| Popis: |
Exposure of Alloy 82 welds to hydrogen containing, de-oxygenated aqueous environments at temperatures below 150°C can result in embrittlement, manifested by a significant reduction of its resistance to cracking. The embrittlement is brought about by nano-scale niobium and titanium rich carbonitrides at grain boundaries which act as hydrogen traps. The presence of stresses may then result in low temperature crack propagation (LTCP).The work reported in this paper provides a better understanding of the effect of grain boundary micro- and meso-structure on LTCP susceptibility. The grain boundary morphology of an Alloy 82 weld microstructure was characterised using image analysis methods, and microstructure-faithful grain boundary profiles imported into Abaqus Finite Element (FE) software. A 2D model of the grain boundary meso-structure was generated using cohesive elements to simulate intergranular LTCP. Stress-assisted diffusion was used to calculate the rate of hydrogen ingress along grain boundaries. Changes in local hydrogen concentrations were then used to obtain the failure energy of each cohesive element. This controls the energy required to advance a crack locally. Grain boundary morphology, crystallography, and coverage of intergranular precipitates can also be included in the cohesive element model.The model was compared to in-situ observations of crack propagation in fatigue pre-cracked Alloy 82 microstructures exposed to 54C hydrogenated water. A windowed-autoclave test facility in conjunction with slow strain rate testing was used to observe a propagating crack along grain boundary segments. Digital image correlation (DIC) was then employed to estimate local crack growth rates and obtain fracture pathway information. Results obtained by in-situ observations were then used to calibrate cohesive model parameters. Post-mortem fractographic assessment was carried to out to determine modes of failure. |
