The role of Laves phase on microstructure evolution and creep strength of novel 9%Cr heat resistant steels

The influence of Laves phase (type Fe2W) formation and growth kinetics on the creep strength at 650 °C of two different 9%Cr heat resistant steels of the authorsʹ design was investigated. The microstructure evolution was characterized using transmission electron microscopy in the scanning mode (STEM). Kinetic modeling was carried out using the software DICTRA. STEM investigations revealed that the Laves phase precipitates tend to form clusters, have an irregular shape and are often located close to M23C6 carbides, along martensite lath boundaries or sub-grain boundaries. DICTRA simulations showed that the growth kinetics of Laves phase was high in the first thousand hours of creep, reaching its equilibrium volume fraction after 7000–10,000 h. Simultaneous competitive growth of M23C6 carbides and Laves phase was simulated showing that Laves phase grows very slowly to reach the final equilibrium phase fraction only after almost 13,000 h, while M23C6 reached the equilibrium phase fraction already during tempering. Best creep results were obtained for the 9%Cr alloy with low carbon content, reduced sub-grain growth, very slow coarsening of MX carbonitrides and slow growth of Laves phase.