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

摘要

Alloy 690 is a high-chromium nickel-base alloy usedin various primary coolant system components and forsteam generator tubes in many nuclear power plants. Athermal treatment is typically applied for Alloy 690 com-ponents, giving the material a better resistance to stresscorrosion cracking, compared to mill-annealed condition,due to ne grain boundary carbides of type_(M23)C_6. Theeffect of thermal aging at 400 °C was investigated in fourdifferent heats of Alloy 690. After 5000 h and 10 000 hof aging, one of the materials showed a signicant in-crease in hardness, whereas the three other heats did notshow a measurable effect of aging. Microstructural andhardness characterizations were performed, on both ini-tial and aged material, by applying optical microscopy,scanning electron microscopy, energy-dispersive X-rayspectroscopy, microhardness and nanoindentation mea-surements, in order to study the microstructural featurescausing the observed increase in hardness.