Hardening (computing)

Boron

Metallurgy

Materials science

Carbide

Layer (electronics)

Creep

Grain boundary

Base metal

Organic chemistry

Microstructure

Austenite

Composite material

Chemistry

Welding

Energy Materials

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 steelCreep deformationBoronBoron nitrideM23C6 carbideBoundary hardening

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