Long-term creep testing and microstructure evaluation of P91 steel weld joints

Trial weld joints were made from wrought and cast modified Cr-1Mo-V steel using GTAW & SMAW methods. Creep testing was carried out at temperature range from 525 degrees C to 625 degrees C and stresses from 50 to 240 MPa. Time to rupture of welds made from tube segments and cast plates reached almost 30 000 hours and 20 000 hours respectively. Creep strength was evaluated according the Larson-Miller parametric equation and microstructure was investigated using both light and electron microscopy. Creep rupture strength of both weld joints tested at temperatures below 600 degrees C falls into the +/- 20% scatter band of the creep rupture strength of the parent material. At 600 degrees C and 625 degrees C the creep strength dropped by 27% and 30% for the plate weld and the tube weld respectively. All ruptures occurred in fine grain and intercritically reheated heat affected zones either in the parent material or in the weld metal. Observation of thin foils prepared from selected regions of the weld joints revealed differences in precipitation processes and the structure recovery causing decrease of dislocation density in some regions. Fine ferritic grains with low density of fine carbonitride precipitate occurred in critical localities. Soft grains were deformed and cavities at grain boundaries initiated the crack propagation.