Creep behavior of P91B steel in the presence of a weld joint

The paper presents creep test data on standard P91B steel specimens made from two distinct regions of a welded plate over a range of stresses (50-190 MPa) and temperatures (600-650 °C). The analysis of test data revealed that the samples having a weld zone within the gage length (cross-weld samples) have lower long term rupture strength than the samples made of the base metal. Estimated weld strength factors (WSF) of this steel were found to be higher than those reported for P91 steel. The study also showed that the effect of welding on loss of rupture ductility is much more prominent than its effect on the reduction in rupture strength. In presence of welded zone the extent of local deformation in ruptured samples was not as prominent as in the samples without weld. Creep damage tolerance factors (λ) were estimated from the creep strain versus time plots. This also showed that the magnitude of λ is significantly reduced in the presence of welding. Examination of microstructure and measurement of density revealed that this difference is primarily due to the formation of cavities in the heat affected zones of welded specimens. In the lower stress regime a few test specimens without any welded region did not fail even after very long creep exposure. Diameters of these specimens were found to have increased in spite of measureable increase in length due to creep. This unusual effect has been attributed to oxide scale growth. It shows up when the increase in diameter due to the growth of oxide scale becomes greater than the decrease in diameter due to the accumulation of creep strain.