Effect of Water on Mechanical Properties and Stress Corrosion Behavior of Alloy 600, Alloy 690, EN82H Welds, and EN52 Welds

Abstract

The fracture toughness and tensile properties of alloy 600 (UNS N06600), alloy 690 (UNS N06690), and their welds (EN82H [UNS N06082] and EN52 [UNS N06052]) were characterized in 54°C and 338°C water with an elevated hydrogen content. Results were compared with air data to evaluate the effect of low- and high-temperature water on the mechanical properties. In addition, the stress corrosion cracking (SCC) behavior of EN82H and EN52 welds was evaluated in 360°C water. Elastic-plastic (JIC) fracture toughness testing revealed that the fracture resistance of all test materials was exceptionally high in 54°C and 338°C air and 338°C water, demonstrating that fracture properties essentially were unaffected by the high-temperature water environment. In 54°C water, however, JIC values for EN82H and EN52 welds were reduced by an order of magnitude, and alloy 690 showed a fivefold decrease in JIC. Scanning electron fractography revealed that the degraded fracture properties were associated with a fracture mechanism transition from ductile dimple rupture to intergranular cracking. The latter was associated with a hydrogen-induced cracking mechanism. The fracture toughness for alloy 600 remained high in 54°C water, and microvoid coalescence was the operative mechanism in low-temperature air and water. Tensile properties for all test materials essentially were unaffected by the water environment, except for the total elongation for EN82H welds, which was reduced significantly in 54°C water. Constant-load testing of precracked weld specimens in 360°C water resulted in extensive intergranular SCC in EN82H welds, whereas no SCC occurred in EN52 welds under comparable test conditions.