Nucleation Stages of Isothermal Transformation in Titanium and Titanium Free Micro-Alloyed Steels

This work is focused on nucleation stages during isothermal austenite transformation in two types ofVanadium micro-alloyed steels. Isothermal treatment wascarried out in the temperature range 350 to 600C.Metallographic evaluation using optical and scanningelectron microscopy (SEM) enabled determination of thenucleation curves of isothermally decomposed austenite.Three curves are found to be relevant to this initiation stageof transformation: first curve is related to grain boundarynucleated ferrite (GBF), second curve is related to intragranularly nucleated ferrite (IGF) and the third to pearlite(P) curve. GBF and IGF curves are divided into two regions,which represents the high temperature and the lowtemperature segments as consequence of either displacivenature of transformation or diffusion transformation.Addition of Ti to V – micro-alloyed steel in this work seemsto be balanced by a slightly higher C and Mn content,leading to limited effect on nucleation stage of austenitedecomposition.