Effect of Boron on Microstructures and Creep Properties of Welded Joint for High Cr Heat Resisting Steels
Masaaki TABUCHI, Hiromichi HONGO and Yukio TAKAHASHI
Abstract:Creep strength of high Cr steel welds is decreased for long-term services at high temperatures due to Type-Ⅳ failure occurs in fine-grained 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 the 9Cr and 12Cr heat resisting steel welds. It was found that the creep strength of HAZ of these steels could be improved by addition of about 0.01% boron with low nitrogen. For the 9Cr steel, while formation of the fine-grained HAZ structures was inevitable independent of boron and nitrogen contents, the creep strength of HAZ and welded joint could be improved by utilizing the grain boundary strengthening effect of boron. For the 12Cr steel with 0.01% boron and low nitrogen, formation of the fine-grained microstructures during weld thermal cycle was suppressed and the creep strength of HAZ and welded joint was considerably improved. Key Words:High Cr Ferritic Steel, Welded Joint, Creep, Type-Ⅳ failure, Boron, Nitrogen Content