Improved Utilization of Added B in 9Cr Heat-Resistant Steels Containing W

Abstract

For the construction of ultra super critical (USC) power plant, 9Cr-3W base ferritic heat-resistant steels with relatively high B and no N have been investigated. Authors have been revealed in the previous report that the addition of 139ppm B significantly improves creep strength of the steels, whereas most of added B forms unidentified borides, which are deemed almost ineffective to creep strength. The effect of improved heat treatment on creep strength and distribution of B in precipitates is investigated to effectively utilize and decrease added B. As a result of the analysis of the extracted residue and characterization of precipitates using field emission Auger electron spectroscopy (FE-AES), most of added B still forms borides in the 92ppm B added steel. These composites are almost dissolved and the B content in M₂₃C₆ carbides is significantly increased by normalizing at 1423K. It is also found by FE-AES analysis that B content in M₂₃C₆ carbides near prior-austenite grain boundaries is relatively higher than that inside grains. Creep strength at 923K for the 92ppm B added steel normalized at 1423K is not improved at short times, but it is remarkably improved to almost the same level as the 139ppm B added steel at long times. This excellent creep strength is achieved resulting in improving microstructural stability through the effective utilization of added B by high-temperature normalizing.