Boron strengthening in ferritic 9Cr3W3CoVNbBN steel with improved crossweld creep performance

Ferritic 9-12 wt.% chromium steels are favored grades for thick-walled high-temperature components in thermal power plants. These steel grades have two major limitations in high temperature service. First, a drop in creep strength observed after about 10.000 hours of creep exposure at service temperatures around 600°C is observed and attributed to the dissolution of finely dispersed V-rich nitrides and the precipitation of coarse particles of modified Z-phase, [(Cr,V,Nb)N]. Second, welded joints of almost all ferritic steel grades are highly susceptible to premature creep failures in the fine-grained heat-affected zone, so-called Type IV Cracking. These failures can be attributed to a loss of strength in the heat-affected zone of up to 50% compared to base material. In the present work, the development of a 9Cr3W3 CoVNb steel with addition of boron and controlled nitrogen content is described. Preliminary creep testing results up to 24.000 hours at 650°C indicate significant improvement in creep strength compared to well established ferritic 9Cr grades such as P91 and P92. Improved creep strength is attributed to a lower driving force for the precipitation of modified Z-phase particles. Crosswelds of the new 9Cr3W3CoVNbBN steel also show improved creep behavior at 650°C. Creep rupture strength is comparable to the mean base material creep strength of the best commercially available grade P92.