Type IV creep damage occurs in welded joints of high chromium steel in power plant. The evaluation method of residual life of the damaged component is demanded. One of them is the methods based on the number density of voids that is defined as the number of voids divided by the area of observation. The parameter represents the life fraction of components and is influenced by the stress tri-axiality, but the influence is hidden by the influence of the area and magnification of observed metallographic structure. The error of the parameter is large if the area of observation is not large enough, because the voids do not uniformly distribute. The parameter varies according to the magnification because voids can’t be seen if magnification of observed photograph of metallographic structure is small. The appropriate areas and magnification are required for obtaining the appropriate parameters, for studying the influence of stress tri-axiality and for evaluating the residual life. In this study, the creep tests with the specimens of simulated fine-grain HAZ of Mod.9Cr-1Mo steel were carried out and samples with Type IV creep damage were obtained. The distribution of voids under the uniform stress was measured under the different conditions of magnifications and areas. We obtained the mean and the standard deviation of the distribution and proposed the area of observation appropriate for allowable error of the number density of voids that depends on the magnification.
ASME Grade 122 (11Cr-0.4Mo-2W-CuVNb) has been used for boiler components in ultra-supercritical (USC) thermal power plants at approximately 873 K. The creep life of the welded joints in this steel decreased as a result of Type IV creep damage that forms in the heat-affected zone (HAZ) under long-term use at high temperatures. In this study, we investigated the relationship between microstructural change and the evolutions of two nonlinear acoustic characterizations with electromagnetic acoustic resonance (EMAR) throughout the creep life in the welded joints and the correlation between two nonlinear acoustic characterizations. One was resonant frequency shift and other three-wave mixing. EMAR was a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). We used bulk wave EMAT, which transmits and receives shear wave propagating in thickness direction of a plate specimen. Creep tests of thick welded joints specimens were interrupted at several time steps at 873 K, and 90 MPa. Two nonlinear acoustic parameters and ultrasonic attenuation decreased from the start to 50% of creep life. After slightly increased, they rapidly increased from 80% of creep life to rupture. We interpreted these phenomena in terms of dislocation recovery, recrystallization, and restructuring related to the initiation and growth of creep void, with support from the SEM and TEM observation. This noncontact resonance-EMAT measurement can monitor the evolution of nonlinear acoustics throughout the creep life and has a potential to assess the Type IV creep damage advance and to predict the creep life of high Cr ferritic heat resisting steels.
Creep damage evaluation by EBSD(Electron BackScatter Diffraction pattern) method was conducted on the austenitic stainless steel SUS304HTB as the boiler tube material. Creep damage was imposed under the condition of 650℃/130MPa and 700℃/75MPa with various interruption hours. The longitudinal mid-section of specimen was cut and observed by EBSD equipment and then Phase maps, IPF(Inverse Pole Figure) maps, KAM(Kernel Average Misorientation) maps and GROD(Grain Reference Orientation Deviation) maps were obtained. Phase maps showed the increasing α-phase transformed from γ-phase at the grains oriented with (111) as the normal direction with the increasing time fraction or the accumulated macroscopic creep strain. The misorientation parameters obtained from KAM maps and GROD maps showed increasing trend with increasing time fraction. The distribution function of GROD was regressed on the basis of continuously distributed dislocation theory and its representative coefficient parameter AG showed the good correlation with time fraction or accumulated creep strain. By measuring AG value near creep voids, creep strains were evaluated in the vicinity of creep voids and did not show significant strain concentration compared with the non-voided area. However, the good correlation between the void length and the creep strain evaluated from AG was observed. The AG parameter was considered as an effective quantity obtained from EBSD observation to evaluate creep strain intensity at the grains with and without creep voids irrespectively.
