This study focuses on a thermo-mechanical fatigue (TMF) life evaluation method for Ni-23Cr-7W alloy for boiler pipes and tubes. The inelastic strain range applied in the TMF test was partitioned into the creep strain and the plastic strain components by using the steady-state creep rate obtained from the creep tests and the cyclic stress-strain curve obtained from the fatigue tests. The TMF life was then predicted by the strain range partitioning method (SRP). The predicted lives based on the SRP method with the creep strain obtained by using the cyclic stress-strain curve at maximum temperature of the TMF loading showed good agreement with the actual lives within a factor of 2 scatter bands. This life prediction method could be applied to other heat of the Ni-23Cr-7W alloy and other thermal cycle of TMF loading. All material data required for the life prediction can be obtained by the isothermal fatigue tests. Therefore, the TMF life of the Ni-23Cr-7W alloy can be predicted from the results of the isothermal fatigue test by using this method.
In creep damage assessment of a welded joint by a finite element analysis, it is important to characterize creep strength property of constituent materials (base metal, heat affected zone(HAZ), weld metal) in the welded joint. In order to clarify the creep strength property of a long-term used Mod.9Cr-1Mo steel welded joint, miniature specimens with 1mm diameter and 5mm parallel length were machined from the constituent materials, and creep tests of these miniature specimens were performed in inert gas atmosphere. Creep strength of the base metal and the HAZ were similar to those of the new welded joint previously reported, while the creep strength of the weld metal remarkably decreased compared to that of the new weld metal. In addition to these tests, creep tests were conducted using miniature and standard size welded joint specimens. Creep rupture time of the miniature specimens was much shorter than that of the standard size specimens. From the results of finite element creep analyses, it was indicated that creep strain at the HAZ of the miniature specimen accumulated faster than that of the standard size specimen due to lower triaxiality factor. It was demonstrated that rupture time of a standard size welded joint specimen is predicted from a miniature welded joint specimen by applying the previously proposed method for new welded joints.
Creep damage simulation combining a void nucleation model and damage mechanics was applied to large-size uniaxial cross weld specimen and circumferentially welded pipe of mod.9Cr-1Mo steel subject to internal pressure and bending at 650°C．The void nucleation model depending on both creep strain rate and triaxiality factor considers the increase of number density of creep void with a size of FGHAZ (fine-grained heat affected zone) grain of the welded joint. Damage mechanics model considers both the strain softening of the FGHAZ material and damage due to the increase of creep voids. The critical value of the damage parameter was defined by the number density of creep void corresponding to the formation of micro cracks. In the case of large-size uniaxial cross weld specimens with X- and U-shaped weld configuration, damage progress in the FGHAZ was successfully expressed by considering the hardness distribution in the FGHAZ in the form of distribution of creep strain rate. In the case of circumferentially welded pipe, the damage simulation using the same material data as the large-size uniaxial cross weld specimen gave the damage initiation at the middle of thickness and the following damage progress towards inside surface of the pipe. This tendency well corresponded to the experimental results, and the damage area in the circumferential direction at the same rupture life consumption was almost the same in both simulation and experiment.
In order to investigate the applicability of hydrogen as a tracer for detecting Type IV creep damage, the hydrogen thermal desorption analysis (TDA) was applied to the small plate-type specimen (10×6×0.5 mm) containing weld metal (WM), heat affected zone (HAZ) and base metal (BM). The specimens were taken from the large cross weld specimens of ASME Gr.122 steel with various degrees of creep damage. Hydrogen charging was carried out by means of cathodic electrolysis, and the hydrogen-charged specimen was subjected to the TDA for measuring the hydrogen evolution curve. Experimental results revealed that the hydrogen desorption characteristic was changed depending on not only the degree of creep damage but also the distance from outer surface. The amount of desorbed hydrogen, CH, increased with consuming creep life from the early stage of creep damage, and this increase was more pronounced at the distances of around 5.5 and 20 mm from the surface. This result indicated that the creep damage was preferentially initiated and accumulated at those regions. From the individual analyses of WM, HAZ and BM specimens, it was also found that the increase in CH was caused principally by the creep damage accumulated in HAZ, especially, FGHAZ and ICHAZ. Additionally, the change in hydrogen thermal desorption characteristic was well correlated with the area fraction of creep voids and the small punch creep strength.
