Combined effects of hydrogen and thermal aging on nickel-based Alloy 690

Thermally treated nickel-based Alloy 690 is the material of choice for the steam generator tubes of many nuclear power plants. Thermal aging of Alloy 690 has been shown to increase intergranular carbide precipitation and at aging temperatures below 450˚C a short-range atomic level ordering of the crystal lattice can take place. The atomic ordering causes lattice contraction and increases hardness and localized strain at grain boundaries. Aging induced processes can lead to a decreased resistance to primary water stress corrosion cracking of this alloy. Six Alloy 690 material conditions, all exhibiting different levels and combinations of intergranular carbide precipitation, were evaluated in the study. Thermal desorption spectroscopy and mechanical loss spectroscopy were used to investigate the correlation between hydrogen uptake and the aging-induced phenomena of the atomic ordering and carbide precipitation, with the aim to better understand the role thermal aging and hydrogen uptake play in the primary water stress corrosion cracking of thermally treated Alloy 690.