IMPROVEMENT ON TOUGHNESS OF WELD HEAT AFFECTED ZONE OF CU-CONTAINING LOW ALLOY STEEL OF LONG SCALE FORGING FOR OFFSHORE APPLICATIONS BY OPTIMIZING CHEMICAL COMPOSITION

摘要

Copper-containing low alloy steel based on ASTM A707 5L grade is widely used for structural parts of offshore wells. Applications of the steel for Ultra-deepwater development require excellent low temperature toughness from the viewpoint of marine accident prevention. However it is difficult to stably obtain good weld joint toughness because the welding condition is inevitably scattering. With those backgrounds, this paper focuses on metallurgical factors controlling the HAZ toughness of A707 modified steel. Potential factors considered are the grain size, M-A and precipitates. A challenge is demonstrated to improve the HAZ toughness by optimizing the Cu and Mn contents. In this study, we investigated mechanical properties including crack tip opening displacement (CTOD) and we observed microstructure using welding tests or various weld heat cycle specimens. The weld heat affected zone (HAZ) of a conventional material had good toughness for the low heat input condition. However it was remarkably decreased for the high heat input condition due to the precipitating martensite-austenite constituent (M-A) in local brittle zones (LBZ). The weld test results indicated the importance of suppressing the formation of M-A in order to improve toughness in the HAZ of the steel. Thereby, we challenged the optimization of chemical composition for HAZ toughness improvement. Cu had no bad influence on the HAZ toughness. It was demonstrated that the HAZ toughness is recovered by good use of Cu precipitates in SC cycle. Moreover the area fraction of M-A is decreased in keeping with Mn content, which leads to the improvement of the ICCG HAZ toughness. Based on our study, the recommended amounts of Cu and Mn are more than 1.0 mass % and less than 0.6 mass %, respectively, to ensure the HAZ toughness, especially ICCG HAZ toughness.