Hydrogen Induced Mechanical Property Behavior of Dissimilar Weld Metal Interfaces

Abstract

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.

Effect of Hydrogen on the Mechanical Properties of ASTM A182 F22 and ASTM A36 Steels Welded Joint Using Inconel 625 as Filler and Buttering Metal

“…They found that, even at very low cooling rates, this martensite is formed and exhibited levels of hardness around 330 to 450 VHN. Fenske et al 21 , mention that the high Ni content can reduce the A C1 temperature, making it lower than the PWHT temperature, favoring reaustenitize in discontinuous PMZs with microstructure of the base metal, and, consequently, generating virgin martensite after cooling. The high hardness of martensite makes it more susceptible to embrittlement by hydrogen.…”

Butter-weld Interface Microstructural Analysis Employing NCS and LAMS Sections

“…Considering the PWHT of 6 hours at 677ºC, represented in Figure 4 by the yellow point, there is a favoring in the formation of Fe 3 C, Cr 7 C 3 and Mo 2 C. Among these carbides mentioned, Fe 3 C and Cr 7 C 3 commonly found in the interface of dissimilar welded joints, are particularly sensitive to hydrogen embrittlement. These carbides may act as hydrogen trapping sites and may be present at the dissimilar interface of the ARBL-Inconel 625 23 , providing a low energy fracture path mainly along the zone M 22,30,31 .…”