One of new material issues of thermal barrier coatings (TBCs) on hot section components in gas turbine engines is the attack by molten calcium-magnesium-alumino-silicates (CMAS). In this work the influence of the top coat spraying condition on the CMAS damage kinetics in air plasma sprayed (APSed) TBC specimens was investigated by employing a synthetic CMAS product. Experimental works were carried out with three different top coats prepared from powders with different particle sizes and shapes. It was found that the depth of CMAS infiltration increased proportionally to the square root of exposure time in all specimens. The infiltration rate was analyzed based on a mercury porosimetry measurement, which suggested that the physical CMAS infiltration into the APSed top coat by the virtue of capillary action may play an intrinsic role in the CMAS damage. The reaction rate between the CMAS and the ceramic top coat was also influenced by the spraying condition.
This study presents creep characteristics of Mod. 9Cr-1Mo steel with various sized specimens and environment. Tensile creep tests were performed using three different sizes of specimen and three different types of testing environment. Creep specimens are a bulk specimen which has φ 6mm and 30mm in gage length, a miniature specimen which has φ 2mm and 10mm in gage length and a thin plate specimen which has 0.76mm in thickness, 1.5mm in width and 7.62mm in gage length. Three different types of testing environment are air, 99.99% Ar gas and vacuum. In the same environmental condition, there was no effect of specimen size on time to rupture. Time to rupture of Mod. 9Cr-1Mo steel in Ar gas was shorter than that in air and vacuum. Oxide thickness is not dominant factor in time to rupture. Fracture mode at specimen surface in Ar gas might be dominant factor in shorter time to rupture. Effect of specimen size and environment on creep strength of Mod. 9Cr-1Mo steel was evaluated on the basis of thinning effect.
Development of creep damage assessment methods of Mod.9Cr-1Mo welded joints which are widely used for pipes in USC power plants are an urgent subject. Miniature specimens with 1mm diameter and 5mm parallel length were machined from a base metal, weld metal and a heat affected zone (HAZ) in a Mod.9Cr-1Mo welded joint. Creep tests of these miniature specimens were performed in inert gas atmosphere by a specially developed creep testing machine. Creep deformation and rupture behaviors of the miniature base metal specimens were almost identical with those obtained from standard size specimens with 10mm diameter confirming that the miniature specimen is available to obtain the same creep strength property as obtained by standard size specimens. Creep strength of the weld metal is comparable with the base metal and that of the HAZ is lower than these metals. Material constants of the Norton’s law were determined from obtained datum by the miniature creep specimens. Creep tests were also conducted with the miniature and the standard size welded joint specimens. Creep rupture times of the miniature welded joint specimens were much lower than those of the standard size specimens. The finite element creep analysis results indicated that the triaxiality factor in the HAZ of the miniature welded joint was smaller than that of the standard size welded joint resulting in faster creep strain rate in the miniature specimen under the same applied stress. Determination procedure of creep deformation property of a HAZ in a welded joint and creep rupture times of the standard size welded joints by creep test results of the miniature welded joint specimens were proposed based on the creep test and the finite element analysis results.
In high Cr ferritic steels, premature failures at the welded joint have recently been a worldwide issue. This failure occurs at fine-grained heat-affected zone (FGHAZ) and it is known as “Type IV creep damage”. Therefore, it is very important to understand the creep properties of local regions of HAZ such as FGHAZ correctly to clarify the mechanism of Type IV creep damage and enhance the high-temperature strength of welded joints. In this study, a TEM disk-type specimen was removed from the coarse-grained HAZ (CGHAZ), FGHAZ, inter-critical HAZ (ICHAZ) and base metal (BM) of Gr.92 welded joint, and then they were subjected to the small punch (SP) creep test. The FGHAZ simulated specimen was also submitted to the SP creep test for comparing results of those with that of actual FGHAZ. In addition, the finite element analysis was conducted to convert the SP creep rupture data into the uniaxial ones. The experimental results revealed that the creep rupture strengths of FGHAZ and FGHAZ simulated specimen were inferior to those of BM, ICHAZ and CGHAZ. The SP creep rupture data of BM and FGHAZ specimen could be successfully converted into the uniaxial creep rupture ones using the load/stress conversion equation “F = 0.47σs”. At the low stress levels, the SP creep rupture data of FGHAZ was in good agreement with that of the welded joint obtained from the uniaxial creep test. This result indicated that the creep strength of FGHAZ itself was one of the important controlling factors of creep strength of welded joint.
Type IV creep damage occurs in welded joints of high chromium steel in power plant. The method based on the number density of voids is one of the methods to evaluate the residual life. The observed number density of void depends on the observation condition; the observation area and the magnification of observed photograph of metallographic structure. In previous study we had studied influence of them and had proposed how to determine the appropriate observation area for the temporary allowable error. In this paper we experimentally show that the appropriate area does not depend on heats or heat treatments. Next, we proposed the method to evaluate the start time of initiation of voids, the initiation rate of voids and the growth rate of voids. The relation of observed number density of voids and the magnification of observed photograph was experimentally taken. Based on it, the distribution of the radius of voids changing with creep time was evaluated. With it these parameters are evaluated. In the future influence of the stress triaxiality on these parameters will be studied.
