Microstructure and oxidation properties of 9Cr–1.7W–0.4Mo–Co ferritic steel after isothermal aging

Microstructural evolutions of 9Cr–1.7W–0.4Mo–Co ferritic heat-resistant steel after isothermal aging were investigated by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The kinetic models are employed to describe the mechanisms of aging hardening and oxidation resistance. The results show that the recovery of martensite lath occurs, and the precipitates with large particle size forms during aging. The alloying elements in the interior of δ-ferrite contribute to the formation of the precipitates located at grain boundaries. The increase of W element gathered on the grain boundaries indicates the forthcoming formation of Laves phase. Relative percentage of hardening firstly increases, and then decreases with the increase of tempering parameter. Both the power exponent, n, and the oxidation rate, k_p, increase with the increase of aging time. In addition, the values of n are less than 2, and the value of k_p is from 0.012 mg/cm²/h up to 0.049 mg/cm²/h with the increase of isothermal aging time. - Highlights: • Alloying elements in δ-ferrite contribute to the precipitates located at grain boundaries. • W gathers on the grain boundaries of δ-ferrite and martensite during aging. • Relative percentage of hardening increases, then decreases with the increase of tempering parameter. • Both the power exponent and the oxidation rate increase with the increase of aging time.