Abstract
Ferritic 9-12 wt.% chromium steels are commonly used for thick-walled high-temperature components in thermal power plants, but they face two major limitations in high-temperature service. Firstly, a reduction in creep strength occurs after approximately 10,000 hours at service temperatures around 600°C, due to the dissolution of finely dispersed V-rich nitrides and the precipitation of coarse particles of the modified Z-phase, [(Cr,V,Nb)N]. Secondly, welded joints of nearly all ferritic steel grades are prone to premature creep failures in the fine-grained heat-affected zone, known as Type IV cracking, which results from a strength loss of up to 50% compared to the base material. This study describes the development of a 9Cr3W3CoVNb steel with added boron and controlled nitrogen content. Preliminary creep testing results up to 24,000 hours at 650°C show a significant improvement in creep strength compared to established ferritic 9Cr grades like P91 and P92, attributed to a reduced driving force for the precipitation of modified Z-phase particles. Crosswelds of the new 9Cr3W3CoVNbBN steel also demonstrate improved creep behavior at 650°C, with creep rupture strength comparable to the mean base material creep strength of the best commercially available grade P92.