Creep damage evaluation by EBSD(Electron BackScatter Diffraction pattern) method was conducted on the ferritic heat resistant Mod.9Cr steel as the turbine piping material. Creep damage was imposed under the condition of 600℃/130MPa and 650℃/100MPa with various interruption hours. The longitudinal mid-section of specimen was cut and observed by EBSD equipment and then Boundary maps, IPF(Inverse Pole Figure) maps, KAM(Kernel Average Misorientation) maps and GROD(Grain Reference Orientation Deviation) maps were obtained. Boundary maps showed the decreasing sub-boundary length having relatively small rotation angles with the increasing time fraction or the accumulated macroscopic creep strain. The misorientation parameters obtained from KAM maps and GROD maps showed decreasing trend with increasing time fraction. The distribution function of GROD was regressed on the basis of continuously distributed dislocation theory and its representative coefficient parameter AG showed good correlation with time fraction or accumulated creep strain. AG multiplied by equivalent grain diameter ld seemed to show slightly higher sensitivity to creep strains. A good correlation between the void length and the creep strain evaluated from AGld was observed. The AGld parameter was considered as an effective quantity obtained from EBSD observation to evaluate creep strain intensity at the grains with and without creep voids irrespectively.
Constant-load creep testing machines have mainly been employed to obtain constitutive equations of creep. Although constant-stress ones would be more preferable to create an accurate constitutive equation of creep, they have not been employed yet because of their complex structures. Furthermore even if constant-stress creep machines were employed, constraint of limbs attached on a gauge length in the uniaxial creep specimen would have been another inevitable issue. That is, extensometer limbs in the specimen usually prevent uniform deformation along the gauge length. In order to release the constraint of circumferential deformation around the extensometer limb, 24 slits were introduced into each limb in this study. Firstly, Finite Element analyses using Norton’s creep law were performed to examine the validity of introducing these slits for the uniform deformation. The analytic results showed that the creep specimen with slits could release the circumferential constraint and raise the uniform deformation along the gauge length. It was also found that there was an optimum depth of the cutting slit to deform uniformly. Subsequently, employing SUS304 austenitic stainless steel, uniaxial creep tests were conducted at two initial stress levels. Rupture lives using specimens with the slit were shorter than those using the conventional specimen when the constant-stress creep machine was employed. Each creep test was interrupted four times to measure the distribution of deformation in the specimen, and then it was found that uniform deformation continued up to the middle of the tertiary stage of creep. The distribution of creep strain depended on an amount of deformation regardless of the type of specimen and testing machine. FE analyses showed that the distribution of strain was also influenced by a small variation of diameter in the specimen and a small superimposed bending load.
Changes of misorintation in grains were studied by Scanning Electron Microscopy and Electron Back Scattering Diffraction (SEM/EBSD) for austenitic stainless steel, SUS 316 that were crept under 100 MPa at 973K. Misorientation of the specimens was calculated by Kernel Average Misorientation (KAM), Grain Average Misorientation (GAM) and Grain Orientation Spread (GOS). The values calculated by KAM, GAM and GOS increased with the strain increasing linearly during the transient creep stage. The tendency corresponded to the changes in total dislocation density estimated by Transmission Electron Microscopy (TEM). In the transient creep stage, good linearity between geometrically necessary (GN) dislocation estimated from KAM values and dislocation density by TEM was revealed. Compared the increments among misorientations calculated by KAM, GAM and GOS, one of GOS was the highest and the others were almost the same. Increments of KAM values were lower than those reported in the previous papers. Because data was obtained from the crept specimens in this study but data were obtained from specimens deformed in room temperature or moderate temperature. It was suggested that the increment of GN dislocation become lower due to dynamic recovery during creep test in this study. When the pitches in the EBSD measurement were taken account into, the increment of GAM values in this study was equal to one reported in the previous paper where the data was obtained from the crept specimens of stainless steel with the similar grain size.
