Study on Life Extension of Steam Turbine Casing Material by Weld Repair in Actual Equipments

75% of our thermal power generation plants have been operating for over 100,000 hours, which has exceeded their designed lifetime, since the start of the service. As a result, extending the life for aged thermal power plant equipment has become increasingly important for reducing the plant costs. In particular, a turbine casing used for a long time often has defects such as cracks. The defective parts are removed, and then, if its remaining wall thickness is below the requirement, the material will be replaced.

HEPCO has established weld repair techniques for the removed defective parts and life assessment technology for a weld zone in actual equipment, and has aimed at cutting costs by extending the equipment replacement period.

We have established the basic technique for weld repair of CrMoV cast steel as of 2006. Since 2007, we have increased the steel kind (CrMo cast steel) and evaluated the safety of welded parts for actual equipment.

1. (1) The establishment of the optimal weld method that can reduce residual stress
   With regard to the weld repair method requiring no Post Weld Heat Treatment(PWHT), our section has had conducted application tests and experiments to establish the optimal weld conditions for turbine casing materials and then found the conditions that can reduce the residual stress.
2. (2) The evaluation of response characteristics for weld cracks
   When long-term aged materials are welded, the cracks are considered to be generated easily. Therefore, we have conducted tests to check the response characteristics of deteriorated materials on the optimal weld condition. As a result, the weldability is good and we have confirmed that no cracks are generated.
3. (3) The evaluation of material characteristics at welded repair parts
   In order to understand the material characteristics at welded repair parts, we have conducted creep tests and low-cycle fatigue tests at welded parts. As a result, we have found that the welded parts have enough strength in actual equipment.
4. (4) The weld repair test in the use of discarded materials
   We have identified the factors for damage through the thermal stress analysis (see Fig.1) , selected the weld repair method and evaluated the safety for welding, and conducted weld repair tests (Photo-1), using the test material as actual equipment, with the optimal weld method. As a result, we have found that no weld cracks were generated, and the residual stress was reduced to 20% or less compared to the conventional weld method.