We studied the relationship between microstructural change and the evolutions of two nonlinear acoustic characterizations with electromagnetic acoustic resonance (EMAR) during creep in Cr-Mo-V steels and the correlation between two nonlinear acoustic characterizations. One was higher harmonic generation and other resonant frequency shift. EMAR was a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). We used axial-shear-wave EMAT, which transmits and receives SH wave propagating in the circumferential direction of a cylindrical specimen. The EMAT had a capability for easily detecting two nonlinear acoustic characterizations for cylindrical one. To measure higher harmonics, excitation of the EMAT at half or one third of the resonance frequency caused the standing wave to contain only the second or third-harmonic component, which was received by the same EMAT to determine the second or third-harmonic amplitudes. In measurement of resonant frequency shift, as the excitation level increases, resonance frequency was shifted and the shift manifested the elastic nonlinearity. Measured two nonlinear acoustic characterizations showed the peaks at 30% and the minimum at 50% of the total life, respectively. We interpreted these phenomena in terms of dislocation mobility and restructuring, with support from the SEM and TEM observation. This noncontact resonance-EMAT measurement can monitor the evolution of the surface-shear-wave nonlinearity throughout the creep life and has a potential to assess the damage evolution and to predict the creep life of metals.
Most boiler component structures are mainly fabricated by welding which are likely to be regions of crack initiation and propagation. In such weldment, it is known that the multi-axial stress fields appear due to the plastic constraint induced by the differences in material micro-structure between the weld metal, heat affected zone and base metal. Additionally, the technique of predicting the life of creep crack initiation and growth have not been clearly developed for weldments of high Cr steels due to many factors such as the variation in micro-structure and the residual stresses caused by welding and thermal cycles. The present paper indicates that in order to predict the total fracture life for weldment of high Cr steels, it is important to clarify the incubation process and path on a structural mechanical basis in order correctly determine the incubation time of creep crack growth. It may be useful to look at the role of Q* as correlating parameters to identify how best to rationalize the correlating parameter that can be used to predict both creep crack initiation and growth in weldments.
Creep strength of high Cr ferritic steel welds decreases than base metals due to Type-IV creep damages formed in the heat affected zone (HAZ) during long-term use at high temperatures. In the present study, creep tests of ASME Gr.91 and Gr.122 steel welded joints were interrupted at several time steps at 600°C. Microstructural changes and damage behaviors in the fine-grained HAZ of welds during creep were investigated by using SEM, TEM, and EBSD. KAM, grain boundary length and hardness of fine-grained HAZ decreased till 0.2 of life and saturated in the Gr.91 steel weld, whereas they decreased after 0.5 of life in the Gr.122 steel weld. It was found that the recovery of dislocation structures in the fine-grained HAZ was completed at early stage of life for the Gr.91 steel weld, whereas it occurred in the later stage of life for the Gr.122 steel weld. These differences of microstructural changes was considered to relate to the differences of Type-IV creep damage behavior; early initiation of creep voids at 0.2 of life in the Gr.91 steel weld and later damage evolution after 0.5 of life in the Gr.122 steel weld.
The micro-macro combined creep damage simulation based on the grain-boundary-fracture-resistance model was applied to the evaluation of small defects distribution in fine-grained heat-affected zones of longitudinal welds in the actual size elbow of modified 9Cr-1Mo (9Cr-1MoVNb) steel subject to internal pressure at 923K. On the basis of simulation results, a prediction scheme for final rupture life of welds was discussed using the damage mechanics concept and effective stress. The applicability of nonlinear fracture mechanics was also discussed assuming the crack length from the microscopic simulation results. Obtained results are summarized as follows : As results of the simulation showed, peaks of the number density of small defects in the subsurface could be predicted, showing good correspondence with the observed results. Final failure life prediction based on the damage mechanics concept was found to be applicable, by assuming both the final failure surface connecting the weakest grain boundaries, and effective stress against this surface. Fracture mechanics approach was also found to be applicable when assuming the crack length from the high peaks of the simulated small defect in the last stage of creep life.
The effect of sampling location on SPC(Small Punch Creep) tests were investigated for weld joints to establish evaluation method of Type IV creep behavior. The SPC specimen shape was 10mm diameter and 0.5mm thick round disc prepared from weld joints of 2.25Cr-1Mo low alloy steel. It was found that the center of SPC specimen should be 2mm apart from the weld interface as the recommended sampling location. Creep damage was imposed for large weld joint specimens by axial creep loading at 620°C, 52MPa with the interrupted time fraction of 0.34, 0.45, 0.64 and 0.82.SPC samples were prepared from those damaged specimens following the recommended way described in this paper. Among the various SPC tests conducted, good relationships were found for the test condition of 625°C, 200N. Namely, good relationships were obtained both between minimum deflection rate and creep life fraction, and between rupture time and creep life fraction. Consequently, creep life assessment of Type IV fracture by SPC tests could be well conducted using the sampling location and the test condition recommended in this paper.
