In this study, the creep strength of welded joints of Grade 91 Type 1 and Type 2 steels was evaluated. It was determined that impurity elements in the Type 1 steel reduced its creep strength. This reduction was attributed to an increase in the amount of residual carbides in the fine-grain heat-affected zone during welding.

This study evaluates various nondestructive testing methods for detecting creep damage and assessing residual life in Grade 91 steel welds. Three primary detection techniques were investigated: phased array ultrasonic testing (PAUT), eddy current testing with high-temperature superconductor direct current and superconducting quantum interference device (ECT•HTS-dc-SQUID), and replica observation. PAUT detected creep damage between 60-80% of creep life, while ECT•HTS-dc-SQUID showed detection capability between 80-90% of creep life. Replica observation revealed creep voids only in the final stages before rupture. Additionally, three strain measurement methods were evaluated: capacitive strain sensors (providing continuous monitoring during creep exposure), laser displacement meters (used during test interruptions), and SPICA strain measurement. Both capacitive sensors and laser meters produced results comparable to conventional extensometer measurements. The SPICA method proved particularly effective in measuring heat-affected zone (HAZ) strain after creep exposure, revealing higher strain values in the HAZ compared to base and weld metal, with a consistent increase during creep exposure.

The effects of chemical composition and heat treatment on the creep properties of ASME Grade 91 type steel were experimentally investigated using materials whose chemical compositions and heat treatment conditions in the steel making process were completely controlled. Regarding chemical composition, only the Al, Cr, and Ni contents were systematically varied while keeping the contents of the other elements and heat treatment conditions constant. Regarding heat treatment, the normalizing and tempering temperatures were varied while keeping the contents of chemical components constant. The creep tests of materials were performed for approximately up to 50,000 h at 650°C. The creep strength of Grade 91 type steel decreased with increasing Al content under the test conditions of short-term to long-term range. On the other hand, the effect of Cr content on the creep life of Grade 91 type steel depended on the stress or time range, and the creep strength of the steel decreased at high Cr contents under test conditions of only the longterm range. No effect of Ni content on the creep life of the materials was observed in the test data obtained in this study. As creep tests are currently being conducted at 625°C and 60 MPa, which are conditions closer to the actual service conditions of main steam piping at ultra-super critical power plants, the creep deformation data at present indicate that the above trends hold true in the long-term range. Regarding the effect of heat treatment, the creep life of the materials tended to increase with increasing normalizing temperature or decreasing tempering temperature. The results obtained in this work indicate that within the scope of the material standards for Grade 91 type steel, the effect of chemical composition on creep life is greater than that of heat treatment.

The creep strength of the base metals and welded joints of ASME Grade 91 type steel under actual service conditions was investigated using long-term used materials in this study. Creep tests were conducted on the materials used for hot reheat or main steam piping at power plants. The remaining creep life of each material under actual service conditions was evaluated using the Larson-Miller parameter for the test result. Then, the creep life of each material under the service condition was estimated as a summation of the service time at the plants and the remaining creep life. The estimation results were useful for examining the validity of the life evaluation formula in the long-term region because it is extremely difficult to obtain creep rupture data under such conditions owing to the long test duration. The estimated creep lives were compared with creep life evaluation curves, which were regulated for Grade 91 type steel in Japan. Regarding the base metals, the estimation results suggest that Grade 91 pipe-type steel tends to exhibit a shorter life than the 99% confidence lower limit of the evaluation curve of the material. This finding indicated that the life evaluation formula for the Grade 91 type steel base metals should be reviewed. On the other hand, the estimation results suggest that the welded joints of Grade 91 type steel generally exhibit a longer life than the 99% confidence lower limit of the evaluation curve of the material, indicating that there is no need to review the life evaluation formula for the Grade 91 type steel welded joints.

