Microstructural Characterization of Precipitation in an Isothermally Aged Nb-containing Microalloyed Steel

In order to control the grain size by the addition of microalloying elements, it is important to realize that the sequence of casting, reheating, control rolling and subsequent weld cycles may produce a complex series of precipitation reactions. To characterize these reactions in a quantitative manner, it is essential to attempt to define the structure, morphology, composition, size and distribution of precipitate phases produced during the thermal processes. The role of precipitate particles is to refine austenite grains by the pinning of the boundaries during normalizing heat treatments or during thermomechanical working, and to harden the ferrite grains on cooling after transformation. The goal of present work is to characterize chemically, structurally and microstructurally the precipitate phases formed in a Nb-containing microalloyed steel during isothermal aging by means of X-ray diffraction (XRD), scanning and transmission electron microscopes (SEM and TEM) and EDS microanalysis.