Study on Remaining Life Assessment of 12Cr Steel for Boilers
Thermal Power Research Section
Outline of the Study
In recent years, thermal power plants require higher-temperature and pressure steams for higher-efficiency operation. Due to this requirement, high-Cr ferritic heat resisting steels are increasingly used in boiler tubes and piping. In our new thermal power plants, they are also used for main steam pipes, etc. Since 12Cr steels (ka-SUS410J3 series) are relatively new materials, we have no accurate methods to assess their remaining lives. This study will establish remaining life assessment techniques for them. The 12Cr steel pipes can receive creep damage within the weld zone, therefore the life assessment on the outer surface is insufficient. In addition to this conventional assessment approach, the application of the ultrasonic noise method will be investigated to detect internal flaws. The ultrasonic noise method is characterized by higher resolution than the conventional ultrasonic flaw detection method and the ability to assess internal creep damage. In this study, creep tests are performed with large test specimens to construct life assessment standards based on structure and hardness changes up to fractures (on the outer surface) and ultrasonic noise data (inside). With this achievement, the period for replacing large equipment can be found, and replacement-cost reduction and reliability improvement of maintenance management are expected.
HEPCO has conducted this study in conjunction with Ishikawajima-Harima Heavy Industries Co., Ltd. (IHI) and Electric Power Development Co., Ltd. (J-POWER).
Fig-1 Principle of ultrasonic noise method (from the material by IHI)
Achievements
The large weldment creep test specimens were prepared from a 12Cr steel
weld joint that is equal to the actual equipment (outer diameter (406.4mm)
thickness (82mm)) in size by doing girth weld and cutting with the same
groove and conditions. The creep test started with the conditions of the
temperature (625 to 650
) and the stress (50 to 100MPa), using 4 conditions. The following findings were obtained.
- The initial hardness (creep consumption: 0%) is higher than the base material at weld metal, but the hardness at both weld metal and base material declines at the same level at fracture (creep consumption: 100%).
- The creep damage of the specimen is first found inside, while the outer surface shows creep voids at the end of life.
- Using the ultrasonic noise method, internal damage to 12Cr steel can be detected.
Plan for the Future
The tests are paused in each test specimen several times to observe the defects. All the creep tests will be completed by the end of FY 2009 (maximum duration is 15,000h) to obtain the data. The cutting surfaces of some test specimens are observed to confirm the state of internal creep damage. Finally, the relationship between the change in material property and creep life consumption rate will be examined and the resulting correlation presents the 12Cr steel life assessment curve.