Low-carbon/medium-nitrogen 316 stainless steel (316FR) is a principal candidate for the high-temperature structural materials of a demonstration fast reactor plant. Thermo-mechanical fatigue damage is one of critical issues to be known for the design and reliability of the high-temperature materials subjected to thermal cycles. Early growth of small cracks in order of micron-meters in size can provide some essential information for life and the remaining life prediction to these failures. Thus, many efforts have been made; however, there are a lot of matters to be understood. This paper is dealing with how cracks propagate under thermo-mechanical fatigue conditions. Special attentions are paid to the roles of strain rate and thermal cycles on small crack propagation behaviors in 316FR stainless steel. The experimental results revealed a difference in crack growth rates between the small and long cracks under the creep-fatigue condition : the small cracks exhibited growth rates remarkably higher than long cracks at a given fatigue J integral range. The results also indicated that the small crack propagation rate under the isothermal low cycle fatigue increased with the decreasing of the strain rate due to creep effect. It was also shown from the results that the role of irreversible creep strain, as well as that of irreversible plastic strain, was essential in small crack propagation process under the in-phase type thermo-mechanical loading.
In order to clarify the relationship between stepwise S-N curve and fracture mode for austenitic stainless steel under high temperature and high cycle fatigue conditions, rotating bending fatigue tests were carried out on stainless steel SUS321-B at 700°C. Under high stress amplitudes and short fatigue life region in the stepwise S-N curve, surface fracture occurred where the crack initiation site was located at the specimen surface, but internal fracture occurred under low stress amplitudes and long fatigue life region. Two types of fracture modes were recognized in the internal fracture depending on the position of the crack initiation sites, that is, being located at the subsurface of the specimen and the interior of the specimen where fish-eye pattern was observed. At the crack initiation sites, multiple inclusions composed of oxides and carbonitride were recognized by EPMA analysis. As the results of the observation of change in deflection at the loading point during fatigue test, it was recognized that the specimen hardened remarkably at the early stage of the fatigue test but a tendency of softening appeared after large number of stress cycles such as 107 cycles. It was considered that characteristic of the stepwise S-N curve at 700°C was brought by changes in deformation behavior and fracture mode. As for the result that surface fracture did not occur under long life region, an effect of oxidized surface layer on preventing for a fatigue crack from initiating at the specimen surface was pointed out, but any evidence to support this opinion was not obtained.
Mod.9Cr-1Mo steel has been widely used for elevated temperature components e.g. power boiler, chemical plant because of its superior strength properties at elevated temperatures. However some failure accident has recently happened in the weldments of power boiler steam pipes and it's due to the creep damage so-called Type IV damage. Many research have being carried out to prevent Type IV failures. On the other hand, Mod.9Cr-1Mo steel is also a candidate material for demonstration fast breeder reactor in Japan. A flow induced vibration is assumed in the piping due to the coolant liquid sodium flowing with high velocity inside the pipe. Therefore, the high cycle fatigue properties of this material at elevated temperatures have to be clarified to evaluate the integrity of the piping against a flow induced vibration. In this research, a series of high cycle fatigue tests were performed at room temperature, 400°C and 550°C. As a result, the fatigue limit appeared at room temperature and 400°C, while it didn't appear within 108 cycles at 550°C. This may be due to the suppression of crack initiation by the oxide film formed on the specimen surface at 550°C. Life prediction was also discussed beyond 108cycles.
This study discusses multiaxial low cycle fatigue strength of Mod.9Cr-1Mo steel under proportional and non-proportional loadings at room and high (823K) temperatures. Strain-controlled multiaxial low cycle fatigue tests were perfoemed using a hollow cylinder specimen including interruption tests. Strain paths employed were a push-pull straining, a reversed torsion straining and a circle straining. The former two are the proportional loadings where principal directions of stress and strain are unchanged. The latter one is the non-proportional loading where axial and shear strains have 90 degree phase difference and principal directions of stress and strain are changed continuously in a cycle. Behaviors of cyclic deformations and failure lives and an evaluation of the failure life are discussed. Surface cracks and microstructures in fatigued specimens are also observed by a digital microscope and a transmission electron microscope (TEM) in order to investigate mechanisms of cyclic deformation, failure and reduction in failure life due to non-proportional loading.
A simple and clear method of evaluating stress and strain ranges under non-proportional multiaxial loading where principal directions of stress and strain are changed during a cycle is needed for assessing multiaxial fatigue. This paper proposes a simple method of determining the principal stress and strain ranges and the severity of non-proportional loading with defining the rotation angles of the maximum principal stress and strain in a three dimensional stress and strain space. Based on the method proposed, non-proportional stress and strain ranges are derived and applicability of the range to the life evaluation of type 304 stainless steel under 15 proportional and non-proportional strain paths are discussed. The strain range taking account of intensity of loading path reducing life can be suitable parameter for multiaxial fatigue life evaluation under non-proportional loading.
