Effect Of Thermal Aging On The Microstructure And Mechanical Properties Of High-Ni And Ni-Base Alloys

The long-term exposure of metallic structures, systems and components to power plant operating temperatures can lead to thermal aging and subsequent degradation. In a VVER-type pressurized water reactor, high-nickel alloys are used in some high-strength applications, such as fasteners in the primary circuit collector and reactor pressure vessel. In western pressurized water reactors, Ni-base alloys are the material of choice for the steam generator tubes. A reduction in the mechanical properties and cracking resistance of these alloys, due to thermal aging, has been identified as a degradation mode associated with some high nickel and nickel-base materials. In this study four industrial heats of thermally treated Alloy 690 and a XH35BT bolt have been aged in air at 400 °C up to 10kh and 15kh, respectively. They have been characterized in both their as-supplied and aged conditions using complementary techniques, including optical and electron microscopy, Vickers microhardness and, for some materials, electrical resistivity and X-ray diffraction measurements. An increased understanding of the thermal aging degradation mechanism would prove beneficial to operators and regulators alike, by demonstrating the long-term reliability of these components. Moreover, there is little publicly available information on the thermal aging behavior of high-Ni alloys in conditions relevant to the nuclear industry, and on the role of manufacturing details on the behavior of Ni-base alloys.