Creep test to the rupture of the dissimilar weld joint made of COST FB2 and COST F martensitic steels was carried out at temperatures ranging from 550°C to 650°C in the stress range from 70 to 220 MPa. Creep rupture strength was evaluated using Larson-Miller parameter. Assessment of microstructure, changes of precipitates and dislocation substructure were correlated with the creep strength. Critical zones of creep damage were determined. At lower temperatures and higher stresses the weld joint ruptured in the base material of COST F steel unaffected by welding, while at higher temperatures and lower stresses rupture occurred in the intercritical heated and fine-grained parts of heat affected zone (HAZ) of steel COST F. The processes of recovery in connection with precipitation of new particles of Laves phase and their coarsening is the main causes of decrease in creep rupture strength in the HAZ of steel COST F.Keywords: Martensitic steels, welding, creep testing, microstructure
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