We investigated the evolutions of two nonlinear acoustic characterizations: resonant frequency shift and three-wave mixing, with electromagnetic acoustic resonance (EMAR) throughout the creep life in the welded joints of ASME Grade 91, one of high Cr ferritic heat resisting steels. EMAR is a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). We used bulk- shear-wave EMAT, which transmits and receives shear wave propagating in thickness direction of a plate specimen. Creep tests of thick welded joints specimens were carried out at 873 K, and 90 MPa and interrupted at several time steps. The two nonlinear acoustic parameters and ultrasonic attenuation decreased from the start to 30% of creep life. After that they rapidly increased from 40% of creep life to rupture. We interpreted these phenomena in terms of dislocation recovery, recrystallization, and restructuring and the initiation and growth of creep void, with support from the SEM and TEM observation.
The applicability of Modified 9Cr-1Mo steel (ASME Grade 91 steel) as the main structural material in advanced loop-type sodium cooled fast reactor has been explored to enhance the safety, the credibility and the economic competitiveness of fast reactor plants. It is well-known that the steel exhibits cyclic softening behavior. Decrease of tensile and creep strength in softened materials has been already reported by other researchers. This paper discusses the relationship between cyclic softening conditions and high temperature material properties. Grade 91 steel has softened by repeat of plastic strain. The softening behavior can be evaluated by the softening rate. Decrease of tensile and creep strength in softened materials can be evaluated by the softening rate and it depends on the cyclic softening conditions. As a result, the cyclic loading conditions which produce little tensile and/or creep strength degradation have been identified.
Crystal orientations of creep damaged Type 316 stainless steel (SUS316L) were measured by 3 organizations using the same specimens, passed in a round robin, in order to investigate the scatter in material damage assessment using the electron backscatter diffraction (EBSD) technique. Two misorientation parameters, KAM (Kernel average misorientation) and GOS (Grain orientation spread), were calculated using mapping data of measured crystal orientations. It was shown that KAM and GOS correlated well with the degree of creep strain. However the scatter in GOS was smaller than in KAM. The GOS parameter showed an excellent correlation with both of the creep strain and creep damage ratio. It was concluded that the GOS parameter can be used for evaluation of the creep damage. The empirical relationship between GOS parameter and the degree of the creep damage can be shared regardless of the SEM/EBSD system used.
In advanced ultra-super critical (A-USC) power generation system, the steam condition becomes severe for conventional structural materials, i.e., high chromium heat resistant steels, because the steam temperature and pressure are 973 K and 35 MPa, respectively. It means that dissimilar welding between nickel base alloys and the heat resistant steels is inevitable. However, because the dissimilar welding engenders interface fracture at high-temperature creep conditions, understanding a relation between creep behavior and ductility of the weld metal as well as the base metals is important to ascertain safety of the plant. Alloy 82 weld metal decreased ductility with increasing creep lifetime because of Ni3Nb in the matrix and two kinds of GB precipitates: M23C6, with Cr, Ni, and Fe; MX with Ni, Nb, Cr and Mn. On the other hand, high B-9Cr steel showed high ductility even after long term creep test. Therefore, the difference of mechanical property leads to incompatibility of strain at the interface between steel and weld metal, resulting in interfacial crack in dissimilar welding condition.
Thermal barrier coatings (TBC) are widely used in gas turbine engines as protective coatings. TBC systems usually consist of a ceramic top coating (TC) and an intermediate metallic bond coating (BC) on a Ni-based superalloy substrate. Thermally grown oxide (TGO), however, forms at the interface between the TC and BC during high-temperature operation. A delamination can occur at the interface because of the TGO formation and growth. Previous studies had successfully improved delamination resistant properties drastically, due to a wedge-like TGO formation in a high-temperature environment, by adding Cerium (Ce) to a conventional CoNiCrAlY bond coat material. However, relationship between the microstructural variation of the wedge-like TGO and the delamination resistance with increasing high-temperature exposure time have not been clarified. In this work, Ce-content BC was formed by a cold spray technique. Following the high-temperature exposure process at 1100℃, wedge-like TGO generation was confirmed in the TBC with the Ce-content BC. The wedge-like TGO shows an increase in growth with an increase in the high-temperature exposure time, and it was confirmed to grow throughout the BC at 100 h. The delamination resistance of the high-temperature exposed TC specimens at 1100ºC were evaluated at various times by using static four-point bending tests. From the results of four-point bending tests, the Ce-content BC indicated high delamination resistance. The residual strain in the TGO was calculated from the Raman peak shift change of Al2O3. The residual strain of the Ce-content BC is relatively small compared to the TBC with convention bond coat. It is expected to be due to the stress dispersed by the generation of the wedge-like TGO. This effect contributes to the improvement of delamination resistance.
