Establishing safe operating limits for equipment operating in hydrogen service remains a concern for the petrochemical industry. A methodology to prioritize equipment, inform future inspections, and guide inspection discovery path forward decisions has been in development for the past several years. The approach is multi-tiered and considers the process conditions that lead to risk of damage, the potential extent of damage, and the effect of applied and residual stresses on the rate of damage growth. The key metric for Fe-C steels and Fe-C-0.5Mo steels is the damage index, which is calculated from a damage rate equation that has been calibrated to select laboratory test data and reported HTHA incidents documented in API 941. The damage rate and significance of damage is handled by considering damage as both diffuse (continuum assessment) and localized (crack/flaw assessment). This approach was originally intended as a means for prioritizing inspection. However, once margins are established to account for uncertainties in operating conditions and material variation, so-called time-dependent Nelson curves can be generated for use in design. This paper provides an overview of the HTHA damage modeling technique and results of recent experimental work, including component-level testing used for validation of the model.