Influence of precipitates evolutions in δ-ferrite and austenite matrix on mechanical properties of alumina-forming austenitic steel

Alumina-forming austenitic (AFA) steel with great application potential in components of power plants has been developed due to its excellent high-temperature properties and corrosion resistance. Evolution of precipitates was characterized using multiple methods, and its effect on mechanical properties of Fe–20Ni–14Cr–3Al alumina-forming austenitic (3Al-AFA) steel after isothermal-aging at 973K was also assessed. The results showed that the particle size and volume fraction of precipitates (mainly Laves, B2–NiAl and σ phase) increase with the duration of isothermal-aging. The volume fraction of δ-ferrite increases significantly also with the extension of isothermal-aging time, which is regarded as a harmful factor to mechanical properties of AFA steel. The strength and Vickers hardness firstly increase and then decrease with the extension of isothermal-aging time, which is inversely proportional to the elongation at the initial stage of isothermal-aging. Both strength and elongation decrease with the isothermal-aging time between 500 h and 1000 h due to the δ-ferrite and coarse precipitates therein. Cr element firstly prone to segregate and diffuse in δ-ferrite, which promotes the diffusions of other alloying elements from matrix to δ-ferrite, resulting in the coarsening of precipitates in δ-ferrite region. The highest yield strength (1171 MPa) is achieved when the size of Laves and B2–NiAl precipitates in matrix is about 100–150 nm.