Creep rupture properties and microstructures of 9Cr–3Co–3W-Nd-B steel welded joints

The creep rupture properties of welded joints and microstructures before and after creep rupture were evaluated for the 9Cr–3W–3Co-Nd-B steel, and simulated HAZ test was conducted to evaluate the creep strength and microstructures of HAZ. The creep strengths of welded joints were almost equivalent to the average strength of base metal, and the welded joints ruptured in HAZ in a similar manner as conventional steels in low-stress tests. The results of creep strength evaluation for simulated HAZ exhibited that welded joints of 9Cr–3W–3Co-Nd-B steel were the least strong in the areas heated to a temperature near 1050 °C, indicative of being the cause of HAZ rupture at the long-term region. A reduction in the creep strength was lessened at temperatures immediately above the AC3 transformation temperature, likely due to a combined effect of suppression of grain refinement by austenite memory effect and maintenance of the amount of grain boundary precipitates partly affected by large prior austenite grains. •Creep rupture properties and microstructures before and after creep rupture are evaluated.•A reduction in the creep strength due to welding was successfully lessened compared with conventional steels.•The welded joints were the least strong in the areas heated to near 1050 °C.•A reduction of creep strength was lessened immediately above AC3 transformation temperature.•A combined effect of suppression of grain refinement and maintenance of grain boundary precipitates.