Microstructure evolution and martensitic transformation behaviors of 9Cr-1.8W-0.3Mo ferritic heat-resistant steel during quenching and partitioning treatment

The advanced quenching and partitioning (Q&P) heat treatment has been applied to 9Cr-1.8W-0.3Mo heat resistant steel. The phase transformation during Q&P is measured by a high-resolution differential dilatometer by which the accurate information can be obtained. The transmission electron microscope examination was conducted to study the microstructure evolution after Q&P, and the refined carbon-enriched martensite laths, which were produced during the second martensitic transformation, were observed. The thermodynamics of carbon partitioning was described by a paraequilibrium model according to which the partitioning of carbon from martensite into austenite can be proved. A kinetic model for the second martensitic transformation was developed with the parameters discussed in details. The retardation of onset and end temperature of the second martensitic transformation can be ascribed to the austenite stabilization caused by carbon enrichment.