Hydrogen Induced Mechanical Property Behavior of Dissimilar Weld Metal Interfaces

A series of catastrophic failures between alloy 625 weld metal (referred to as ‘A625’) and AISI 8630 low alloy steel forgings (referred to as ‘8630’) on cathodically-charged subsea equipment demonstrate the need to gain a better understanding of the hydrogen-induced tearing resistance of these interfaces as well as similar types of interfaces also currently used in the field. Other dissimilar metal weld interfaces in use include ASTM A182 F22 (referred to as ‘F22’) welded with A625. Similar metal alternatives are also in use, including F22 welded with low alloy steel (referred to as ‘LAS’). Slow strain rate single-edge notched bend tests under hydrogen charging conditions were used to establish ‘R-curve’ crack growth resistance trends for the F22-A625 and F22-LAS weld metal interfaces. Differences in ‘R-curve’ crack growth behavior between the different weld metal interfaces have been observed and compared to R-curve results from 8630-A625 interfaces. The F22-LAS interface demonstrates the most tearing resistance under slow strain rate after hydrogen charging followed by the F22-A625 and then the 8630-A625 interface. Subtle differences between the weld metal microstructures are described and provide a possible explanation regarding the difference in ‘R-curve’ behavior.