A thermal barrier coating (TBC) system consists of a TBC topcoat (TC), a bond coat (BC), and a substrate. Residual stress acting on a coating is a important parameter for materials mechanics and hence need to be evaluated. We propose a simple method for evaluating the residual stresses acting on each layer of a TBC system by using a curvature method. First, a three-layered model was constructed based on the misfit strain between the coatings and the substrate. This method uses the curvatures of the substrate with a blast treatment, the BC system specimen with the blast and BC, and the TBC system specimen with the blast, BC, and TC instead of a change in curvature during thermal spraying. Next, the residual stresses acting on the TBC system specimen with varying thicknesses were evaluated by using the proposed method. It was confirmed that the proposed method provides reasonable residual stresses in the BC and TC. Finally, the thermal stress after the coating deposition was theoretically evaluated, and the quenching stress of the coating during the deposition was evaluated by using the difference between the experimentally determined residual stress and the theoretical thermal stress. The mechanism for generating the residual stress in the TBC system during thermal spraying was investigated using the quenching stress and thermal stress.
Local wall thinning is one of serious problems in aged power generating and chemical plants. As the thinning grows inside the pipes, it is difficult to detect and evaluate it from the outer surface of pipe. In this study, an evaluation method of semi-ellipsoidal local wall thinning on the inner surface of pipe by the direct-current potential difference method (DC-PDM) was proposed, and the related numerical analyses were carried out. At first, very fast electric field analysis method, i.e., defect-current modification method (DCMM), was extended to the local wall thinning with semi-ellipsoid shape on the inner surface of pipe, and the solution was validated by comparing with that obtained by the finite element method (FEM). Then, the geometry of the local wall thinning was evaluated by the proposed method based on the distribution of potential difference on the outer surface of pipe obtained by FEM. The evaluated results agreed well with the actual geometry. It was found that the DC-PDM is applicable to the evaluation of local wall thinning.
This study describes evaluation of multiaxial low cycle fatigue life of a lead-free solder, Sn-8Zn-3Bi. Strain controlled multiaxial low cycle fatigue tests under proportional and non-proportional loading conditions were carried out using a hollow cylinder specimen at room temperature. The loading conditions employed were a push-pull, a reversed torsion, a box-shape, a step-shape and a circle-shape strain paths. The former two loading conditions are the proportional loading and the latter three loading conditions are the non-proportional loading in which directions of principal stresses and strains are changed in a cycle. The effect of non-proportional loading on the fatigue life of Sn-8Zn-3Bi was investigated. The reduction in low cycle fatigue life due to non-proportional loading was identified, and the life was the shortest for the circle-shape strain path when fatigue lives were correlated by a von Mises equivalent strain range. Fatigue lives of four strain paths other than the reversed torsion strain paths were correlated within a factor of two band by an equivalent inelastic strain range. All of the fatigue lives were evaluated well by a non-proportional strain range considering the effect of additional strain hardening due to non-proportional loading, which was proved to be effective for life evaluation of the solder under non-proportional loading.
This study discusses the effect of additive elements on crack initiation and propagation behavior for Sn-Bi solders at high temperatures. Sn-Bi solders are lower melting point temperature materials, so that it is useful for low temperature soldering. Cyclic push-pull fatigue test with center through holed specimen were conducted at 313K and 353K in order to investigate the crack initiation and propagation behavior from stress concentration part of three kinds of Sn-Bi solders. Crack initiated at the early stage and almost of the life period were crack propagation process for the three kinds of Sn-Bi solders. Crack propagation direction at 353K was the maximum shear direction although the maximum principal direction at 313K. The reason for the crack propagation direction difference might be change of slip system at high temperature. There were additive elements Ag, Cu, Ni and Ge effect on the crack propagation rate although there was no effect on the crack initiation cycle. The crack propagation of Ag, Cu, Ni and Ge added solder seems to have a slower rate than that of Sn-58Bi solder. We also investigated the adaptation of J-integral range parameter which is usual used for crack propagation rate evaluation for conventional steel. The J-integral range parameter evaluates the crack propagation rate independent of the additive elements at high temperatures.
The microstructures of the KA-SUS304J1HTB (ASME code: 2328) were investigated in order to clarify the reason for the onset of accelerating creep. Creep tests were conducted at 873 K for a stress range from 240 MPa to 300 MPa. Creep interrupted tests were also conducted at 873 K for a stress of 240 MPa. Creep deformation of the KA-SUS304J1HTB was divided into the following three regions: transient creep, minimum creep and accelerating creep. The creep life of the KA-SUS304J1HTB consisted mostly of accelerating creep, regardless of the stress conditions. The creep cracks initiated along the grain boundary due to accelerating creep and the number and length of the creep cracks increased in accordance with the creep time. In addition, the results of microstructure observations conducted using the electron channeling contrast image (ECCI) method indicated that dynamic recovery and sub-grain formation around the grain boundaries occurred during the accelerating creep region. On the other hand, dynamic recovery in the grain did not occur under this condition. Increments in the grain orientation spread (GOS) produced by creep were almost the same as those produced by high temperature tensile deformation, although the deformation mode was different. The values calculated using the GOS increased with the strain increasing linearly. However, the large misorientation area calculated by kernel average misorientation (KAM) was only localized near the grain boundaries under the creep condition.