Duplex stainless steels and their deposited weld metal have ferrite and austenite microstructure, which have material properties that are different. In addition, the microstructure of the base metal and weld metal are clearly different; therefore, it affects the hydrogen diffusion and accumulation, and hydrogen-induced cracking behavior at the microstructural scale. In this study, the influence of the microstructure on hydrogen-induced cracking behavior of duplex stainless steel weld metal was investigated. Specimens of duplex stainless steel weld metal were prepared and slow strain rate tensile test was performed after hydrogen charging. Cracks were observed at boundaries of ferrite and austenite phases. In order to clarify the stress and hydrogen concentration distribution at the microstructural scale, a microstructure-based finite element simulation was performed. A finite element model based on a cross sectional observation of the microstructure was designed to calculate the stress and hydrogen concentration distribution. The simulation result showed that the hydrogen accumulation occurs at ferrite/austenite boundaries, which corresponded to the locations where cracks were observed in the experiment. On the other hand, the hydrogen concentration at the accumulation site in the weld metal was low compared to that in the base metal. Therefore, the influence of the phase fraction and the stress-strain curves of the ferrite and austenite phases on the hydrogen concentration was investigated by the proposed numerical simulation. It was demonstrated that both the phase fraction and stressstrain curves have significant influence on the microscopic distribution of hydrogen concentration.

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For two series of API 5L X65 linepipes Pipes A and B , the critical condition for ductile cracking of the linepipe steel and the applicability of the critical condition to an axially notched linepipe were investigated. Static 3-point bending tests for Charpy Vnotch specimens were conducted in order to evaluate the critical condition of ductile cracking from the notch tip by using FEanalyses. At the position of ductile cracking from the notch tip for the Charpy type specimens, the stress triaxiality was approx. 0.6 for both linepipe steels, however the equivalent plastic strain p was different on each linepipe; the p for the ductile cracking was approx. 0.65 for Pipe A and approx. 1.47 for Pipe B. Hydraulic burst tests were then conducted for internally patched linepipes with an axial through-wall TW notch. The results of the FE-analyses for the hydraulic burst tests indicated the following: 1 the position of the ductile cracking at the TW notch tip was not the center of the wall-thickness WT , but a slightly shifted position to the inner surface from the center of WT, 2 the equivalent plastic strain at the position where a ductile crack was initiated for the TW notched linepipe was almost the same as that obtained from the 3-point bending test result for the Charpy V-notch specimen. The present study revealed that the critical strain for ductile cracking from a notch tip for a Charpy type specimen was in good agreement with that for an axially notched linepipe. It was therefore clarified that the critical condition for ductile cracking for linepipes with an actual flaw could be predicted from the results of a small-scale test and FE-analysis to evaluate the relationship between the stress triaxiality and the equivalent plastic strain at the position of the ductile cracking.

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To evaluate the effect of the dual-phase microstructure on hydrogen diffusion behavior, a numerical simulation using a two-dimensional dual-phase model was performed. The effect of the dual-phase microstructure was evaluated by changing the morphological parameters such as the shape and geometrical distribution of the austenite phase using two different phase fractions. The results indicated that the hydrogen does not diffuse through the higher coefficient phase on a preferential basis; instead, the diffusion occurs with the same time dependency in both phases. In addition, the layered pattern of the austenitic phase and the higher phase fraction of the austenite phase reduced the degree of hydrogen diffusion. The hydrogen diffusion in dual-phase steel is significantly affected by the difference in the hydrogen diffusion constants and morphology of the microstructure of the dual-phase steel.