Alloy 800 H溶接熱影響部の高温経年劣化割れ

抄録

Several cases of cracking failures in the HAZ of alloy 800H pipe have been experienced in equipment operating in the temperature range of 550-700°C. The influence of several heat treatments on the cracking sensitivity by cylindrical type restraint cracking test was investigated. And the mechanism of this crack formation was considered.
Main results obtained in this research were as follows.
(1) Intergranular oixidation and cracking were detected at the bottom of notched HAZ area of as-welded specimens in the temperature range of 600-650°C. The area of cracking coincided with the area of γ' phase precipitation.
(2) By PWHT of solution heat treatment, HAZ cracking was not detected in the specimens aged for 3000 hours at 600°C, while only intergranular oxidation was detected. The sensitivity of this cracking can be reduced remarkably by the use of the PWHT of solution heat treatment that reduces the initial weld residual stress.
(3) When the cohesive force between grain boundaries is severely reduced by intergranular oxidation and weld tensile residual stress acts upon grain boundaries, the intergranular cracking occurs.