Creep strength of welded joints for high Cr ferritic steels is decreased for long-term services at high temperatures due to Type IV creep damages that occur in heat affected zone (HAZ). Aiming at improving the creep strength of HAZ, we have investigated the effect of boron and nitrogen content on the microstructures and creep strength of HAZ for the 9Cr and 12Cr steels produced based on the P91 and P122 steel. It was found that the effect of boron appeared remarkably for the 12Cr steel. By increasing boron and decreasing nitrogen, the formation of fine-grained HAZ structure during weld thermal cycle could be suppressed and the creep strength of HAZ was considerably improved. For the 9Cr steel, while the formation of fine-grained HAZ structure could be inevitable independent of boron and nitrogen contents, the creep strength of fine-grained HAZ was improved by grain boundary strengthening effect of boron.

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