Effect of nickel and cobalt addition on the precipitation-strength of 15Cr ferritic steels

The effect of nickel and cobalt addition on the microstructure of 15 mass% chromium (15Cr) steels and their mechanical properties, such as creep strength and impact toughness, was investigated. The microstructure of 15Cr ferritic steel changes from a ferrite single phase to dual phases of ferrite and martensite by the addition of nickel, which is an austenite-stabilizing element. The creep strength and precipitation behavior of intermetallic compounds (Laves phase, χ-phase and μ-phase) during creep exposure at elevated temperature were strongly influenced by the addition of nickel and cobalt. An increase in cobalt content led to an increase in creep strength by the formation of fine particles and a homogeneous distribution of intermetallic compounds precipitated in the ferritic matrix. The creep rupture life of 15Cr steel at 923 K and 140 MPa was approximately 100 times longer than that of conventional 9Cr ferritic steels with a tempered martensitic microstructure strengthened by carbides and carbonitrides.

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► Co addition in 15Cr ferritic steels improved both Charpy impact toughness and creep strength.
► Creep rupture lives of 15Cr ferritic steels are 100 times longer than that of conventional steels.
► Co addition reduced particles size of intermetallic compounds precipitated in ferritic matrix.
► Homogeneous distribution of intermetallic compound precipitations was enhanced by Co addition.