In order to establish a creep damage assessment method for 47Ni-23Cr-23Fe-7W (HR6W), which is a candidate material of A-USC, microstructure observation of creep interrupted specimens and ruptured specimen was conducted, and the creep damage process was examined. Creep tests were conducted under conditions of 800°C, 70 MPa, 700°C, and 100 MPa. For creep damage assessment, an optical microscope was used for replicas sampled from the outer surface of specimens, and crack ratio at grain boundaries was assessed. The results indicated that creep voids and cracks were initiated at grain boundaries from about 0.35 of creep life ratio, and crack ratio increased drastically after creep life ratio of 0.65. This crack ratio was almost the same regardless of the specimen shape Therefore, the method to assess crack ratio using replicas is considered to be an effective method for creep damage assessment of HR6W. An increase in the crack ratio due to an increase in creep life ratio showed the same trend as the change in elongation of creep interrupted specimens. Microstructure observations were conducted with interrupted specimens using SEM-ECCI (Electron Channeling Contrast Imaging) in order to clarify the cause of acceleration creep. The results showed that sub-boundary developed significantly near grain boundaries, which indicates that sub-boundary development may cause acceleration.

In order to establish a induction bending technique for Ni-based alloy HR6W large pipe, induction bending test was conducted on HR6W, which is a piping candidate material of 700°C class Advanced Ultra-Super Critical. In this study, a tensile bending test in which tensile strain was applied and a compression bending test in which compression strain was applied to the extrados side of the pipe bending part. As the results of these two types of induction bending tests, it was confirmed that a predetermined design shape could be satisfied in both bending tests. In addition, the wall thickness of the pipe was equal to or greater than that of the straight pipe section in compression bending. Therefore, if compression bending is used, it is considered unnecessary to consider the thinning amount of the bent portion in the design. Next, penetrant test(PT) on the outer surface of the bending pipes were also confirmed to be acceptable. Subsequently, metallographic samples were taken from the outer surface of the extrados side, neutral side and intrados side of the pipe bending portion. Metallographic observation confirmed that the microstructures were normal at all the three selected positions. After induction bending, the pipe was subjected to solution treatment. Thereafter, tensile tests and creep rupture tests were carried out on samples that were cut from the extrados side, neutral side and intrados side of the pipe bending portion. Tensile strength satisfied the minimum tensile strength indicated in the regulatory study for advanced thermal power plants report of Japan. Each creep rupture strength was the almost same regardless of the solution treatment conditions. From the above, it was possible to establish a induction bending technique for HR 6W large piping.

23Cr-45Ni-7W alloy (HR6W) is a material being considered for use in the high temperature parts of A-USC boilers in Japan. In order to establish an assessment method of creep damage for welded components made using HR6W, two types of internal pressure creep tests were conducted. One is for straight tubes including the circumferential weld and the other is for welded branch connections. The test results for the circumferential welds ensured that the creep rupture location within the area of the base metal, as well as the time of rupture, can be assessed by mean diameter hoop stress. On the other hand, the creep rupture area was observed in the weld metal of the branch connections, although the creep strength of Inconel filler metal 617 was higher than that of HR6W. FE analyses were conducted using individual creep strain rates of the base metal, the heat affected zone and the weld metal to clarify this difference in the failures of these two specimens. Significant stress was only produced in the weld metal as opposed to the base metal, due to the difference in creep strain rates between the welded branch connections and creep crack were initiated in the weld metal. The differences between the two failure types were assessed using the ductility exhaustion method.

The effects of pre-strain on creep properties of Alloy 740 have been investigated. Tensile strain was 7.5% and introduced by room temperature tensile test. Creep tests were conducted under 750 degree C, 275-350MPa. Creep rupture life of pre-strained sample decreased by half compared with as-heat treated sample. Creep behaviors of both samples were almost similar in primary creep stage, but onset of creep rate acceleration of pre-strained sample was faster than those of as-heat treated sample. As a result, minimum creep rate of pre-strained sample were two times larger than that of as-heat treated sample. From the observation of ruptured specimen, pre-strained sample had much more sub cracks than as-heat treated sample. On the other hand, microstructure of both samples was also different. There were MC precipitates on grain boundary in both ruptured specimens, but both size and number of MC precipitates were larger in pre-strained sample although creep life of pre-strained sample was shorter than that of as-heat treated sample. In this paper, the difference of creep behavior will be discussed in terms of both the microstructural change and mechanical damage.