Microstructure and Hydrogen-Induced Failure Mechanisms in Fe and Ni Alloy Weldments

Hydrogen Embrittlement Understood

“…It has also been found that similar morphological features on fracture surfaces are not necessarily driven by the same underlying processes. [10][11][12]15,16,41] Phase transformations certainly occur in some alloy systems in the presence of strain and hydrogen. [37,38,[42][43][44] However, the dilemma is in determining the role of the phase transformation in the hydrogen embrittlement process and in determining when it occurs.…”

“…For example, focusedion beam machining affords the opportunity to extract a volume of material from a site-specific location, including ones on complex fracture surfaces for subsequent examination in a transmission electron microscope. [10] This approach has been used to determine the microstructure that has evolved beneath different types of hydrogeninduced fracture surfaces including flat features on pipeline steels, [11] ''quasi-cleavage'' fracture surfaces on pipeline steels, [12] microvoids, [13] ''quasi-cleavage'' and intergranular surfaces in lath martensite steels, [13,14] and intergranular facets in Ni [15] and Fe. [16] As detection of hydrogen at specific locations remains one of the outstanding issues in hydrogen-induced degradation of metals, it is inferred from observed responses.…”

A micromechanical image-based model for the featureless zone of a Fe–Ni dissimilar weld

“…A series of catastrophic failures of AISI 8630 steel/IN625 Nickel alloy dissimilar-metal welds has highlighted the need for further investigations into the role of hydrogen on the fracture process [1,2].…”

“…Recent TEM and SEM studies [1,2] have revealed detailed information about the microstructure in the vicinity of the fusion boundary of 8630/625 dissimilar welds and the fracture morphologies which alternates between a flat cleavage-like morphology and a highly irregular cratered structure. Figure 1 [3] shows a simplification of the interface microstructure of a 8630/625 dissimilar weld, [4][5][6], i.e.…”

In-situ neutron diffraction measurement of stress redistribution in a dissimilar joint during heat treatment

“…As a result of carbon diffusion from the relatively higher carbon forging into the buttering during PWHT, carbides form within a narrow, planar-solidified band in the weld metal [5][6][7]. These chromium-based M 7 C 3 precipitates have been considered to play a critical role in the embrittlement of the joints, when subject to cathodic protection (CP) [6].…”