IMPROVEMENT OF TYPE IV CREEP CRACKING RESISTANCE OF 9Cr HEAT RESISTING STEELS BY BORON ADDITION

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