We present a novel computational framework to quantify the fracture resistance of welds. In the first stage of the simulation framework, a thermo-metallurgical weld process model is used to predict local fractions of microstructural constituents within weld regions based on steel composition and welding parameters. The resulting phase-fraction maps are used to define heterogeneous material properties, which are subsequently employed in structural integrity assessments using an elastoplastic phase field fracture model. The framework is tailored to predicting the failure of hydrogen transport pipelines, demonstrating its potential to assess the feasibility of repurposing existing pipeline infrastructure for hydrogen transport. First, the welding process model is validated against experimental microhardness maps for vintage and modern pipeline welds, showing an excellent agreement. Coupled hydrogen diffusion-fracture simulations are then conducted to determine the critical pressure at which hydrogen transport pipelines will fail. To this end, the model is enriched with a microstructure-sensitive description of hydrogen transport and hydrogen-dependent fracture resistance. A defect analysis of an X52 pipeline reveals that even small defects in a hard heat-affected zone can drastically reduce the critical failure pressure. This underlines the critical role of hard phases in heat-affected zones in the defect tolerance of pipelines and the need to develop computational tools capable of resolving these. Importantly, simulations spanning numerous relevant conditions (defect size and orientations) are used to build Virtual Failure Assessment Diagrams (FADs), enabling a straightforward uptake of this mechanistic approach in fitness-for-service assessment.