The change in electrochemical property of 12%Cr ferritic steam turbine steels with creep was investigated to examine an applicability of electrochemical material characterization method as a procedure for creep damage assessment. The anodic polarization curve measurement was applied to several creep test specimens with various degrees of damage. Experimental results revealed that a single current peak appeared at around +150-200 mV during the measurement. The electrical charge of the peak (F) could be expressed as a function of the volume fraction (V), the average size (r) and the Cr content (a) of M23C6 carbide, i.e., F = 70890×r×V×a. This result suggested that the peak current density, Ip, mainly reflected the size of M23C6 carbide because two other variables showed no significant change. The Ip increased with accumulating the creep damage, and this increment was more pronounced in the long-term creep region. There was a good correlation between the Ip and the Vickers hardness. It was also found that the applied stress and the creep life fraction of creep damaged samples could be predicted based on the measured Ip. Additionally, the electrochemical property of actual turbine components (high middle pressure rotor and casing) was measured using the portable electrochemical cell for on-site measurement. The results obtained in this study indicated that this electrochemical material characterization method was a potential procedure for creep damage assessment for turbine components.
Electromagnetic acoustic resonance (EMAR) is a contactless resonant method with an electromagnetic acoustic transducer (EMAT). This method enables not only to measure exact ultrasonic attenuation of measured sample but also to eliminate nonlinear acoustic effect between the sample and transducer. In this study, the EMAR was applied to investigate the relationships between nonlinear acoustic characterizations; resonant frequency shift, three-wave mixing and birefringence acoustoelasticity and microstructural changes induced by tensile plastic strain in a low-carbon steel, JIS-S25C. Furthermore, we developed a single bulk-shear-wave EMAT which was composed of three-layer elongated coils and a pair of permanent magnets to measure in three-wave mixing. The EMAT transmits and receives shear wave propagating in thickness direction of a plate specimen. Three nonlinear acoustic parameters and ultrasonic attenuation increased with increase in tensile plastic strain. This phenomenon is interpreted as resulting from microstructure changes, especially, dislocation density and crystal misorientation. This is supported by X-ray observations for dislocation density and EBSD (electron backscattering diffraction) for the misorientation.
Recent introduction of renewable energy systems has impelled the land based gas turbine power generation systems to play two different types of roles: one is a role as base load system, the other is a role as power adjusting system. To cope with the latter role, it is necessary to take into account of thermal fatigue failure of the gas turbine components, where transient thermal stress is often a critical issue to be concerned. In this study, non-stationary induced thermal fatigue failure behavior of a superalloy was explored. At first, a new test equipment was developed so that high cycle non-stationary thermal fatigue loading can be superimposed on stationary low cycle thermo-mechanical fatigue (TMF) loading. By means of the testing equipment, the early growth of small cracks around the cooling holes was investigated under the superimposed condition in a directionally solidified Ni-base superalloy, where some cooling holes were artificially introduced for a simulation. The experimental works clearly demonstrated that the crack growth rate under the superimposed condition was significantly accelerated, compared with that under the stationary low cycle TMF loading. This behavior was discussed based on fracture mechanistic view point.
Header stub welds for thermal power plants undergo creep-fatigue damage due to thermal expansion and contraction, which are caused by cyclic startup and shutdown of the plant. In this study, creep-fatigue tests were conducted using header stub mock-up specimens of 47Ni-23Cr-23Fe-7W alloy to investigate the creep-fatigue damage process. It was discovered that the cracks were initiated from the outer surface of the tube near the bond line, and they propagated toward the inner side in both fatigue and creep-fatigue tests. Transgranular cracks were observed in fatigue tests, whereas cracks were found to be progressed along the grain boundary in creep-fatigue tests. The failure mechanism of 47Ni-23Cr-23Fe-7W alloy header stub mock-up specimens was composed of two steps. At first, cracks progressed on the surface near the bond line, and then cracks progressed toward the inner surface. Crack initiation and propagation behaviors of the 47Ni-23Cr-23Fe-7W alloy header stub mock-up specimen were the same as those of the 2.25Cr-1Mo steel header mock-up specimen.