A thermal barrier coating (TBC) is employed in gas turbine hot parts. The TBC system is complicated materials system because it consists of a substrate, a blast layer, a bond-coat and a TBC top-coat. In order to understand the influence of the TBC top-coat on the high temperature low-cycle fatigue of the substrate, the fatigue tests at 405°C and 900°C were carried out on the four specimens, a bare substrate, a substrate with blasting, a specimen with blasting and bond-coating, and a TBC specimen with full process. It is understood from the test result that each three treatment decreases the life of the substrate and the life of the TBC specimen is the shortest. It was also confirmed that the coatings share the load for the substrate, which was positive factor for the substrate. However, under the low-cycle fatigue condition, the blasting decreases the life of a bare specimen because of roughness effect. The coatings also decrease the life of the substrate with blasting because of coating cracks. It was found that the coating damage was dominant for the life of the substrate under the low-cycle fatigue.
In this study, the influence of bond coating process for a thermal barrier coating (TBC) on thermally grown oxide (TGO) growth was investigated, which used in advanced gas turbine system. The bond coats (BCs) were sprayed by different spray techniques, namely the low pressure plasma spraying (LPPS) and the cold spraying (CS). Yttria Stabilized Zirconia (YSZ) top coat was subsequently fabricated on the BC layer by air plasma spraying (APS). The coatings were oxidized isothermally at 1000°C for different time periods up to 1000h. The TGO growth was measured considering oxidation rate, k, and impedance measurement. The results showed that the TGO growth of the TBC with CS-BC is less than that of the TBC with LPPS. In the case of the TBC with CS-BC, formation of mixed oxides is less than the LPPS one, which has harmful influence on delamination of the top coating. Resultantly, the CS-BC has much higher resistivity than the LPPS. We observed that the bond coating process affected the generation and growth behavior of the TGO, especially high temperature oxidation rate. The present study concluded that the CS-BC system is able to generate better quality TGO.
In this study, the effects of heat treatment were clarified, and then the optimum heat treatment condition was investigated for the cold sprayed Ni base superalloy IN738LC coatings. In order to characterize the microstructure of heat treated cold spray coatings, SEM and TEM observation, EBSD analyses were carried out. And also, in order to evaluate mechanical properties, small punch (SP) tests and four-point bending tests were conducted. And as for an as-sprayed IN738LC coating, heat treatment (1121°C/2h + 843°C/24h) was applied. Prior to heat treatment, the as-sprayed coating had nano-order crystalline in the vicinity of the interface between coating and substrate. However, after heat treatment, the nano-order grains changed for micron-order grains. This grain growth can induce the improvement of the material properties such as ductibility and adhesion strength etc., and release of residual stress. And then, optimum heat treatment condition was investigated. At first, effects of heat treatment were considered. It was thought that the changes of configuration of γ' precipitation phases and crystal grain were important for characteristics of heat treated coatings. As a result of heat treatments, it is clear that the crystal grain of as-sprayed coating was mainly changed by solution heat treatment, and γ' phase precipitated in solution heat treatment and grew in aging treatment. Therefore, the discussion about optimal heat treatment condition was focused on the conditions of solution heat treatment. The coatings were heat-treated at higher temperature or for longer time compared with standard heat treatment of virgin IN738LC. It was clear that solution treatment at higher temperature was especially effective for miniaturization and homogenization of γ' phases, and growth of crystal grains. In the coating strength tests and adhesion strength tests, the specimen with higher temperature heat-treatment demonstrated better results than that of the specimen with standard heat treatment.
A new testing system for high temperature structural materials and components used for gas turbine has been developed. The system consists of a “can” type of combustion system and a servo-electro hydraulic material testing system, and it can apply thermo-mechanical loadings to a test sample in a simulated gas turbine combustion environment. By means of this new apparatus, failure behavior of a thermal barrier coating (TBC) specimen was studied under a condition of cyclic burner heating on which a constant tensile load was superimposed. The present testing system is expected to provide useful understandings on the damage evolution of the TBCs under specific conditions.
This paper presents tensile properties of a new developed polyimide thin film used in electronic devices. Tensile tests were performed to determine Young's modulus, proportional limit, yield stress, ultimate tensile strength and elongation of the polyimide film at three strain rates and three temperatures. Effects of strain rate and temperature on the tensile properties were discussed. There was a little anisotropy of tensile properties caused by injection direction in the polyimide film. Young's modulus, proportional limit, yield stress and ultimate tensile strength increased with increasing the strain rate. Only elongation decreased with the strain rate. Young's modulus, proportional limit, yield stress and ultimate tensile strength decreased with increasing temperature but elongation increased. Applicability of a viscoelastic model for describing the stress-strain curves of the polyimide film was proposed.