In order to characterize the creep and/or fatigue crack initiation and growth behavior of Ni-base superalloys, an investigation into damage behavior based on the EBSD (Electron BackScattered Diffraction) method using notched specimens has been carried out. The misorientation in the vicinity of notches increased with the increase in the creep or creep-fatigue damage characteristically depending on a CC (Conventional Casting) or a DS (Directionally Solidified) superalloy. The stress holding time clearly influenced the creep and/or fatigue crack growth behavior and the appearance of misorientation development. However, it was shown that the relationship between the average misorientation in grain and the RNOD (Relative Notch Opening Displacement) was independent of the creep and/or fatigue conditions. It is concluded that the misorientation analysis of damaged samples based on the EBSD method allows the prediction of the initiation and the growth behaviors of the creep and/or fatigue crack.
In single crystal Superalloys, fatigue crack prefers to propagate on crystallographic slip plane at lower temperature, while prefers to propagate perpendicular to the loading axis independent of crystal orientation at higher temperature. The former cracking mode is called as shearing mode cracking, and the latter is called as mode-I cracking. The aim of this work is to understand the determining factor of cracking mode in single crystal superalloys, and we focus on the influence of γ/γ’ microstructure and anomalous increase of yield strength of γ’ phase at elevated temperature on deformation behavior around crack-tip. First, the relation between macroscopic tensile strength and γ/γ’ microstructural phase strength was studied by using unit-cell FE simulations, and yield strength of γ phase and critical resolved shear stress (CRSS) of γ’ phase was evaluated. Second, a constitutive equation which considered γ/γ’ microstructure was made by using Eshelby’s inclusion theory. Finally, FE simulations were performed to understand the influence of γ/γ’ microstructure on crack tip deformation behavior. A series of FE simulations revealed that γ/γ’ microstructure and anomalous increase of yield strength in γ’ phase at elevated temperature strongly affect the slip deformation behavior around crack-tip.
Damage behaviors of a thermal barrier coated (TBCed) Ni-base superalloy IN738LC were studied under high temperature low-cycle fatigue (LCF) condition. In this work, the HVOF process was selected as the spray process for bond coat. The CoNiCrAlY alloy was coated by 100 μm in thickness as the bond-coat on the Ni-base superalloy IN738LC substrate. And the 8YSZ was coated by 250 μm in thickness as the top-coat on the bond-coat. The LCF tests of TBCed IN738LC were carried out at 900℃ in air. The LCF lives of the TBCed IN738LC were compared with those of bare IN738LC and metal protective coated IN738LC specimen. The results of the LCF tests at 900℃ indicated that the LCF strength of the TBCed IN738LC is better than those of bare and metal protective coated IN738LC. The effect of thermal oxidation on the LCF life of the TBCed IN738LC was also investigated. The effect of thermal oxidation on the LCF life of the TBCed IN738LC was not significant. It was revealed from an observation of the damage morphologies that the cracking morphologies by LCF loadings were changed by the thermal oxidation.
The use of thermal barrier coatings (TBC) for turbine blades in gas turbine engines has allowed higher engine operating temperatures exceeding 1200℃ at the surface of the TBC top coat. More recently, it has been recognized that at these high temperatures the TBCs can be damaged by calcium-magnesium-alumino-silicates (CMAS) resulting from siliceous minerals (dust, sand, ash) containing the intake air and from unclean fuels such as a syngas and biomass gas. In the present study, the CMAS damage was simulated in laboratory employing a synthetic CMAS product on Air Plasma splayed (APS) and Electron Beam Physical Vapor Deposition (EB-PVD) thermal barrier coatings. The changes in microstructure and mechanical characteristics were characterized after isothermal exposure at 1200-1250℃ for various times. Microstructural observation showed the ingression of the CMAS into the TBC top coat, the dissolution of the TBC top coat into the CMAS and the phase transformation of the TBC top coat. It was found from micro-indentation testing of the TBC top coat that shorter exposure time increased the hardness, but longer exposure time decreased the toughness. Based on these results, the effect of the CMAS product on the delamination behavior of the TBC top coat was assessed.