Internal pressure creep tests were conducted on boiler tube of KA-SUS304J1HTB under stress of 60 to 80 MPa at 750°C, and the creep test results were compared with those of uniaxial tensile creep test. Increase in outer diameter of tubular specimens was recognized at about 0.4 of creep life ratio, independent of stress condition, and acceleration creep stage was clearly observed. Creep rupture elongation of 5 to 8% of internal creep test was smaller than those of uniaxial tensile creep test in the range of 17 to 43%. Many cracks were observed on the cross section near the fracture part. Number density of cracks was highest in the area close to outer surface, and it decreased to about half in the area close to inner surface of the tube. Voids and cracks were mainly observed at the interface between sigma phase and γ-matrix. Large misorientation of austenite matrix phase was observed in the vicinity of sigma-phase, and many dislocations were also observed in the same area. It has been concluded that formation of void and crack was enhanced by non-uniform local deformation at the interface between sigma phase and γ-matrix due to difference in deformability of them.
Type IV creep damage initiates in Fine Grain Heat Affected Zone (FGHAZ) in welded joints that has been subjected to the stress at high temperature for years. The shape of grain boundaries in FGHAZ is complex and complicated before it was subjected to creep. As creep goes on, the shape of grain boundaries becomes simpler and the average grain size becomes larger because of the recovery and the recrystallization. However average grain size is one of indications of creep damage, the complexity of grain boundary shape has not been researched as the indication of creep damage. The grain boundary is the fractal and the complexity of the shape of grain boundaries is represented by the fractal dimension. In this study, we carried on some creep tests with the simulated FGHAZ of Mod. 9Cr-1Mo steel and measured the fractal dimension of grain boundaries of them. The shapes of the test piece were the notched bar, the smooth bar and the smooth square bar. An optical microscope and SEM were used to observe the metallographic structure. Box-counting method was used to measure the fractal dimension.In the smooth test pieces, the average grain size became larger and the fractal dimension became smaller with creep. In the notched test piece, the average grain size became larger but the fractal dimension did not change with creep. The fractal dimension can be an indication of creep damage under uniaxial stress. As the indication of type IV creep that initiates under multi-axial stress, the fractal dimension of grain boundaries needs to be more researched.
Recently, layered transitional metal dichalcogenide (TMD) materials which are bound by vdW forces between the layers have received much attention owing to their excellent properties. In this study, we focused on layered molybdenum disulfide (MoS2) thin film fabrication by mist CVD. Thin films were fabricated by using a precursor solution containing a mixture of hexaammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24・4H2O) and thiourea (CH4N2S) dissolved in methanol. As a results, raman peaks around 380 cm-1 (E12g) and 410 cm-1 (A1g) were observed from the samples prepared at 300 - 400 °C. Remarkably, the growth temperature of MoS2 thin-films by mist CVD is comparatively lower temperature than that of the traditional CVD. Furthermore, it was found that high quality MoS2 on entire substrate was fabricated under the condition of sufficient thermal decomposition time.
This study aims to stabilize the resistance welding of carbon fiber reinforced thermoplastic (CFRTP) using spread or woven carbon fiber heating element. The joined CFRTP parts was sateen weave CF/PPS laminates, and the material used for the resistance heating element was spread carbon fiber and plain weave carbon fiber. The effects of processing conditions such as aspect ratio of fusion joining part, applied current, conduction time, and also material conditions such as resistance heating element were investigated experimentally. The contents of evaluation were temperature distribution of resistance heating element by using a thermography, welding area ratio measured by image analysis and monitoring of electricity. From the experimental results, in the case of width size change, it was suitable for resistance welding of large area. On the other hand, in the case of length size change, it was not suitable for resistance welding because the edge effect occurred around the electrode. Subsequently, in the case of using the spread carbon fiber as resistance heating element, it was found that the thermal distribution was unevenness because of uneven distribution of carbon fibers.