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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 quantit...

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.

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 ...

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.

Ni-based superalloys are commonly used in hot gas path parts such as turbine blades because of their higher strength under high temperature. Their high strength results from an intermetallic compound such as a cubical γ' phase precipated in a γ phase. However, the γ' phase tends to show coarsening and lafting. The re-heat process has been adopted to repair the creep damage in the Ni-based superalloys. The re-heat process can restore the γ' phase by carrying out thermal cycles with an appropriate combination of temperature and duration. In this report, we focused on creep damage under the bending load. First, the creep constitutive law was developed to calculate creep strain more accurately. Then, we investigated the relationships between the effect of the re-heat process and creep crack initiation under the bending load through a mock-up creep test.The time-dependent hardening law was selected as a creep constitutive law. The calculated creep deformation showed relatively good correspondence with the results of uni-axial and mock-up creep tests. Also, creep crack initiation seemed to be controlled by the re-heat process at an appropriate stage. Re-heated specimens showed critical creep deformation, corresponding to creep crack initiation, about half that of virgin specimens. This seemed to be because of transgranular precipitation, not all of which can be removed using the re-heat process. In uni-axial creep tests, we found some critical value for cumulative creep strain, which causes harmful precipitation during the re-heat process. Therefore, we conclude that creep damage in Ni-based superalloys can be repaired effectively using the re-heat process before cumulative creep strain reaches a critical value that is restrained by the initiation of harmful precipitation or creep cracks.

In order to confirm the validity of the model for creep damage mechanism constructed from a series of experiments on reheat pipe elbow of mod. 9Cr-1Mo steel with a practical size, microstructural observation and uniaxial creep rupture tests were carried out using the welded joint of the hot reheat pipe elbows (No. 5 and No. 25) taken from a practical plant after long-term service. The obtained results are as follows : (1) Nucleation and growth of creep voids were observed in the fine-grained HAZ region of the inside-elbow region, where the triaxiality factor of stress was high compared to the other regions in the HAZ. The phenomenon of creep damage in the long-term used elbows was consistent with that in the practical sized experimental elbow used for the model construction. (2) The degree of creep damage observed for No. 5 and No. 25 elbows corresponded to the early stage of creep damage. But the creep damage of No. 5 elbow was more serious than that of No. 25 elbow due to the difference in degree of circularity between the two elbows. (3) Considering that the long-term used elbows underwent creep damage to some extent, the creep rupture strength except for the repair-welded specimen exhibited high enough to exceed the demand strength estimated from the life assessment curve for the welded joint of high chromium ferritic steels presented by Nuclear and Industrial Safety Agency.

The effects of oxygen partial pressure ( p O 2 ) of the preoxidation atmosphere on the growth of thermally grown oxide (TGO) and thermal cycle life of plasma-sprayed thermal barrier coatings (TBCs) were investigated. The p O 2 of the preoxidation atmosphere was controlled by using a solid-state electrochemical oxygen pump system. The purity and microstructure of continuous Al 2 O 3 layer formed on the bond coat during preoxidation at 1050 °C were highly influenced by the p O 2 of the atmosphere. The specimen preoxidized at 1050 °C under a p O 2 of 10 −14 to 10 −15 atm, which is around the dissolution pressure of (Co, Ni)(Al, Cr) 2 O 4 spinel, showed the lowest growth rate of TGO and the longest thermal cycle life.

The effects of La2O3 addition on thermal conductivity, phase stability and thermal cycle life of Y2O3 stabilized ZrO2 plasma sprayed coatings were investigated. Although low thermal conductivity as well as high resistance to sintering was achieved by La2O3 addition, it tended to also result in lower phase stability and thermal cycle life of the coatings. Optimization of the composition and structure of the coatings improved these properties, and the optimized coatings showed prolonged thermal cycle life.

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.

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).

BN coated SiC (Hi-Nicalon) fiber-reinforced β'-SiAlON based composites have been fabricated by reactive melt infiltration. β'-SiAlON was produced in-situ by the reaction of β-Si 3 N 4 and AIN reactants in the preforms with Y-Al-Si-O molten glass. The wettability of the fibers to the molten glass was improved by infiltration and pyrolysis of perhydropolysilazane. The reaction between the fiber and molten glass was depressed using a controlled N 2 atmosphere. The mechanical properties of the composites at room temperature were significantly influenced by the composition of the nitride reactants introduced into the preforms. The increase of the amount of β'-SiAlON formed in the matrix probably increased the residual tensile radial stress between the fiber and matrix, resulting in the improvement of the mechanical properties. The high temperature mechanical performance could be improved by increasing the packing of nitride reactants into the fabrics before melt infiltration of the glass because of the decrease in the amount of residual glass.

In order to establish the creep life assessment method for modified 9Cr-1Mo steel welded joints on hot reheat piping of generating power plants, internal pressure creep tests conducted with full size components are analysed using the finite element method (FEM). As a result, it is shown that the creep damage distribution in weld heat affected zone (HAZ) can be predicted by stress distribution in HAZ, and that average stress in HAZ can be a representative value for creep strength of welded joints.

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 © 2004 by ASME

An experimental investigation is reported of the zero pressure gradient boundary layer along a 90 degree streamwise corner. Turbulent and non-turbulent conditional sampling measurements in the outer intermittent region of the boundary layer as well as a comprehensive set of conventional measurements were made in a cross section 1.7m downstream from the leading edge. The results of a turbulent zone-averaged secondary velocity field indicate a pair of closed streamwise vortex patterns near the corner, as in conventional measurements. A large difference was found in the outer part of the boundary layer, where the turbulent zone-averaged secondary flow moves from the wall to the free stream direction. On the contrary, the non-turbulent zone-averaged counterpart moves from the free stream to the wall and it includes almost no streamwise vorticity, indicating that the secondary flow of the second kind is generated only when the flow is turbulent.

An experimental investigation of the zero pressure gradient turbulent boundary layer along a 90 degree streamwise corner is reported. Turbulent non-turbulent conditional sampling measurements in the outer intermittent region of the boundary layer as well as a comprehensive set of conventional measurements were made in a cross section at 1.7 m down stream from the leading edge. The results of a turbulent zone averaged secondary velocity field indicate a set of closed streamwise vortex patterns near the corner, as in the conventional measurements. A large difference was found in the outer part of the boundary layer, where the turbulent zone averaged secondary flow moves from the wall to the free stream direction. On the contrary, the non-turbulent zone averaged counterpart moves from the free stream to the wall and which includes almost no streamwise vorticity, indicating that the secondary flow of the second kind is generated only when the flow is turbulent.