Improving the Type IV Cracking Resistance of 9Cr Ferritic Steels by Boron Addition

摘要

Significant improvement in the creep properties ferritic steels have been reported by controlled addition of boron and reducing the nitrogen content to avoid the formation of boron nitride [1]. During high temperature exposure, boron preferentially segregates to precipitates and thus delays their coarsening, which in turns delays formation of creep cavities at the coarse precipitates and thus leads to improvement in the creep properties. The type IV fracture is also associated with the formation of creep cavities at the large precipitates present in the fine grained heat affected zone (FGHAZ) of the weld joints and hence it is expected that the resistance to the Type IV cracking should be better for the boron containing ferritic steels than for the conventional steels. According creep tests of base metals and weld joints of high-B, low-N 9CrWCoVNb steels were carried out at 923K to find out the effect of born addition on the Type IV cracking behaviour of the steels. Results from this study are presented here.