To ensure a reliable power supply with a minimum environmental impact in the future, further increases in efficiency and flexibility of fossil-fired power plants are major challenges to address in recent R&D activities. In order to counterbalance substantial fluctuations in the electricity grid due to the rising share of renewable resources, frequent start-ups and shut-downs of turbomachinery will be claimed by the market. Hence, along with the development of advanced materials for elevated steam parameters, contemplations with respect to altered boundary conditions of established materials, e.g., for bolted pipe flange connections, are required. In this paper, a selection of results from a recently finished research work on stress relaxation will be presented. Together with European power plant component manufacturers, a newly developed test rig comprising a scaled intermediate pressure (IP) steam turbine pipe flange now allows investigations under near-service conditions. Before, throughout, and after performance of the experiments, an extensive measurement setup enables the examination of effects that cannot be studied in standardized relaxation tests. Within this work, a loss in bolt pretension of up to almost 50% over a comparatively short period of experimental time was observed. By means of numerical calculations, creep deformation in the transition areas between pipe and flange plate sections was identified to be a major originator of these pretension drops. Particularly, good correlations between experiments and finite element method (FEM) simulations could be achieved through the implementation of enhancements concerning the fitting strategy of the Graham–Walles-type creep model which was used.