PURPOSE: To increase stress corrosion crack resistance by heat-treating an alloy 718 structural member at a specified condition and annealing it at a temp. enough to melt δ precipitate.

CONSTITUTION: An alloy 718 structural member used in a unclear reactor environment by using water is annealed at about 1093±8°C for about 1hr ±10min and is cooled at a speed of about 56±29°C/hr down to about 718±8°C. Successively, the alloy is subjected to aging at about 718±8°C for 4hr ±10min, cooling down to about 621±8°C at a speed of about 56±28°C/hr, aging at about 621±8°C for 16hr±10min and cooling to a room temp. By this method, a δ precipitated part is melted, the δ precipitate does not exist at a boundary, and element homogeneity at the boundary is increased. Thus, stress corrosion crack resistance of the material is increased, and a crack propagation speed of the material is decreased.