Microstructures of isothermally transformed Fe-Nb-C alloys

Abstract

The microstructures of an Fe-Nb-C alloy, with and without boron, developed by the γ → α transformation during isothermal holding were examined mainly by metallographic techniques. Ferrite formed above about 750°C is equiaxed or polygonal, while Widmanstätten ferrite is developed at lower transformation temperatures increasingly replacing polygonal ferrite, as commonly observed in low alloy steels. The carbide precipitation changes with transformation temperature. Above about 800°C, the structure is not uniform, i.e. almost carbide free ferrite is formed at first and later coarse interphase precipitation takes place. Interphase precipitation occurs readily around 800°C, while uniform precipitation of NbC from supersaturated ferrite is the principal structural change around 730°C. At the lower temperatures, ferrite often contains a high density of dislocations and coarse cementite. The observed structural changes are explained in terms of the γ → α reaction kinetics and the partition of carbon to austenite during transformation.