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This paper presents a numerical scheme for modelling hydrogen assisted stress corrosion crack (HASCC) extension along centerline of gas tungsten arc (GTA) welds of austenitic stainless steel 21Cr-6Ni-9Mn (21-6-9). FEM based cohesive zone modelling (CZM) is used to examine crack extension through the fusion zone (FZ). Diffusion of hydrogen through the lattice is analyzed by finite difference method incorporating effects of hydrostatic stress (σ_h). J versus crack extension curves have been obtained. The predicted results compare well with published experimental data. Results include effects of size and shape of FZ and heat affected zone (HAZ) on J-∆a variations, temporal variations of hydrogen concentration and σ_h along the crack line ahead of the tip.

Austenitic stainless steels are widely employed in machines and structures as they have very good resistance against HASCC. But, weld joints of the material are weaker against HASCC due to retained δ-ferrite. Several experiments have shown that the dominant sites for initiation of micro-cracks are the ferrite and ferrite-austenite boundaries in the weld microstructure. The micro-cracks gradually develop into macro-cracks, which grow subsequently both along, and perpendicular to, the initial crack line/plane. A 2-D analysis of such a crack growth along the weld centerline is the objective of this study.

 

The analysis of the problem is difficult because of existence of three distinct material zones, i.e. FZ, HAZ and the base metal (BM). Both tensile and H₂ diffusion properties differ from one zone to another. There is not much published data on the properties except some experimental results on variation of J with crack extension. The study of the problem is further complicated by the fact that the corrosion affects the crack extension and the later, in turn, affects the diffusion and corrosion. The two phenomena are therefore coupled. The analysis of such a problem through homogenous material in the presence of HASCC has been reported earlier by several investigators. Both sequential and coupled analysis have been reported in the literature. A sequential analysis of a crack propagating along the weld centerline is considered in this paper. Due to non-availability of exhaustive material properties/data, e.g., tensile strength, % elongation, diffusivity parameters, reduction of cohesive strength with hydrogen concentration, etc., the appropriated data are adjusted here with few iterations to get the best predictions for J vs. ∆a variations. In the modelling, variation of tensile properties across the HAZ has been linearly interpolated between BM (σ_y= 485MPa) and FZ (σ_y= 675MPa). The case studies presented concerns internal hydrogen assisted corrosion (IHAC) in CT specimen with crack along the centerline of the weld. After fabricating the specimens with a pre-crack, they were kept in hydrogen bath for a charging time of 29 days to reach a uniform hydrogen concentration of 230 ppm(by weight) and then tested at a loading rate of 0.4 mm/min.

 

Computation, modelling, simulation