Mechanisms for boron effect on microstructure and creep strength of ferritic power plant steels

Abstract

<title/>

Boundary and sub-boundary hardening are shown to be the most important strengthening mechanism in creep of 9%Cr steel. Soluble boron reduces the coarsening rate of M₂₃C₆ carbides near prior austenite grain boundaries during creep, enhancing the boundary and sub-boundary hardening for long times at 650°C. 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 normalising heat treatment, which significantly reduces soluble boron and soluble nitrogen concentrations. The boundary and sub-boundary hardening are significantly reduced in the fine grained region of the heat affected zone (HAZ) of Gr.92 welded joints, promoting type IV fracture. Soluble boron produces substantially the same microstructure between base metal and HAZ in 9Cr steel welded joints, resulting in no type IV fracture at 650°C.

Keywords:

• 9Cr steel
• Creep deformation
• Boron
• Boron nitride
• M23C6 carbide
• Boundary hardening