DEVELOPMENT OF A MICROSTRUCTURALLY-FAITHFUL MESO-SCALE MODEL OF LOW TEMPERATURE CRACK PROPAGATION IN ALLOY 82 WELD METAL

Exposure of Alloy 82 welds to hydrogen containing, de-oxygenated aqueous environments at temperatures below 150°C can result in a reduction of fracture toughness. The work reported in this paper aims to provide a better understanding of the effect of grain boundary micro- and meso-structure on Low Temperature Crack Propagation (LTCP) susceptibility. Grain boundary morphology of an Alloy 82 weld microstructure was characterised using an image analysis routine, and microstructurally-faithful grain boundary profiles imported into Abaqus Finite Element (FE) software. A 2D model of the grain boundary meso-structure was then generated using cohesive elements to simulate crack development, allowing simulation of intergranular fracture whilst avoiding determination of the stress singularity at the crack tip. The model was then compared to in-situ observations of crack propagation in Alloy 82 microstructures exposed to 54C hydrogenated water, using a windowed-autoclave test facility. Slow strain rate tests with a dissolved hydrogen concentration ranging from 49cc/kg to 69cc/kg showed a propagating crack captured during testing. Digital image correlation (DIC) was then used to estimate crack growth rates and fracture pathways, combined with post-mortem fractographic assessment. Tensile testing of hydrogen pre-charged miniature samples also showed evidence of intergranular embrittlement as well as separation along slip planes. The fracture surface showed a mixed mode failure, with regions of intergranular fracture. Intergranular cracking results are used to inform the microstructurally-faithful finite element model.