Effect of thermal aging on microstructure and hardness of industrial heats of Alloy 690

Alloy 690 is a high-chromium nickel-base alloy used in various primary coolant system components and for steam generator tubes in many nuclear power plants. A thermal treatment is typically applied for Alloy 690 components, giving the material a better resistance to stress corrosion cracking, compared to mill-annealed condition, due to fine grain boundary carbides of type M23C6. The effect of thermal aging at 400 ◦C was investigated in four different heats of Alloy 690. After 5000 h and 10 000 h of aging, one of the materials showed a significant increase in hardness, whereas the three other heats did not show a measurable effect of aging. Microstructural and hardness characterizations were performed, on both initial and aged material, by applying optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, microhardness and nanoindentation measurements, in order to study the microstructural features causing the observed increase in hardness.