Effect of Boron on Microstructure and Creep Strength ofAdvanced Ferritic Power Plant Steels

The boundary and sub-boundary hardening is shown to be the most important strengthening mechanism in creep of 9Cr steel. The soluble boron reduces the coarsening rate of M23C6 carbides along boundaries near prior austenite grain boundaries during creep, enhancing the boundary and sub-boundary hardening for up to long times. The enhancement of boundary and sub-boundary hardening retards the onset of acceleration creep, which decreases the minimum creep rate and improves the creep life.Excess addition of boron and nitrogen promotes the formation of boron nitrides during normalizing heat treatment, which significantly reduces soluble boron and nitrogen concentrations and offsets the benefit due to boron and nitrogen.