Hydrogen-related disbonding of weld overlay of corrosion resistant alloys

For the manufacture of hydrogenation pressure vessels of heavy wall sections, cladding with a layer or multilayer of corrosion resistant alloy on the inside wall of the vessel by Submerged Arc Welding with a strip electrode has become a common practice. However, the occurrence of disbonding of the overlay from the base steel, usually Cr-Mo low alloy steel, has remained a problem, especially during cooling down of the vessel in an emergency. There are three major factors attributing to the disbonding. One is the hydrogen accumulation at the interface between the overlay and the base steel as a result of the difference in solubility and diffusivity of hydrogen between the overlay of austenitic structure and the base steel of ferritic structure. The second factor is the residual tensile stresses developed in the overlay during cooling down of the vessel from operation due to the difference in thermal expansion between the dissimilar materials. Finally, the susceptible microstructure along the interface to the disbonding plays another principal role. Nevertheless, the influence of the microstructure, and the actual mechanism of disbonding, remain obscure.;A dissimilar material bond was employed to evaluate the level of residual tensile stresses in the overlay. Tensile stresses exist perpendicular and parallel to the interface in the overlay near the interface.;A systematic investigation on the formation of a crack-susceptible structure was carried out, and the mechanisms for the formation of such a structure are proposed. A so-called Type II grain boundary adjacent and parallel to the interface in the overlay side has been confirmed as detrimental. A phase change during solidification and/or the presence of inclusions from the base steel due to dilution effect segregating at the front of planar grains in the initial stage of solidification are responsible for the formation of Type II grain boundaries.;A proper selection of overlay materials and the welding procedure can eliminate or reduce the chance for the formation of Type II grain boundaries. Hence the hydrogen related disbonding can be minimized.;A simulation of hydrogen accumulation in the Type II grain boundary is established based on a Finite Differential Method. A level of more than 120 ppm of hydrogen can be reached at the Type II boundary. This well exceeds the solubility of hydrogen in ferrite (...