Thermo-Mechanical Coupled Analysis and Fastening Force Evolution of Exhaust Manifold Connecting Bolts

A research framework integrating material testing, constitutive fitting, thermo-mechanical coupled simulation, and local refined analysis was developed for 06Cr15Ni25Ti2MoAlVB superalloy bolts. High-temperature tensile and creep tests were performed to establish the Ramberg–Osgood elastic-plastic model and Norton–Bailey creep model. Based on CFD-derived thermal boundary conditions, a thermo-mechanical-creep coupled finite element model was constructed to investigate bolt temperature, global stress response, local thread stress concentration, and fastening-force evolution under rated operating conditions. The results show that the maximum temperature of the exhaust manifold is about 885 °C, while the maximum bolt temperature reaches about 250 °C. The global model predicts a maximum bolt equivalent stress of approximately 513 MPa near the initial threaded contact region. The refined thread model reveals severe stress concentration at the root of the first engaged thread, with a peak stress of about 905 MPa. After 1000 h of high-temperature holding and cooling, bolt fastening force decreases irreversibly by an average of 17.6%, with a maximum reduction of 28.27%. The results provide a reference for fastening design, fastening-force retention evaluation, and life assessment of bolted connections in high-temperature exhaust systems.