (EBSD) method, by measuring over a million data points located across almost the whole zone of the tested specimens. It has been shown under various test conditions such as differing temperatures and strain rates that the average misorientation increases linearly with the increase of creep strains. As a result, it is confirmed that it is possible to assess the remaining creep fracture life from its average misorientation value regardless of the testing temperature and strain rate. In addition, the deformation and fracture mechanisms of various types of mechanical damage, such as tensile, impact, and creep conditions, were discussed by evaluating the characteristics of misorientation distribution. As a result, it has been revealed that creep damage affects a large area of the material, but it is localized near grain boundaries, which is completely different from that of tensile or impact damage. In conclusion, detailed investigation into the cross section of fracture samples using the EBSD misorientation analysis allows for the qualitative estimation of the fracture mode under various applied stress conditions, the cause of the fracture.

Analysis of damage behaviour based on EBSD method under creep–fatigue conditions for polycrystalline nickel base superalloys

Abstract In order to detect creep–fatigue damage before the crack initiation stage, an investigation into damage behaviour based on the electron backscattered diffraction (EBSD) method for a polycrystalline nickel base superalloy has been carried out. The average misorientation within grains increases linearly up to the initiation of cracks with the increase in creep strains. Even if stress distributions exist by stress concentration, assessment in all the damage areas allowed the evaluation of creep damage regardless of geometrical influence. Furthermore, the influence of strain rates, introducing fatigue and testing temperatures are hardly observed in the misorientation analysis. Misorientation almost corresponded to inelastic strain regardless of those influences. Quite a small misorientation caused by fatigue led to an equivalent result between creep conditions and creep–fatigue conditions. It is concluded that the misorientation analysis of damaged materials based on the EBSD method allows the quantitative estimation of creep strain and the assessment of remaining creep fracture life.

Geometrical influence for creep damage evaluation of IN738LC using electron backscatter diffraction

Abstract The Ni-based superalloy IN738LC is an outstanding heat-resistant material, but it does have the disadvantage of being brittle. Therefore creep degradation such as deformation of components or initiation of microscopic creep voids only appear at the end of its available creep life. To date, electron backscatter diffraction (EBSD) in conjunction with scanning electron microscopy has been used for damage evaluation. Some results showed that the misorientation was correlated with the magnitude of macroscopic plastic or creep strains. In this study, by using the EBSD method, the characteristics of the change and distribution tendencies of crystallographic misorientation on creep life have been investigated. In addition, the geometrical influence of stress concentrated areas for misorientation analysis has been investigated, which is important to evaluate damage in practical components like blades. As a result, the grain reference orientation deviation parameter which corresponds to the spread of creep strain has been found to be useful to estimate creep life among the misorientation parameters. Also, misorientation analysis allows the evaluation of creep damage before the appearance of obvious defects without being subjected to the geometrical influence of the stress concentrated areas.

Creep damage evaluation of IN738LC based on the EBSD method by using a notched specimen

The Ni-base superalloy IN738LC developed as a heat-resistant material has excellent high temperature strength, but at the same time it has a brittle property. As a result, any evidence of creep degradation such as deformation of components or initiation of creep voids has hardly made an appearance by the end of its available creep life. This paper investigates the possibility that creep damage can be evaluated by using the electron backscatter diffraction (EBSD) method to analyze misorientation which is considered to be caused by creep strain. The use of an in-situ observational testing machine with a notched specimen allowed creep-EBSD round tests which consisted of creep tests in the in-situ obser- vational testing machine and EBSD observations using scanning electron microscopy (SEM) alternately against an identical specimen. The outcome of these experiments was that crystal orientation change tendencies corresponding to the creep strain were continuously observed. It has been found that there was a good correlation between the creep strains and the misorientations referenced to the average orientation in the grain, the parameter of Grain Reference Orientation Deviation (GROD) defined by TSL EBSD system. Consequently, this study shows that the creep damage could be estimated by analyzing misorientations using the EBSD system automatically. Also, it has been found that crystal orientation changes appeared around notches in the direction of maximum shearing stress, which implies that the misorientation development was caused by the crystallographic lattice rotation.

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

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