Nondestructive Testing and Evaluation Method of Thermal Aging of CF-8M Cast Stainless Steel

The main pipe of PWR is the key component of the steam supply system of nuclear power plant, and it is also an important part of the reactor coolant pressure boundary. The main pipe of the coolant system of light water reactor is made of cast stainless steel parts, which has high material strength, high toughness and good corrosion resistance. However, there is thermal aging embrittlement during long-term operation at 280-325 °C, resulting in the increase of hardness and tensile strength of stainless steel, and the significant decrease of impact energy and toughness. Therefore, the safety margin of the main pipeline should be analyzed and evaluated to ensure the safe and reliable operation, which is an important link in the aging management of the main pipeline. Especially when the nuclear power unit in service is ready to prolong its life, the requirements of thermal aging detection and evaluation of main pipeline are put forward. Therefore, it is necessary to use nondestructive method to detect the thermal aging of the main pipeline in service and evaluate the thermal aging state. The thermoelectric power of nuclear grade CF-8M static cast stainless steel main pipe material was measured after thermal aging test at 400 °C for 10 000 h. The relationship of mechanical properties (impact energy) and thermoelectric power with aging parameters after different thermal aging time was investigated, and the corresponding relationship between normalized parameters of thermal aging influencing factors and mechanical properties, thermoelectric power value was obtained. The experimental results show that the impact energy decreases rapidly in the initial stage, and the decreasing trend of impact energy tends to be gentle after thermal aging to 8 000 h. During the experiment, the thermoelectric power increases linearly with the increase of logarithmic aging time. With the increase of thermoelectric power, the impact energy begins to decrease rapidly, and then decreases slowly and gradually tends to saturation. The variation of impact energy with thermoelectric power is similar to that of impact energy with thermal aging time. Based on the linear relationship between the normalized parameters and thermoelectric power, the degree of performance degradation after thermal aging for service parts can be evaluated by the thermoelectric power. This experimental study provides a test basis for thermoelectric power detection and thermal aging evaluation of main pipe components on-site. © 2022 Atomic Energy Press. All rights reserved.