Creep lives of high Cr ferritic heat resisting steel weldments decrease due to Type IV fracture, which occurs as a result of the formation and growth of creep voids and cracks on grain boundaries in the fine-grained heat affected zone (HAZ). Because boron is considered to suppress the coarsening of grain boundary precipitates and growth of creep voids, we have investigated the effect of boron addition on the creep properties of 9Cr steel weldments. Four kinds of 9Cr-3W-3Co-VNb steels with boron content varying from 47 to 180ppm and with nitrogen as low as 20ppm were prepared. The steel plates were welded by gas tungsten arc welding and creep tested at 923K. It was found that the fine-grained HAZ did not exist in the present steel weldments ; that was quite different from the conventional high Cr steels such as P91 and P92 steel. Boron addition had the effect to suppress coarsening of grain boundary carbides during creep not only in base metal but also in HAZ. As a result of these phenomena, the welded joints of present steels showed no Type IV fractures and much better creep lives than those of the conventional high Cr steel weldments.
